HIGHLY ALLOYED STAINLESS STEEL FORGINGS MADE WITHOUT SOLUTION ANNEAL

Abstract

The post-forging solution anneal step normally carried out on hot forgings made from highly alloyed metals can be eliminated while still avoiding the formation of deleterious intermetallic phases by adopting a number separate features in connection with the way the forging is made.

Description

RELATED APPLICATION

This application is related to application U.S. Ser. No. 15/371,455, filed Dec. 7, 2016, for HIGHLY ALLOYED STAINLESS STEEL FORGINGS MADE WITHOUT SOLUTION ANNEAL, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

FIG. 1 schematically illustrates a conventional process for producing multiple hot forgings in series. As shown in this figure, billet 10, which is typically derived by sectioning forging stock obtained from the foundry, is heated to forging temperature in heater 12. If desired, pyrometer 14, which is aimed at the side wall of the billet as it exits heater 12, can be used to monitor and record the billet's temperature as it exits the heater for record purposes. Heated billet 10 is then moved to a forging station 16 where a series of forging dies forge the billet into the desired shape, thereby producing the hot forging. In some case, two or more forgings can be made from a single billet. In other instances, a single forging is made from each billet.

The heated hot forging so made is then rapidly quenched from its elevated temperature, usually by immersion in quench tank 18 containing water or other cooling liquid, while any flash that might have been created is deposited in a scrap bin, not shown. Completed metal parts are then typically made by subjecting the hot forging so made to some sort of shaping operation, such as machining or the like.

Although FIG. 1 shows only one hot forging being made, it will be understood that in actual commercial practice, the process of FIG. 1 is continuous in the sense that multiple hot forgings are made in series by repeating the process shown there over and over again. In addition, the process may be automated in the sense that a suitable automatic control system (not shown) is employed to control one or more operations of the process including movement of billet 10 through each station of the process as well as the operation of one or more pieces of equipment in each station.

Highly alloyed metals, i.e., metal alloys containing substantial amounts of additional metal elements other than their base metal elements, exhibit many desirable properties including superior strength and corrosion resistance. However, they can be very sensitive to the formation of undesirable intermetallic phases when heated to elevated temperature.

This is illustrated in FIG. 2, which is an isothermal time-temperature-transformation (TTT) diagram for such an alloy, in particular an AISI-2205 duplex stainless steel. When such an alloy is maintained at conditions of time and temperature within the envelop of its particular TTT curve the individual elements forming the alloy tend to segregate from one another, with some of these elements combining with one another to form discrete intermetallic phases. Thus, each curve in FIG. 2 shows how much (i.e., 1%, 3%, 5%, and 10%) of the deleterious intermetallic sigma phase will form in this alloy when held at a particular temperature for a particular time. For example, FIG. 2 shows that holding this alloy at a temperature of about 860° C. for a soak period of about 2 minutes leads to the precipitation of 1% sigma phase in the alloy. Likewise, holding this alloy at this same temperature for about 7 minutes at 860° C. causes the formation of 5% sigma phase.

Extrapolation of the upper portion of TTT-curves to very long times yields an upper critical temperature above which intermetallic phases are thermodynamically unstable. Extrapolation of the lower portion of TTT-curves to very long times leads to a lower critical temperature below which intermetallic phases do not form for kinetic reasons. The temperature range defined by the upper and lower critical temperatures is called the critical temperature range for intermetallic phase formation. If the alloy is held at a temperature above the critical range, all of the elements in the alloy including those already present in intermetallic phases, tend to redistribute themselves into a uniform solid solution. Meanwhile, once the alloy is at a temperature below the critical range, the elements in the alloy are completely immobile with respect to one another no matter how long the alloy is held at that temperature.

The presence of these intermetallic phases in more than insignificant amount is detrimental to the properties of the alloys of interest in this disclosure. These intermetallic phases, which are typically rich in chromium and molybdenum, adversely affect the corrosion resistance of the areas immediately surrounding these phases which become depleted in these elements. In addition, these intermetallic phases can also substantially lower the impact resistance of the alloy.

For this reason, the ASTM A182 standard specification, as well as the NORSOK M-650 supply chain qualification standard, require that forgings made from duplex and super duplex stainless steels be subjected to a post-forge solution anneal. The NORSOK M-650 standard also requires post-forge solution annealing of forgings made from super austenitic 6-moly alloys, i.e., austenitic stainless steels with at least 6% molybdenum. Because of these requirements, it is standard practice in industry to subject hot forgings made from these and other highly alloyed metals to a conventional solution anneal after the forging has cooled to room temperature, or at least a “safe” temperature below the alloy's lower critical temperature. Normally, post-forge solution annealing is done by heating the forging up to an elevated temperature above its upper critical temperature, maintaining the forging at this elevated temperature long enough to dissolve any intermetallic phases that might be present, and then cooling the forging to below its lower critical temperature rapidly enough so that formation of new intermetallic phases is avoided or at least minimized.

This rapid cooling step is illustrated in FIG. 3, which shows continuous cooling transformation curves, or “CCT curves,” for this alloy (2205). For example, FIG. 3 shows that, if this alloy is cooled from 950° C. to below 600° C. within roughly 25 minutes according to the cooling regime represented by the solid line in this figure, it will develop about 1% deleterious sigma phase. On the other hand, if the alloy is cooled by the cooling regimes represented by the other lines in this figure, it will develop about 3%, 5% or even 10% of this deleterious sigma phase depending on which cooling rate is followed.

Because of the time and complexity involved, post-forge solution annealing is expensive. In addition, it may also lead to various technical and commercial problems such as surface oxidation, lower mechanical properties due to grain growth, added production time and cost and negative environmental impact including consumption of energy and cooling water. Accordingly, it would be desirable to eliminate this step, if possible.

SUMMARY

In accordance with this invention, it has been found that the post-forging solution anneal step normally carried out on hot forgings made from highly alloyed metals can be eliminated while simultaneously avoiding problems associated with detrimental intermetallic phases by adopting a number of specific features when the forging is made.

Thus, this invention provides an improvement in continuous, automatic processes for making multiple hot forgings in series from multiple billets made from a highly-alloyed metal, the improvement wherein the hot forgings are made without subjecting these hot forgings to post-forging solution anneal.

In addition, this invention also provides an improvement in processes for making metal parts which are useful in one or more applications including chemical processing, scrubbers, pulp mills, bleach washers, food processing and oil field piping, in which process a hot forging made from a highly-alloyed metal is shaped into a metal part, the improvement comprising shaping the hot forging into the metal part without subjecting this hot forging to post-forging solution anneal.

In addition, this invention also provides an improvement in metal parts which are (a) made by shaping a hot forging of a highly-alloyed metal and (b) useful in one or more applications including chemical processing, scrubbers, pulp mills, bleach washers, food processing and oil field piping, the improvement wherein the metal part is made without subjecting the hot forging to post-forging solution anneal.

Finally, this invention also provides a process for making a metal part from a billet, the process comprising

• (a) heating a billet of a highly-alloyed metal up to its forging temperature in a manner so that the heated billet is essentially free of intermetallic phases,
• (b) forging the billet in a manner so that the forging obtained is essentially free of intermetallic phases,
• (c) cooling the forging to below its critical temperature range CTR rapidly enough so that it is essentially free of intermetallic phases, and
• (d) forming the metal part by machining the hot forging of step (c) without subjecting this hot forging to post-forging solution anneal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a conventional hot forging process; and

FIG. 2 is an isothermal time-temperature-transformation (TTT) diagram for an AISI-2205 duplex stainless steel alloy; and

FIG. 3 is a diagram of the continuous cooling transformation curves, or “CCT curves,” for the alloy of FIG. 2; and

FIG. 4 is a thermal history diagram illustrating the relationship between time and temperature in the manufacture of a hot forged product in accordance with the invention; and

FIG. 5a is a photomicrograph of the microstructure of a hot forging produced without a post-forging solution anneal in accordance with this invention; and

FIG. 5b is a photomicrograph of the microstructure of a comparative hot forging otherwise identical to that of FIG. 5a but which was produced with a post-forging solution anneal in accordance with conventional practice.

DETAILED DESCRIPTION

Definitions

Unless otherwise clear from context, the following terms used in this disclosure will have the following meanings:

“Automatic” and “automatically” as they relate to processes for making multiple hot forgings from multiple billets means that one or more automatic control systems are used to control at least some portion of the operation of the process including the movement of billets and forgings through each station of the process as well as the operation of the equipment in each station. Such processes can be fully automatic meaning that all portions of the operation of the process are controlled by the automatic control systems or semi-automatic meaning that some but not all portions of the operation of the process are controlled by the automatic control systems.

“Billet” means the piece of metal on which the forging steps of the inventive process are carried out. Normally, a billet is obtained by subdividing a piece of forging stock into sections of appropriate size.

“Discharged to waste” means that the billet or forging being referred to does not become, or form part of, a final product produced by the inventive process. It does not mean that the billet or forging is abandoned completely, as most such billets and forgings will be used for scrap or some other purpose.

“Essentially free of intermetallic phases” means a concentration of intermetallic phases in a metal product which is so small that it does not adversely affect the properties of the metal product in any significant way. Many commercial metal products, including intermediate products, are made to have a desired set of properties as determined by product specifications for that particular metal product. With respect to these products, a metal product which is essentially free of intermetallic phases will be understood to mean a metal product which, although possibly containing intermetallic phases which may adversely affect its properties, contains these intermetallic phases in concentrations which are so small that the metal product still meets its product specifications.

“Forging stock” means a metal product which has been obtained by subjecting an ingot to one or more metal working operations such as hot or cold rolling, forging or the like to reduce its thickness dimension. In some instances, metal working will be done at the foundry, while in other instances, metal working will be done at a separate forging shop. Typically, forging stock will be in the form of a rod, bar or strip whose length exceeds its thickness.

“Highly alloyed metal” means a metal alloy which is formed from a base metal such as Fe or Ni and which includes a substantial amount of one or more other metal elements such that the metal alloy tends to form intermetallic phases when heated to elevated temperature.

“Hot forging” means a metal product whose shape has been obtained, at least in part, by subjecting a metal billet which has been heated to a forging temperature above its upper critical temperature for intermetallic phase formation to substantial localized compressive forces. These substantial localized compressive forces are normally delivered by a hammer or other suitable implement, but may also be delivered by deforming the billet between two mating dies. Specific forging operations include roll forging, swaging, cogging, open-die forging, closed-die forging, impression-die forging, press forging, automatic hot forging, radial forging, and upset forging. For the sake of clarity, “hot forging” does not imply that the forging is hot—only that temperature at which forging was carried out, the forging temperature, was above the critical temperature mentioned above.

“Ingot” means the metal product obtained when a molten metal is solidified. When such a product is continuously cast, “ingot” will also be understood to include longitudinal sections of such a product. “Ingot” is intended to distinguish products which are obtained by reducing the thickness of an ingot by some form of hot or cold working procedure such as hot or cold rolling, forging, and the like.

Highly Alloyed Metals

The inventive forging process is carried out on highly alloyed metals.

Thus, in one embodiment, the inventive forging process is carried out on ferrous based alloys which contain significant amounts of additional metal elements other than Fe. Examples include martensitic stainless steels, ferritic stainless steels, austenitic stainless steels, highly alloyed austenitic stainless steels, super austenitic stainless steels, and austenitic-ferritic stainless steels such as lean duplex, duplex, super duplex, and hyper duplex stainless steels.

Steels of particular interest in this regard are those exhibiting a CP value of 500 or more. The CP value is a relative indication of the kinetics of precipitation of intermetallic phases in an alloy. It is described in U.S. Pat. No. 5,494,636, the disclosure of which is incorporated herein by reference. The CP value of an alloy can be calculated using the following formula, wherein the percentages are weight percentages based on total alloy weight:

CP=20×% Cr+0.3×% Ni+30×% Si+40×% Mo+5×% W+10×% Mn+50×% C−200×% N.

Alloys having CP values of less than 500 are not especially prone to developing deleterious intermetallic phases rapidly when heated to elevated temperatures. On the other hand, those which exhibit CP values of 500-700 show some tendency to do so, while those exhibiting CP values of 700-750 are even more prone to do so. Meanwhile alloys having CP values of 750-800 and above are especially prone to develop these deleterious intermetallic phases when heated to elevated temperatures. In accordance with this invention, billets made from all such alloys and especially those exhibiting CP values of 500-700, 701-750 and 751-800 and above can be used as raw materials for the inventive process to make hot forgings exhibiting little or no deleterious intermetallic phases, even though such forgings have been made without a post-forging solution anneal.

Of special interest in this regard are the superstainless steels, i.e., stainless steels which contain about 19 to 26 wt. % Cr and 3 to 8 wt. % Mo.

Basically, there are two types of superstainless steels, those that exhibit an austenitic phase structure and those that exhibit a duplex phase structure. Superstainless steels exhibiting an austenitic phase structure normally contain about 19 to 25 wt. % Cr and 5 to 8 wt. % Mo and are sometimes referred to as “super-austenitic 6-moly alloys.” Examples of such steels include AISI-N08367 (alloy 6XN or AL6XN), AISI-531254 (alloy 254), AISI-N08925 (alloy 1925hMo) and AISI-531266 (alloy B66). Superstainless steels exhibiting a duplex phase structure normally contain about 24 to 26 wt. % Cr and 3 to 5 wt. % Mo and are sometimes referred to as “superduplex” stainless steels. Examples of such steels include AISI-S32750 (alloy 2507) and AISI-S32760 (alloy Zeron 100).

Additional metal alloys on which the inventive forging process can be carried out are the nickel based alloys which contain at least about 2 wt. % Mo and at least 18 wt. % Cr. Specific examples include alloys AISI-N0820 (alloy C20 or “Carpenter 20”), AISI-N08031 (alloy 31) and AISI-N08825 (alloy 825).

Still other alloys on which the inventive forging process can be carried out are the “super-austenitic 7 moly” alloys, examples of which include AISI-S32654 (alloy 654) and AISI-S31277 (alloy 27-7Mo), the “highly alloyed austenitic” stainless steels such as AISI-N08904 (alloy 904L), the “lean duplex” alloys such as AISI-S32101 (alloy LDX 2101), regular duplex alloys such as AISI-S32205 (alloy 2205), the “hyper duplex” alloys such as AISI-S33207 (alloy SAF 3207) and the well-known “conventional” austenitic stainless steels such as AISI-S31600 (alloy 316) and AISI-S31700 (alloy 317).

Finally, also of special interest are all of the alloys identified in ASTM A182 which are said to require solution annealing. See, especially, Table 1 of this ASTM test method.

Starting Material

In accordance with this invention, it has been found that hot forgings made from highly alloyed metals which exhibit a desirable combination of properties including mechanical strength and superior corrosion resistance can be produced without subjecting the forging to solution annealing after it has been made, as previously thought necessary, by adopting a number of separate features in connection with the way the forging is made.

The first of these features, which may be regarded as optional and conventional but is still important, relates to the manner in which the billet on which the inventive process is carried out is selected. In accordance with this feature, only those billets that are essentially free of intermetallic phases are selected for this purpose. In other words, billets which are not essentially free of intermetallic phases are rejected for carrying out this invention, and as further discussed below.

As indicated above in connection with FIG. 2, when a highly-alloyed metal such as a duplex stainless steel is maintained within its critical temperature range for a sufficiently long time (i.e., within the envelop defined by its particular TTT curve), some of the elements forming the alloy will combine with one another to form discrete intermetallic phases. These intermetallic phases, if present in more than insignificant amounts, are the underlying reason why a hot forging made from such an alloy exhibits poor properties. Therefore, it would seem to make sense to start with forging stock which had already been fully solution annealed at the foundry or forging shop to remove these intermetallic phases.

However, in some instances in commercial practice, the forging stock is not necessarily in a fully solution annealed state when obtained from the foundry or forging shop. For many alloys, the elevated temperatures commonly used to hot work an ingot, or to hot-roll barstock, of the alloy into forging stock are roughly the same as the elevated temperatures needed to solution anneal the alloy. In addition, it is common practice in the foundry or forging shop to rapidly quench forging stock as part of its manufacturing operation. As a result, the assumption is normally made that the forging stock obtained is already essentially free of intermetallic phases.

However, the hot working temperatures actually used in particular foundry operations may be less than the minimum temperature required to achieve effective solution anneal. In addition, rapid quenching may not have been rapid enough. So, there is a real risk that such forging stock will contain substantial amounts of intermetallic phases, since its thermal history may have been insufficient to remove all of these phases.

In other instances in commercial practice, the forging stock is solution annealed at the foundry or forging shop before being shipped to the customer. Sometimes, the foundry or forging shop assures the customer that this forging stock is free of intermetallic phases because it was solution annealed before being shipped. However, in these situations, there is still a risk that the solution annealing process actually carried out was insufficient to remove essentially all of the intermetallic phases that might have been present.

In still other instances in commercial practice, the foundry or forging shop provides the customer with a certified analysis of the composition, properties and phase structure of the forging stock being delivered based on actual analytical tests carried out on this particular piece of forging stock or on representative samples of this forging stock. In these situations, the risk that the forging stock received still contains intermetallic phases is less.

In accordance with this first optional feature of the invention, care is taken to ensure that only those billets that are essentially free of intermetallic phases are selected for use in this invention by rejecting billets which are not essentially free of these phases. In practical terms, this cannot be done solely by relying on the processing history and/or assurances provided by the foundry and/or forging shop relating to solution anneal of the forging stock from which these billets are derived. Rather, one or more additional steps are necessary to conclude that this forging stock, and hence the billets derived from this forging stock, are in fact essentially free of these intermetallic phases.

In accordance with one of these additional steps, a certified analysis of the phase structure of the billet, the forging stock from which the billet is derived, or at least representative samples of this forging stock are necessary before it can be assumed that the billet selected for use in the inventive hot forging process is, in fact, essentially free of intermetallic phases. The party carrying out the inventive hot forging process can, itself, obtain such a certified analysis. Additionally or alternatively, the party carrying out the inventive hot forging process may also rely on such a certified analysis obtained from its foundry/supplier in those circumstances in which the party finds it reasonable to do.

Regardless of the particular procedure adopted, it is important that the starting material of the inventive process, i.e., the billet on which the inventive hot forging process is practiced, be selected to be essentially free of the intermetallic phases that give rise to poor alloy properties in the ultimate hot forging product produced.

In certain embodiments of this invention, still another step that can be taken to help insure that the hot forgings produced by the inventive process are essentially free of intermetallic phases is to restrict the maximum thickness of the hot forging which is produced. Because of inherent heat transfer restrictions, the larger a forging becomes, the more difficult it is to rapidly cool its core. What this means in practical terms is that, as the thickness of a forging becomes larger, cooling the core of the forging after solution anneal rapidly enough to prevent intermetallic phases from forming becomes more difficult or even impossible. So, in some embodiments of this invention, the maximum thickness or diameter of the forging being made is restricted to a value which is small enough to avoid this heat transfer problem from occurring.

Accordingly, in these embodiments of the invention, the maximum thickness or diameter of the hot forging being made, and hence the maximum thickness or diameter of the billet from which it is made, is restricted to 25 centimeters, 20 centimeters, 15 centimeters, 12 centimeters, 9 centimeters, 6 centimeters, 5 centimeters, 4 centimeters or even 3 centimeters.

Heating the Forging Stock at the Foundry or Forging Shop

As indicated above, one way of ensuring that only those billets that are essentially free of detrimental intermetallic phases are selected for use in this invention is to confirm that an appropriate solution anneal is carried out at the foundry or forging shop on the forging stock from which the billet is derived. This is illustrated in FIG. 4, which is a thermal history diagram illustrating the relationship between time and temperature in the manufacture of hot forgings in accordance with this invention.

Starting at point 20 in which the forging stock is at or near room temperature, the forging stock is subjected to a solution anneal procedure at the foundry or forging shop in which it is first heated to point 22 which is above the critical temperature range CTR of the alloy. It is then maintained at this temperature for a suitable period of time to ensure that essentially all of the deleterious intermetallic phases that might be present in the alloy redissolve. At that time, when point 24 is reached, the forging stock is then rapidly cooled, typically by quenching with water or other cooling liquid, back down to room temperature at point 26. As can be seen from this figure, during this rapid quenching procedure, the forging stock spends a certain amount of time, denoted by Δt₁, in the critical temperature range CTR of the alloy.

As discussed above, it is desirable in accordance with this invention to ensure that the billets on which the inventive process is carried out are essentially free of detrimental intermetallic phases. What this means in terms of the forging stock illustrated in FIG. 4 is that the period of time the alloy is held above its critical temperature range CTR, as represented by line extending between points 22 and 24, is long enough to redissolve any deleterious intermetallic phases that might have been originally present in this forging stock and, in addition, that Ati is short enough so that essentially no detrimental intermetallic phases form as this forging stock cools through its critical temperature range CRT.

Heating Step of this Invention

Turning now to this invention, the heating step of the inventive process is represented by the line extending between points 28 and 30 in FIG. 4. In accordance with a second feature of this invention, this heating step is done in a manner so that when it is completed, which will normally be when the heated billet is removed from heater 12 in FIG. 1, the heated billet obtained is essentially free of deleterious intermetallic phases.

Preferably, this is done by following a number of specific practices, as further discussed below. The first of these practices is to heat the billet to its forging temperature as rapidly as possible, at least during the time period the billet is within its critical temperature range denoted by Δt₂ in this figure. For this purpose, heating will normally be done by resistance or induction heating, since radiant heating inside a furnace is just too slow.

As indicated above, in current commercial practice, especially when the billet being used has not been solution annealed at the foundry or forging shop, it is uncertain whether the billet is essentially free of deleterious intermetallic phases. For this reason, it is not uncommon for these billets to be heated up to forging temperatures slowly, as this prevents cracking during heat up of billets containing these intermetallic phases. Indeed, one manufacturer expressly recommends that at least one of its highly-alloyed metal products, super duplex 2507 stainless steel, be heated slowly for this reason. See, http://smt.sandvik.com/en/materials-center/material-datasheets/billets/sandvik-saf-2507/. During this slow heating, additional intermetallic phases will likely form when the billet is within its critical temperature range.

In contrast to this approach, in the inventive process, the billet is heated up as rapidly as possible to avoid formation of any new intermetallic phases to the greatest extent possible and hence ensure that the heated billet is essentially free of deleterious intermetallic phases. One way this can be done in a continuous, automatic process such as illustrated in FIG. 1 is to determine a minimum acceptable heating rate for each particular alloy being processed and to discharge to waste any billet whose heating rate fails to achieve this minimum.

For any particular alloy, for example for duplex alloy 2205 whose TTT and CCT curves are illustrated in FIGS. 2 & 3, a determination can be made that the rate of billet heating should be some predetermined minimum such as, for example, at least 400° F./min. (204° C./min.), at least 500° F./min (260° C./min.), at least 600° F./min. (333° C./min.), at least 700° F./min. (371° C./min.), or even at least 800° F./min. (427° C./min.) during the time period over which the temperature of the billet is in its critical temperature range CTR, or at least some predetermined portion of its critical temperature range CTR) such as, for example, from 800° C. (1472° F.) to 900° C. (1652° F.) for the alloy of FIGS. 2 and 3. Every billet whose actual heating rate fails to achieve this minimum is then automatically discharged to waste, thereby ensuring that formation of new intermetallic phases is avoided, reliably and consistently, time after time, for each billet being heated.

Determining billet heating rate for this purpose can conveniently be done using the temperature of the front or rear face of the billet as it is being transported through heater 12. For this purpose, a pyrometer focused on the center of the billet face can be used. Alternatively or additionally, a thermocouple attached to the center of the face or received in a hole drilled in the face can be used. In a continuous automatic process such as shown in FIG. 1, the automatic control system of the process, or a separate automatic control system, can be programmed to cause an automatically-operated gate or other suitable piece of equipment to discharge to waste each billet whose heating rate fails to conform to the predetermined heating rate minimum.

Another approach that can be used for ensuring that formation of new intermetallic phases is avoided during billet heat-up is to control billet temperature at the end of this heating step. One way this can be done is to determine minimum and maximum acceptable billet temperatures at the end of this heating step and to discharge to waste any billet whose actual temperature is less than this minimum or greater than this maximum.

With respect to any particular alloy, for example, alloy whose TTT and CCT curves are illustrated in FIGS. 2 and 3, a determination can be made that the target temperature of the billet at the end of this heating step should be some predetermined value such as, for example, 1,900° F. (1,038° C.), 2,000° F. (1,093° C.), 2,100° F. (1,149° C.), 2,200° F. (1,204° C.), 2,300° F. (1,260° C.), 2,400° F. (1,316° C.) or even 2,500° F. (1,371° C.) and, in addition, that minimum actual temperature at the end of this cycle should not be less than this target temperature by a first predetermined temperature difference such as, for example, 150° F. (83° C.), 100° F. (56° C.), 75° F. (42° C.), 50° F. (28° C.) or even 25° F. (14° C.). Every billet whose actual temperature at the end of the heating step is below this minimum is then automatically discharged to waste, thereby further ensuring that formation of new intermetallic phases is avoided, reliably and consistently, time after time, for each billet being heated.

To determine billet actual temperature at the end of the heating step, the measured temperature of the side wall of the billet as it exits its heater, as described above in connection with FIG. 1, can be used. Alternatively or additionally, the measured temperature of the front or rear face of the billet, as described immediately above, can also be used.

Still another approach that can be used for ensuring that formation of new intermetallic phases is avoided during billet heat-up is to compare the measured temperature of the side wall of the billet as it exits its heater with the measured temperature of the front or rear face of the billet as it exits its heater and to discharge to waste all billets in which the difference between these two measured temperatures exceeds a predetermined maximum. For example, with respect to the particular alloy whose TTT and CCT curves are illustrated in FIGS. 2 and 3, a determination can be made that the difference between these two measured temperatures should not exceed a second predetermined temperature difference such as, for example, 200° F. (111° C.), 150° F. (83° C.), 100° F. (56° C.), 75° F. (42° C.), or even 50° F. (28° C.). Every billet in which the difference between these two measured temperatures exceeds this predetermined maximum is then automatically discharged to waste, thereby further ensuring that formation of new intermetallic phases is avoided reliably and consistently time after time for each billet being heated.

A still further approach that can be used for ensuring that formation of new intermetallic phases is avoided during billet heat-up is to continue heating the billet above its critical temperature range long enough to redissolve any intermetallic phases that may be present. For example, keeping the temperature of the billet above its upper critical temperature for a longer period of time than the billet was within its critical temperature range during billet heat-up (or at least within the most critical portion of this critical temperature range) will generally ensure that any deleterious intermetallic phases that might have formed during heat-up are eliminated before the forging step begins.

For example, with respect to the particular alloy whose TTT curve is illustrated in FIG. 2, heating the billet at a temperature which is above the upper arm of its critical temperature range CTR (e.g., from 1000° C. to 1250° C.) for a period of times which is at least 3, at least 4, at least 5, at least 6, at least 7 or even at least 8 times as long as the period of time the temperature of the billet is within the most critical portion of its critical temperature range CTR (e.g., 800° C. to 900° C.) will generally ensure that any intermetallic phases that may still be present in the billet have redissolved.

Although this continued heating approach requires additional time and heating, it is nonetheless also effective in producing heated billets which are essentially free of deleterious intermetallic phases.

From the above, it can be seen that many different approaches can be used in this invention for minimizing the formation of intermetallic phases during billet heat-up including (a) rapid billet heating, (b) controlling billet temperature at the end of the heating step, (c) controlling the difference between the measured temperatures at the side wall and front or rear face of the billet as it exits the heater, and (d) heating the billet above its critical temperature range long enough to redissolve any intermetallic phases that may be present. When the inventive process is carried out in a continuous, automatic mode for producing multiple hot forgings, in addition to approach (a) in which the billet is rapidly heated, one or more of approaches (b), (c) and (d) can also be used. Preferably, the following combinations of approaches are used: (a)+(b), (a)+(c), (a)+(d), (a)+(b)+(c), (a)+(b)+(d), (a)+(c)+(d), and (a)+(b)+(c)+(d).

Avoiding Excessive Billet Heating

As appreciated in the art, billets which have been heated too hot may be difficult to forge and, in addition, may undergo excessive surface oxidation.

In accordance with another optional feature of this invention, steps can be taken to ensure that the billets are not overheated during the billet heating step of this invention. This can be done, for example, by discharging to waste all billets whose actual temperature at the end of the heating step exceeds a predetermined maximum.

For example, with respect to the particular alloy whose TTT and CCT curves are illustrated in FIGS. 2 and 3, a determination can be made that the maximum temperature of the billet at the end of the heating step should not exceed this target temperature by a third predetermined temperature difference such as, for example, 200° F. (111° C.), 150° F. (83° C.), 100° F. (56° C.), 75° F. (42° C.), or even 50° F. (28° C.). Every billet whose actual temperature at the end of the heating step is above this maximum is then automatically discharged to waste, thereby ensuring that the billet has not been overheated.

Another approach that can be used for avoiding billet overheating is to discharge to waste any billet which remains in heater 12 for a holding time which exceeds a predetermined maximum.

For example, with respect to the particular alloy whose TTT and CCT curves are illustrated in FIGS. 2 and 3, a determination can be made that the holding time for billet 10 in heater 12 should not exceed some predetermined maximum such as, for example, 120 seconds, 90 seconds, 75 seconds, 50 seconds, 40 seconds, or even 30 seconds. Every billet in which the actual holding time exceeds this predetermined maximum is then automatically discharged to waste, thereby further ensuring that excessive surface oxidation of the forging produced is avoided.

Forging the Billet

In the next step of the inventive process, the heated billet is converted into a forging by the application of substantial hot working. This can be done by any known hot working technique including roll forging, swaging, cogging, open-die forging, closed-die forging, impression-die forging, press forging, automatic hot forging, radial forging, and upset forging.

In FIG. 4, this forging step is represented by the line extending from points 30 to 32. As shown there, this forging step begins at point 30, when the heated billet is removed from its heating source and ends at point 32 when rapid cooling of the forging produced begins. In accordance with still another feature of the inventive hot forging process, this hot forging step is accomplished in such a way that essentially no intermetallic phases form during this step. This, in turn, is accomplished by insuring that the temperature of this billet/forging, or at least its core, does not drop below the upper boundary of its critical temperature range CTR at any time during this entire forging step.

In commercial practice, this forging step normally involves a number of different operations including removing the billet from its heating source, transferring the heated billet to the forging apparatus, hot forging the billet, removing the forging so formed from the forging apparatus, transferring the forging to its rapid cooling station and initiating rapid cooling by contact with water or other cooling liquid. For a variety of reasons, including lengthy forging operations, large forgings and inefficient processing, completion of this forging step can take several tens of minutes to hours or even longer. However, as illustrated in FIG. 4, as soon as the billet is removed from its heating source at point 30 it begins to cool rapidly. As a result, it is not uncommon in conventional practice that the temperature of the billet/forging, or at least a substantial portion of the billet/forging, drops below the upper boundary of its critical temperature range CRT for a not-insignificant period of time.

If this does occur, in some instances in conventional practice, nothing is done based on the notion that any deleterious intermetallic phases which have been introduced at this time can be removed by the subsequent solution anneal that is always done on these products. More commonly, however, the forging is reheated to cause at least some of the deleterious intermetallic phases which have formed to redissolve before rapid cooling begins.

In accordance with this feature of the inventive process, this conventional practice of relying on subsequent solution anneal and/or reheating the billet/forging during its forging step is avoided as being unnecessary, as keeping the temperature of the billet/forging above the upper boundary of its critical temperature range CRT at all times during this entire forging step insures that no deleterious intermetallic phases form during this time.

In theory, the most straightforward way of insuring that the temperature of billet/forging remains above the upper boundary of its critical temperature range CRT at all times during this entire forging step would appear to be to monitor the temperature of the billet at various times and/or stages of this forging step. In practice, however, this can prove to be impractical for a variety of reasons. Therefore, in terms of process control, the easiest way of insuring that the temperature of billet/forging remains at this desired level is to monitor the time between the start and end of this forging step, i.e., the period of time which elapses between points 30 and 32 in FIG. 4. In addition to monitoring this elapse of time, in some embodiments of this invention, the temperature of the billet at point 30, i.e., the temperature of the billet when it leaves heater 12, can also be monitored as well.

In this regard, for each particular hot forging that will be made, the length of time it will take that billet/forging to cool after leaving its heating source to a temperature defined by the upper boundary of its critical temperature range, CRT, can be readily calculated. Therefore, the easiest way of insuring that the temperature of the billet/forging remains above this upper boundary at all times during the entire forging step is by insuring that rapid quenching of forging begins before this length of time has expired.

Thus, it is contemplated that the entire forging step of the inventive process, from beginning to end, will be carried out in less than 3 minutes, more commonly in less than 2 minutes, less than 90 seconds, less than 75 seconds, less than 60 seconds, less than 45 seconds or even less than 30 seconds. Of course, carrying out this step so quickly normally requires that the billet/forging be fairly small, as a practical matter, which is the case for many hot forgings made from the heavily alloyed metals contemplated by this invention.

In this regard, skilled metallurgists understand that, because of inherent heat transfer limitations, the rate at which the core of a metal workpiece heats or cools is normally slower than the rate at which surface of the workpiece heats or cools. In addition, skilled metallurgists further understand this difference becomes greater as the size of the workpiece becomes larger. Furthermore, skilled metallurgists also understand that a forge hammer or other hot working implement can act as a heat sink, in effect rapidly sucking the latent heat out of the particular surfaces of a billet which are struck by these implements, thereby causing these billet surfaces to cool very rapidly. For these reasons, skilled metallurgists understand that, in the inventive process as in many other metallurgical processes in which a workpiece is being heated or cooled, the temperature of the interior or core of the billet/forgings being processed may be different from the temperature at its surface.

Accordingly, it is important to understand that, in the present invention, when it is said that the temperature of billet/forging remains above the upper boundary of its critical temperature range CRT at all times during this entire forging step, it does not mean that every portion of the billet/forging always remains above this temperature. Rather, what it means is that it is possible that the temperature of some portion of the billet/forging at some point in time during the forging step could drop below this temperature for some period of time. However, if this does happen, nonetheless, the time over which this occurs as well as the portion of the billet/forging in which this temperature drop occurs is so small that its effect on the product hot forging obtained is insignificant in the sense that this product will still meet its applicable product specifications.

Thus, it will be appreciated that, in some embodiments of this invention, especially when the parts being made are small, the temperature of all portions of the billet/forging will remain above the upper boundary of the critical temperature range CRT at all times during this entire forging step. In contrast, in other embodiments, especially when the parts are larger, the core of the billet/forging will remain above the upper boundary of the critical temperature range at all times, while some or all of the outer surfaces of billet forging may drop below this temperature for periods of time which are too short to enable deleterious intermetallic phases to form to any significant degree. It is also possible that in still other embodiments, even the core of the billet/forging may drop below this temperature for a very short period of time. However, this will likely occur only when the billet/forging is very small and, in addition, is less desirable than the other embodiments of the invention in which the temperature of the core, and preferably the temperature of the entire billet/forging is kept above the upper boundary of the critical temperature range CRT at all times during this entire forging step.

Other steps for insuring that the temperature of the billet/forging remains above the upper boundary of its critical temperature range CRT at all times during the entire forging step of the inventive process can also be used, including maintaining the equipment used for carrying out this step, or at least some of it, at elevated temperature which is high enough to prevent rapid cooling from ambient conditions.

From the above, it can be seen that minimizing the time over which the forging step of the inventive process is carried out is yet another approach that can be undertaken by this invention to ensure that the forging obtained at the end of this step is essentially free of deleterious intermetallic phases. For this purpose, when the inventive process is carried out in a continuous, automatic mode for producing multiple forgings in series, the automatic control system of the process, or a separate automatic control system, can be programmed to discharge to waste any forging with respect to which the forging step of the inventive process, as measured from the time when the billet is removed from its heating source to the time when rapid cooling begins, takes longer than a predetermined maximum period of time. For example, with respect to the particular alloy whose TTT and CCT curves are illustrated in FIGS. 2 and 3, a determination can be made that the total time for this forging step should not is some predetermined maximum such as, for example, 120 seconds, 90 seconds, 75 seconds, 50 seconds, 40 seconds, or even 30 seconds. Every billet in which the actual time for this forging step exceeds this predetermined maximum is then automatically discharged to waste, thereby further ensuring that formation of new intermetallic phases is avoided, reliably and consistently, time after time, for each hot forging being made.

Rapid Cooling

Once the forging step of the inventive process is completed, the forging obtained is rapidly cooled to a temperature which is below its critical temperature range CTR. In accordance with still another feature of the inventive process, the rate at which the forging is quenched is fast enough to prevent intermetallic phases from forming in any significant amount.

In FIG. 4, this rapid cooling step is represented by the line extending from points 32 to 34, although in actual practice the forging will normally be rapidly cooled down to a temperature approaching room temperature, as represented by point 36. As shown in FIG. 4, this rapid cooling step begins at point 32, when the hot forging is first contacted with a cooling medium and ends at point 34 when the hot forging has cooled to a temperature which is below the lower limit of its critical temperature range CTR. In accordance with this feature of the inventive process, cooling of the hot forging in this cooling step is accomplished so that the time the hot forging is within its critical temperature range CTR, which is denoted by Δt₄ in this figure, is so short that deleterious intermetallic phases do not have an opportunity to form, at least to any significant degree.

This can be done in any conventional way such as by contacting the hot forging with water or other cooling liquid, either by immersing the hot forging in the cooling liquid, by directing jets or sprays of the cooling liquid at the hot forging, or other suitable procedure.

In some embodiments of this invention, this is accomplished by immersing each hot forging into the cooling liquid individually or as a small number of small forgings having been made from the same billet, rather than as a large number of forgings which are typically quenched together after solution annealing.

In this regard, in conventional practice, the normal way of rapidly quenching hot forgings which are relatively small in size is to arrange a group of the hot forgings in a tray or basket or other holding device and then immerse the tray and all of its contents in the cooling liquid. This approach inherently slows the rapid cooling process down, because the close packing of the hot forgings with respect to one another plus the mass of the tray or other holder makes it more difficult for the cooling liquid to touch and hence remove heat from the surfaces of each hot forging. These problems are avoided if each hot forging is individually immersed into the cooling liquid, which speeds the rate at which cooling of each forging occurs.

Thus, in accordance with this feature of the inventive process, it is contemplated that when the inventive process is carried out to make multiple hot forgings which are relatively small in the sense of having a maximum thickness or diameter of 25 centimeters, 20 centimeters, 15 centimeters, 12 centimeters, 9 centimeters, 6 centimeters, 5 centimeters, 4 centimeters or even 3 centimeters, these hot forgings are rapidly cooled by immersing each in a pool of cooling water or other liquid individually. Normally, multiple hot forgings of this type will be made serially, i.e., one after the other, and so it is further contemplated that these individual hot forgings will be individually immersed in cooling liquid in the same serial fashion, as this not only speeds the rate at which cooling occurs, as mentioned above, but also minimizes the lag time between completion of forging and initiation of rapid cooling for each forging. In addition, to ensure that cooling will be fast enough, it is desirable that the temperature of the water or other cooling liquid used for this purpose be maintained at or below some predetermined maximum such as, for example, 175° F. (79° C.), 150° F. (66° C.), 125° F. (52° C.), 100° F. (38° C.), or even 75° F. (24° C.).

In other situations, multiple forgings are made from the same billet, with multiple billets being processed serially to make these multiple hot forgings. In this situation, because the multiple forgings that are made from the same billet at essentially the same time, the group of forgings made from the same billet can be rapidly cooled together. However, in this instance, it is still preferable to rapidly cool each group of such forgings individually, in the order each group is made.

From the above, it can be seen that rapidly cooling the hot forging to below its critical temperature range CTR is yet another feature that can be undertaken by this invention to ensure that the hot forgings ultimately produced are essentially free of undesirable intermetallic phases. For this purpose, when the inventive process is carried out in a continuous, automatic mode for producing multiple billets in series, the automatic control system of the process, or a separate automatic control system, can be programmed to discharge to waste any forging in which rapid cooling is not accomplished fast enough. For example, a determination can be made that the maximum temperature of the water or other cooling liquid in holding tank 20 should not exceed some predetermined maximum such as, for example, 175° F. (79° C.), 150° F. (66° C.), 125° F. (52° C.), 100° F. (38° C.), or even 75° F. (24° C.). Every forging in which the temperature of the water or other cooling liquid in which the forging is immersed exceeds this predetermined maximum is then automatically discharged to waste, thereby further ensuring that formation of new intermetallic phases is avoided, reliably and consistently, time after time, for each hot forging being made.

Inventive Process as a Whole

From the foregoing, it can be seen that intermetallic phases can form during any stage in the manufacture of a hot forged product, starting with how the forging stock from which the product is derived is processed in the foundry or forging ship and ending with how the product is rapidly cooled after forging. In addition, it can also be seen that an important aspect of the inventive process is that in each of these manufacturing stages, care is taken to eliminate or at least minimize the amount of these deleterious intermetallic phases that form.

To this end, it should be appreciated that what is most important in carrying out the inventive process is that the ultimate hot forged product obtained is essentially free of these deleterious intermetallic phases in the sense that it meets its applicable product specifications. For example, hot forgings made by the inventive process, when formed from the AISI-2205 duplex stainless steel of FIGS. 2 and 3 as well as other highly alloyed steels like the super austentic alloys and the super duplex alloys having a CP value of at least 500, reliably and consistently exhibit a weight loss of no greater than 4 g/m² and no pitting when tested per ASTM-G48, as required by the NORSOK M650 standard.

Accordingly, it should also be appreciated that it is not essential that each step of the inventive process be carried out to minimize formation of these intermetallic phases to the greatest extent possible or avoid formation of these intermetallic phases altogether. Rather, all that is necessary is that a combination of features be adopted, as discussed in this disclosure, so that the concentration of these deleterious intermetallic phases in the hot forged product ultimately produced is low enough so that it still meets its applicable product specifications.

Machined Parts

As well understood in industry, metal parts which are useful in a variety of different applications including chemical processing, scrubbers, pulp mills, bleach washers, food processing, oil field piping, for example, are made from hot forgings by some sort of shaping operation, usually machining. In this context, machining will be understood to mean a process for shaping an object by removing material from a workpiece by some type of cutting or grinding operation. Examples include turning, milling, drilling, tapping, surface grinding, cylindrical grinding, belt grinding, electrical discharge machining, electrochemical machining, electrochemical grinding, chemical milling, ultra-sonic machining, electron beam machining and the like.

In accordance with this invention, such metal parts are made from the hot forgings of this invention, which have been made without a post-forging solution anneal.

A first advantage of this approach is that the overall cost of producing such parts is decreased significantly, since the post-forging solution anneal steps normally carried out in their manufacture has been eliminated by this invention.

A second advantage of this approach is that a higher quality part can normally be obtained. This is illustrated in FIGS. 5a and 5b, which are photomicrographs showing the microstructures of two hot forgings made in the following working examples. FIG. 5a shows the microstructure at the center of a hot forging made by the inventive process, i.e., without a post-forging solution anneal, while FIG. 5b shows the microstructure at the center of an otherwise identical hot forging made with a post-forging solution anneal. By comparing these two photomicrographs, it can be seen that the hot forging of this invention exhibited a finer grain structure than the comparative hot forging made with a conventional post-forging solution anneal. As further described in the following working examples, this difference in microstructure translates to a significant improvement in properties of the hot forgings made by the inventive process.

EXAMPLES

Two hot forgings made in the same production lot were recovered and tested. This production lot was carried out in general accordance with the procedure of FIG. 1, except that the process controls discussed above in connection with this invention for avoiding intermetallic phases were used. Both hot forgings were made from billets of Alloy 254 (AISI-S31254) super austenitic stainless steel about 5 cm. in diameter and 10 cm. long which had been cut from the same solution annealed forge rod. Since they were made in the same production lot, both had been subjected to same heat treatment, which had been carried out in general accordance with the procedure of FIG. 4.

One of these hot forgings was subjected to a conventional post-forging solution anneal in which the forging, after cooling to room temperature by water quenching, was heated and maintained at a temperature of 1177° C. (2150° F.) for at least 30 minutes, after which it was rapidly quenched by immersion in water. The other hot forging, representing this invention, was recovered in its as-quenched condition—i.e., it was recovered without a post-forging solution anneal. These hot forgings were then sectioned, and photomicrographs taken of the metal at the center of each forging.

FIG. 5a is the photomicrograph of the hot forging made in accordance with this invention, which had been produced without post-forging solution anneal. As can be seen, its grain structure was fine and uniform, suggesting excellent mechanical properties.

Meanwhile, FIG. 5b is the photomicrograph of the comparative hot forging which had been produced with a conventional post-forging solution anneal. As can be seen, its grain structure was much larger and less uniform, suggesting lesser mechanical properties.

In order to compare the mechanical properties of forgings produced by this invention (“as-quenched forgings”) with those of otherwise identical forgings made with a conventional post-forge solution-anneal, the same procedure described above in connection with FIGS. 5a and 5b was repeated in seven different forging runs. Each forging run was conducted in generally the same way as described above, i.e., in general accordance with the procedure of FIG. 1 using the process controls of this invention. Forgings were removed from the seven forging lots in the as quenched state, while the remaining forgings of each lot were subjected to a post-forge solution annealing process. The respective seven pairs of as-quenched and solution-annealed forgings were then tested. The results are shown in the following Tables 1, 2 3 and 4. In these tables, the values in the column with the header “As Quenched” and in the column under the header “Solution Annealed” are the comparable values for forgings from the same forging lot. At the bottom of each table, the average property value is shown along with the change in properties from solution-annealed to as-quenched.

TABLE 1
Tensile Strength, ksi
Forging Lot	As Quenched	Solution Annealed
1	120	101
2	106.5	100
3	112.5	102
4	116	104
5	110.5	101
6	111	98
7	107	102
average	112	101
increase	11%

TABLE 2
0.2% Yield Strength, ksi
Forging Lot	As Quenched	Solution Annealed
1	74	48.6
2	55.5	49.9
3	66	50.5
4	71	51
5	63	51
6	66.5	47.3
7	67	53
average	67	50.2
increase	36%

TABLE 3
Elongation, %
Forging Lot	As Quenched	Solution Annealed
1	44	57
2	53	56
3	46.5	52
4	46.5	60
5	43.5	58
6	47.5	59
7	47.5	59
average	47	57
decrease	18%

TABLE 4
Area Reduction, %
Forging Lot	As Quenched	Solution Annealed
1	81.5	83
2	82.5	84
3	81.5	83
4	82	82
5	84.5	82
6	81.5	81
7	82.5	81
average	82.3	82.3
increase	0%

As can be seen from these tables, the as-quenched forgings of this invention were considerably stronger, in terms of both Tensile Strength and 0.2% Yield Strength, than their post-forging solution annealed counterparts. As a result of this higher strength, the as-quenched forgings of this invention were somewhat less ductile, in terms of their Elongation, than their post-forging solution annealed counterparts. However, these measured elongation values are considerably above the minimum required value of 35% for elongation of alloy 254 forgings as listed in the ASTM A182 standard specification for forgings. Reduction of Area (Table 4) values for as-quenched forgings were essentially the same as the values for the corresponding solution-annealed forgings, which indicates that the as-quenched forgings had high ductility.

This observation confirms that hot forgings made in accordance with this invention, and hence shaped metal parts made from these hot forgings, will exhibit significantly better mechanical properties than their conventional counterparts.

Although only a few embodiments of this invention have been described above, many modifications can be made without departing from the spirit and scope of this invention. All such modifications are intended to be included within the scope of this invention, which is to be limited only by the following claims.

Claims

1. In a continuous, automatic process for making multiple hot forgings in series from multiple billets made from a highly-alloyed metal, the improvement wherein the hot forgings are made without subjecting these hot forgings to post-forging solution anneal.

2. The process of claim 1, wherein only those billets which are essentially free of deleterious intermetallic phases are selected as the billets to be forged.

3. The process of claim 1, wherein the hot forgings are made by a forging process including a heating step in which the billets are heated by means of resistance or induction heating from below the critical temperature range of the highly-alloyed metal to a forging temperature above this critical temperature range.

4. The process of claim 3, wherein the heating step is carried out by (a) automatically discharging to waste any billet whose heating rate fails to achieve a predetermined minimum.

5. The process of claim 4, wherein the heating step is further carried out by (b) automatically discharging to waste any billet whose temperature at the end of this step is below a predetermined minimum or above a predetermined maximum.

6. The process of claim 4, wherein the heating step is further carried out by (c) automatically discharging to waste any billet in which the difference between the measured temperature of the side wall of the billet at the end of this step and the measured temperature of the front or rear face of the billet at the end of this step exceeds a predetermined maximum.

7. The process of claim 4, wherein the heating step is further carried out by (d) heating the billet above its critical temperature range long enough to redissolve any intermetallic phases that may be present.

8. The process of claim 4, wherein the heating step is further carried out by discharging to waste all billets whose actual temperature at the end of the heating step exceeds a predetermined maximum, or by discharging to waste all billets which are heated in the heating step for a holding time which exceeds a predetermined maximum.

9. The process of claim 1, wherein the hot forgings are made by a forging process which includes (1) a heating step in which the billets are heated from below the critical temperature range of the highly-alloyed metal to a forging temperature above this critical temperature range rapidly enough so that the heated billets obtained are essentially free of deleterious intermetallic phases, (2) a forging step in which the billets are forged into forgings, and (3) a cooling step in which the forgings so made are cooled from above their critical temperature range to below their critical temperature range.

10. The process of claim 9, wherein step (1) is carried out in a heater, wherein step (3) is carried out by contact of the forging so made with water or other cooling liquid, and further wherein step (2) is carried out by automatically discharging to waste any forging whose forging step is carried out for a period of time which is longer than a predetermined maximum, this period of time beginning when the billet is removed from its heater in step (1) and ending when the forging so made is contacted with water or other cooling liquid in step (3).

11. The process of claim 9, wherein step (2) is carried out rapidly enough so that the forgings obtained are essentially free of deleterious intermetallic phases, and further wherein step (3) is carried out rapidly enough so that the cooled forgings obtained are essentially free of deleterious intermetallic phases.

12. The process of claim 11, wherein step (1) is carried out in a heater, wherein step (3) is carried out by contact of the forging so made with water or other cooling liquid, and further wherein step (2) is carried out by automatically discharging to waste any forging whose forging step is carried out for a period of time which is longer than a predetermined maximum, this period of time beginning when the billet is removed from its heater in step (1) and ending when the forging so made is contacted with water or other cooling liquid in step (3).

13. The process of claim 9, wherein cooling is carried out by immersing the forging so made in water or other cooling liquid, said process further comprising automatically discharging to waste any forging which is immersed in water or other cooling liquid having a temperature exceeding a predetermined maximum.

14. The process of claim 1, wherein the highly-alloyed metal has a CP value of at least 500, wherein the CP value of the alloy is given by the following formula:

CP=20×% Cr+0.3×% Ni+30×% Si+40×% Mo+5×% W+10×% Mn+50×% C−200×% N.

15. The process of claim 14, wherein the highly-alloyed metal is a superstainless steel containing about 19 to 26 wt. % Cr and 3 to 8 wt. % Mo.

16. The process of claim 14, wherein the highly-alloyed metal is a nickel based alloy containing at least about 4 wt. % Mo.

17. In a process for shaping a hot forging which is made from a highly-alloyed metal into a metal part which is useful in one or more applications including chemical processing, scrubbers, pulp mills, bleach washers, food processing and oil field piping, the improvement comprising shaping the hot forging into the metal part without subjecting this hot forging to post-forging solution anneal.

18. The process of claim 17, wherein shaping is accomplished by machining the hot forging.

19. The process of claim 17, wherein the highly-alloyed metal has a CP value of at least 500, wherein the CP value of the alloy is given by the following formula:

CP=20×% Cr+0.3×% Ni+30×% Si+40×% Mo+5×% W+10×% Mn+50×% C−200×% N.

20. A process for making a metal part from a hot forging of a highly-alloyed metal, the process comprising

(a) heating a billet of a highly-alloyed metal up to its forging temperature in a manner so that the heated billet is essentially free of intermetallic phases,

(b) forging the billet in a manner so that the forging obtained is essentially free of intermetallic phases,

(c) cooling the forging so obtained to below its critical temperature range CTR rapidly enough so that the forging obtained is essentially free of intermetallic phases, and

(d) forming the metal part by machining the hot forging of step (c) without subjecting this hot forging to post-forging solution anneal.

21. The process of claim 20, wherein the highly-alloyed metal has a CP value of at least 500, wherein the CP value of the alloy is given by the following formula:

CP=20×% Cr+0.3×% Ni+30×% Si+40×% Mo+5×% W+10×% Mn+50×% C−200×% N.

22. The product of the process of claim 21.

23. In a metal part which is (a) made by shaping a hot forging of a highly-alloyed metal and (b) useful in one or more applications including chemical processing, scrubbers, pulp mills, bleach washers, food processing and oil field piping, the improvement wherein the metal part is made without subjecting the hot forging to post-forging solution anneal.

24. The metal part of claim 23, wherein the metal part is made by machining the hot forging.

25. The metal part of claim 24, wherein the highly-alloyed metal has a CP value of at least 500, wherein the CP value of the alloy is given by the following formula:

CP=20×% Cr+0.3×% Ni+30×% Si+40×% Mo+5×% W+10×% Mn+50×% C−200×% N.

26. A metal part made by machining a hot forging made by the process of claim 1.