Method of producing a steel moulding

Abstract

The invention relates to a method of producing a steel moulding using a sinter powder with a base of iron containing at least one non-ferrous metal selected from a group comprising Mn, Cr, Si, Mo, Co, V, B, Be, Ni and Al, the rest being Fe and unavoidable impurities resulting from the manufacturing process, comprising the steps of preparing the sinter powder, compacting the sinter powder to produce a green compact in a mould, sintering the green compact under a reducing atmosphere and then cooling and hardening, characterised in that the total proportion of non-ferrous metals in the sinter powder is selected from a range with a lower limit of 1% by weight and an upper limit of 60% by weight, and the sinter powder is sintered to an at least approximately completely austenitic structure, and hardening takes place by subjecting the steel moulding to mechanical load so that the austenitic structure is transformed at least partially to a martensitic structure.

Description

The invention relates to a method of producing a steel moulding using a sinter powder with a base of iron which contains at least one non-ferrous metal selected from a group comprising manganese, chromium, silicium, molybdenum, cobalt, vanadium, boron, beryllium, nickel and aluminium, the rest being iron with the inevitable impurities resulting from the manufacturing process, comprising the steps of preparing the sinter powder, compacting the sinter powder to form a green compact in a mould, sintering the green compact under a reducing atmosphere and then cooling and hardening it, as well as a sintered moulding with a moulding body, at least part of which is made from a sinter powder with a base or iron containing at least one non-ferrous metal selected from a group comprising manganese, chromium, silicium, molybdenum, cobalt, vanadium, boron, beryllium, nickel and aluminium, the rest being iron with the inevitable impurities resulting from the manufacturing process.

In order to prevent distortions and deformation when quenching metal components with water and oil, patent specification DE 11 2004 001 875 T5 proposes a method of producing a thin individual component, comprising the steps of heating the thin individual component, followed by subjecting the thin individual component to a quenching and isothermal process using pressing moulds as a means of cooling the thin individual component. This is preferably used to produce steel components containing at least 0.4% by weight of carbon. The isothermal conditions cause a reaction which converts the pattern structure into a bainitic structure. The steel used is an S53C steel containing nickel and a steel based on a composition with an improved quenching property and which enables adequate hardness to be obtained by slow cooling, and this steel contains 0.7% by weight of carbon, 1% by weight silicium, 0.6% by weight manganese, 1.5% by weight chromium and 0.3% by weight molybdenum. This DE-T5 also describes a process of producing martensite based on continuous quenching, but which is followed by a step of heating at 150° C. for 120 minutes. The bainite structure is preferred because shorter quenching is needed according to the explanations given in this DE-T5, which results in the required toughness without having to run a heating step and which prevents any secular change in dimension. The disadvantage of the method described in the DE-T5 is that either the pressing moulds have to be air-cooled for a longer period once the components have hardened or it is necessary to heat the actual mould, thereby incurring extra expense to produce the mould and work with it.

The objective of this invention is to propose a method of producing hardened, sintered precision components and a sintered component produced by it.

This objective is achieved by the invention using the method described above, whereby the proportion of non-ferrous metals in the sinter powder is selected from a range with a lower limit of 1% by weight and an upper limit of 60% by weight and the sinter powder is sintered almost completely to an austenitic structure, and hardening take place by subjecting the steel moulding to mechanical pressure to at least partially transform it from an austenitic to a martensitic structure, and, independently of this by the sintered moulding, for which the total proportion of the at least one non-ferrous metal in the sinter powder is selected from a range with a lower limit of 1% by weight and an upper limit of 60% by weight, and the moulding body has a martensitic structure at least in the surface or in the regions close to the surface or in the surface regions obtained by a reaction induced by high pressure.

The process of producing high-precision sintered components normally includes a finishing step which does not involve the removal of material, for example calibration. To this end, these sintered components are placed in a calibrating die and processed under pressure to obtain the final shape. The method proposed by the invention offers an advantage in this respect in that surface hardening takes place at the same time as this mechanical transformation during this calibration process, thereby obviating the need for an additional hardening step in the processing sequence. It is also of advantage if the component is additionally subjected to temperature during the hardening step, thereby preventing the undesired occurrence of re-crystallisation. An additional cost advantage can also be obtained as a result due to the shorter cycle times on the one hand and due to reduced processing at temperature on the other hand. Furthermore, it is not necessary to temper or cool the die or mould in which this transformation to a martensite structure takes place when subjected to mechanical load, in particular pressure, because the component can not be deformed in any event due to the fact that the component surfaces lie against the mould surfaces. This means that the method proposed by the invention can also be used to produce sintered components with a complex geometry without any risk of distortion to the component.

The total proportion of the at least one non-ferrous metal in the iron-based sinter powder may also be selected from a range with a lower limit of 5% by weight and an upper limit of 55% by weight or may be selected from a range with a lower limit of 18% by weight and an upper limit of 27% by weight.

In order to speed up the martensitic reaction, mechanical load may be applied by operating at a pressure corresponding to the range of −10% of the pressure threshold and the maximum resistance to pressure of the respective material (measured in accordance with DIN 50106) and/or at a temperature selected from a range with a lower limit of 20° C. (room temperature) and an upper limit of 180° C. if the sintered mouldings are subjected to pressure in the cold state, or which is selected from a range with a lower limit of 180° C. and an upper limit of 550° C. if the sintered mouldings are subjected to pressure accompanied by heat. This further reduces the cycle time and thus increases productivity.

The pressure at which the sintered component can be subjected to mechanical load may specifically be selected from a range with a lower limit corresponding to the pressure at −10% of the pressure threshold and an upper limit corresponding to the pressure at +30% of the pressure threshold of the respective material (measured in accordance with DIN 50106) or from a range with a lower limit corresponding to the pressure at −5% of the pressure threshold and an upper limit corresponding to the pressure at +20% of the pressure threshold of the respective material (measured in accordance with DIN 50106).

During the process of applying mechanical load in the cold state, the temperature may also be specifically selected from a range with a lower limit of 40° C. and an upper limit of 150° C. or a lower limit of 60° C. and an upper limit of 100° C.

During the process of applying mechanical load accompanied by heat, the temperature may be selected from a range with a lower limit of 200° C. and an upper limit of 500° C. or from a range with a lower limit of 250° C. and an upper limit of 350° C.

In one variant of the method, a carburizing gas is added to the reducing atmosphere for the sintering process or a carburizing gas is used as the reducing atmosphere. This enables the carbon content in at least the superficial regions of the green compact to be increased during sintering, which is conducive to the subsequent formation of martensite.

In this respect, it may be of advantage to run the sintering process in two stages, namely in the form of what is referred to as pre-sintering, which takes place at a temperature below the temperature of the second sintering step, followed by what is referred to as high-temperature sintering. This enables higher carbon contents to be obtained without the risk of brittle cracking during the hardening reaction, thereby generally enabling greater strength to be imparted to the sintered component. Accordingly, the temperature applied during pre-sintering may be selected from a range with a lower limit of 60% and an upper limit of 80% of the temperature of the second sintering step for example. For example, pre-sintering may be run at a temperature selected from a range with a lower limit of 600° C. and an upper limit of 1000° C. and the high-temperature sintering may be run at a temperature selected from a range with a lower limit of 1100° C. and an upper limit of 1450° C.

In yet another variant of the method proposed by the invention, the steel moulding is produced with a density of max. 7.3 g/cm³, at least at the core. This enables the properties of the steel moulding to be optimised in that there is a certain residual elasticity at the core, whilst an appropriate mechanical strength is imparted to superficial areas due to the hardening reaction. Furthermore, the weight of the steel moulding can be reduced. By superficial regions is meant those regions which extend to a component depth of 0.5 mm.

With a view to increasing the proportion of carbon, it is of advantage to add a proportion of graphite to the sinter powder Instead of or in addition to the carburizing gas, and this is selected from a range with a lower limit of 0.1% by weight and an upper limit of 5% by weight. This again promotes the process of producing an at least almost complete formation of martensite, at least in regions close to the surface.

In particular, the proportion of graphite may also be selected from a range with a lower limit of 0.1% by weight and an upper limit of 3% by weight or from a range with a lower limit of 0.5% by weight and an upper limit of 2% by weight.

In order to obtain higher densities in the green compact already, it is of advantage if up to 8% by weight of pressing agents and/or up to 2% by weight of a binding agent, in particular an organic one, are added to the iron-based powder. What this also achieves is that a higher porosity is obtained in the sintered component as these agents are burned off during sintering and pre-sintering, which makes subsequent compaction during calibration easier. In particular, this makes it easier to press sinter powders that are intrinsically difficult to press, in particular steel powders containing chromium. Above a total of 10% by weight of agents, the porosity can become too high, which can lead to lower final densities of the finished sintered component under certain circumstances.

The proportion of pressing agent may be specifically selected so that it is also up to a max. 2.5% by weight or up to a max. 1.5% by weight and the proportion of binding agent may be up to a max. of 0.75% by weight or a max. 0.5% by weight.

In order to reduce costs on the one hand and with a view to optimising properties and producing sintered components with properties opposite those of the raw materials used on the other hand, the method may be operated in such a way that an additional sinter powder is placed in the mould and this is compacted jointly with the iron-based sinter powder, or, in another variant of the method, a semi-finished moulding is produced in a first step, this is placed in the pressing mould and at least certain areas of it are coated with the steel powder with an iron base, e.g. by spraying, and sintered jointly with the iron-based steel powder, or, in yet another variant of the method, a semi-finished moulding is made from the iron-based sinter powder in a first step and the semi-finished moulding is joined to another finished moulding made from a sinter powder that is different from the sinter powder of the first semi-finished moulding in another step. This being the case, those surfaces which will be subjected to higher loads in the application for which the sintered component will be used can be selectively coated with the iron-based sinter powder and then hardened by a martensite reaction, in other words specific properties can be obtained to suit the intended application.

To provide a clearer understanding, the invention will be described in more detail on the basis of examples.

Firstly, it should be pointed out that individual features or combinations of features which become apparent from the illustrated and described examples of embodiments may be taken as representing individual inventive solutions and solutions proposed by the invention in their own right.

All the figures relating to ranges of values in the description should be construed as meaning that they include any and all part-ranges, in which case, for example, the range of 1 to 10 should be understood as including all part-ranges starting from the lower limit of 1 to the upper limit of 10, i.e. all part-ranges starting with a lower limit of 1 or more and ending with an upper limit of 10 or less, e.g. 1 to 1.7, or 3.2 to 8.1 or 5.5 to 10.

As explained above, the invention relates to the manufacture of components of sintered steel made from an austenitic material which forms martensite during moulding and thus hardens. The surface may be compacted or alternatively, components which do not undergo any surface compaction can be produced, or the surface density may also be reduced. By preference, however, the surface is compacted. The method proposed by the invention offers new possibilities for moulding high-precision sintered components which are able to withstand high stress. To this end, there are several variants of the method used to produce the compact.

For example, whole components may be pressed from sinter powder with a base of iron.

Another option is to produce what are referred to as component composites. To this end, the pressing die may be filled with at least two or more different sinter powder mixtures which are then jointly compacted or a component composite is made by a multi-stage powder pressing process, whereby a semi-finished component is pressed, and optionally also sintered, from a sinter powder that is different from the iron-based sinter powder and the iron-based sinter powder is pressed onto it in another pressing step, after which they are sintered jointly.

Another option for producing component composites is to shape a green compact to close to the final contour from another sinter powder by pressing the powder in a mould and optionally also sintering it, and then applying the sinter powder with an iron base to at least those regions of the steel component or sintered moulding which will be subjected to higher loads during the service life of the component by coating or spraying methods known from the prior art, after which this coated and optionally sintered green compact is then sintered. It goes without out saying that in this case, it would also be possible to coat the entire surface of the green compact with the iron-based powder. Instead of using a green compact shaped close to the final contour, however, it would also be possible to produce a semi-finished part from a solid material which is not manufactured by a sintering process but is made using a casting or punching process.

Another option would be to join two or more components pressed in separate work steps using a known method, e.g. sinter joining or sintering and brazing or similar. In the case of sinter joining, it is possible to join two green compacts or two sintered parts to one another or to join one sintered component to a green compact, in which case it would also be possible to join more than two parts and the enumeration of options for two parts is then adapted accordingly. In any event, at least one of the two or more parts to be joined is at least partially contains the sinter powder with an iron base or is made from it.

It should be pointed out that the other sinter powder may also be a sinter powder with a base of iron but if this is the case, it is based on a different composition. Alternatively, however, it would also be possible to use a sinter powder known from the prior art as the other sinter powder, for example with a base of Cu, e.g. bronze.

By adapting the sintering process accordingly, a structure is produced in some regions of the iron-based sinter powder which becomes harder under mechanical load. The ability to harden is achieved due to the fact that a soft, primarily austenitic structure is obtained after the sintering process, which then reacts when subjected to mechanical load, in particular pressure, and the reaction brings about a transformation to a martensitic structure. This structural transformation results in hardening in the moulded region. Moulding may be achieved in various ways, for example by cross-compaction (transverse rollers) or by axial compaction (axial rollers) or by a multi-stage final pressing (e.g. calibration).

Various different finishing processes may be used.

General Description of the Process

1.) Mixing the Powder

In the case of component composites made from other sinter powders, the mixing of powder for portions of the component which do not harden during moulding may take place in a manner known from the prior art. To this end, iron-based powder mixtures may be used with a total of up to 10% by weight of metallic non-ferrous alloying elements, optionally up to 5% by weight of graphite and/or optionally up to 3% by weight of pressing agents and optionally up to 0.5% by weight of organic binders. These mixtures are produced in a conventional manner from pure iron powder or pre-alloyed or alloyed-on iron powders serving as the base material, to which alloying elements and optionally other agents are added. Alternatively, a so-called master mixture in a highly concentrated form is pre-mixed, possibly at a temperature and/or using solvents, and then admixed with iron powder or individual elements are mixed in by adding them directly to the iron powder.

The binding agents used may include resins, silanes, oils, polymers or adhesives. Amongst the pressing agents which may be used are waxes, stearates, silanes, amides and polymers, for example.

By using other non-ferrous alloying elements such as chromium, copper, nickel, manganese, silicium, molybdenum and vanadium, the properties of such sintered components made from an iron-based powder can be improved accordingly, in a manner already known from the prior art. For example, an alloy with molybdenum will prevent brittleness during tempering in the case of chromium steels. The hardening capacity and toughness are improved as a result. Furthermore, resistance to creep at higher temperatures can be increased. Adding nickel will improve moulding ability under cold conditions. Manganese can increase tensile strength and yield strength. Silicium will prevent the precipitation of cementite from the martensite during tempering.

Since the main effect of these alloying elements is known from the prior art, further explanation is unnecessary here.

The proportion of non-ferrous alloying elements may also be selected from a range with a lower limit of 0.2% by weight and an upper limit of 8% by weight, in particular from a range with a lower limit of 1% by weight and an upper limit of 6% by weight.

This being the case, the proportion of copper used may be selected from a range with a lower limit of 0% by weight and an upper limit of 6% by weight, in particular from a range with a lower limit of 0.1% by weight and an upper limit of 4% by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 2% by weight.

The proportion of chromium may be selected from a range with a lower limit of 0% by weight and an upper limit of 5% by weight, in particular from a range with a lower limit of 0.1% by weight and an upper limit of 4% by weight, preferably from a range with an upper limit of 0.2% by weight and an upper limit of 3% by weight.

The proportion of nickel may be selected from a range with a lower limit of 0% by weight and an upper limit of 8% by weight, in particular from a range with a lower limit of 0.1% by weight and an upper limit of 4% by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 2% by weight.

The proportion of manganese may be selected from a range with a lower limit of 0% by weight and an upper limit of 10% by weight, in particular from a range with a lower limit of 0.1% by weight and an upper limit of 5% by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 2% by weight.

The proportion of molybdenum may be selected from a range with a lower limit of 0% by weight and an upper limit of 3% by weight, in particular from a range with a lower limit of 0.1% by weight and an upper limit of 1.5% by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 0.85% by weight.

The proportion of silicium may be selected from a range with a lower limit of 0% by weight and an upper limit of 5% by weight, in particular from a range with a lower limit of 0.1% by weight and an upper limit of 2% by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 0.5% by weight.

The proportion of vanadium may be selected from a range with a lower limit of 0% by weight and an upper limit of 8% by weight, in particular from a range with a lower limit of 0.1% by weight and an upper limit of 2% by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 0.5% by weight.

The proportion of graphite may also be selected from a range with a lower limit of 0% by weight and an upper limit of 2% by weight, in particular from a range with a lower limit of 0.1% by weight and an upper limit of 1.5% by weight, preferably from a range with a lower limit of 0.2% by weight and an upper limit of 0.8% by weight.

Examples of typical mixtures are:

• • Fe (pre-alloyed with 0.85% by weight Mo)+0.1% by weight−0.3% by weight C+0.4% by weight−1.0% by weight pressing agents and optionally binding agents
  • Fe+1% by weight−3% by weight Cu+0.5% by weight−0.9% by weight C+0.3% by weight−0.8% by weight of pressing agents and optionally binding agents
  • Astaloy CrM (Cr+Mo pre-alloyed iron powder)+1% by weight−3% by weight Cu+0.1% by weight−1% by weight C+0.3% by weight−1.0% by weight pressing agents and optionally binding agents.

However, any other standard compositions used in the sintering industry may also be used.

The iron-based sinter powder which hardens during moulding or the corresponding alloys are mixed using conventional mixing techniques. During production, particular attention is paid to the properties of the highly alloyed powder, in particular the fact that the materials are substances which are very hard and not readily compressible or not compressible at all. One option is to use ferro-alloys containing up to at least approximately 60% by weight of one or more alloying elements from the group comprising Mn, Cr, Si, Mo, Co, V, B, Be, Ni and Al. Alternatively, however, it is possible to use aqueous, gaseous or oil-sprayed iron-based powders, in which case higher contents of one or more elements from the group comprising Mn, Cr, Si, Mo, Co, V, B, Be, Ni and Al are added.

The total content of this non-ferrous metal in the sinter powder with a base of iron may also be specifically selected from a range with a lower limit of 15% by weight and an upper limit of 55% by weight, in particular from a range with a lower limit of 20% by weight and an upper limit of 50% by weight or from a range with a lower limit of 25% by weight and an upper limit of 40% by weight.

Accordingly, the proportion of manganese in the sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 35% by weight, in particular from a range with a lower limit of 5% by weight and an upper limit of 25% by weight or from a range with a lower limit of 10% by weight and an upper limit of 15% by weight.

The proportion of chromium in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 20% by weight, in particular from a range with a lower limit of 4% by weight and an upper limit of 15% by weight or from a range with a lower limit of 7% by weight and an upper limit of 12% by weight.

The proportion of silicium in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 10% by weight, in particular from a range with a lower limit of 1% by weight and an upper limit of 8% by weight or from a range with a lower limit of 3% by weight and an upper limit of 6% by weight.

The proportion of molybdenum in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 10% by weight, in particular from a range with a lower limit of 2% by weight and an upper limit of 8% by weight or from a range with a lower limit of 4% by weight and an upper limit of 6% by weight.

The proportion of cobalt in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 10% by weight, in particular from a range with a lower limit of 1% by weight and an upper limit of 7% by weight or from a range with a lower limit of 2.5% by weight and an upper limit of 5% by weight.

The proportion of vanadium in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 10% by weight, in particular from a range with a lower limit of 2.4% by weight and an upper limit of 8.1% by weight or from a range with a lower limit of 3.2% by weight and an upper limit of 6.5% by weight.

The proportion of boron in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 5% by weight, in particular from a range with a lower limit of 1% by weight and an upper limit of 4% by weight or from a range with a lower limit of 2% by weight and an upper limit of 3 by weight.

The proportion of beryllium in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 5% by weight, in particular from a range with a lower limit of 1.5% by weight and an upper limit of 4.3% by weight or from a range with a lower limit of 2.3% by weight and an upper limit of 3.8% by weight.

The proportion of nickel in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 35% by weight, in particular from a range with a lower limit of 5% by weight and an upper limit of 25% by weight or from a range with a lower limit of 10% by weight and an upper limit of 15% by weight.

The proportion of aluminium in the finished sinter powder mixture with a base of iron when ready for moulding may be selected from a range with a lower limit of 0% by weight and an upper limit of 10% by weight, in particular from a range with a lower limit of 2% by weight and an upper limit of 7.8% by weight or from a range with a lower limit of 3.9% by weight and an upper limit of 6.2% by weight.

Examples of appropriate mixtures are:

18% by weight Mn+2.5% by weight Al+3.5% by weight Si+0.5% by weight V+0.3% by weight B, the rest being Fe
or
24% by weight Mn+3% by weight Al+2.5% by weight Si, the rest being Fe
or
14% by weight Mn, 5% by weight Ni+3% by weight Al+3% by weight Si, the rest being Fe.

These mixtures are made up and homogenised using appropriate mixing methods known from powder metallurgy. It is also possible to use techniques known from the prior art for processing binding agents or the known process of diffusion alloying used to obtain a uniform distribution, especially in the case of fine powders.

2) Pressing

The iron powder mixtures produced by the methods described above are compacted and shaped by means of coaxial pressing methods. In this respect, care should be taken to ensure that allowance is already made for the changes in shape and design which occur during the process of producing the pressing die. Depending on the pouring density and theoretical density of the powder mixtures, pressing pressures of 600 Mpa to 1200 Mpa are used.

In order to produce component composites from conventional powders with segments or regions made from alloys which harden during moulding, it is possible to use double or multiple powder filling techniques. Using these methods, different powders can be introduced into different regions of the mould and then shaped jointly by pressing the powder. Using such methods, sintered components may also be placed in the powder pressing mould and then powder pressed “around” them.

The compacts obtained by these different methods (also referred to as green compacts) are the starting point for the subsequent pressing steps.

It would also be possible to use pressing methods known from sintering the industry other than coaxial pressing methods, such as isostatic pressing methods, etc.

3) De-Waxing+Sintering

The compacts may also be pre-sintered using a heat treatment involving an atmosphere based on gases which produce at least partial carburization. To this end, reducing atmospheres are obtained using nitrogen-hydrogen mixtures with hydrogen in a proportion of up to 50% by volume. The proportion of hydrogen may also be 0% by vol to 100% by vol or 1% by vol to 60% by vol or 2% by vol to 40% by vol.

Carburizing gases (endo-gas, methane, propane and similar) may optionally also be used. The temperature for pre-sintering may be between 600° C. and 1050° C., for example, and the pre-sintering time may be between 10 minutes and 2 hours for example.

Pre-sintering causes organic binding agents and lubricants to be burned off and makes it easier to produce a bond between the particles. A lower hardness level can be achieved due to incomplete dissolution of individual alloying elements. The hardness of the sintered component may be adjusted so that a high degree of moulding is obtained during the subsequent compaction process (calibration) with an excess of up to 30%. Especially in the case where hardness is less than 140 HB 2.5/62.5, a surprisingly high degree of moulding ability was observed.

Alloying elements involving oxygen in particular are difficult to process during pre-sintering. By selecting the processing parameters accordingly within the specified ranges, a massive build-up of oxygen can be at least largely prevented during pre-sintering so that this does not have a negative effect on moulding ability.

In the pre-sintered state, Cr—Mo pre-alloyed powders are also easier to calibrate.

During pre-sintering, the powder grains are sintered to only a limited degree, resulting in a somewhat weak sintered bond.

Furthermore, by pre-sintering at a temperature of below 1100° C., the graphite is only incompletely diffused into the iron matrix material.

The temperatures applied during the actual sintering process are typically between 1100° C. and 1350° C. or higher depending on the alloying system used and the sintering time is between 10 minutes and 2 hours, in particular between 29 minutes and 60 minutes.

After sintering and pre-sintering, the sintered component is cooled, to which end it is preferable to set a cooling rate selected from a range with a lower limit of 10° C./minute and an upper limit of 250° C./minute, in particular selected from a range with a lower limit of 30° C./minute and an upper limit of 200° C./minute, for example selected from a range with a lower limit of 50° C./minute and an upper limit of 150° C./minute.

4) Transformation to a Martensitic Structure

All of the known moulding methods may be used, as explained above. In any event, the moulding process is set up so that the predominantly austenitic structure (present in at least the peripheral regions) is transformed at least partially to produce martensite, preferably up to at least 99%.

The mechanically applied pressure may be a pressure selected from one of the ranges specified above.

Moulding may optionally also take place at an increased temperature. This being the case, the temperature for cold or warm moulding may be selected from the ranges specified above. The sintered component may be heated prior to moulding for this purpose and/or may be processed using a tempered mould. Another option is one whereby the sintered component is not cooled to room temperature after moulding and instead is moulded at this temperature, in which case there is no need for additional tempering of the component or mould.

Subjecting the sintered component to mechanical load enables surface hardness values of between 400 HV5 and 750 HV5 to be obtained.

5) Thermo-Chemical Finishing Process

Due to the outstanding properties obtained after hardening, it is not usually necessary to perform any additional heat treatment.

However, it is optionally nevertheless possible to apply heat treatment with a view to further optimising the properties (e.g. baking or tempering). The components are often thermally de-greased beforehand. If sinter-hardened materials are used for component composites, a non-carburizing process may be used, such as inductive hardening.

6) Mechanical Processing

All the mechanical finishing or coating processes known from the prior art may be used.

EXAMPLE 1

Surface-Compacted Gear

Composition of the sinter powder: 18% by weight Mn+3.5% Si+2.5% by weight Al+0.5% by weight V+0.3% by weight B+1% by weight pressing agent, the rest being Fe

Pressing pressure applied to obtain the compact: 800 Mpa (6.8 g/cm³ density)

Temperature during sintering: 1280° C.

Sintering time: 45 minutes

Composition of the reducing atmosphere: N2/H2 (60% by vol/40% by vol)

Surface compaction by rolling the toothing: practical theoretical density up to a depth of 0.5 mm for a surface hardness >400 HV-5

In comparative dynamic tests, the finished sintered gear exhibited better durability properties than sintered gears with the same geometry made from conventional sinter powder and dynamically hardened following surface compaction.

EXAMPLE 2

Surface-Compacted Composite Sprocket Wheel with a Functional Surface Hardened During Moulding

A coating of sinter powder which can be hardened during moulding was sprayed onto the functional surface of a compact made from conventional sinter powder and a composite component produced by sintering which was then partially compacted and thus moulded and hardened.

Composition of the sinter powder for the base component: 2% by weight Cu+0.7% by weight C+0.8% by weight pressing agent, the rest being Fe

Pressing pressure applied to produce the compact: 600 Mpa (6.9 g/cm² density)

Composition of the sinter powder for the functional surface: 14% by weight Mn+5% by weight Ni+3% by weight Al+3% by weight Si+6% by weight pressing agent+2% by weight binding agent, the rest being Fe

Coating density of the sinter powder for the functional surface once sprayed on: 1.2 mm

Temperature during sintering: 1250° C.

Sintering time: 45 minutes

Composition of the reducing atmosphere: N2/H2 (95% by vol/5% by vol)

Coating density of the sinter powder for the functional surface after sintering: 0.5 mm

Surface compaction by rolling the functional surface: practical theoretical density up to a depth of 0.2 mm for a surface hardness >400 HV-5

The finished composite sprocket wheel exhibited significantly better wear resistance than conventionally produced sprocket wheels.

The embodiments illustrated as examples represent possible variants of the method proposed by the invention and it should be pointed out at this stage that the invention is not specifically limited to the variants specifically illustrated, and instead the individual variants may be used in different combinations with one another and these possible variations lie within the reach of the person skilled in this technical field given the disclosed technical teaching. Accordingly, all conceivable variants which can be obtained by combining individual details of the variants described and illustrated are possible and fall within the scope of the invention.

Claims

1. Method of producing a steel moulding using a sinter powder with a base of iron containing at least one non-ferrous metal selected from a group comprising Mn, Cr, Si, Mo, Co, V, B, Be, Ni and Al, the rest being Fe and unavoidable impurities resulting from the manufacturing process, comprising the steps of preparing the sinter powder, compacting the sinter powder to produce a green compact in a mould, sintering the green compact under a reducing atmosphere and then cooling and hardening, wherein the total proportion of non-ferrous metals in the sinter powder is selected from a range with a lower limit of 1% by weight and an upper limit of 60% by weight, and the sinter powder is sintered to an at least approximately completely austenitic structure, and hardening takes place by subjecting the steel moulding to mechanical load so that the austenitic structure is transformed at least partially to a martensitic structure.

2. Method as claimed in claim 1, wherein the mechanical-load is obtained by applying a pressure which is at least as high as the pressure at −10% of the pressure threshold of the respective material (measured in accordance with DIN 50106).

3. Method as claimed in claim 1, wherein the mechanical load is produced at a temperature selected from a range with a lower limit of 20° C. and an upper limit of 180° C. or selected from a range with a lower limit of 180° C. and an upper limit of 550° C.

4. Method as, claimed in claim 1, wherein a carburizing gas is added to the reducing atmosphere for the sintering process or a carburizing gas is used as the reducing atmosphere.

5. Method as claimed in claim 1, wherein the steel moulding is produced with a core density of 7.3 g/cm3 maximum.

6. Method as claimed in claim 1, wherein graphite is added to the sinter powder in a proportion selected from a range with a lower limit of 0.1% by weight and an upper limit of 5% by weight.

7. Method as claimed in claim 1, wherein up to 8% by weight of pressing agent and/or up to 2% by weight of binding agent, in particular an organic one, is added to the iron-based powder.

8. Method as claimed in claim 1, wherein an additional sinter powder is placed in the mould and compacted together with the iron-based sinter powder

9. Method as claimed in claim 1, wherein a semi-finished moulding is produced in a first step and placed in the mould, and at least certain regions of it are coated with the steel powder with an iron base and sintered jointly with the steel powder with an iron base.

10. Method as claimed in claim 1, wherein a semi-finished moulding is produced from the sinter powder with an iron base in a first step and the semi-finished moulding is joined to another semi-finished moulding made from a sinter powder that is different from the sinter powder of the first semi-finished moulding in another step.

11. Sintered moulding with a moulding body made from a sinter powder with an iron base containing at least one non-ferrous metal selected from a group comprising Mn, Cr, Si, Mo, Co, V, B, Be, Ni and Al, the rest being Fe with unavoidable impurities caused by the production process, wherein the total proportion of the at least one non-ferrous metal in the sinter powder is selected from a range with a lower limit of 1% by weight and an upper limit of 60% by weight and the moulding body has a martensitic structure at least at the surface or in regions close to the surface or in surface regions induced by a reaction under high mechanical load.

12. Sintered component as claimed in claim 11, wherein part of the moulding body is produced from another sinter powder that is different from the sinter powder with an iron base.