Method for nitriding metal in salt bath and metal manufactured using the same

Provided is a method for nitriding a metal in a salt bath by using a non-cyanide salt and a nitrided metal manufactured using the same. The method includes the steps of: immerging at least one salt selected from the group consisting of KNO3, KNO2, Ca(NO3)2, NaNO3 and NaNO2 into the salt bath; melting the salt by heating and maintaining the molten salt at a predetermined temperature; and submerging the metal in the salt bath. Nitriding in non-cyanide salts, such as potassium nitrate (KNO3), potassium nitrite (KNO2), sodium nitrate (NaNO3), sodium nitrite (NaNO2), calcium nitrate (Ca(NO3)2) and their mixtures, is capable of solving an environmental pollution problem and reducing a cost. Also, the method is capable of increasing nitrided depth of the metal two to six times compared to conventional nitriding methods. As a result, the method can be carried out in various application fields.

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Description
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for nitriding metal in a salt bath and nitrided metal manufactured using the same; and, more particularly, to a method for nitriding iron or steels by using non-cyanide salt bath, and nitrided iron or steels manufactured using the same.

2. Background of the Related Art

Steels have been widely used for machine parts because of their inherent properties. To be used for machine parts, steels are usually first heat-treated to impart thereto strength, toughness and durability, all of which are the qualities machine parts require. In addition, for machine parts that are often exposed to corrosive environment, surfaces thereof are further heat-treated to impart thereto corrosion resistance.

Nitriding is one of the methods for processing the metal surface to impart thereto a corrosion resistance thereof. The nitriding methods include gas nitriding using NH3 gas, salt bath nitriding using KCN, KCNO etc., gas nitrocarburizing (carbo-nitriding) using a mixture of NH3 gas and RX gas, i.e., endothermic gas, and ion nitriding involving an insertion of a mixture of N2 and H2 gas into plasma.

Generally, although nitriding is applied to steels to improve their abrasion(wear) resistance and fatigue resistance, it can also be carried out to improve the corrosion resistance thereof.

Of the nitriding methods mentioned hereinabove, the salt bath nitriding is most widely used for a variety of machine parts including automobile components, because properties of chemicals for the salt bath and their melting points can be freely controlled to provide stability through a wide range of process temperatures without eroding the surface of the object being treated. To be more specific, in addition to its excellent thermal conductivity, soaking properties and easily controllable processing conditions, it is cheaper to design and maintain, compared with other nitriding methods. For example, it is easy to operate the salt bath, and the heating rate is 4 times faster in the salt bath than in the atmosphere. The salt bath is especially suitable to heat-treatment of high speed steel which is sensitive to crystal(grain) growth. When a material treated in a salt bath comes into a contact with the atmosphere, a film including the salt bath constituents is formed on the surface thereof, the film preventing oxidation by preventing the material from a direct contact with the atmosphere. Furthermore, the surface of the material thus treated is rather clean, making the salt bath an ideal heat-treatment for both mass production and small-lot-sized production.

Cyanide-containing salt is generally used for a salt bath nitriding method, producing cyanide ions inside the bath. Since the cyanide ion is classified as a toxic chemical, it must be carefully and tightly controlled, and this can be an expensive proposition. Also, there is a problem of a cost involved for processing waste water and gas.

Further, the nitriding treatment in a molten salt including cynides is a nitrocarburizing (carbo-nitriding) method involving a simultaneous penetration of carbon and nitrogen. It has a shortcoming in that although the surface hardness of the material thus treated improves significantly, the tensile strength gets only slightly enhanced. The conventional salt bath nitriding method using a cyanide salt also has a problem that its applications are limited to molds or gears since the depth to which the material can be nitrided is limited.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a method for nitriding a metal using non-cyanide salts, and a nitrided metal manufactured using the same.

It is another object of the present invention to provide a salt-bath nitriding method for nitriding a metal, in which nitrogen penetrates into the metal, and a nitrided metal manufactured using the same.

It is yet another object of the present invention to provide a salt bath nitriding method for nitriding a metal, capable of increasing hardness and tensile strength of the metal to be treated, and a nitrided metal manufactured using the same.

It is still another object of the present invention to provide a salt bath nitriding method for nitriding a metal, capable of maximizing a nitriding depth, and a nitrided metal manufactured using the same.

In accordance with one aspect of the present invention, there is provided a method for nitriding a metal in a salt bath, the method including the steps of: a) immersing a non-cyanide salt into the salt bath; b) melting the salt by heating and maintaining the molten salt at a predetermined temperature; and c) submerging the metal in the salt bath.

In the present invention, it is preferred that the non-cyanide salt includes at least one selected from a group consisting of NaNO3, NaNO2, KNO3, KNO2 and Ca(NO3)2, and the metal is one of iron and steels.

At this time, the predetermined temperature is within a range of 400° C. to 700° C., and the submerging time is within a range of 1 minute to 24 hours.

In the present invention, when iron is nitrided in the salt bath including at least one of the group consisting of KNO3, KNO2, Ca(NO3)2, NaNO3, and NaNO2, the iron can be nitrided into a depth of 0.1 mm to 3.0 mm from its surface.

In the present invention, when a steel is nitrided in the salt bath including at least one of the group consisting of KNO3, KNO2, Ca(NO3)2, NaNO3, and NaNO2, the steel can be nitrided into a depth of 0.1 mm to 3.0 mm from its surface.

The steel includes ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel, alloy steel and IF steel.

The ultra-low carbon steel nitrided by the present invention has the surface hardness ranging from more than 120 Hv to equal to or less than 450 Hv. The low carbon steel has the surface hardness being more than 200 Hv to equal to or less than 410 Hv. The medium carbon steel has the surface hardness being more than 130 Hv to equal to or less than 420 Hv. The high carbon steel has the surface hardness being more than 150 Hv to equal to or less than 400 Hv. The alloy steel has the surface hardness being more than 200 Hv to equal to or less than 410 Hv. IF steel has the surface hardness being more than 165 Hv to equal to or less than 400 Hv. The surface hardness of the steels nitrided by the present invention can be improved to a maximum of 420 Hv. The surface hardness of the iron nitrided by the present invention is also improved.

The ultra-low carbon steel nitrided by the present invention has the tensile strength ranging from more than 35 kgf/mm2 to equal to or less than 110 kgf/mm2. The low carbon steel has the tensile strength ranging from more than 45 kgf/mm2 to equal to or less than 110 kgf/mm2. The medium carbon steel has the tensile strength ranging from more than 45 kgf/mm2 to equal to or less than 100 kgf/mm2. The high carbon steel has the tensile strength ranging from more than 60 kgf/mm2 to equal to or less than 95 kgf/mm2. The alloy steel has the tensile strength ranging from more than 55 kgf/mm2 to equal to or less than 110 kgf/mm2. The tensile strength of IF steel and iron can be improved by the nitriding method of the present invention.

The salt-bath nitriding method of the present invention can be applied to the iron, the IF steel, the carbon steel including the ultra-low carbon steel having a carbon content of at least 0.0001 wt % to less than 0.13 wt %, the low carbon steel having a carbon content of at least 0.13 wt % to less than 0.2 wt %, the medium carbon steel having a carbon content of at least 0.21 wt % to less than 0.51 wt %, and the high carbon steel having a carbon content of at least 0.51 wt % to less than 2.0 wt %, the steel having a chrome content of 0.1 wt % to 1.5 wt %, the steel having a molybdenum content of 0.05 wt % to 0.5 wt %, the steel having a nickel content of 0.1 wt % to 10 wt %, the steel having a manganese content of 0.1 wt % to 2.0 wt %, the steel having a boron content of 0.001 wt % to 0.1 wt %, the steel having a titanium content of 0.01 wt % to 0.1 wt %, the steel having a vanadium content of 0.05 wt % to 0.15 wt %, the steel having a niobium content of 0.005 wt % to 0.1 wt %, and the steel having an aluminum content of 0.005 wt % to 0.1 wt %. Also, the salt-bath nitriding method of the present invention can be applied to the alloy steel including at least two kinds of the steels suggested above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph illustrating relationship between a nitriding time and a hardness profile in a steel nitrided in accordance with a first embodiment of the present invention;

FIG. 2 is a graph illustrating relationship between the nitriding time and the hardness profile in the steel nitrided in accordance with the first embodiment of the present invention;

FIG. 3 is a graph illustrating relationship between a nitriding temperature and the hardness profile in the steel nitrided in accordance with the first embodiment of the present invention;

FIG. 4 is a graph illustrating relationship between the nitriding time and the surface hardness of the steel nitrided in accordance with the fourth embodiment of the present invention;

FIG. 5 is a graph illustrating relationship between the nitriding temperature and time and the hardness profile in the steel nitrided in accordance with the fourth embodiment of the present invention;

FIG. 6 is a graph illustrating relationship between the nitriding time and the hardness profile in the steel nitrided in accordance with the fourth embodiment of the present invention;

FIG. 7 is a graph illustrating the hardness profile in the steel nitrided in accordance with the fifth embodiment of the present invention;

FIG. 8 is a graph illustrating the hardness profile in the steel nitrided in accordance with the sixth embodiment of the present invention; and

FIG. 9 is a graph illustrating relationship between a mixture ratio of a mixed salt and the hardness profile in the steel nitrided in accordance with the sixth embodiment of the present invention.

 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in more detail.

In nitriding of a metal, the present invention incorporates therein the nitrogen dissolution principle involving a non-cyanide molten salt, more particularly, NaNO3, NaNO2, KNO3, KNO2, Ca(NO3)2 and mixtures thereof as a molten salt, as opposed to a conventional nitriding method such as a nitrocarburizing (carbo-nitriding) method involving the use of cyanides, e.g., KCN and NaCN, as the molten salt wherein carbon and nitrogen are simultaneously diffused into the metal.

The method for nitriding the metal in accordance with the present invention involves immersing at least one salt from a group consisting of NaNO3, NaNO2 KNO3, KNO2 and Ca(NO3)2 into a salt bath, melting the salt and maintaining of the molten salt at a predetermined temperature ranging from 400° C. to 700° C.

Subsequently, the metal to be nitrided is submerged in the bath for 1 minute to 24 hours.

During this time, nitrogen, oxygen and nitrogen oxides are generated from the non-cyanide molten salts of the present invention, NaNO3, NaNO2 KNO3, KNO2, Ca(NO3)2 and mixtures of thereof, by the following reaction formulae 1 to 3.

The following reaction formula 1 represents nitrogen formation reaction in the molten salt bath of NaNO3 and NaNO2.


NaNO3→NaNO2+½O2


2 NaNO2→Na2O+NO2+NO


2NaNO2+2NO→2NaNO3+N2  [Reaction formula 1]

The following reaction formula 2 represents nitrogen formation reaction in the molten salt bath of KNO3 and KNO2.


KNO3→KNO2+½O2


2KNO2→K2O+NO2+NO


2KNO2+2NO→2KNO3+N2  [Reaction formula 2]

The following formula 3 shows nitrogen formation reaction in the molten salt bath of Ca(NO3)2.


Ca(NO3)2→CaO+2NO2+½O2


2NO2→2O2+N2  [Reaction Formula 3]

As shown in Table 1, those metals nitrided, including carbon steel (including ultra-low carbon steel, low carbon steel, medium carbon steel and high carbon steel), alloy steel, IF steel and iron using the salt-bath nitriding method in accordance with the present invention are nitrided to a depth of 0.1 mm to 3.0 mm from the surface. The range of nitrided depth/diffusion layer thickness obtained through the present invention is 2 to 6 times larger than that obtained using the conventional nitriding methods, meaning that a nitrided/diffusion layer formed using the nitriding method of the present invention extends from the surface to the metal inner part, and consequently the surface hardness and tensile strength of the metal also improve compared to those of the metal nitrided using the conventional nitriding method. Reference for the table 1 are as follows:

K. Funatani, “Low-Temperature Salt Bath Nitriding of Steels”, Metal Science and Heat Temperature, Vol. 46, No. 7, PP. 277-281 (2004).

TABLE 1 Thickness of Temperature diffusion Nitriding method (K) Type of Steel layer (μm) Nitride process by 953 Low carbon 3000 the present steel invention 913 IF steel 1500 Tufftride TFI 853 1015 800 853 1045 780 853 34Cr4 480 853 X210Cr12 160 Tufftride NSI 843 1015 780 843 SCM435 171 “Soft” Nitriding in 843 SS2250 353 gas medium “Soft” Nitriding in 793 38CrMoAl 78–97 gas medium — 40Cr 63–80 Gas Nitriding 773 SAE9254 49 Plasma Nitriding 793 722M24 72 (Pused) 793 (DC) 722M24 Plasma Nitriding 833 En40B 100 813 En19 110 793 Nitraps 46 823 36CrMo 100 793 36CrMo + 0.1Y 200 823 36CrMo + 0.1Ce 215 Low-temperature 753 SKD61 150 salt bath Nitriding 843 SKD61 106 (palsonite) 753 SCM435 141 843 SCM435 200

Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

First Embodiment

In accordance with the first embodiment of the present invention, steel is nitrided using the NaNO3 molten salt. The nitrided steel includes ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel.

Each of the ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel is submerged in the NaNO3 molten salt bath for 2 hours at a temperature of 500° C.

Table 2 shows changes in surface hardness and tensile strength of the samples nitrided in the molten salt bath, wherein the hardness was measured using a Vickers hardness tester under a load of 1 kgf.

In case of ultra-low carbon steel, the surface hardness increases by 119% and the tensile strength increases by 47%. In case of low carbon steel, the surface hardness increases by 47% and the tensile strength increases by 19%.

In case of medium carbon steel, the surface hardness increases by 32% and the tensile strength increases by 18%. In case of high carbon steel, the surface hardness increases by 28% and the tensile strength increases by 16%. In case of alloy steel, the surface hardness increases by 24% and the tensile strength increases by 17%.

That is, in case of steel, the surface hardness increases by 20% to 120% and the tensile strength increases by 15% to 50%.

The differences in the amount of increases shown in the surface hardness depending on the steel type can be attributed to the differences in the nitrogen diffusion rate associated with each type of steels determined by the carbon content therein.

TABLE 2 Change of Hardness Change of Tensile (Hv) Strength (kgf/mm2) In- After In- Type Before After creasing Before nitrid- creasing of nitriding nitriding rate nitriding ing rate steel process process (%) process process (%) Ultra 128 280 119 34 50 47 low carbon steel Low 194 286 47 62 74 19 carbon steel Medium 183 241 32 56 66 18 carbon steel High 230 294 28 73 85 16 carbon steel Alloy 226 281 24 71 83 17 steel

FIG. 1 is a graph the showing the hardness distribution in the thickness direction of the ultra-low carbon steel before (As) and after nitriding in the NaNO3 molten salt bath at 500° C. for 30 minutes, 1 hour, 2 hours and 5 hours, respectively.

The nitrided depth or the diffusion depth increases with increasing nitriding time, and the hardness decreases with increasing distance from the surface because the nitrogen concentration decreases with increasing distance from the surface. When the steel is nitrided for 5 hours, it can be seen that the steel is nitrided to a depth of about 0.6 mm from the surface.

FIG. 2 shows the hardness distribution along the thickness direction of low carbon steel nitrided in the NaNO3 molten-salt bath at 680° C. for 3, 6, 12 and 24 hours, respectively, wherein the hardness is measured using a Vickers hardness tester under a load of 3 kgf.

As shown in FIG. 2, the nitrided depth or the diffusion depth of the steel increases with increasing nitriding time. The nitrided depth of the steel after nitriding for 24 hours is about 3 mm, which is 6 times deeper than that obtained from the conventional nitriding method.

Also, the surface hardness after nitriding is 450 Hv, which is more than 4 times higher than that of the non-treated specimen.

Accordingly, the nitriding method of the present invention can increase the nitrided depth of the steel by 2 to 6 times compared to the conventional cyanide-based salt bath nitriding method.

FIG. 3 shows hardness distributions along the thickness direction of the ultra-low carbon steel before and after nitriding in the NaNO3 molten-salt bath at 500° C. and 600° C. for 3 hours. The nitrided depth of the steel nitrided at 600° C. is 3 times deeper than that of the steel nitrided at 500° C. The surface hardness of the steel nitrided at 600° C. is 100 Hv higher than that of the steel nitrided at 500° C. That is, the surface hardness and nitrided depth of steel increase with increasing nitriding temperature.

Table 3 shows changes in tensile strength of ultra low carbon steel depending on the nitriding temperature wherein the samples are nitrided for 3 hours at 450° C., 500° C., 550° C. and 600° C., respectively, using the salt-bath nitriding method of the first embodiment of the present invention.

As shown in FIG. 3, in case of the nitriding temperature of 450° C., the tensile strength increases by 5%. As the temperature increases, the tensile strength of the steel also increases. Accordingly, when the temperature is 600° C., the tensile strength increases by 134%.

TABLE 3 Nitriding Nitriding Tensile Increasing temperature time strength rate Division (° C.) (h) (kgf/mm2) (%) Before — — 34.8 0 nitriding After 450 3 36.6 5 nitriding 500 50.8 46 550 64.5 85 600 81.4 134

That is, since it is possible to simultaneously improve the hardness and the tensile strength by nitriding the steel according to the first embodiment, the present invention can be applied to diverse fields including diverse components and structural members.

Second Embodiment

In accordance with the second embodiment of the present invention, steel is nitrided by using the NaNO2 molten salt.

Steels including ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel are submerged in the salt bath at 450° C. for 2 hours.

Table 4 shows changes in surface hardness and tensile strength of the samples nitrided in the molten salt bath, wherein the surface hardness is measured using a Vickers hardness tester under a load of 1 kgf.

For ultra-low carbon steel, the surface hardness increases by 54% and the tensile strength increases by 21%. For low carbon steel, the surface hardness increases by 32% and the tensile strength increases by 15%.

For medium carbon steel, the surface hardness increases by 19% and the tensile strength increases by 13%. For high carbon steel, the surface hardness increases by 18% and the tensile strength increases by 12%.

For alloy steel, the surface hardness increases by 17% and the tensile strength increases by 14%.

That is, in case that steels are nitrided by the molten salt bath nitriding method of the second embodiment of the present invention, the surface hardness increases by 15% to 60%, and the tensile strength increases by 10% to 25%.

Accordingly, the molten salt bath nitriding method in accordance with the second embodiment of the present invention also increases the surface hardness and tensile strength of the steels.

TABLE 4 Change of Hardness Change of Tensile (Hv) Strength (kgf/mm2) In- In- Type creasing After creasing of Before After rate Before nitrid- rate steel nitriding nitriding (%) nitriding ing (%) Ultra- 128 197 54 34 41 21 low carbon steel Low 194 257 32 62 71 15 carbon steel Medium 183 218 19 56 63 13 carbon steel High 230 271 18 73 82 12 carbon steel Alloy 226 265 17 71 81 14 steel

Third Embodiment

In accordance with the third embodiment of the present invention, steels are nitrided using the KNO2 molten salt.

The steels including ultra-low carbon steel, low carbon steel, high carbon steel and alloy steel are submerged in the molten salt bath at 480° C. for 2 hours.

Table 5 shows changes in hardness and tensile strength of the samples submerged in the molten salt bath, wherein the surface hardness is measured using a Vickers hardness tester under a load of 1 kgf.

For ultra-low carbon steel, the surface hardness increases by 45% and the tensile strength is increases by 15%. For low carbon steel, the surface hardness increases by 25% and the tensile strength increases by 11%.

For high carbon steel, the surface hardness increases by 17% and the tensile strength increases by 10%. For alloy steel, the surface hardness increases by 12% and the tensile strength increases by 11%.

That is, when the steels are nitrided using the molten salt bath nitriding method of the third embodiment of the present invention, the surface hardness increases by 10% to 50%, and the tensile strength increases by 10% to 20%.

Accordingly, the molten salt bath nitriding method in accordance with the third embodiment of the present invention also increases the surface hardness and the tensile strength of the steels.

TABLE 5 Change of Hardness Change of Tensile (Hv) Strength (kgf/mm2) In- In- Type creasing After creasing of Before After rate Before nitrid- rate steel nitriding nitriding (%) nitriding ing (%) Ultra- 128 186 45 34 39 15 low carbon steel Low 194 243 25 62 69 11 carbon steel High 230 268 17 73 80 10 carbon steel Alloy 226 252 12 71 97 11 steel

Fourth Embodiment

In the fourth embodiment of the present invention, steel is nitrided using the KNO3 molten salt.

The steel to be nitrided is Interstitial-Free (IF) steel, which includes carbon (C) of 0.003 wt %, manganese (Mn) of 1.23 wt %, aluminum (Al) of 0.037 wt %, titanium (Ti) of 0.027 wt %, phosphorus (P) of 0.050 wt %, nitrogen (N) of 0.002 wt % and sulfur (S) of 0.008 wt %.

The IF steel is nitrided in the KNO3 molten bath at 560° C., 580° C., 600° C., 620° C. and 640° C., respectively.

FIG. 4 shows the surface hardness of the IF steel nitrided in the KNO3 molten bath as functions of time and temperature.

As shown in FIG. 4, as the nitriding time and temperature increase, the surface hardness increases under most temperature conditions. Although the increase of the hardness can be explained as solution strengthening, the present invention is not limited to this theory.

However, when the nitriding time in the KNO3 molten salt at 620° C. exceeds 8 hours, or the nitriding time in the KNO3 molten salt at 640° C. exceeds one hour, the surface hardness decreases. It is understood that this decrease in the surface hardness is caused by the formation of the nitrided layer in the grain boundaries of the IF steel.

In Table 6, the surface hardness values of the IF steel nitrided by the third embodiment of the present invention are given. When the IF steel is nitrided at temperatures of 560° C. to 640° C., the surface hardness increases by 75% to 130%.

TABLE 6 Change of Hardness (Hv) Change of Hardness after nitriding for 16 h. (Hv) after nitriding for 1 h. Increasing Increasing Nitriding Before After rate Nitriding Before After rate Temperature nitriding nitriding (%) Temperature nitriding nitriding (%) 560° C. 165 289 75 620° C. 165 336 104 580° C. 165 329 99 640° C. 165 355 115 600° C. 165 379 130

FIG. 5 shows the hardness distribution along the thickness direction of the IF steel nitrided by the fourth embodiment of the present invention.

The IF steel is nitrided in the KNO3 molten salt at 560° C. for 16 hours and at temperatures of 560° C., 580° C., 600° C. and 620° C. for 8 hours.

Referring to FIG. 5, the hardness of the IF steel decreases with increasing depth from the surface because the nitrogen concentration decreases with increasing distance from the steel surface. When the nitrided depth is defined as the distance between the surface and the position where the hardness value is equaled to 110% of that of the center of the IF steel before nitriding, the nitrided depth formed in each condition ranges from about 1.38 mm to 1.5 mm, which is 3 to 5 times thicker than the thickness of the nitrided layer formed using the conventional method.

FIG. 6 is a graph showing hardness distribution along the thickness direction of the IF steel nitrided in the KNO3 molten salt at 640° C. for 1 hour, 2 hours, 4 hours, 8 hours and 16 hours.

As shown in the FIG. 6, for IF steel, as the nitriding time increases, the difference in hardness between the surface and the interior decreases, resulting in the IF steel having, as well as an increased surface hardness, an increased bulk hardness, as a consequence of nitrogen diffusing into the interior and the difference in concentration thereof between the surface and the interior decreasing. In other word, the nitriding method in accordance with the present invention will lead to an IF steel having an increased surface and bulk hardness, resulting from a nitrogen diffusing into the interior at a higher diffusion rate.

Fifth Embodiment

In the fifth embodiment of the present invention, steel is nitrided using the Ca(NO3)2 molten salt.

The steel to be nitrided in the fifth embodiment is low carbon steel.

Since Ca(NO3)2 is highly hygroscopic at a room temperature, including combined water, it is preferred to use Ca(NO3)2 after removing moisture by heating for a predetermined time.

The fifth embodiment of the present invention includes the process of removing moisture by heating Ca(NO3)2 for 4 hours at 100° C. to 150° C., heating Ca(NO3)2 to 580° C. to form the Ca(NO3)2 molten-salt bath and submerging the low carbon steel in the bath for 3 hours.

FIG. 7 is a graph showing the surface hardness profile in low carbon steel nitrided by the fifth embodiment of the present invention.

As shown in FIG. 7, the low carbon steel nitrided by the fifth embodiment is nitrided to a depth of 0.5 mm from the surface, and has the surface hardness that is 2 times higher than the surface hardness (As) of the steel before nitriding.

Sixth Embodiment

In the sixth embodiment of the present invention, steel is nitrided using a molten mixture of KNO3 and NaNO3.

In the sixth embodiment of the present invention, the low carbon steel is nitrided in the molten mixture of KNO3 and NaNO3 whose mixture ratios are 1:1, 8:2 and 2:8.

Table 7 shows the surface hardness values of steels nitrided by the sixth embodiment of the present invention. Various types of steel are submerged in the molten mixture of KNO3 and NaNO3 whose ratio is 1:1 for 12 or 24 hours at 650° C.

At this time, the hardness is measured using a Vickers hardness tester under a load of 3 kg.

The hardness values of the steels nitrided in the mixture of KNO3 and NaNO3 increase by 69% to 251% depending on the steel type.

TABLE 7 Change of Hardness (Hv) Nitriding Increasing Time Before After rate Type of steel (h) nitriding nitriding (%) Ultra-low 24 128 449 251 carbon steel low carbon 12 194 406 109 steel Medium carbon 12 183 391 114 steel High carbon 24 230 389 69 steel Alloy steel 24 226 387 71

Various steels are submerged in the mixture of KNO3 and NaNO3 whose ratio is 1:1 at 580° C., and changes in surface hardness and tensile strength of the nitrided steels depending on nitriding time are measured.

As shown in Table 8, nitriding in accordance with the fifth embodiment of the present invention increases the hardness and the tensile strength of all the steels. The hardness and tensile strength increase with increasing nitriding time.

TABLE 8 Change of Hardness Change of Tensile (Hv) strength (kgf/mm2) Nitriding Increasing Increasing Type of Time Before After rate Before After rate steel (h) nitriding nitriding (%) nitriding nitriding (%) Ultra- 3 120 283 136 35 48 37 low 12 120 421 251 35 92 163 carbon steel low 3 200 283 42 45 55 22 carbon 12 200 403 102 45 79 76 steel Medium 3 130 181 39 45 57 27 carbon 12 130 398 206 45 88 84 steel High 3 150 201 34 60 76 27 carbon 12 150 391 161 60 87 45 steel Alloy 3 200 274 37 55 75 36 steel 12 200 409 105 55 90 64

FIG. 8 is a graph showing the hardness profiles of steel nitrided at 680° C. for 200 minutes in the KNO3 bath, the NaNO3 bath, the 50% KNO3-50% NaNO3 mixture bath at 680° C. for 200 minutes.

The hardness was measured using a Vickers hardness tester.

In FIG. 8, the steel nitrided in the mixture bath has a nitrided depth of 1.5 mm and a surface hardness of 160 Hv, which is higher than that of the steel nitrided in the single salt baths and 3 times higher than that of the steel before nitriding.

FIG. 9 is a graph showing the hardness profiles of the low carbon steel nitrided in the 80% KNO3-20% NaNO3 bath and 20% KNO3-80% NaNO3 bath at 650° C. for 4 hours, respectively.

As shown in FIG. 9, the surface hardness of the steel nitrided in the mixture baths is about 2 times higher than that of the steel before nitriding.

The present invention can solve an environmental pollution problem and can reduce a cost for nitriding steels by using molten non-cyanide salts, such as sodium nitrate (NaNO3), sodium nitrite (NaNO2), calcium nitrate (Ca(NO3)2) and their mixtures.

Since the present invention can increase the nitrided depth or nitrogen-diffusion depth of steels two to six times higher than that obtained using conventional nitriding methods, thereby nitriding the inner part as well as the surface of the metal, its applications are extended to various fields.

Since the present invention can be applied to bulk hardening as well as surface hardening of steels by increasing hardness and tensile strength of the metal, it is possible to apply the present invention to many fields including light and highly strong automobile components and diverse structural members which require improved wear resistance, corrosion resistance and fatigue life.

The present application contains subject matter related to Korean patent application No. 2006-0049077, filed in the Korean Intellectual Property Office on May 30, 2006, the entire contents of which are incorporated herein by reference.

The terms and words used in the present specification and claims should not be construed to be limited to the common or dictionary meaning, because an inventor defines the concept of the terms appropriately to describe his/her invention as best he/she can. Therefore, they should be construed as a meaning and concept fit to the technological concept and scope of the present invention.

Therefore, the embodiments and structure described in the present specification are nothing but one preferred embodiment of the present invention, and do not represent all of the technological concept and scope of the present invention. Therefore, it should be understood that many equivalents and modified embodiments that can substitute those described in this specification exist.

Claims

1. A method for nitriding a metal in a salt bath, comprising the steps of:

a) immersing at least one salt selected from a group consisting of KNO3, KNO2, Ca(NO3)2, NaNO3 and NaNO2 into the salt bath;
b) melting the salt by heating and maintaining the molten salt at a predetermined temperature; and
c) submerging the metal in the salt bath.

2. The method as recited in claim 1, wherein the predetermined temperature is within a range of 400° C. to 700° C.

3. The method as recited in claim 1, wherein, in the step c), a submerging time is within a range of 1 minute to 24 hours.

4. The method as recited in claim 2, wherein the metal is one of iron and steels.

5. The method as recited in claim 3, wherein the metal is one of iron and steels.

6. A metal nitrided in a salt bath including at least one selected from a group consisting of KNO3, KNO2, Ca(NO3)2, NaNO3 and NaNO2, wherein the metal is iron and the iron is nitrided to a depth of 0.1 mm to 3.0 mm from the surface.

7. A metal nitrided in a salt bath including at least one selected from a group consisting of KNO3, KNO2, Ca(NO3)2, NaNO3 and NaNO2, wherein the metal is steel and the steel is nitrided to a depth of 0.1 mm to 3.0 mm from the surface.

8. The metal as recited in claim 7, wherein the steel is at least one selected from a group consisting of ultra low carbon steel, low carbon steel, medium carbon steel, high carbon steel, alloy steel and IF steel.

9. The metal as recited in claim 8, wherein the ultra low carbon steel has a surface hardness being more than 120 Hv to equal to or less than 450 Hv.

10. The metal as recited in claim 8, wherein the low carbon steel has a surface hardness being more than 200 Hv to equal to or less than 410 Hv.

11. The metal as recited in claim 8, wherein the medium carbon steel has a surface hardness being more than 130 Hv to equal to or less than 420 Hv.

12. The metal as recited in claim 8, wherein the high carbon steel has a surface hardness being more than 150 Hv to equal to or less than 400 Hv.

13. The metal as recited in claim 8, wherein the alloy steel has a surface hardness being more than 200 Hv to equal to or less than 410 Hv.

14. The metal as recited in claim 8, wherein the IF steel has a surface hardness being more than 165 Hv to equal to or less than 400 Hv.

15. The metal as recited in claim 9, wherein the ultra-low carbon steel has a tensile strength being more than 35 kgf/mm2 to equal to or less than 110 kgf/mm2.

16. The metal as recited in claim 10, wherein the low carbon steel has a tensile strength being more than 45 kgf/mm2 to equal to or less than 110 kgf/mm2.

17. The metal as recited in claim 11, wherein the medium carbon steel has a tensile strength being more than 45 kgf/mm2 to equal to or less than 100 kgf/mm2.

18. The metal as recited in claim 12, wherein the high carbon steel has a tensile strength being more than 60 kgf/mm2 to equal to or less than 95 kgf/mm2.

19. The metal as recited in claim 13, wherein the alloy steel has a tensile strength being more than 55 kgf/mm2 to equal to or less than 110 kgf/mm2.

20. The metal as recited in claim 14, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

21. The metal as recited in claim 15, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

22. The metal as recited in claim 16, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

23. The metal as recited in claim 17, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

24. The metal as recited in claim 18, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

25. The metal as recited in claim 19, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

26. The metal as recited in claim 14, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

27. The metal as recited in claim 15, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

28. The metal as recited in claim 16, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

29. The metal as recited in claim 17, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

30. The metal as recited in claim 18, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

31. The metal as recited in claim 19, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

32. The metal as recited in claim 14, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

33. The metal as recited in claim 15, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

34. The metal as recited in claim 16, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

35. The metal as recited in claim 17, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

36. The metal as recited in claim 18, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

37. The metal as recited in claim 19, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

38. The metal as recited in claim 14, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

39. The metal as recited in claim 15, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

40. The metal as recited in claim 16, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

41. The metal as recited in claim 17, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

42. The metal as recited in claim 18, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

43. The metal as recited in claim 19, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

44. The metal as recited in claim 14, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

45. The metal as recited in claim 15, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

46. The metal as recited in claim 16, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

47. The metal as recited in claim 17, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

48. The metal as recited in claim 18, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

49. The metal as recited in claim 19, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

50. The metal as recited in claim 14, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

51. The metal as recited in claim 15, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

52. The metal as recited in claim 16, wherein a titanium content of the steel ranges from 0.1 wt % to 0.1 wt %.

53. The metal as recited in claim 17, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

54. The metal as recited in claim 18, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

55. The metal as recited in claim 19, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

56. The metal as recited in claim 14, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

57. The metal as recited in claim 15, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

58. The metal as recited in claim 16, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

59. The metal as recited in claim 17, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

60. The metal as recited in claim 18, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

61. The metal as recited in claim 19, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

62. The metal as recited in claim 14, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

63. The metal as recited in claim 15, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

64. The metal as recited in claim 16, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

65. The metal as recited in claim 17, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

66. The metal as recited in claim 18, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

67. The metal as recited in claim 19, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

68. The metal as recited in claim 14, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

69. The metal as recited in claim 15, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

70. The metal as recited in claim 16, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

71. The metal as recited in claim 17, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

72. The metal as recited in claim 18, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

73. The metal as recited in claim 19, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

74. The metal as recited in claim 15, wherein the ultra-low carbon steel has a carbon content ranging from at least 0.0001 wt % to less than 0.13 wt %.

75. The metal as recited in claim 16, wherein the low carbon steel has a carbon content ranging from at least 0.13 wt % to less than 0.2 wt %.

76. The metal as recited in claim 17, wherein the medium carbon steel has a carbon content ranging from at least 0.21 wt % to less than 0.51 wt %.

77. The metal as recited in claim 18, wherein the high carbon steel has a carbon content ranging from at least 0.51 wt % to less than 2.0 wt %.

Patent History
Publication number: 20070193659
Type: Application
Filed: Oct 27, 2006
Publication Date: Aug 23, 2007
Applicant: ILJIN LIGHT METAL CO., LTD. (Kyunggi-Do)
Inventors: Hee Won Jung (Kyunggi-Do), Dong Nyung Lee (Seoul), Young June Park (Kyunggi-Do), Dong Sam Kim (Kyunggi-Do), Kyu Hwan Oh (Seoul), Yinzhong Shen (Daejeon City)
Application Number: 11/588,370
Classifications
Current U.S. Class: Nitriding (148/228); Ferrous (i.e., Iron Base) (148/318)
International Classification: C23C 8/26 (20060101);