A high-toughness wear-resistant alloy steel product and its preparation method

Abstract

The present invention relates to an alloy steel product prepared by a method comprising the following steps: 1) forging a raw alloy steel; 2) subjecting the forged product to a multi-element co-penetration treatment; and 3) quenching the product after the multi-element co-penetration treatment. The raw alloy steel comprises, by mass percentage, C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, B: 0.001-0.005%, Ti: 0.1-0.5%, Al: 0.03-0.1%, S: ≤0.015%, P: ≤0.015%, with the remainder being Fe and unavoidable impurities. The alloy steel product has high surface hardness, good wear resistance, and high core toughness, and is suitable for use as bucket teeth for excavators, particularly conical bucket teeth.

Description

Technical Field

[0001] The present invention belongs to the technical field of alloys, and in particular relates to a high-toughness wear-resistant alloy steel product and a preparation method thereof. Background Art

[0002] Bucket teeth are one of the most severely worn parts of an excavator. They can be divided into earth teeth, rock teeth, and conical teeth according to their use. Earth teeth are mainly used in light working environments such as excavating soil and river sand, and require high wear resistance; rock teeth are used for heavy-duty operations such as rock, crushed stone, and gravel, and have higher requirements for wear resistance; and conical teeth are suitable for drilling rock formations with low hardness and need to have greater impact resistance. They are required to have not only high wear resistance but also a certain degree of impact toughness. Conical bucket teeth are subjected to complex forces during operation. When in contact with materials, they are subjected to both impact and bending moments. During service, the tip of the bucket teeth is subjected to greater impact and sliding abrasive wear, and furrows and deformation often appear on the tip surface, causing surface wear or shedding. Bucket tooth breakage and poor wear resistance will lead to frequent downtime and production suspension for bucket tooth replacement, affecting the working efficiency of the excavator.

[0003] Excavator bucket teeth are typically cast from high-manganese steel or low-alloy steel. High-manganese steel has good toughness, but requires significant impact to work-harden and fully develop its wear resistance. Low-alloy steel bucket teeth, after quenching, have high hardness and good wear resistance, but they have poor toughness and are prone to breakage.

[0004] At present, bucket teeth at home and abroad are usually prepared by heat treatment or surface strengthening. CN102242314A discloses a multi-element alloy toughened, wear-resistant medium manganese steel and its preparation process. The composition of the alloy steel by mass percentage is: C: 0.9-1.3, Mn: 8-10, Si: 0.2-0.6, Cr: 1.0-2.5, W: 0.5-2.0, V: 0.1-0.5, Ti: 0.1-0.3, Re≤0.15, B≤0.01, S≤0.04, P≤0.07, and the rest is Fe. It is treated with a composite modifier of rare earth ferrosilicon alloy, ferrotitanium alloy and ferrovanadium alloy. Through the control of the process and parameters such as smelting, lost foam negative pressure forming, water toughening, tempering, etc., the wear-resistant steel parts such as electric shovel bucket teeth, semi-autogenous grinding machine liners, conveyor scrapers, etc. made of this alloy steel have a service life more than twice that of high manganese steel. However, this alloy steel contains a large amount of precious alloy elements and requires the use of rare earth composite modifiers. The preparation process is complex and difficult to promote and apply on a large scale. CN102453911A discloses a surface strengthening method for excavator bucket teeth, which is characterized by the following process steps: (1) removing the surface oxide layer and oily foreign matter from the surface of the excavator bucket teeth; (2) placing the excavator bucket teeth in a laser processing system consisting of a synchronous powder feeder, a six-axis five-linkage processing machine tool, and a cross-flow gas laser generator for laser cladding, and cladding a tungsten carbide alloy powder with excellent wear resistance and good toughness on the bucket teeth to form a dense metallurgical coating. This method clads the surface of the bucket teeth with wear-resistant hard alloy, and the cladding layer will fall off under the action of large impact, and cannot play a wear-resistant role.

[0005] Therefore, it is necessary to develop a wear-resistant bucket tooth with high toughness. Summary of the Invention

[0006] The present invention provides an alloy steel product, which is prepared by a method comprising the following steps:

[0007] 1) Forging the raw alloy steel into shape;

[0008] 2) subjecting the forged product to multi-element co-infiltration treatment;

[0009] 3) The product after multi-element co-penetration treatment is quenched.

[0010] The element composition of the raw alloy steel is calculated by mass percentage as follows: C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, B: 0.001-0.005%, Ti: 0.1-0.5%, Al: 0.03-0.1%, S: ≤0.015%, P: ≤0.015%, and the balance is Fe and unavoidable impurities.

[0011] In certain embodiments, step 1) comprises:

[0012] The raw alloy steel is heated to 1050-1150° C. and forged under a forging pressure of 800-1600t.

[0013] In certain embodiments, step 2) comprises:

[0014] The forged product is kept at a temperature of 1100-1250°C for 4-8 hours for multi-element co-infiltration treatment.

[0015] In certain embodiments, step 3) comprises:

[0016] a) Cooling the product after multi-element co-infiltration treatment to 860-920°C;

[0017] b) keeping the product obtained in the previous step in a first salt bath at 300-350° C. for 3-10 minutes;

[0018] c) keeping the product obtained in the previous step in a second salt bath at 400-500° C. for 30-60 minutes;

[0019] d) cooling the product obtained in the previous step with water.

[0020] In certain embodiments, step b) is to keep the product obtained in the previous step in a first salt bath at 310-350° C. for 3-10 minutes.

[0021] In certain embodiments, step c) is to keep the product obtained in the previous step in a second salt bath at 420-500° C. for 30-60 min.

[0022] In certain embodiments, the first salt bath is composed of 30-60% sodium nitrate (NaNO3) and 40-70% potassium nitrate (KNO3). In certain embodiments, the second salt bath is composed of 60-90% potassium chloride (KCl) and 10-40% chromium chloride (CrCl3).

[0023] In the present invention, step 3) utilizes the residual heat after the multi-element co-penetration treatment to quench the product after the multi-element co-penetration treatment.

[0024] In certain embodiments, the multi-component co-penetration agent used in the multi-component co-penetration treatment consists of C powder, Ti powder, Cr powder, Mo powder, Al powder, and NH4Cl powder.

[0025] In certain embodiments, the composition of the multi-component co-penetration agent is: C powder 5-20%, Ti powder 10-40%, Cr powder 15-35%, Mo powder 10-20%, Al powder 5-15%, and NH4Cl powder 5-25%.

[0026] In certain embodiments, the surface hardness of the steel alloy product is ≥64HRC, such as about 65HRC, about 66HRC, about 67HRC, about 68HRC, about 69HRC, about 70HRC. In certain embodiments, the core hardness of the steel alloy product is ≥50HRC, such as about 51HRC, about 52HRC, about 53HRC, about 54HRC, about 55HRC, about 56HRC. In certain embodiments, the impact energy KV2 of the steel alloy product is ≥40J, such as about 44J, about 45J, about 46J, about 47J, about 48J, about 49J, about 50J, about 51J, about 52J, about 55J, about 58J, about 60J, about 62J, about 65J, about 67J, about 68J. In certain embodiments, the dynamic load wear amount of the alloy steel product is ≤0.20g, such as about 0.19g, about 0.18g, about 0.17g, about 0.16g, about 0.15g, about 0.14g, about 0.13g, about 0.12g, about 0.11g, about 0.1g, about 0.9g, about 0.8g, about 0.7g. In certain embodiments, the impact energy KV2 of the alloy steel product is ≥45J. In certain embodiments, the dynamic load wear amount of the alloy steel product is ≤0.19g.

[0027] In certain embodiments, the alloy steel product is a bucket tooth.

[0028] The present invention also provides an excavator or a loader comprising the alloy steel product.

[0029] The present invention also provides a method for preparing an alloy steel product, comprising:

[0030] 1) Forging the raw alloy steel into shape;

[0031] 2) subjecting the forged product to multi-element co-infiltration treatment;

[0032] 3) The product after multi-element co-penetration treatment is quenched.

[0033] The element composition of the raw alloy steel is calculated by mass percentage as follows: C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, B: 0.001-0.005%, Ti: 0.1-0.5%, Al: 0.03-0.1%, S: ≤0.015%, P: ≤0.015%, and the balance is Fe and unavoidable impurities.

[0034] In certain embodiments, step 1) of the method comprises:

[0035] The raw alloy steel is heated to 1050-1150° C. and forged under a forging pressure of 800-1600t.

[0036] In certain embodiments, step 2) of the method comprises:

[0037] The forged product is kept at a temperature of 1100-1250°C for 4-8 hours for multi-element co-infiltration treatment.

[0038] In certain embodiments, the multi-component co-penetration agent used in the multi-component co-penetration treatment in the method consists of C powder, Ti powder, Cr powder, Mo powder, Al powder, and NH4Cl powder.

[0039] In certain embodiments, the composition of the multi-component co-penetration agent in the method is: C powder 5-20%, Ti powder 10-40%, Cr powder 15-35%, Mo powder 10-20%, Al powder 5-15%, and NH4Cl powder 5-25%.

[0040] In certain embodiments, step 3) of the method comprises:

[0041] a) Cooling the product after multi-element co-infiltration treatment to 860-920°C;

[0042] b) keeping the product obtained in the previous step in a first salt bath at 300-350° C. for 3-10 minutes;

[0043] c) keeping the product obtained in the previous step in a second salt bath at 400-500° C. for 30-60 minutes;

[0044] d) cooling the product obtained in the previous step with water.

[0045] In certain embodiments, step b) of the method is to keep the product obtained in the previous step in a first salt bath at 310-350° C. for 3-10 minutes.

[0046] In certain embodiments, step c) of the method is to keep the product obtained in the previous step in a second salt bath at 420-500° C. for 30-60 minutes.

[0047] In certain embodiments, the first salt bath is composed of 30-60% sodium nitrate (NaNO3) and 40-70% potassium nitrate (KNO3). In certain embodiments, the second salt bath is composed of 60-90% potassium chloride (KCl) and 10-40% chromium chloride (CrCl3).

[0048] In the present invention, step 3) utilizes the residual heat after multi-element co-penetration to quench the product after the multi-element co-penetration treatment.

[0049] As used herein, the term "about" is understood to mean within the normal tolerance range of the art, for example, within 2 standard deviations of the mean. Specifically, the term "about" is understood to mean within + / -10%, + / -9%, + / -8%, + / -7%, + / -6%, + / -5%, + / -4%, + / -3%, + / -2%, + / -1%, + / -0.5%, + / -0.4%, + / -0.3%, + / -0.2%, + / -0.1% of the value. Unless otherwise apparent from the context, all numerical values ​​provided herein are modified by the term "about."

[0050] Beneficial effects of the present invention

[0051] The invention optimizes the ratio of chemical elements, reduces the amount of carbon and alloy elements, and adopts forging, multi-element co-penetration and heat treatment to prepare a high-toughness, high-wear-resistant alloy steel product.

[0052] The alloy steel product provided by the present invention has high surface hardness, good wear resistance, and high core toughness. It is suitable for use as bucket teeth of excavators, especially conical bucket teeth, and can meet the use requirements of conical bucket teeth in coal mines, ground mines, frozen soil and other working conditions.

[0053] The present invention utilizes the residual heat after multi-element co-penetration to perform quenching treatment on the product after multi-element co-penetration treatment, which can reduce energy consumption and cost and is conducive to realizing industrial production. DETAILED DESCRIPTION

[0054] The embodiments of the present invention will be described in detail below with reference to the examples. However, it will be understood by those skilled in the art that the following examples are only intended to illustrate the present invention and should not be construed as limiting the scope of the present invention. Where specific conditions are not specified in the examples, conventional conditions or conditions recommended by the manufacturer were used. Where the raw materials, equipment, or instruments used are not specified by the manufacturer, they are all commercially available conventional products.

[0055] Example 1

[0056] The raw alloy steel used to prepare the high-toughness, wear-resistant conical bucket teeth in this embodiment has an elemental composition, by mass percentage, of C: 0.25%, Si: 0.50%, Mn: 5.0%, Ni: 0.6%, Cr: 0.30%, B: 0.0015%, Ti: 0.15%, Al: 0.03%, S: 0.015%, P: 0.015%, and the remainder is Fe and unavoidable impurities.

[0057] The raw alloy steel is heated to 1130°C and then forged using a 1000t press. The forged bucket teeth are then subjected to a multi-component co-penetration treatment at 1200°C for 5 hours. The multi-component co-penetration agent is composed of 20% C + 40% Ti + 15% Cr + 10% Mo + 5% Al + 10% NH4Cl powders that are fully stirred and mixed. The bucket teeth after the multi-component co-penetration treatment are quenched using the residual heat after the multi-component co-penetration treatment. Specifically, the bucket teeth after the multi-component co-penetration treatment are cooled to 910°C and then directly placed in a first salt bath at 350°C for 3 minutes. They are then placed in a second salt bath at 500°C for 30 minutes. Finally, they are water-cooled to obtain high-toughness, wear-resistant conical bucket teeth. The first salt bath consists of 30% sodium nitrate (NaNO3) and 70% potassium nitrate (KNO3), and the second salt bath consists of 90% potassium chloride (KCl) and 10% chromium trichloride (CrCl3).

[0058] Example 2

[0059] The raw alloy steel used to prepare the high-toughness, wear-resistant conical bucket teeth in this embodiment has an elemental composition, by mass percentage, of C: 0.50%, Si: 0.30%, Mn: 4.0%, Ni: 1.0%, Cr: 0.20%, B: 0.0010%, Ti: 0.50%, Al: 0.10%, S: 0.010%, P: 0.008%, and the remainder is Fe and unavoidable impurities.

[0060] The raw alloy steel is heated to 1150°C and then forged using an 800t press. The forged bucket teeth are then subjected to a multi-component co-penetration treatment at 1250°C for 8 hours. The multi-component co-penetration agent is composed of 10% C + 10% Ti + 35% Cr + 10% Mo + 10% Al + 25% NH4Cl powders that are fully stirred and mixed. The bucket teeth after the multi-component co-penetration treatment are quenched using the residual heat after the multi-component co-penetration treatment. Specifically, the bucket teeth after the multi-component co-penetration treatment are cooled to 860°C and then directly placed in a first salt bath at 350°C for 7 minutes. They are then placed in a second salt bath at 420°C for 50 minutes. Finally, they are water-cooled to obtain high-toughness, wear-resistant conical bucket teeth. The first salt bath consists of 50% sodium nitrate (NaNO3) and 50% potassium nitrate (KNO3), and the second salt bath consists of 70% potassium chloride (KCl) and 30% chromium trichloride (CrCl3).

[0061] Example 3

[0062] The raw alloy steel used to prepare the high-toughness, wear-resistant conical bucket teeth in this embodiment has an elemental composition, by mass percentage, of C: 0.60%, Si: 0.40%, Mn: 8.0%, Ni: 0.9%, Cr: 0.70%, B: 0.0045%, Ti: 0.50%, Al: 0.09%, S: 0.010%, P: 0.010%, and the remainder is Fe and unavoidable impurities.

[0063] The raw alloy steel is heated to 1080°C and then forged using a 1500t press. The forged bucket teeth are then subjected to a multi-component co-penetration treatment at 1130°C for 7 hours. The multi-component co-penetration agent is composed of 10% C + 30% Ti + 30% Cr + 15% Mo + 10% Al + 5% NH4Cl powders that are fully stirred and mixed. The bucket teeth after the multi-component co-penetration treatment are quenched using the residual heat after the multi-component co-penetration treatment. Specifically, the bucket teeth after the multi-component co-penetration treatment are cooled to 870°C and then directly placed in a first salt bath at 330°C for 5 minutes. Then, they are placed in a second salt bath at 450°C for 40 minutes. Finally, they are water-cooled to obtain high-toughness and wear-resistant conical bucket teeth. The first salt bath is composed of 60% sodium nitrate (NaNO3) and 40% potassium nitrate (KNO3), and the second salt bath is composed of 65% potassium chloride (KCl) and 35% chromium trichloride (CrCl3).

[0064] Example 4

[0065] The raw alloy steel used to prepare the high-toughness, wear-resistant conical bucket teeth in this embodiment has an elemental composition, by mass percentage, of C: 0.40%, Si: 0.50%, Mn: 6.0%, Ni: 0.7%, Cr: 0.55%, B: 0.0030%, Ti: 0.30%, Al: 0.05%, S: 0.015%, P: 0.010%, and the remainder is Fe and unavoidable impurities.

[0066] The raw alloy steel is heated to 1100°C and then forged using a 1200t press. The forged bucket teeth are then subjected to a multi-component co-penetration treatment at 1190°C for 6 hours. The multi-component co-penetration agent is composed of 15% C + 20% Ti + 20% Cr + 20% Mo + 15% Al + 10% NH4Cl powders that are fully stirred and mixed. The bucket teeth after the multi-component co-penetration treatment are quenched using the residual heat after the multi-component co-penetration treatment. Specifically, the bucket teeth after the multi-component co-penetration treatment are cooled to 890°C and then directly placed in a first salt bath at 340°C for 10 minutes. They are then placed in a second salt bath at 470°C for 60 minutes. Finally, they are water-cooled to obtain high-toughness, wear-resistant conical bucket teeth. The first salt bath is composed of 45% sodium nitrate (NaNO3) and 55% potassium nitrate (KNO3), and the second salt bath is composed of 80% potassium chloride (KCl) and 20% chromium trichloride (CrCl3).

[0067] Example 5

[0068] The raw alloy steel used to prepare the high-toughness, wear-resistant conical bucket teeth in this embodiment has an elemental composition, by mass percentage, of C: 0.20%, Si: 0.60%, Mn: 7.0%, Ni: 0.5%, Cr: 0.80%, B: 0.0050%, Ti: 0.10%, Al: 0.08%, S: 0.012%, P: 0.012%, and the remainder is Fe and unavoidable impurities.

[0069] The raw alloy steel is heated to 1050°C and then forged using a 1600t press. The forged bucket teeth are then subjected to a multi-component co-penetration treatment at 1100°C for 8 hours. The multi-component co-penetration agent is composed of 5% C, 25% Ti, 15% Cr, 20% Mo, 15% Al, and 20% NH4Cl powders that are thoroughly stirred and mixed. The bucket teeth are then quenched using the residual heat after the multi-component co-penetration treatment. Specifically, the temperature of the multi-component co-penetration-treated bucket teeth is reduced to 920°C. The teeth are then directly placed in a first salt bath at 310°C for 8 minutes, then placed in a second salt bath at 430°C for 40 minutes, and finally water-cooled to obtain high-toughness, wear-resistant conical bucket teeth. The first salt bath is composed of 40% sodium nitrate (NaNO3) and 50% potassium nitrate (KNO3), and the second salt bath is composed of 60% potassium chloride (KCl) and 40% chromium trichloride (CrCl3).

[0070] After testing, the physical and mechanical properties of the high-toughness wear-resistant conical bucket teeth prepared in Examples 1-5 are shown in Table 1. The wear test was carried out using an MLD-10 dynamic load abrasive wear testing machine. The dynamic load wear test parameters were: impact energy 2J, impact time 1h, impact frequency 100 times / min, and abrasive (5-10) mesh quartz sand. The impact energy was measured according to GB / T 229-2020, and the hardness was measured according to GB / T 230.1-2018.

[0071] Table 1 Physical and mechanical properties of high toughness and wear-resistant tapered bucket teeth

[0072]

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to preferred embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or some technical features can be replaced by equivalents without departing from the spirit of the technical solutions of the present invention. They should all be included in the scope of the technical solutions claimed for protection by the present invention.

Claims

1. An alloy steel product prepared by a method comprising the following steps: 1) Forging the raw alloy steel into shape; 2) subjecting the forged product to multi-element co-infiltration treatment; 3) The product after multi-element co-penetration treatment is quenched. The element composition of the raw material alloy steel is calculated by mass percentage as follows: C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, B: 0.001-0.005%, Ti: 0.1-0.5%, Al: 0.03-0.1%, S: ≤0.015%, P: ≤0.015%, and the balance is Fe and unavoidable impurities. Wherein step 1) comprises: The raw alloy steel is heated to 1050-1150° C. and forged under a forging pressure of 800-1600t; Step 2) comprises: subjecting the forged product to a multi-component co-penetration treatment at a temperature of 1100-1250° C. for 4-8 hours, wherein the multi-component co-penetration agent used in the multi-component co-penetration treatment comprises C powder, Ti powder, Cr powder, Mo powder, Al powder, and NH 4 Cl powder; Step 3) includes: a) Cooling the product after multi-element co-infiltration treatment to 860-920°C; b) keeping the product obtained in the previous step in a first salt bath at 300-350° C. for 3-10 minutes; c) keeping the product obtained in the previous step in a second salt bath at 400-500° C. for 30-60 minutes; d) cooling the product obtained in the previous step with water.

2. The alloy steel product according to claim 1, wherein the composition of the multi-component co-penetration agent is: C powder 5-20%, Ti powder 10-40%, Cr powder 15-35%, Mo powder 10-20%, Al powder 5-15%, and NH4Cl powder 5-25%.

3. The alloy steel product of claim 1, wherein step b) is to keep the product obtained in the previous step in a first salt bath at 310-350°C for 3-10 minutes.

4. The alloy steel product of claim 1, wherein step c) is to keep the product obtained in the previous step in a second salt bath at 420-500°C for 30-60 minutes.

5. The alloy steel product of claim 1, wherein the first salt bath consists of 30-60% sodium nitrate and 40-70% potassium nitrate.

6. The alloy steel product of claim 1, wherein the second salt bath consists of 60-90% potassium chloride and 10-40% chromium trichloride.

7. The alloy steel product according to any one of claims 1 to 6, having one or more of the following characteristics: i) Surface hardness ≥ 64HRC, ii) Core hardness ≥ 50HRC, iii) Impact energy KV2 ≥ 40J, iv) Dynamic load wear ≤ 0.20g.

8. The alloy steel product according to claim 7, wherein the impact energy KV2 is ≥ 45 J.

9. The alloy steel product according to claim 7, wherein the dynamic wear loss is ≤ 0.19 g.

10. The alloy steel product according to any one of claims 1 to 6, which is a bucket tooth.

11. An excavator or a loader comprising the alloy steel product according to any one of claims 1 to 10.

12. A method for preparing an alloy steel product, comprising: 1) Forging the raw alloy steel into shape; 2) subjecting the forged product to multi-element co-infiltration treatment; 3) The product after multi-element co-penetration treatment is quenched. The element composition of the raw material alloy steel is calculated by mass percentage as follows: C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, B: 0.001-0.005%, Ti: 0.1-0.5%, Al: 0.03-0.1%, S: ≤0.015%, P: ≤0.015%, and the balance is Fe and unavoidable impurities. The step 1) comprises: heating the raw alloy steel to 1050-1150° C. and forging the raw alloy steel under a forging pressure of 800-1600t; Step 2) comprises: subjecting the forged product to a multi-component co-penetration treatment at a temperature of 1100-1250° C. for 4-8 hours, wherein the multi-component co-penetration agent used in the multi-component co-penetration treatment comprises C powder, Ti powder, Cr powder, Mo powder, Al powder, and NH 4 Cl powder; Step 3) includes: a) Cooling the product after multi-element co-infiltration treatment to 860-920°C; b) keeping the product obtained in the previous step in a first salt bath at 300-350° C. for 3-10 minutes; c) keeping the product obtained in the previous step in a second salt bath at 400-500° C. for 30-60 minutes; d) cooling the product obtained in the previous step with water.

13. The method of claim 12, wherein the composition of the multi-component co-penetration agent is: C powder 5-20%, Ti powder 10-40%, Cr powder 15-35%, Mo powder 10-20%, Al powder 5-15%, and NH4Cl powder 5-25%.

14. The method of claim 12, wherein step b) comprises keeping the product obtained in the previous step in a first salt bath at 310-350°C for 3-10 minutes.

15. The method of claim 12, wherein step c) comprises keeping the product obtained in the previous step in a second salt bath at 420-500°C for 30-60 minutes.

16. The method of claim 12, wherein the first salt bath consists of 30-60% sodium nitrate and 40-70% potassium nitrate.

17. The method of any one of claims 12 to 16, wherein the second salt bath consists of 60-90% potassium chloride and 10-40% chromium trichloride.

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