CN116445853A - High-toughness wear-resistant alloy steel product and preparation method thereof - Google Patents

Abstract

The invention relates to an alloy steel product, which is prepared by a method comprising the following steps: 1) forging the alloy steel as a raw material; 2) subjecting the forged and formed product to multi-component co-infiltration treatment; 3) processing the multi-component co-infiltration process The product is subjected to quenching treatment, and the element composition of the raw material alloy steel is calculated by mass percentage as 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 alloy steel product has high surface hardness, good wear resistance, and high toughness at the core, and is suitable for bucket teeth of excavators, especially tapered bucket teeth.

Description

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

technical field

The invention belongs to the field of alloy technology, and in particular relates to a high-toughness wear-resistant alloy steel product and a preparation method thereof.

Background technique

Bucket teeth are one of the most severely worn parts of excavators, and can be divided into earth moving teeth, rock teeth and bevel teeth according to their usage. Earthwork teeth are mainly used in light working environments such as excavating soil and river sand, and require high wear resistance; rock teeth are used in heavy-strength operations such as rocks, gravel, and gravel, and have higher requirements for wear resistance; The teeth are suitable for drilling rock formations with low hardness, and need to have greater impact resistance, not only high wear resistance, but also certain impact toughness. The conical bucket teeth are subjected to complex forces during work. When contacting materials, they not only bear the impact but also bear the bending moment. During the service process, the tip of the bucket tooth is subject to large impact and sliding abrasive wear, and furrows and deformations often appear on the surface of the tip, resulting in surface wear or shedding. Bucket tooth breakage and non-wear resistance will lead to frequent shutdowns, stop production to replace bucket teeth, and affect the working efficiency of the excavator.

The bucket teeth currently used in excavators are generally cast from high manganese steel or low alloy steel. High manganese steel has good toughness, but it needs a large impact force to produce work hardening, so that its wear resistance can be fully exerted; low alloy steel bucket teeth have higher hardness and wear resistance after quenching. Good, but its toughness is poor and it is prone to fracture.

At present, bucket teeth at home and abroad are usually prepared by heat treatment or surface strengthening. CN102242314A discloses a multi-component alloy strengthening and toughening, wear-resistant medium manganese steel and its preparation process. The composition mass percentage of the alloy steel 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, the rest is Fe, using rare earth ferrosilicon alloy, titanium iron alloy and vanadium iron alloy The composite modificator is used for processing, through the process and parameter control of smelting, lost foam negative pressure forming, water toughening treatment, tempering and so on. Wear-resistant steel parts such as bucket teeth of electric shovels, liners of semi-autogenous mills, and scrapers of conveyors prepared with this alloy steel have a service life more than twice that of high manganese steel. However, this alloy steel contains more precious alloy elements, and requires the use of rare earth composite modifiers, and the preparation process is complicated, making it difficult to be popularized and applied on a large scale. CN102453911A discloses a method for strengthening the surface of excavator bucket teeth, which is characterized in that the process steps are as follows: (1) remove surface oxide layer and oily foreign matter from the surface of excavator bucket teeth; Laser cladding is performed in a laser processing system composed of a powder feeder, a six-axis five-link processing machine tool, and a cross-flow gas laser generator. A tungsten carbide alloy powder with excellent wear resistance and good toughness is deposited on the bucket teeth. Cladding metallurgically bonded dense coatings. In this method, the wear-resistant cemented carbide is clad on the surface of the bucket teeth, and the cladding layer will fall off under the action of a large impact, which cannot play the role of wear resistance.

Therefore, it is necessary to develop a wear-resistant bucket tooth with high toughness.

Contents of the invention

The invention provides an alloy steel product prepared by a method comprising the following steps,

1) forging the raw material alloy steel into shape;

2) Perform multi-component co-infiltration treatment on forged products;

3) Quenching the product after multi-component co-infiltration treatment,

The elemental composition of the raw material alloy steel is C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, and 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.

In certain embodiments, step 1) comprises:

The raw alloy steel is heated to 1050-1150° C., and forged under the condition of a forging pressure of 800-1600 t.

In certain embodiments, step 2) comprises:

The forged product is kept at a temperature of 1100-1250° C. for 4-8 hours to carry out multi-component co-infiltration treatment.

In certain embodiments, step 3) comprises:

a) Cool the product after the multi-component co-infiltration treatment to 860-920°C;

b) keeping the product obtained in the previous step in the 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.

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

In some 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 minutes.

In certain embodiments, the first salt bath consists of 30-60% sodium nitrate (NaNO ₃ ) and 40-70% potassium nitrate (KNO ₃ ). In certain embodiments, the second salt bath consists of 60-90% potassium chloride (KCl) and 10-40% chromium trichloride (CrCl ₃ ).

In the present invention, step 3) quenches the product after multi-component co-infiltration treatment by using the waste heat after multi-component co-infiltration treatment.

In some embodiments, the multi-component co-infiltration agent used in the multi-component co-infiltration treatment consists of C powder, Ti powder, Cr powder, Mo powder, Al powder, and NH ₄ Cl powder.

In some 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% %, NH ₄ Cl powder 5-25%.

In certain embodiments, the alloy steel product has a surface hardness ≥ 64HRC, such as about 65HRC, about 66HRC, about 67HRC, about 68HRC, about 69HRC, about 70HRC. In some embodiments, the core hardness of the alloy steel product is ≥50HRC, such as about 51HRC, about 52HRC, about 53HRC, about 54HRC, about 55HRC, about 56HRC. In some embodiments, the impact energy KV ₂ of the alloy steel 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 alloy steel product has a dynamic wear amount of ≤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 alloy steel product has an impact energy KV ₂ ≥ 45J. In certain embodiments, the dynamic wear of the alloy steel product is ≤0.19g.

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

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

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

1) forging the raw material alloy steel into shape;

2) Perform multi-component co-infiltration treatment on forged products;

3) Quenching the product after multi-component co-infiltration treatment,

The elemental composition of the raw material alloy steel is C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, and 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.

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

The raw alloy steel is heated to 1050-1150° C., and forged under the condition of a forging pressure of 800-1600 t.

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

The forged product is kept at a temperature of 1100-1250° C. for 4-8 hours to carry out multi-component co-infiltration treatment.

In certain embodiments, the multi-component co-infiltration agent used in the multi-component co-infiltration treatment in the method consists of C powder, Ti powder, Cr powder, Mo powder, Al powder, and NH ₄ Cl powder.

In some 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%, NH ₄ Cl powder 5-25%.

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

a) Cool the product after the multi-component co-infiltration treatment to 860-920°C;

b) keeping the product obtained in the previous step in the 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.

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

In some 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.

In certain embodiments, the first salt bath consists of 30-60% sodium nitrate (NaNO ₃ ) and 40-70% potassium nitrate (KNO ₃ ). In certain embodiments, the second salt bath consists of 60-90% potassium chloride (KCl) and 10-40% chromium trichloride (CrCl ₃ ).

In the present invention, in step 3), the residual heat after multi-component co-infiltration is used to quench the product after multi-component co-infiltration.

As used herein, the term "about" is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. In particular, the term "about" is to be understood as +/-10%, +/-9%, +/-8%, +/-7%, +/-6%, +/-5% of the stated value , +/-4%, +/-3%, +/-2%, +/-1%, +/-0.5%, +/-0.4%, +/-0.3%, +/-0.2%, + /-0.1% or less. Unless otherwise evident from the context, all numerical values provided herein are modified by the term "about".

Beneficial effects of the present invention

The invention optimizes the ratio of chemical elements, reduces the amount of carbon and alloy elements, and prepares a high-toughness, high-wear-resistant alloy steel product by means of forging, multi-component co-infiltration and heat treatment.

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

The invention adopts the waste heat after multi-component co-infiltration to quench the product after multi-component co-infiltration treatment, which can reduce energy consumption and cost, and is beneficial to realize industrialized production.

Detailed ways

Embodiments of the present invention will be described in detail below in conjunction with examples, but those skilled in the art will understand that the following examples are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Those who do not indicate the specific conditions in the examples are carried out according to the conventional conditions or the conditions suggested by the manufacturer. The raw materials, equipment or instruments used without indicating the manufacturer are all conventional products that can be obtained from the market.

Example 1

The raw material alloy steel used to prepare the high-toughness and wear-resistant conical bucket teeth in this example has an elemental composition 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 balance is Fe and unavoidable impurities.

Heat the above-mentioned raw material alloy steel to 1130°C, and then use a 1000t press to forge it; then, the forged bucket teeth are subjected to multi-component co-infiltration treatment at a temperature of 1200 °C, and the holding time is 5 hours. %Ti+15%Cr+10%Mo+5%Al+10%NH ₄ Cl powder is fully stirred and mixed; use the waste heat after multi-component co-infiltration to quench the bucket teeth after multi-component co-infiltration treatment, specifically, Cool the bucket tooth after multi-component co-infiltration treatment to 910°C, then put it directly into the first salt bath at 350°C, keep it warm for 3 minutes, then put it into the second salt bath at 500°C, keep it warm for 30 minutes, and finally Water cooling to obtain high toughness and wear-resistant tapered bucket teeth. Wherein the first salt bath is composed of 30% sodium nitrate (NaNO ₃ ) and 70% potassium nitrate (KNO ₃ ), the second salt bath is composed of 90% potassium chloride (KCl) and 10% chromium trichloride (CrCl ₃ ) .

Example 2

The raw material alloy steel used to prepare the high-toughness wear-resistant conical bucket tooth in this example, in terms of mass percentage, has an elemental composition 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 balance is Fe and unavoidable impurities.

Heat the above-mentioned raw material alloy steel to 1150°C, and then use an 800t press to forge it; then, the forged bucket teeth are subjected to multi-component co-infiltration treatment at a temperature of 1250 °C, and the holding time is 8 hours. The multi-component co-infiltration agent is composed of 10%C+10 %Ti+35%Cr+10%Mo+10%Al+25%NH ₄ Cl powder is fully stirred and mixed; use the residual heat after multi-component co-infiltration to quench the bucket teeth after multi-component co-infiltration treatment, specifically, Cool the bucket tooth after multi-component co-infiltration treatment to 860°C, then put it directly into the first salt bath at 350°C, keep it warm for 7 minutes, then put it into the second salt bath at 420°C, keep it warm for 50 minutes, and finally Water cooling to obtain high toughness and wear-resistant tapered bucket teeth. Wherein the first salt bath is composed of 50% sodium nitrate (NaNO ₃ ) and 50% potassium nitrate (KNO ₃ ), and the second salt bath is composed of 70% potassium chloride (KCl) and 30% chromium trichloride (CrCl ₃ ) .

Example 3

The raw material alloy steel used to prepare the high-toughness wear-resistant tapered bucket teeth in this example, in terms of mass percentage, has an elemental composition 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 balance is Fe and unavoidable impurities.

Heat the above-mentioned raw material alloy steel to 1080°C, and then forge it with a 1500t press; then, the forged bucket teeth are subjected to multi-component co-infiltration treatment at a temperature of 1130 °C, and the holding time is 7 hours. The multi-component co-infiltration agent is composed of 10%C+30 %Ti+30%Cr+15%Mo+10%Al+5%NH ₄ Cl powder is fully stirred and mixed; use the waste heat after multi-component co-infiltration to quench the bucket teeth after multi-component co-infiltration treatment, specifically, Cool the bucket tooth after multi-component co-infiltration to 870°C, then put it directly into the first salt bath at 330°C, keep it warm for 5 minutes, and then put it into the second salt bath at 450°C During the process, heat preservation for 40 minutes, and finally water cooling to obtain high toughness and wear-resistant tapered bucket teeth. The first salt bath consists of 60% sodium nitrate (NaNO ₃ ) and 40% potassium nitrate (KNO ₃ ), the second salt bath consists of 65% potassium chloride (KCl) and 35% chromium trichloride (CrCl ₃ ) .

Example 4

The raw material alloy steel used to prepare the high-toughness wear-resistant conical bucket teeth in this example has an elemental composition 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 balance is Fe and unavoidable impurities.

Heat the above-mentioned raw material alloy steel to 1100°C, and then forge it with a 1200t press; then, the forged bucket teeth are subjected to multi-component co-infiltration treatment at a temperature of 1190 °C, and the holding time is 6 hours. The multi-component co-infiltration agent is composed of 15%C+20 %Ti+20%Cr+20%Mo+15%Al+10%NH ₄ Cl powder is fully stirred and mixed; use the waste heat after multi-component co-infiltration to quench the bucket teeth after multi-component co-infiltration treatment, specifically, Cool the bucket tooth after multi-component co-infiltration treatment to 890°C, then put it directly into the first salt bath at 340°C, keep it warm for 10 minutes, then put it into the second salt bath at 470°C, keep it warm for 60 minutes, and finally Water cooling to obtain high toughness and wear-resistant tapered bucket teeth. The first salt bath consists of 45% sodium nitrate (NaNO ₃ ) and 55% potassium nitrate (KNO ₃ ), the second salt bath consists of 80% potassium chloride (KCl) and 20% chromium trichloride (CrCl ₃ ) .

Example 5

The raw material alloy steel used to prepare the high-toughness wear-resistant tapered bucket teeth in this example has an elemental composition 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 balance is Fe and unavoidable impurities.

Heat the above-mentioned raw material alloy steel to 1050°C, and then use a 1600t press to forge it; then, the forged bucket teeth are subjected to multi-component co-infiltration treatment at a temperature of 1100 °C, and the holding time is 8 hours. The multi-component co-infiltration agent is composed of 5%C+25 %Ti+15%Cr+20%Mo+15%Al+20%NH ₄ Cl powder is fully stirred and mixed; use the residual heat after multi-component co-infiltration to quench the bucket teeth after multi-component co-infiltration treatment, specifically, Reduce the temperature of the bucket tooth after the multi-component co-infiltration treatment to 920°C, then put it directly into the first salt bath at 310°C, keep it warm for 8 minutes, and then put it into the second salt bath at 430°C In the salt bath, heat preservation for 40 minutes, and finally water-cooled to obtain high toughness and wear-resistant tapered bucket teeth. Wherein the first salt bath is composed of 40% sodium nitrate (NaNO ₃ ) and 50% potassium nitrate (KNO ₃ ), and the second salt bath is composed of 60% potassium chloride (KCl) and 40% chromium trichloride (CrCl ₃ ) .

After testing, the physical and mechanical properties of the high-toughness and wear-resistant tapered bucket teeth prepared in Examples 1-5 are shown in Table 1, wherein the MLD-10 dynamic load abrasive wear tester was used for the wear test, and the dynamic load wear amount test The parameters are: impact energy 2J, impact time 1h, impact frequency 100 times/min, abrasive (5-10) mesh quartz sand, impact energy is measured according to GB/T 229-2020, and hardness is measured according to GB/T 230.1-2018.

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

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: the present invention can still be Modifications to the specific implementation of the invention or equivalent replacement of some technical features; without departing from the spirit of the technical solution of the present invention, should be included in the scope of the technical solution claimed in the present invention.

Claims (17)

1. An alloy steel product prepared by a method comprising the steps of, 1) forging the raw material alloy steel into shape; 2) Perform multi-component co-infiltration treatment on forged products; 3) Quenching the product after multi-component co-infiltration treatment, The elemental composition of the raw material alloy steel is C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, and 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. 2. The alloy steel product of claim 1, wherein step 1) comprises: The raw alloy steel is heated to 1050-1150° C., and forged under the condition of a forging pressure of 800-1600 t. 3. The alloy steel product of claim 1, wherein step 2) comprises: The forged product is kept at a temperature of 1100-1250° C. for 4-8 hours to carry out multi-component co-infiltration treatment. 4. The alloy steel product according to claim 3, wherein the multi-component co-infiltration agent used in the multi-component co-infiltration treatment is composed of C powder, Ti powder, Cr powder, Mo powder, Al powder, and NH ₄ Cl powder. 5. The alloy steel product according to claim 4, wherein the multi-component co-penetrating agent is composed of: C powder 5-20%, Ti powder 10-40%, Cr powder 15-35%, Mo powder 10-20%, Al Powder 5-15%, NH ₄ Cl powder 5-25%. 6. The alloy steel product of claim 1, wherein step 3) comprises: a) Cool the product after the multi-component co-infiltration treatment to 860-920°C; b) keeping the product obtained in the previous step in the 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, Preferably, step b) is to keep the product obtained in the previous step in the first salt bath at 310-350°C for 3-10 minutes; Preferably, 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; Preferably, the first salt bath is composed of 30-60% sodium nitrate (NaNO ₃ ) and 40-70% potassium nitrate (KNO ₃ ); Preferably, the second salt bath is composed of 60-90% potassium chloride (KCl) and 10-40% chromium trichloride (CrCl ₃ ). 7. The alloy steel product of any one of claims 1 to 6, having one or more of the following characteristics: i) Surface hardness ≥ 64HRC, ii) Heart hardness ≥ 50HRC, iii) Impact energy KV ₂ ≥ 40J, iv) Dynamic load wear ≤ 0.20g. 8. The alloy steel product according to claim 7, whose impact energy KV ₂ ≥ 45J. 9. The alloy steel product according to claim 7, whose dynamic load wear amount is ≤0.19g. 10. The alloy steel product according to any one of claims 1 to 7, which is a bucket tooth. 11. An excavator or loader comprising an alloy steel product according to any one of claims 1-10. 12. A method for preparing an alloy steel product, comprising: 1) forging the raw material alloy steel into shape; 2) Perform multi-component co-infiltration treatment on forged products; 3) Quenching the product after multi-component co-infiltration treatment, The elemental composition of the raw material alloy steel is C: 0.2-0.6%, Si: 0.3-0.6%, Mn: 4-8%, Ni: 0.5-1%, Cr: 0.2-0.8%, and 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. 13. The method of claim 12, wherein step 1) comprises: The raw alloy steel is heated to 1050-1150° C., and forged under the condition of a forging pressure of 800-1600 t. 14. The method of claim 12, wherein step 2) comprises: The forged product is kept at a temperature of 1100-1250° C. for 4-8 hours to carry out multi-component co-infiltration treatment. 15. The method of claim 14, wherein the multi-component co-infiltration agent used in the multi-component co-infiltration treatment consists of C powder, Ti powder, Cr powder, Mo powder, Al powder, and NH ₄ Cl powder. 16. The alloy steel product according to claim 15, wherein the multi-component co-penetration agent is composed of: C powder 5-20%, Ti powder 10-40%, Cr powder 15-35%, Mo powder 10-20%, Al Powder 5-15%, NH ₄ Cl powder 5-25%. 17. The method of claim 12, wherein step 3) comprises: a) Cool the product after the multi-component co-infiltration treatment to 860-920°C; b) keeping the product obtained in the previous step in the 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, Preferably, step b) is to keep the product obtained in the previous step in the first salt bath at 310-350°C for 3-10 minutes; Preferably, 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; Preferably, the first salt bath is composed of 30-60% sodium nitrate (NaNO ₃ ) and 40-70% potassium nitrate (KNO ₃ ); Preferably, the second salt bath is composed of 60-90% potassium chloride (KCl) and 10-40% chromium trichloride (CrCl ₃ ).