RU2790841C1 - Method for surface treatment of heat-resistant stainless steel - Google Patents

Abstract

FIELD: steel processing.

SUBSTANCE: methods for chemical-thermal treatment of steel, namely, methods for depassivation of steel surface with a chromium content of 16% or more, to be used when hardening steel in vacuum furnaces. The method includes preparation for depassivation and depassivation of the surface of the part, whereas the preparation the surface of the part includes cleaning from contamination, and depassivation is carried out in stages: 1) heat treatment of the part in a vacuum hardening furnace with heating to a temperature of 1000-1050°C, the residual pressure in the furnace chamber is not more than 2 mbar; 2) exposure of the part until it is completely warmed up, the residual pressure in the furnace chamber is not more than 2 mbar; 3) cooling of the part in nitrogen or argon with a purity of at least 99.99%, the residual pressure in the furnace chamber during heating and exposure is not more than 2 mbar; 4) moving the part for 30 minutes into a vacuum chemical-thermal treatment oven; 5) treatment of the part with a saturated solution of ferric chloride FeCl₃; 6) additional oxidation, which includes heating the part in a nitrogen atmosphere to a temperature of 500°C, the residual pressure in the furnace chamber is 0.3 bar; 7) after equalizing the temperature of the part, replacing nitrogen in the furnace chamber with nitrous oxide, exposure for 2 hours; 8) replacement of nitrous oxide with ammonia in the furnace chamber; 9) repeating steps 6-8; 10) nitriding without removing the part from the furnace.

EFFECT: more adapted steel surface for further nitriding.

2 cl, 1 dwg, 11 tbl

Description

The invention relates to methods of chemical-thermal treatment of steel, and in particular to methods of depassivation of the surface of steel and can be used when hardening steel in vacuum furnaces.

The invention is known from the prior art (SU1295775A1, 01/07/1985), which describes a method for chemical-thermal treatment of titanium-based alloys, including oxidation at 800-850°C in a vacuum of 3⋅10 ^-1 - 3⋅10 ^-4 mm Hg, which differs the fact that, in order to increase wear resistance by increasing the thickness of the diffusion layer while maintaining the quality of the surface, after oxidation, additional nitriding is carried out at a temperature below the temperature of the α+β→β transformation.

Also known is the invention (EP0544987A1, 1993-06-09), which describes a method for processing alloyed steels and refractory metals, for example, Ti, Zr and Nb, especially for depassivation and subsequent thermochemical surface treatment in a process chamber under pressure and temperature, characterized in that that a gas mixture selected from the group N2, H2 or NH3 is introduced into the process chamber for the purpose of depassivation, at an absolute pressure of more than 1 bar and a temperature from 100°C to 1000°C. At the second stage of the process, a gas or gas mixture selected from the group of N-, C- or B-containing gases is introduced into the process chamber for the purpose of thermochemical surface treatment at a temperature of 100°C to 1000°C at an absolute pressure greater than or equal to 1 bar .

The closest to the claimed technical solution is the invention (TW428046B, 2001-04-01), which describes the process of vacuum nitriding without layers of compounds, which includes the following procedures: preparation, pumping to vacuum, heating, depassivating and nitriding. At the stage of preparation, the billet is placed in a nitriding furnace equipped with a closed chamber. After the chamber has been evacuated to a vacuum state, nitrogen is introduced for the heating step. And then nitrous oxide is added to remove the passivation film from the workpiece surface due to oxidation in the depassivation step. Finally, an atmosphere containing ammonia, nitrous oxide and nitrogen is introduced into the nitriding process chamber. The total atmospheric flow rate is below 10 l/min/m (liter per minute per square meter). Thus, no composite layer is formed on the workpiece. This facilitates the subsequent hard composite coating process, and also increases the speed of nitriding, saves energy and improves usability.

The disadvantage of these technical solutions is the difficult nitriding of steel due to insufficient adaptation to the class of stainless steels.

The task to be solved by the claimed invention is the development of a faster and more efficient method of steel depassivation at minimal cost and without the use of reagents harmful to equipment and the environment.

This task according to independent claim 1 is solved due to the fact that the method of surface treatment of heat-resistant stainless steel with a chromium content of 16% or more, consisting of the preparation and depassivation of steel, while the preparation includes cleaning the surface of the part from contamination, and depassivation includes the following stages: 1) heat treatment in a vacuum hardening furnace by heating to a temperature of 1000-1050°C, while the residual pressure in the chamber should not exceed 2 mbar; 2) exposure of the part until it is completely warmed up, while the residual pressure in the furnace chamber should not exceed 2 mbar; 3) cooling of parts in nitrogen or argon with a purity of at least 99.99%; 4) within 30 minutes transfer to a vacuum chemical-thermal treatment oven; 5) treatment of parts with a saturated solution of ferric chloride (FeCl3); 6) additional oxidation, which includes heating parts in a nitrogen atmosphere to a temperature of 500°C, at a residual pressure in the furnace chamber of 0.3 bar; 7) after equalizing the temperature of the part, replacing nitrogen in the furnace chamber with nitrous oxide and holding for two hours; 8) recovery, including the replacement of nitrous oxide with ammonia in the furnace chamber; 9) repeat steps 6-8; 10) nitriding without taking the part out of the furnace where the oxidation took place.

The task according to independent point 2 is solved due to the fact that the method of surface treatment of heat-resistant stainless steel with a chromium content of less than 16% consists of two stages: preparation and depassivation of steel, while the preparation includes cleaning the surface of the part from contamination, and depassivation includes the following stages: 1) heat treatment in a vacuum hardening furnace by heating to a temperature of 1000-1050°C, while the residual pressure in the chamber should not exceed 2 mbar; 2) exposure of the part until it is completely warmed up, while the residual pressure in the furnace chamber should not exceed 2 mbar; 3) cooling of parts in nitrogen or argon with a purity of at least 99.99%; 4) within 30 minutes transfer to a vacuum chemical-thermal treatment oven; 5) additional oxidation, which includes heating parts in a nitrogen atmosphere to a temperature of 500°C, at a residual pressure in the furnace chamber of 0.3 bar; 6) after equalizing the temperature of the part, replacing nitrogen in the furnace chamber with nitrous oxide and holding for two hours; 7) recovery, including the replacement in the furnace chamber of nitrous oxide with ammonia; 8) repeat steps 5-7; 9) nitriding, without removing the part from the furnace where the oxidation took place.

The technical result is to obtain a more adapted steel surface for further nitriding.

In this invention, the passivation layer is a thin protective film on the surface of the steel.

The term "depassivation" means the destruction of the passivating layer on the surface, or a decrease in its passivating ability, which prevents the desired chemical reactions from occurring.

The term austenitization is the process of heating steel to a temperature where austenite becomes the main structure.

The term "vacuum hardening furnace" is a furnace that is designed to carry out heat treatment in a vacuum or in an airless atmosphere.

A low-alloy section is a section with a low content of carbon and other inclusions that improve and supplement the original properties of the steel.

Temperature equalization is the process when the temperature of the parts becomes equal to the temperature in the furnace chamber.

Nitriding is a process of diffusion saturation of the surface layer of steels and alloys with nitrogen.

The method of surface treatment of heat-resistant stainless steel consists of the following steps: preparation and depassivation.

It has been previously established that all stainless steels are heavily alloyed with chromium. All steels with a high chromium content, before gas nitriding, must undergo a special treatment to remove the oxide film that prevents saturation with nitrogen.

The sequence of the method for steel with a chromium content of 16% or more:

1. Stage of preparation.

At this stage, the surface of the part is cleaned of possible contaminants. Processing is carried out with an alcohol solution or a saturated solution of ferric chloride (FeCl3).

2. Stage of depassivation.

2.1. Heat treatment of a part in a vacuum hardening furnace. Any brand of furnace can be used here, which is able to withstand the specified parameters in terms of temperature and vacuum. The processing mode of the part should include heating to the steel austenitization temperature, it is recommended to use a temperature of 1000-1050°C. The residual pressure in the furnace chamber must not exceed 2 mbar.

2.2. Detail excerpt. It is carried out on the basis of the complete heating of the part, that is, when the temperature of the entire part is the same and has reached the desired value. The temperature is determined by the calculation method, or using a thermocouple. For different steels and different forms of steels, exposure is carried out differently. The holding time during austenitization for each steel can be found in the reference literature. The residual pressure during holding in the furnace chamber must not exceed 2 mbar.

2.3. Part cooling. It is carried out in the same vacuum hardening furnace by the method of convection cooling (heat extraction in an inert gas environment). The refrigerant gas enters the chamber by means of pumping. Given the simplicity and prevalence in the industry, it is proposed to use nitrogen with a purity of at least 99.99%. Alternatively, argon of the same purity can be used.

2.4. Moving the part into a vacuum oven for chemical-thermal treatment of any brand. After complete cooling of the part, that is, when the temperature of the part becomes equal to the temperature of the cooling medium, they must be transferred within 30 minutes to a vacuum chemical-thermal treatment furnace for additional oxidation followed by nitriding. Complete cooling of the parts is determined by the calculation method or using a thermocouple.

2.5. Before carrying out the oxidation process, it is necessary to treat the parts with a saturated solution of ferric chloride (FeCl3).

2.6. One additional oxidation cycle includes heating the parts in a nitrogen atmosphere to a temperature of 500°C, with a residual pressure of 0.3 bar in the furnace chamber. Oxidation at this stage is called additional, because the oxide layer is already on the surface of the parts and even more oxygen is added to it so that the oxide layer becomes loose.

2.7. After equalizing the temperature of the parts, nitrogen is replaced with nitrous oxide, that is, the pump pumps out a mixture of gases from the chamber, and the volumes taken are replenished by supplying nitrous oxide from the cylinder. The temperature of the parts is determined by the calculation method or using a thermocouple. Next, a two-hour exposure at a given temperature and pressure is performed. At this stage, the oxide film grows, which leads to the appearance of discontinuities and porosity on it.

2.8. Recovery stage. At this stage, the nitrous oxide is replaced by ammonia, in the same way as in the previous stage. The oxidized surface of the part is reduced by ammonia hydrogen to a metallic state. At a residual pressure of 0.3 bar and a temperature of 600°C, ammonia decomposes into hydrogen and atomic nitrogen. Hydrogen begins to restore low-alloyed areas of the oxide film to the state of iron, and nitrogen diffuses deep into the metal, creating centers for further nitriding.

2.9. The oxidation cycle described in steps 2.6-2.8 is repeated a second and, if necessary, a third time, depending on the end goal.

2.10. After the completion of oxidation, without taking the part out of the furnace, nitriding is carried out by known methods. For each steel, nitriding modes are selected differently. For the same steel, the nitriding regimes may differ, since they are selected depending on which nitride phases need to be obtained and to what depth.

The sequence of the method for steel with a chromium content of less than 16%:

2. Stage of depassivation.

2.1. Heat treatment of a part in a vacuum hardening furnace. Any brand of furnace can be used here, which is able to withstand the specified parameters in terms of temperature and vacuum. The processing mode of the part should include heating to the steel austenitization temperature, it is recommended to use a temperature of 1000-1050°C. The residual pressure in the furnace chamber must not exceed 2 mbar.

2.2. Detail excerpt. It is carried out on the basis of the complete heating of the part, that is, when the temperature of the entire part is the same and has reached the desired value. The temperature is determined by the calculation method, or using a thermocouple. For different steels and different forms of steels, exposure is carried out differently. The holding time during austenitization for each steel can be found in the reference literature. The residual pressure in the furnace chamber during holding must not exceed 2 mbar.

2.3. Part cooling. It is carried out in the same vacuum hardening furnace by the method of convection cooling (heat extraction in an inert gas environment). The cooling gas enters the chamber by means of an inlet. Given the simplicity and prevalence in the industry, it is proposed to use nitrogen with a purity of at least 99.99%. Alternatively, argon of the same purity can be used.

2.4. Moving the part into a vacuum oven for chemical-thermal treatment of any brand. After complete cooling of the part, that is, when the temperature of the part becomes equal to the temperature of the cooling medium, they must be transferred within 30 minutes to a vacuum chemical-thermal treatment furnace for additional oxidation followed by nitriding. Complete cooling of the parts is determined by the calculation method or using a thermocouple.

2.5. One additional oxidation cycle includes heating the parts in a nitrogen atmosphere to a temperature of 500°C, with a residual pressure of 0.3 bar in the furnace chamber. Oxidation at this stage is called additional, because the oxide layer is already on the surface of the parts and even more oxygen is added to it so that the oxide layer becomes loose.

2.6. After equalizing the temperature of the parts, nitrogen is replaced with nitrous oxide, that is, the pump pumps out a mixture of gases from the chamber, and the volumes taken are replenished by supplying nitrous oxide from the cylinder. The temperature of the parts is determined by the calculation method or using a thermocouple. Next, a two-hour exposure at a given temperature and pressure is performed. At this stage, the oxide film grows, which leads to the appearance of discontinuities and porosity on it.

2.7. Recovery stage. At this stage, the nitrous oxide is replaced by ammonia, in the same way as in the previous stage. The oxidized surface of the part is reduced by ammonia hydrogen to a metallic state. At a residual pressure of 0.3 bar and a temperature of 600°C, ammonia decomposes into hydrogen and atomic nitrogen. Hydrogen begins to restore low-alloyed areas of the oxide film to the state of iron, and nitrogen diffuses deep into the metal, creating centers for further nitriding.

2.8. The oxidation cycle described in steps 2.5-2.7 is repeated a second and, if necessary, a third time, depending on the end goal.

2.9. After the completion of oxidation, without taking the part out of the furnace, nitriding is carried out by known methods. For each steel, nitriding modes are selected differently. For the same steel, the nitriding regimes may differ, since they are selected depending on which nitride phases need to be obtained and to what depth.

Below are the experimental data of the implementation of the invention.

Production of samples of heat-resistant stainless steel using this method.

1. Material selection and sample preparation. The following brands were selected for testing the technology:

Steels 40X13 and 20X13 are the most common martensitic steels.

Steels 12X18H10T and 08X16H9 are the most common austenitic stainless steels. Many enterprises are interested in nitriding these steels, including PJSC NZHK, JSC Neftemash and many others.

Steel 13Kh11N2 V2MF (EI961) is a representative of the martensitic-ferritic class. It is actively used in nuclear and fusion reactors under development, in stationary gas turbine plants, and in the aviation industry. Nitriding of this steel is an interesting task for Sukhoi Company JSC.

2. Preparation of samples for testing.

Based on the experience of operating vacuum furnaces and conducting metallographic studies, the optimal size of the samples was determined. Samples should be 2-5 mm thick and have a test area of 6-10 square centimeters.

Samples were made and labeled. To prevent the influence of surface irregularities on the results, the surfaces of all samples were subjected to a grinding operation.

3. Preparation of equipment for nitriding.

For testing, the equipment of the Thermomet-M vacuum heat treatment workshop was used, since it meets the standard requirements of modern vacuum thermal treatment workshops. Vacuum furnaces and analytical equipment were used, on which the following preparatory work was carried out:

- for the BMI V-63 vacuum hardening furnace, thermocouples, pressure measuring instruments and flowmeters were checked, the furnace chamber was annealed and degassed, and equipment was replaced;

- for the BMI V-53 vacuum furnace, thermocouples, pressure measuring devices and flowmeters were verified, the furnace chamber was annealed and degassed, and equipment was replaced;

- Verification was carried out for Tukon 2500 microhardness tester. Abrasive consumables were purchased for the Polylab P12 grinding and polishing machine.

4. Preparation of the test program.

At this stage, four fundamental processes were carried out, which made it possible to evaluate the effectiveness of these processes in vacuum furnaces (table 1).

Before each process, some parts will undergo preliminary preparation (Table 2).

For each process, a batch of samples was used according to Table 3.

5. Sample preparation.

When choosing a control method, it was necessary to take into account the characteristics of the oxide layer, which must be depassivated.

The first characteristic of the oxide layer is its thickness, which is directly related to the chromium content in the steel (Fig. 1). Figure 1 shows that for the studied steels, the thickness of the oxide film lies in the nanoscale range. It is extremely difficult to fix such values on the existing equipment.

In addition to the size of the oxide film, the rate of its recovery after depassivation is of importance. Practice has shown that on a completely cleaned sample in an air atmosphere, the oxide film is re-formed within a few hours. Therefore, the depassivation step should take place immediately before nitriding, or directly during nitriding.

In view of these reasons, the method of indirect control was chosen. At the end of each test, a comprehensive nine-hour nitriding was carried out. As a result, if the surface was depassivated, then a nitrided layer formed on it, as evidenced by an increase in the surface hardness and a change in the structure and color of the surface.

The change in color was recorded visually, the change in structure and microhardness was recorded with a Tukon 2500 hardness tester.

The microhardness of nitrided samples was measured at loads of 5 and 2.5 kg. The data were compared with the original hardness of the samples presented in table 4.

Measurement under two loads allows to evaluate the presence and depth of the formed layer without the use of microstructural analysis.

6. Thermochemical treatment of prototypes of heat-resistant stainless steel in a vacuum furnace.

Based on this invention, the following modes of additional oxidation were formed (table 5).

6.1. First stage of testing.

The first stage included checking the effectiveness of depassivation by the preliminary oxidation method, as well as assessing the interaction of this method with others.

To test Mode No. 1 (table 5), samples of all studied steel grades were selected. Two samples each underwent vacuum depassivation in a V-63 TNS furnace.

The vacuum depassivation mode included heating to a temperature of 1000°C. The residual pressure in the furnace chamber did not exceed 2 mbar at the stage of heating and holding. The exposure was carried out at the rate of complete heating of the parts and amounted to 30 minutes. After exposure, the parts were cooled in an atmosphere of gaseous nitrogen. Nitrogen was used with a purity of at least 99.9% and a pressure of 4 bar. After complete cooling, half of the samples were machined on a Polylab P12 machine for 15 minutes.

In addition, samples that did not undergo vacuum depassivation in the V-63 TNS furnace underwent mechanical processing.

All processed samples, together with samples that did not undergo any additional processing, were loaded into the B-53RN furnace and went through the process corresponding to mode No. 1 (table 5).

After the process, each sample was tested for hardness by the Vickers method at a load of 5 kg and 2.5 kg. The results are presented in table 6.

Visual control confirms the presence of a layer on the material EI961 and in a certain way on materials 40X13 and 20X13. There are no visual signs of successful nitriding on 12Kh18N10T materials. Steel 08X16H9 has signs of a nitrided layer, and they have increased hardness, but the coverage area is less than 50%.

6.2. Second stage of testing.

The second stage included checking the effectiveness of depassivation by the preliminary triple oxidation method, as well as assessing the interaction of this method with others.

To test Mode No. 2 (table 5), samples of all the studied steel grades were selected. Two samples each underwent vacuum depassivation in a V-63 TNS furnace.

The vacuum depassivation mode included heating to a temperature of 1000°C. The residual pressure in the furnace chamber did not exceed 2 mbar at the stage of heating and holding. The exposure was carried out at the rate of complete heating of the parts and amounted to 30 minutes. After exposure, the parts were cooled in an atmosphere of gaseous nitrogen. Nitrogen was used with a purity of at least 99.9% and a pressure of 3 bar. After complete cooling, half of the samples were subjected to mechanical processing on a Polylab P12 machine for 15 minutes.

In addition, samples that did not undergo vacuum depassivation in the V-63 TNS furnace underwent mechanical processing.

All processed samples, together with samples that did not undergo any additional processing, were loaded into the B-53RN furnace and went through the process corresponding to mode No. 2 (table 5).

After the process, each sample will pass the Vickers hardness test at a load of 5 kg and 2.5 kg. The results are presented in table 7.

Visual control confirms the presence of a layer on the material EI961, 40X13 and 20X13. On the material 12X18H10T and 08X16H9 there are signs of a nitrided layer, but it is uneven.

6.3. The third stage of testing.

The third stage included checking the effectiveness of complex depassivation, including preliminary double oxidation and increased emission of oxygen-containing gases during the nitriding process itself.

To test Mode No. 3 (table 5), samples of all the studied steel grades were selected. Two samples each underwent vacuum depassivation in a V-63 TNS furnace.

The vacuum depassivation mode included heating to a temperature of 1000°C. The residual pressure in the furnace chamber did not exceed 2 mbar at the stage of heating and holding. The exposure was carried out at the rate of complete heating of the parts and amounted to 30 minutes. After exposure, the parts were cooled in an atmosphere of gaseous nitrogen. Nitrogen was used with a purity of at least 99.95% and a pressure of 2 bar.

The vacuum depassivated samples, together with the samples that did not undergo any additional processing, were loaded into the B-53RN oven and went through the process corresponding to mode No. 3 (table 5).

At this stage, samples undergoing mechanical processing were not tested.

After carrying out, each sample was tested for hardness by the Vickers method at a load of 5 kg and 2.5 kg. The results are presented in table 8.

Visual control confirms the presence of a layer on the material EI961, 40X13 and 20X13. There are no visual signs of successful nitriding on materials 12Kh18N10T and 08Kh16N9.

The third process went through part of the samples 08X16H9, there is a nitriding layer on the samples, but it is in its infancy, the hardness is determined only at a load of 1 kg.

6.4. The fourth stage of testing.

The fourth stage included an assessment of the effectiveness of vacuum depassivation and mechanical treatment without preliminary oxidation.

To test Mode No. 4 (table 5), samples of all the studied steel grades were selected. Two samples each underwent vacuum depassivation in a V-63 TNS furnace.

The vacuum depassivation mode included heating to a temperature of 1000°C. The residual pressure in the furnace chamber did not exceed 2 mbar at the stage of heating and holding. The exposure should be carried out at the rate of complete heating of the parts and amounted to 30 minutes. After exposure, the parts were cooled in an atmosphere of gaseous nitrogen. Nitrogen was used with a purity of at least 99.95% and Ball pressure.

Some of the samples did not undergo vacuum depassivation, but were subjected to mechanical processing on a Polylab P12 machine for 15 minutes.

Vacuum depassivated samples, mechanically processed samples, together with samples that did not undergo any additional processing, were loaded into a B-53RN furnace and went through the process corresponding to mode No. 4 (table 5).

After the process, each sample passed the Vickers hardness test at a load of 5 kg and 2.5 kg. The results are presented in table 9.

Visual control confirms the presence of a layer on those materials EI961, 40X13 and 20X13 that have been mechanically processed, samples EI961, 40X13 and 20X13 that have not been mechanically processed do not detect a nitrided layer. There are no visual signs of successful nitriding on materials 12X18H10T and 08X16H9.

7. Analysis of the results of thermochemical treatment of prototypes of heat-resistant stainless steel.

7.1. Analysis of the results for steel 13X11H2 V2MF (EI961).

Steel EI961 belongs to the ferrite-martensitic class. Its chromium content is the lowest of the studied materials, which should have made depassivation easier.

Carrying out a test cycle showed that for successful nitriding of this steel, it is sufficient to use preliminary oxidation in a vacuum furnace with nitrous oxide, according to mode No. 1. This operation, regardless of combination with other methods of depassivation, gives a full-fledged result. All samples after preliminary oxidation successfully passed the nitriding stage. At the same time, the degree of surface nitriding was more than 95%, the nitriding depth was 80 μm, and the increase in hardness was 350 HV5. The use of three-cycle and complex oxidation gives similar results.

Separate use of mechanical treatment or vacuum depassivation without preliminary oxidation does not give stable positive results.

7.2. Analysis of results for steel 40X13.

Steel 40X13 belongs to the martensitic class. The chromium content in it is slightly higher than in the previous grade, but part of it is bound in the form of carbides.

Carrying out a test cycle showed that for successful nitriding of this steel, preliminary oxidation according to mode No. 1 is no longer enough. For complete depassivation, it is necessary to use either triple oxidation according to mode No. 2, or complex oxidation according to mode No. 3. The samples that underwent oxidation according to mode No. 2 and No. 3 successfully passed the nitriding stage. At the same time, the degree of surface nitriding was more than 95%, the nitriding depth was 60 μm, and the increase in hardness was more than 300 HV5.

Separate use of mechanical treatment or vacuum depassivation without preliminary oxidation does not give stable positive results.

7.3. Analysis of results for steel 20X13.

Steel 20X13 belongs to the martensitic class. The chromium content in it is the same as in the previous steel, but there is less carbon.

Carrying out a test cycle showed that for successful nitriding of this steel, preliminary oxidation according to mode No. 1 is no longer enough. For complete depassivation, it is necessary to use either triple oxidation according to mode No. 2, or complex oxidation according to mode No. 3. The samples that underwent oxidation according to mode No. 2 and No. 3 successfully passed the nitriding stage. At the same time, the degree of surface nitriding was more than 95%), the nitriding depth was 60 μm, and the increase in hardness was more than 300 HV5.

Separate use of mechanical treatment or vacuum depassivation without preliminary oxidation does not give stable positive results.

7.4. Analysis of the results for steel 12Kh18N10T.

Steel 12X18H10T belongs to the class of austenitic steels. The highest chromium and nickel content of the samples tested make this steel the most resistant to nitriding.

Carrying out a test cycle showed that the depassivation of this steel by mechanical means, by the vacuum depassivation method and by preliminary oxidation is inefficient. Satisfactory results are obtained by repeated preliminary oxidation.

7.5. Analysis of the results for steel 08X16H9.

Steel 08X16H9 belongs to the class of austenitic steels, like the previous steel, therefore it also has the highest content of chromium and nickel from the tested samples, this makes the steel the most resistant to nitriding.

Carrying out a test cycle showed that the depassivation of this steel by mechanical means, by the vacuum depassivation method and by preliminary oxidation is inefficient. Satisfactory results are obtained by repeated preliminary oxidation.

8. Metallographic analysis of prototypes.

The main method for determining the effectiveness of depassivation techniques was the method of measuring the microhardness of the surface of machined parts. The method of metallographic analysis was used to actually confirm the results obtained by microhardness measurements.

8.1. Metallographic analysis of prototypes of the first stage.

At the first stage, work was carried out to evaluate the effectiveness of the main depassivation methods, which are rationally used for nitriding in vacuum furnaces.

Optical microscopy and hardness measurements are carried out on the TUKON 2500 complex. Optical microscopy was carried out at x200 and x500 magnifications. The hardness measurement was carried out at a load of 2 kg with exposure at a load of 30 seconds. The research results are presented in Table 10. In Table 10, the first digit of the sample number indicates the steel grade, the second and third digits are randomly generated.

Comparing the results of metallographic analysis and measurements of microhardness, the statement was proved that a uniform increase in surface hardness after nitriding clearly indicates successful depassivation and, as a consequence, the formation of a uniform nitrided layer.

For all studied steels, it was confirmed that nitriding without prior depassivation does not lead to the formation of a nitrided layer.

The results of microstructural analysis of samples from steel EI961 revealed that both the vacuum depassivation method and the method of additional oxidation according to modes No. 2 and No. 3 can be effectively used for successful nitriding of this steel. The mechanical treatment method does not allow achieving stable and uniform results on the entire surface, and the combination of vacuum depassivation and additional oxidation proved to be the most effective method.

The results of microstructural analysis of samples from steel 40X13 showed that the use of a separate method of vacuum depassivation loses its effectiveness. It was also noted that the use of the mechanical treatment method does not ensure the uniformity of the formed layer, while the method of additional oxidation according to modes No. 2 and No. 3 gives stable in area, but less durable and deep layers. The complex of vacuum depassivation with subsequent additional oxidation according to modes No. 2 and No. 3 proved to be the most effective.

The results of microstructural analysis of samples made of steel 20X13 showed that a stable and uniform result could only be obtained by combining the vacuum depassivation method and additional oxidation according to modes No. 2 and No. 3.

8.2. Metallographic analysis of prototypes of the second stage.

At the second stage, work was carried out to develop the final technology for depassivation of the surface of stainless heat-resistant steels.

Optical microscopy and hardness measurements are carried out on the TUKON 2500 complex. Optical microscopy was carried out at x200 and x500 magnifications. Hardness measurement was carried out at a load of 2 kg with exposure at a load of 30 seconds. The research results are presented in Table 11. In Table 11, the first digit of the sample number indicates the steel grade, the second and third digits are randomly generated.

The results of the microstructural analysis of the samples in comparison with the results of microhardness measurements confirmed the high efficiency of depassivation by a complex of vacuum depassivation and additional oxidation for steels EI961, 40X13 and 20X13.

For austenitic steels, it was found that increasing the post-oxidation cycles without using additional influencing factors does not increase the overall effect of depassivation to an insufficient extent. Approximation of the results shows that a sufficient number of cycles for complete depassivation is impractical.

It was found that surface treatment of austenitic steels with a saturated solution of FeCl3 immediately before the start of additional oxidation makes it possible to obtain the required results when using a complex of vacuum depassivation and additional oxidation according to mode No. 6.

During the tests, some regularities were revealed:

- the most universal and effective method is preliminary oxidation;

- the combination of several methods made it possible to form a vision of a technology that is universal for all grades, confirmed by a positive intermediate result on all five steel grades.

- at the same time, it was found that for each material it is rational to select its own unique method, which will be the most economically feasible for solving a specific problem.

After nitriding, the microhardness of the samples was monitored and the microstructure of individual samples was analyzed. Then the following conclusions were made:

1. The vacuum depassivation treatment method is effective for depassivation of steels with a chromium content of not more than 16%. For steels with a higher concentration of chromium, a combination of this method with others is required.

2. The efficiency of the additional oxidation method increases with the cyclic repetition of the oxidation and reduction operations. At the same time, increasing the number of cycles to more than five does not significantly increase the depassivating effect, but significantly lengthens the process and makes it too expensive.

3. For steels with a chromium content of more than 16%), immediately before additional oxidation, it is necessary to pre-clean the surface with a saturated solution of FeCl3. This greatly improves the depassivation efficiency.

Claims (2)

1. A method for treating the surface of a part made of heat-resistant stainless steel with a chromium content of 16% or more, including preparation for depassivation of the surface of the part and depassivation of the surface of the part; in the first stage, heat treatment of the part is carried out in a vacuum hardening furnace by heating to a temperature of 1000-1050°C, and the residual pressure in the furnace chamber is provided not exceeding 2 mbar; at the second stage, the part is kept until it is completely heated, while the residual pressure in the furnace chamber is provided not exceeding 2 mbar; at the third stage, the part is cooled in a nitrogen or argon medium with a purity of at least 99.99%, and the residual pressure in the furnace chamber during heating and holding is provided not exceeding 2 mbar; at the fourth stage, the part is transferred to a vacuum chemical-thermal treatment furnace for 30 minutes; at the fifth stage, the item is treated with a saturated solution of ferric chloride FeCl ₃ ; at the sixth stage, additional oxidation is carried out, including heating the part in a nitrogen atmosphere to a temperature of 500 ° C, and the residual pressure in the furnace chamber is provided equal to 0.3 bar; at the seventh stage, after equalizing the temperature, the parts are replaced in the furnace chamber with nitrous oxide for nitrogen and kept for two hours; at the eighth stage, the surface of the part is restored, including the replacement of nitrous oxide with ammonia in the furnace chamber; at the ninth stage, the stages from the sixth to the eighth are repeated; at the tenth stage, nitriding is carried out without removing the part from the furnace in which oxidation was carried out. 2. A method for surface treatment of a heat-resistant stainless steel part according to claim 1, characterized in that the ninth step is repeated at least two times.

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