A boronized bearing steel and a method for producing the same

Abstract

The application relates to the technical field of bearing steel, in particular to a boronized bearing steel and a preparation method thereof. The boronized bearing steel comprises a base body and a boronized layer; the base body is 20CrNiMo steel and comprises the following components in percentage by weight: C 0.18-0.22wt%, Si 0.25-0.32wt%, Mn 0.75-0.83wt%, Cr 0.48-0.55wt%, Ni 0.50-0.70wt%, Mo 0.40-0.50wt%, V 0.18-0.21wt%, and the balance is Fe and inevitable impurities; the boronized layer is obtained through salt bath boronizing, the boronizing agent for the salt bath boronizing is mainly composed of Na2B4O7, Al and KCl, and the thickness of the boronized layer is 20-80 mu m. The application forms a boronized layer with a certain thickness on the surface of the base body through specific base body components and boronizing agents, and the surface hardness, wear resistance, strength and toughness of the bearing steel are significantly improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of bearing steel, in particular to a boronized bearing steel and a preparation method thereof. BACKGROUND

[0002] A roller bit is a main rock breaking tool in oil and gas development engineering, which breaks stratum rock in a way of impact, crushing and shearing. Among them, the bearing is a main part in the roller bit, and the roller bit cannot continue to work due to the failure of the bearing to a great extent in the working process. The service life of the bearing directly determines the service life of the roller bit.

[0003] In the use process of the bearing, in addition to the brittle fracture failure, there are two important failure phenomena, that is, wear failure and corrosion failure. The wear failure and the corrosion failure are mostly caused by the damage of the bearing surface or the position close to the surface, because the bearing surface is more severe than the internal working environment, which often leads to early damage. Therefore, to improve the service life of the bearing, further surface strengthening measures must be taken on the basis of the overall high strength and toughness of the bearing steel. Among them, the surface chemical heat treatment process is to form a compound layer (permeation layer) with special physical and chemical properties on the surface of the material, and the bonding strength between the permeation layer and the matrix is higher through metallurgical bonding, so as to endow the material surface with excellent properties such as high wear resistance and high corrosion resistance, and the surface chemical heat treatment process is often used for bearing material treatment.

[0004] In order to improve the wear resistance of the bearing steel, boronizing is one of the effective surface layer treatment methods. Compared with the general chemical heat treatment method, the boronizing layer obtained by the boronizing process has the characteristics of high hardness and good wear resistance. However, the boronizing layer has high brittleness. Especially for some low-carbon alloy steels (20CrNiMo, etc.), after boronizing and quenching and low-temperature tempering treatment, the toughness will be reduced to a certain extent. SUMMARY

[0005] In order to solve the problem that the existing boronizing process reduces the toughness of the low-carbon bearing steel to a certain extent, the application provides a boronized bearing steel and a preparation method thereof. Taking 20CrNiMo low-carbon bearing steel as the research object, a boronizing layer with a certain thickness is formed on the surface of the substrate through specific substrate components and boronizing agents, so that the surface hardness, wear resistance, strength and toughness of the bearing steel are significantly improved.

[0006] In a first aspect, the application provides a boronized bearing steel, comprising a base body and a boronized layer; the base body is 20CrNiMo steel, comprising the following components in percentage by weight: C 0.18-0.22wt%, Si 0.25-0.32wt%, Mn 0.75-0.83wt%, Cr 0.48-0.55wt%, Ni 0.50-0.70wt%, Mo 0.40-0.50wt%, V 0.18-0.21wt%, the balance being Fe and inevitable impurities; the boronized layer is obtained by salt bath boronizing, the boronizing agent of the salt bath boronizing is mainly composed of Na2B4O7, Al and KCl, the weight ratio of the Na2B4O7, Al and KCl is 1:0.06-0.10:0.5-0.6, and the thickness of the boronized layer is 20-80μm.

[0007] The application takes 20CrNiMo steel as the base body, and the control of the carbon content is the key to improve the hardenability and hardenability of the wear-resistant cast steel. If the carbon content is too low, pearlite structure with low hardness and poor wear resistance will appear during quenching, and even if boronizing, the corresponding bearing steel is also difficult to have high wear resistance. If the carbon content is too high, a large amount of high-carbon martensite with high brittleness will appear during quenching, thereby reducing the strength and toughness of the bearing steel, and the boronized layer is also prone to cracking or even peeling during use.

[0008] Silicon dissolves into the bearing steel, although it can strengthen the base body, but it will also make the base body brittle to some extent, so the content of silicon in the bearing steel needs to be strictly controlled. Manganese can significantly improve the hardenability of the bearing steel, but excessive manganese will increase the residual austenite, thereby affecting the wear resistance of the bearing steel. Chromium can improve the hardenability and tempering stability of the bearing steel, but if the chromium content is too high, it will combine with carbon to form chromium carbide, further reducing the carbon content in the base body. Therefore, the amounts of carbon, silicon, manganese and chromium in the base body of the application need to be strictly controlled within the set range, so as to obtain a base body with excellent wear resistance and toughness.

[0009] The addition of nickel can refine the grains, distort the lattice of the bearing steel and hinder the dislocation movement, reduce the brittle transition temperature of the bearing steel, thereby improving the strength and toughness of the bearing steel, but too high content of nickel will reduce the high temperature resistance of the bearing steel. The application appropriately increases the content of molybdenum, which not only effectively promotes the uniform hardening of the bearing steel during quenching, but also compensates for the high-temperature resistance of high-content nickel, but high molybdenum content will also increase the brittleness of the bearing steel. Therefore, a certain amount of vanadium is also added to the base body of the application. Vanadium can further refine the grains, thereby cooperating with nickel to improve the toughness of the bearing steel, and in combination with the high-temperature stability characteristics of vanadium, the high-temperature strength of the bearing can be effectively improved. Thus, by limiting the contents of nickel, molybdenum and vanadium, the application ensures that the bearing steel has high toughness while having good high-temperature resistance, laying a good foundation for high-temperature heat treatment during salt bath boronizing process.

[0010] In addition, the salt bath boronizing process is adopted in the application, which can improve the utilization rate of boronizing agent compared with solid boronizing, and is safer compared with gas boronizing, and is more conducive to the practical application of industry. The boronizing agent uses sodium tetraborate as a boron source, which can react with aluminum to form an alumina-aluminum boride composite ceramic film layer, thereby improving the wear resistance and corrosion resistance of the boronized layer. The sodium tetraborate can also react with potassium chloride in a molten state to form sodium chloride and potassium tetraborate. The potassium tetraborate can be used as a flame retardant, a rust inhibitor and an adhesive, which can not only increase the corrosion resistance and structural strength of the boronized layer, but also further increase the bonding stability of the boronized layer and the substrate. On the other hand, the potassium tetraborate can further react with aluminum to form an alumina-aluminum boride composite ceramic film layer, and the generated composite ceramic film layer has higher bending strength and fracture toughness due to the adhesion of potassium tetraborate.

[0011] However, in the application, the aluminum and potassium chloride in the boronizing agent need to be added in an appropriate amount in order to ensure the strength of the substrate. If the content of aluminum and potassium chloride is insufficient, it is difficult to form a uniform, complete and firmly bonded composite ceramic film layer. If the content of aluminum and potassium chloride is too high, the excess components will further penetrate into the substrate and affect the performance of the substrate. Therefore, the weight ratio of each component in the boronizing agent is strictly controlled to promote the effective function of the boronizing agent.

[0012] On this basis, the thickness of the boronized layer will also affect the wear resistance and toughness of the low-carbon bearing steel to some extent. If the thickness of the boronized layer is thin, the boronizing agent has not yet effectively functioned, and the bonding strength between the boronized layer and the substrate is poor, which is difficult to maintain high wear resistance. If the thickness of the boronized layer is thick, it will affect the subsequent quenching of the bearing steel, and the toughness of the boronized bearing steel obtained will decrease. Therefore, the specific substrate components and the boronizing agent are combined with the set thickness of the boronized layer, and the boronized bearing steel obtained has excellent wear resistance and toughness, which can be better applied to the bearing of the roller bit.

[0013] Preferably, the boronizing agent further comprises NH4Cl.

[0014] Preferably, the weight ratio of Na2B4O7 to NH4Cl is 1:0.3-0.5.

[0015] The ammonium chloride can promote the boronizing agent to effectively penetrate into the substrate to perform surface chemical heat treatment on one hand, thereby improving the bonding strength between the boronized layer and the substrate; on the other hand, the ammonium chloride can react with sodium tetraborate to generate boron nitride, and the boron nitride can react with aluminum in a molten state to generate aluminum nitride, which has good thermal shock resistance and can effectively improve the strength and toughness of the boronized layer when compounded in the boronized layer. In addition, since the ammonium chloride decomposes to generate ammonia and hydrochloric acid in a heated state, the aluminum metal in the present application can adsorb the hydrochloric acid to generate aluminum chloride, thereby effectively reducing the generation of hydrogen chloride gas in the boronizing process and reducing the harm to personnel and the environment.

[0016] In actual operation, if the amount of ammonium chloride is insufficient, it is difficult to effectively play the role of permeation and reinforcement, and if the amount of ammonium chloride is too much, the aluminum metal cannot effectively adsorb the hydrogen chloride generated by the decomposition of the ammonium chloride. Therefore, the application needs to strictly control the amount of ammonium chloride added, and when the ammonium chloride is added according to the above weight ratio, the ammonium chloride can effectively act on the boronizing reaction to reduce the escape of hydrogen chloride gas, thereby not only improving the bonding strength between the boronized layer and the substrate, but also further improving the wear resistance, strength and toughness of the boronized layer.

[0017] In a second aspect, the application provides a preparation method of a boronized bearing steel, comprising the following steps:

[0018] Preparation of 20CrNiMo steel blank: a 20CrNiMo steel blank is prepared according to the component proportion of the substrate;

[0019] Heat treatment: after preheating and dehydration treatment, the boronizing agent is crushed to form a mixed powder, the 20CrNiMo steel blank is put into a salt bath furnace, a part of the mixed powder is added first, heated to austenitizing temperature, and the remaining mixed powder is added in batches and stirred uniformly during the heating process. After the mixed powder is melted and no longer expands, stop stirring, cover the furnace cover and heat preservation, and after the heat preservation is completed, directly quench in water to room temperature to obtain a boronized bearing rough material;

[0020] Tempering: the boronized bearing rough material is first cooked in boiling water, then washed with alcohol, and then dried and stored in vacuum to obtain the boronized bearing steel.

[0021] According to the above technical scheme, the boronizing agent is added in batches, which helps the boronizing agent to gradually react with the substrate, thereby improving the reaction efficiency of the two. On this basis, the water quenching method is used to form lath martensite and sheet martensite in the substrate structure, and then low-temperature tempering is performed by boiling water cooking. The temperature of the boiling water is relatively constant, which helps to reduce the influence of the environment on the mechanical properties of the bearing steel. The substrate obtained in this way can better combine with the carburized layer and has higher toughness and strength.

[0022] Preferably, in the step of heat treatment, the austenitizing temperature is 980-1030℃, and the holding time is 3-5h.

[0023] By the above technical solution, the boronizing treatment is realized by active atom diffusion, and the treatment temperature and time will affect the diffusion distance and concentration distribution of the active atom in the metal matrix. Since the diffusion distance of the active atom is limited, the affected area is mainly the transition zone between the infiltration layer and the core of the matrix, and the boron concentration in the transition zone is higher than that in the core of the matrix, and boron has a great influence on the martensite phase change. In addition, the expansion coefficient of boride (FeB and Fe2B) is large, and the matrix structure is extruded during the formation process, and the transition zone will have a certain degree of strain, which will affect the phase change and growth orientation of the transition zone. The phase change product and mechanical properties of the transition zone are important parameters for determining the bonding strength of the boronized layer and the matrix.

[0024] For this purpose, the present application has found through research and experiments that, when the heat treatment temperature is 980-1030℃ and the holding time is 3-5h, the boronized layer and the matrix are more closely bonded, and thus a boronized bearing steel with higher toughness and strength is obtained, so it is preferred.

[0025] Preferably, in the step of heat treatment, the temperature is raised at a rate of 5-10℃ / min.

[0026] By the above technical solution, if the temperature rising rate is too fast, too much austenite will be left, which will reduce the wear resistance of the matrix, and if the temperature rising rate is too slow, brittle martensite will be generated, so it is appropriate to control the temperature rising rate at 5-10℃ / min.

[0027] Preferably, in the step of heat treatment, the remaining mixed powder is added twice, the first time when the system is heated to 650-700℃, and the second time when the system is heated to 800-850℃.

[0028] By the above technical solution, when the system is heated to 650-700℃, sodium tetraborate and aluminum in the boronizing agent are first melted into a liquid state, and at this time the first addition of the remaining mixed powder helps sodium tetraborate to react with metallic aluminum to form a composite ceramic film layer. Subsequently, when the temperature is raised to 800-850℃ for the second time, potassium chloride is melted and reacts with sodium tetraborate, and the generated potassium tetraborate further reacts with the melted metallic aluminum, so that the boronized layer obtained has good wear resistance and toughness.

[0029] Preferably, in the step of tempering, the cooking time is 0.8-1.5h.

[0030] The surface of the boronized bearing crude material obtained by the heat treatment is attached with more salts, the salts can be effectively cleaned by boiling and washing, but if the boiling and washing time is too long, the boronized layer on the surface will be damaged, and the boiling and washing time is preferably 0.8-1.5 hours.

[0031] In a third aspect, the application provides a preparation method of the boronized bearing steel, comprising the following steps:

[0032] Preparation of 20CrNiMo steel billet: the 20CrNiMo steel billet is prepared according to the component proportion of the matrix;

[0033] Heat treatment: Na2B4O7, Al and KCl are crushed into mixed powder after preheating and dehydration, NH4Cl is crushed into NH4Cl powder after preheating and dehydration, the 20CrNiMo steel billet is put into a salt bath furnace, a part of the mixed powder is added first, heated to austenitizing temperature, the remaining mixed powder is added in batches and stirred uniformly during the heating process, NH4Cl powder is added after the mixed powder is melted, and ammonia gas is continuously introduced, the stirring is stopped after the system no longer expands, the furnace cover is covered for heat preservation, and the boronized bearing crude material is obtained after the heat preservation is completed and directly quenched in water to room temperature.

[0034] Tempering: the boronized bearing crude material is first boiled and washed in boiling water, then cleaned with alcohol, and then dried and stored in vacuum to obtain the boronized bearing steel.

[0035] According to the above technical scheme, the ammonium chloride is added after the boronizing agent is completely melted, at this time, the reaction between sodium tetraborate, metal aluminum and potassium chloride in the boronizing agent has been initially completed, the ammonium chloride is prevented from competing with sodium tetraborate and metal aluminum in the early stage of boronizing reaction, and thus the bearing steel obtained has more excellent wear resistance, strength and toughness.

[0036] In summary, the application has the following beneficial effects:

[0037] 1. The application takes 20CrNiMo low-carbon bearing steel as the research object, controls the amount of carbon, silicon, manganese, chromium, nickel, molybdenum and vanadium in the matrix component, uses the boronizing agent composed of Na2B4O7, Al and KCl for salt bath boronizing, and strictly controls the thickness of the boronized layer, so that the boronized bearing steel obtained can significantly improve the surface hardness, wear resistance and other properties while maintaining the high strength and toughness of the matrix, and can be better applied to the bearings of the roller bit.

[0038] 2. The application adds ammonium chloride in the boronizing process to cooperate with other boronizing agents, which can further improve the bonding strength between the boronized layer and the matrix, and improve the wear resistance, strength and toughness of the boronized layer.

[0039] 3. The application can make the substrate better combined with the carburized layer and has higher toughness and strength by batch feeding boronizing agent and adopting water quenching and boiling water washing tempering.

[0040] 4. The application can avoid the competition reaction of ammonium chloride to sodium tetraborate and aluminum in the early stage of boronizing reaction by adding ammonium chloride after the mixed powder is completely melted, so that the bearing steel obtained has more excellent wear resistance and toughness. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 is the OM base body tissue morphology distribution OM diagram in Example 1 of the application; wherein the left graph is enlarged 500 times, and the right graph is enlarged 200 times.

[0042] Figure 2 is the OM layer + base body tissue morphology distribution diagram in Example 1 of the application, specifically two different angle enlarged 200 times schematic diagrams.

[0043] Figure 3 is the OM layer + base body tissue morphology distribution diagram in Example 1 of the application, specifically two different angle enlarged 500 times schematic diagrams.

[0044] Figure 4 is the SEM base body tissue morphology distribution diagram in Example 1 of the application, specifically two different angle enlarged 5000 times schematic diagrams.

[0045] Figure 5 is the SEM base body tissue morphology distribution diagram in Example 1 of the application, specifically two different angle enlarged 2000 times schematic diagrams.

[0046] Figure 6 is the SEM layer + base body tissue morphology distribution diagram in Example 1 of the application, specifically four different angle enlarged 2000 times schematic diagrams.

[0047] Figure 7 is the SEM layer + base body tissue morphology distribution diagram in Example 1 of the application, specifically two different angle enlarged 1000 times schematic diagrams.

[0048] Figure 8 is the hardness test result diagram of Example 1 of the application.

[0049] Figure 9 is the friction and wear experiment result diagram of Example 1 of the application. DETAILED DESCRIPTION

[0050] In order to make the above objectives, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application are described in detail below. In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the spirit of the present application, so the present application is not limited to the specific embodiments disclosed below.

[0051] The raw materials of the present application are all from commercially available products, and the present application is further described in detail below in combination with the drawings, examples and comparative examples.

[0052] Example

[0053] Example 1

[0054] The present embodiment discloses a boronized bearing steel, and a preparation method thereof includes the following steps:

[0055] (1) Preparation of 20CrNiMo steel billet: 2.0 g of C powder, 3.0 g of Si powder, 8.1 g of Mn powder, 5.1 g of Cr powder, 6.6 g of Ni powder, 4.4 g of Mo powder and 2.0 g of V powder are accurately weighed based on 1 kg of the substrate, and the balance is supplemented with Fe powder. The above is mixed and melted in a melting furnace, and then poured into a mold for preparing a bearing to form a 20CrNiMo steel billet;

[0056] (2) Heat treatment: 950 g of borax (Na2B4O7·10H2O), 40 g of aluminum powder and 280 g of potassium chloride powder are preheated and dehydrated, then crushed to form a mixed powder. The 20CrNiMo steel billet is put into a salt bath furnace, one-third of the mixed powder is added first, and the temperature is raised to 1000℃ at a rate of 8℃ / min. During the heating process, the remaining mixed powder is added twice and stirred uniformly. The first time is when the system is heated to 680℃, one-third of the mixed powder is added, and the second time is when the system is heated to 820℃, the remaining one-third of the mixed powder is added. After the mixed powder is melted and no longer expands, stop stirring, cover the furnace cover and keep warm for 4h. After the heat preservation is completed, it is directly quenched to room temperature in water to obtain a boronized bearing rough material;

[0057] (3) Tempering: the boronized bearing rough material is first placed in boiling water for 1h, the surface residual salt is clearly removed with a soft brush, and then cleaned in an ultrasonic cleaning machine with alcohol. After drying, the boronized bearing steel is obtained and stored in vacuum. The microstructure of the boronized + quenched sample is observed by OM and SEM, and the results are shown in Figures 1 to 7 As can be seen from the figure, the matrix structure of the quenched sample is mainly composed of lath martensite and sheet martensite. After measurement, it is known that the thickness of the boronized layer is about 30-50μm.

[0058] Examples 2-4

[0059] Example 2-4 adjusts the amount of raw materials used in the preparation of 20CrNiMo steel billets based on the method of Example 1. The main component table of the obtained substrate is shown in Table 1 below.

[0060] Table 1 Component table of the substrate of Examples 1-4 (unit: %)

[0061]

[0062] Among them, the measured boronizing layer thickness of Example 2 is about 20-55 μm, the measured boronizing layer thickness of Example 3 is about 60-80 μm, and the measured boronizing layer thickness of Example 4 is 40-65 μm.

[0063] Example 5-7

[0064] Examples 5-7 adjust the process parameters for preparing heat-treated and tempered based on the method of Example 1. The corresponding adjustment parameters are shown in Table 2.

[0065] Table 2 Process parameter table of Example 1 and Examples 5-7

[0066]

[0067] Among them, the measured boronizing layer thickness of Example 5 is about 35-60 μm, the measured boronizing layer thickness of Example 6 is about 25-60 μm, and the measured boronizing layer thickness of Example 7 is 25-50 μm.

[0068] Example 8-9

[0069] Examples 8-9 adjust the amount of components of the mixed powder in the heat treatment step based on the method of Example 1.

[0070] Among them, 950g of borax (Na2B4O7·10H2O), 30g of metal aluminum powder, and 250g of potassium chloride powder are preheated and dehydrated to form a mixed powder, and the corresponding obtained boronizing bearing steel has a measured boronizing layer thickness of about 20-55 μm.

[0071] In Example 9, 950g of borax (Na2B4O7·10H2O), 50g of metal aluminum powder, and 300g of potassium chloride powder are preheated and dehydrated to form a mixed powder, and the corresponding obtained boronizing bearing steel has a measured boronizing layer thickness of about 30-65 μm.

[0072] Example 10

[0073] The embodiment adjusts the adding method of the mixed powder in the heat treatment step based on the method of embodiment 1. The difference after adjustment is that after the 20CrNiMo steel blank is put into the salt bath furnace, the mixed powder is put into the salt bath furnace at one time for salt bath boronizing. The obtained boronized bearing steel is measured to have a boronized layer thickness of about 25-65 μm.

[0074] Embodiment 11

[0075] The embodiment adjusts the adding method of the mixed powder in the heat treatment step based on the method of embodiment 1. The difference after adjustment is that after the 20CrNiMo steel blank is put into the salt bath furnace, one half of the mixed powder is added, and the temperature is raised to 1000℃ at a temperature raising speed of 8℃ / min. During the temperature raising process, the remaining one half of the mixed powder is added when the temperature reaches 680℃ for salt bath boronizing. The obtained boronized bearing steel is measured to have a boronized layer thickness of about 30-60 μm.

[0076] Embodiments 12-16

[0077] Embodiments 12-16 adjust the component of the boronizing agent in the heat treatment step based on the method of embodiment 1. The boronizing agent is additionally added with NH4Cl on the basis of the original Na2B4O7, Al and KCl. The specific heat treatment steps are as follows:

[0078] Take 950g borax (Na2B4O7·10H2O), 40g metal aluminum powder and 280g potassium chloride powder to be crushed into mixed powder after preheating and dehydration treatment. Take 0.2-0.6 times of the weight of Na2B4O7 of ammonium chloride powder for preheating and dehydration treatment. Then put the 20CrNiMo steel blank into the salt bath furnace, add one third of the mixed powder, and raise the temperature to 1000℃ at a temperature raising speed of 8℃ / min. During the temperature raising process, add the remaining mixed powder twice and stir uniformly. The first time is to add one third of the mixed powder when the system is raised to 680℃, and the second time is to add the remaining one third of the mixed powder when the system is raised to 820℃. After the mixed powder is melted, add the preheated and dehydrated ammonium chloride powder and continuously introduce ammonia gas. When the system no longer expands, stop stirring, cover the furnace cover, and keep warm for 4h. After the warm-keeping is finished, directly quench in water to room temperature to obtain boronized bearing rough material.

[0079] Among them, 100g of ammonium chloride powder is added in embodiment 12, 150g of ammonium chloride powder is added in embodiment 13, 200g of ammonium chloride powder is added in embodiment 14, 250g of ammonium chloride powder is added in embodiment 15, and 300g of ammonium chloride powder is added in embodiment 16. The obtained boronized bearing steel of embodiments 12-16 is measured to have a boronized layer thickness of about 40-55 μm.

[0080] Example 17

[0081] Example 17 is based on the method of Example 14, and the adding method of boronizing agent in the heat treatment is adjusted. The difference after adjustment is that after the 20CrNiMo steel blank is put into the salt bath furnace, the mixed powder and ammonium chloride powder are mixed uniformly and then put into the salt bath furnace at one time for salt bath boronizing. The obtained boronized bearing steel sample is measured, and the thickness of the boronized layer is about 30-65 pm.

[0082] Comparative Example

[0083] Comparative Example 1

[0084] This comparative example is based on the method of Example 1, and the amount of raw materials used in the preparation of 20CrNiMo steel blank is adjusted. The main component table of the corresponding obtained substrate is shown in Table 1 above.

[0085] After the obtained boronized bearing steel sample is quenched, the substrate structure is mainly composed of lath martensite and the compactness is less than that of Example 1. The thickness of the boronized layer is about 15-70 pm after measurement.

[0086] Comparative Example 2

[0087] This comparative example is based on the method of Example 1, and the component of boronizing agent in the heat treatment step is adjusted. Specifically, aluminum oxide powder is used to replace metal aluminum powder, that is, the boronizing agent is composed of Na2B4O7, Al2O3 and KCl.

[0088] After the obtained boronized bearing steel sample is quenched, although the substrate structure is also mainly composed of lath martensite and lamellar martensite, the compactness of the substrate structure is less than that of Example 1. The thickness of the boronized layer is about 25-90 pm after measurement.

[0089] Comparative Example 3

[0090] This comparative example is based on the method of Example 12, and the component of boronizing agent in the heat treatment step is adjusted. Specifically, the metal aluminum powder is replaced with an equal amount of potassium chloride powder.

[0091] After the obtained boronized bearing steel sample is quenched, the substrate structure is mainly composed of lath martensite and the compactness is less than that of Example 1. The thickness of the boronized layer is about 55-95 pm after measurement.

[0092] Comparative Example 4

[0093] This comparative example is based on the method of Example 12, and the component of boronizing agent in the heat treatment step is adjusted. Specifically, the potassium chloride powder is replaced with an equal amount of metal aluminum powder.

[0094] The obtained boronized bearing steel sample after quenching, the matrix structure is mainly composed of lath martensite and the density is less than that of Example 1, the thickness of the boronized layer is about 80-120 μm after measurement.

[0095] Comparative Example 5

[0096] The comparative example is based on the method of Example 1, the component and amount of the boronizing agent in the heat treatment step are adjusted, specifically, 950 g of borax (Na2B4O7·10H2O), 140 g of metal aluminum powder, and 280 g of potassium chloride powder are preheated and dehydrated, and then crushed to form a mixed powder.

[0097] Comparative Example 6

[0098] The comparative example is based on the method of Example 1, the component and amount of the boronizing agent in the heat treatment step are adjusted, specifically, 950 g of borax (Na2B4O7·10H2O), 40 g of metal aluminum powder, and 380 g of potassium chloride powder are preheated and dehydrated, and then crushed to form a mixed powder.

[0099] Performance test

[0100] The boronized bearing steel obtained in Examples 1-13 and Comparative Examples 1-3 above is used as a sample to perform the following performance tests.

[0101] 1. Hardness: The microhardness of the boronized layer is measured by an HV-1000 micro Vickers hardness tester, the test load is 250 gf, the loading time is 10 s, the dotting position starts from the corner of the sample, and the horizontal direction is spaced by 100 μm, and the vertical direction is spaced by 10 μm, and a total of 10 points are tested for hardness.

[0102] Table 3 shows the test results of the sample of Example 1 at different positions, and the corresponding hardness value curve is shown in FIG. 1. Figure 8 As can be seen from the figure, the microhardness gradually decreases from the boronized layer to the matrix. The average microhardness of the boronized layer is about 1560 HV, the hardness of the transition layer is about 741 HV, and the average microhardness of the matrix is only 457 HV. The results show that the microhardness value is increased by about 3.4 times after boronizing. The average microhardness of each layer of other examples and comparative examples is shown in Table 4 below.

[0103] 2. Wear resistance: The ML-100C abrasive wear tester is used for friction and wear test, the weight is 8 g, the disc rotation speed is 60 revolutions / min, the sample feed is 2 mm / revolution, the stroke is 60 mm, the acceleration and deceleration time is 5 ms, the rotation number is 30 revolutions, i.e. the single friction and wear time is 30 s, and the mass loss is recorded after 180 s.

[0104] Figure 9The weight loss and wear time change rule of the sample of Example 1 during the friction and wear experiment is given, as shown in the figure, the weight loss of the boronized sample is smaller during the experiment, and the weight loss is slightly accelerated only after 90s of wear, but the weight loss is still relatively gentle. Compared with the sample after the boronized layer, the weight loss of the matrix sample from the beginning to the end of the wear experiment showed a linear decreasing trend with the wear time, which showed that the wear resistance of the boronized alloy steel was significantly improved. The mass loss of other examples and comparative examples is shown in Table 4 below.

[0105] 3. Mechanical properties: the upper yield strength R eH , tensile strength R m , total elongation at break A t and reduction of area Z of the sample as a whole were measured according to the detection standard of GB228-2010, and the detection results are shown in Table 4 below.

[0106] Table 3 Hardness measurement results of Example 1 (HV 0.025 )

[0107] HV 1 2 3 4 5 6 7 8 9 10 hardness 1668.7 1527.1 1483.7 741.6 472.7 417.2 488.7 417.1 496.5 450.7

[0108] Table 4 Detection results of Examples 1-13 and Comparative Examples 1-3

[0109]

[0110]

[0111] According to the description of the above examples and comparative examples, combined with Table 4, the detection results of Examples 1-17 and Comparative Examples 1-6 are compared and analyzed, it can be obtained that the matrix structure of the boronized bearing steel quenched according to the application mainly consists of lath martensite and flaky martensite. From the thickness of the boronized layer, the thickness of the boronized layer after boronizing treatment is controlled to be 20-80μm, which is narrower than that of the comparative example. From the microhardness, the average microhardness of the boronized layer is about 1475-1611HV, and the average microhardness of the matrix is about 435-467HV, the microhardness value of the boronized layer is increased by about 3.4 times or more, and the microhardness improvement efficiency is better than that of the comparative example. From the wear resistance, the mass loss of the application is lower; from the mechanical strength, the yield strength of the application is 985-1043MPa, and the tensile strength is 1269-1340MPa, which is significantly better than that of the comparative example; from the toughness, the total elongation at break of the application is 30.7-44.5%, and the reduction of area is 59.2-71.2%, which is also higher than that of the comparative example.

[0112] Therefore, the application takes 20CrNiMo low-carbon bearing steel as the research object, controls the dosage of carbon, silicon, manganese, chromium, nickel, molybdenum and vanadium in the base component, carries out salt bath boronizing by using a boronizing agent composed of Na2B4O7, Al and KCl, and strictly controls the thickness of the boronized layer. The boronized bearing steel obtained thereby can significantly improve the surface hardness, wear resistance and other properties while maintaining the high strength and toughness of the base, and can be better applied to the bearing of the roller bit.

[0113] Further analysis of the test results of Examples 1-7 and Comparative Example 1 shows that slight adjustment of the raw material dosage and heat treatment and tempering process parameters for preparing 20CrNiMo steel billets will affect the performance of the boronized bearing steel. Therefore, the raw material dosage and process parameters in the application need to be strictly controlled.

[0114] Analysis of the test results of Example 1 and Examples 8-9 and Comparative Examples 2-6 shows that the main components of the boronizing agent in the application must be composed of Na2B4O7, Al and KCl in a weight ratio of 1:0.06-0.10:0.5-0.6. Only in this way can the boronized bearing steel significantly improve the surface hardness, wear resistance and other properties while maintaining the high strength and toughness of the base, and be better applied to the bearing of the roller bit. As shown by the test results of Example 1, Example 12 and Examples 2-6, the mechanical properties of the boronized bearing steel are better when a specific proportion of Al, KCl and ammonium chloride is added as the boronizing agent component than when the same amount of two of them is used alone as the boronizing agent component, indicating that the three components together with borax as the boronizing agent achieve a synergistic improvement effect.

[0115] Analysis of the test results of Example 1 and Examples 10-11 shows that the use of batch feeding of the corresponding mixed powder of the boronizing agent can obtain boronized bearing steel with more excellent performance. Further selection of the three-time feeding method can obtain boronized bearing steel with more excellent microhardness, wear resistance, mechanical strength and toughness.

[0116] Analysis of the test results of Example 1 and Examples 12-16 shows that the addition of an appropriate amount of ammonium chloride can further improve the bonding strength between the boronized layer and the base, and the wear resistance, strength and toughness of the boronized layer. The addition amount of ammonium chloride needs to be strictly controlled according to the weight ratio of Na2B4O7 to NH4Cl being 1:0.3-0.5. Only in this way can the ammonium chloride effectively play its role and ensure the wear resistance, strength and toughness of the boronized layer.

[0117] The detection results of example 14 and example 17 are analyzed, it can be obtained that, after the boronizing agent is completely melted, the ammonium chloride is added, at this time, the sodium tetraborate, the metal aluminum and the potassium chloride in the boronizing agent have preliminarily completed the reaction, the contention reaction of the ammonium chloride to the sodium tetraborate and the metal aluminum in the early stage of the boronizing reaction is avoided, and thus the bearing steel has more excellent microhardness, wear resistance, strength and toughness.

[0118] The specific embodiment is only an explanation of the application, and is not a limitation of the application, and those skilled in the art can make modifications to the embodiment without creative contribution according to the needs after reading the specification, but as long as it is within the scope of the claims of the application, it is protected by the patent law.

Claims

1. A boron-infused bearing steel, characterized in that: The base body is 20CrNiMo steel, and includes the following components in percentage by weight: C 0.18-0.22wt%, Si 0.25-0.32wt%, Mn 0.75-0.83wt%, Cr 0.48-0.55wt%, Ni 0.50-0.70wt%, Mo 0.40-0.50wt%, V 0.18-0.21wt%, and the balance of Fe and inevitable impurities; the boronizing layer is obtained by salt bath boronizing, and the boronizing agent of the salt bath boronizing is composed of Na2B4O7, Al and KCl, the weight ratio of the Na2B4O7, Al and KCl is 1:0.06-0.10:0.5-0.6, and the thickness of the boronizing layer is 20-80μm; The preparation method of the boronized bearing steel comprises the following steps: Preparation of 20CrNiMo steel billet: 20CrNiMo steel billet is prepared according to the component proportion of the base body; Heat treatment: after the boronizing agent is preheated and dehydrated, the boronizing agent is crushed to form a mixed powder, the 20CrNiMo steel billet is put into a salt bath furnace, a part of the mixed powder is added first, and the temperature is raised to the austenitizing temperature; during the temperature rising process, the remaining mixed powder is added in batches and stirred uniformly; after the mixed powder is melted and no longer expands, the stirring is stopped, the furnace cover is covered, and the temperature is kept; after the temperature keeping is finished, the temperature is directly quenched to room temperature in water to obtain a boronized bearing crude material; Tempering: the boronized bearing crude material is first boiled in boiling water, then washed with alcohol, and then dried and stored in vacuum to obtain the boronized bearing steel.

2. The boronized bearing steel according to claim 1, characterized in that: In the step of heat treatment, the austenitizing temperature is 980-1030℃, and the temperature keeping time is 3-5h.

3. The boronized bearing steel of claim 1, wherein: In the step of heat treatment, the temperature is raised at a temperature rising speed of 5-10℃ / min.

4. The boronized bearing steel of claim 1, wherein: In the step of heat treatment, the remaining mixed powder is added twice, the first time is when the system is raised to 650-700℃, and the second time is when the system is raised to 800-850℃.

5. The boronized bearing steel of claim 1, wherein: In the step of tempering, the boiling time is 0.8-1.5h.

6. A boron-infused bearing steel, characterized by: The base body is 20CrNiMo steel, and includes the following components in percentage by weight: C 0.18-0.22wt%, Si 0.25-0.32wt%, Mn 0.75-0.83wt%, Cr 0.48-0.55wt%, Ni 0.50-0.70wt%, Mo 0.40-0.50wt%, V 0.18-0.21wt%, and the balance of Fe and inevitable impurities; the boronizing layer is obtained by salt bath boronizing, and the boronizing agent of the salt bath boronizing is composed of Na2B4O7, Al, KCl and NH4Cl, the weight ratio of the Na2B4O7, Al, KCl and NH4Cl is 1:0.06-0.10:0.5-0.6:0.3-0.5, and the thickness of the boronizing layer is 20-80μm; The preparation method of the boronized bearing steel comprises the following steps: Preparation of 20CrNiMo steel blank: according to the component proportion of the matrix, the 20CrNiMo steel blank is prepared; Heat treatment: after preheating and dehydration treatment, Na2B4O7, Al and KCl are crushed to form a mixed powder, and NH4Cl is crushed to form an NH4Cl powder after preheating and dehydration treatment; the 20CrNiMo steel blank is put into a salt bath furnace, a part of the mixed powder is added first, and the temperature is raised to the austenitizing temperature; during the heating process, the remaining mixed powder is added in batches and stirred uniformly; after the mixed powder is melted, the NH4Cl powder is added and ammonia gas is continuously introduced; after the system no longer expands, the stirring is stopped, the furnace cover is covered, and the heat preservation is carried out; after the heat preservation is completed, it is directly quenched in water to room temperature to obtain a boronized bearing rough material; Tempering: the boronized bearing rough material is first placed in boiling water for cooking and washing, then cleaned with alcohol, and then dried and stored in vacuum to obtain the boronized bearing steel.

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