KR19980069660A - Surface treatment method of low carbon alloy steel - Google Patents

Abstract

The present invention relates to a surface treatment method of a low carbon alloy steel, the first step of increasing the amount of alloying elements of the surface layer by diffusion coating chromium on the surface layer of the low carbon alloy steel, the surface by carburizing heat treatment of the material coated by the first step Including a second step of uniformly dispersing the high hardness chromium carbide, the low carbon alloy steel, characterized in that to ensure high hardness, high wear resistance, high corrosion resistance and high fracture toughness inside the low carbon alloy steel surface Provide a surface treatment method.

Description

Surface treatment method of low carbon alloy steel

The present invention relates to a surface treatment method of low carbon alloy steel, and more particularly, in the manufacturing of components such as power generation equipment, industrial machinery, heavy equipment, etc. by diffusing coating chromium (Cr) on the surface layer of low carbon alloy steel and then surface by plasma carburizing heat treatment process The present invention relates to a method for surface treatment of low carbon alloy steel, which ensures high hardness, high wear resistance, and high corrosion resistance, and ensures high fracture toughness inherent in low alloy steel.

In general, as a material requiring corrosion resistance and abrasion resistance, such as power generation equipment, shipbuilding machinery parts, various driving parts, industrial machinery, expensive alloy materials such as expensive stainless steel or high chromium cast iron were used. In particular, in the case of various mechanical parts that require corrosion resistance and wear resistance at the same time, there is a design problem inevitably using expensive materials to satisfy the required characteristics.

In addition, the above-described materials have the characteristics of supply and demand by the order, so that there is a problem that a large number of orders must be manufactured for supply and demand, thereby preparing a large number of materials more than necessary, even though a small amount of material is required, the overall cost of the product Has a problem of weakening the competitiveness,

Therefore, in order to secure the properties of materials required for design by using mass-produced materials, many researchers in the related industry have reviewed the applicability of the technology for surface modification. It was not early to get results.

In addition, as described above, commercially available steels are actually designed to be substantially larger in size in order to lower sufficient fracture toughness and fatigue crack growth rate in terms of fracture toughness. Therefore, many researchers have attempted to introduce diffusion coating and surface carburizing process of chromium as a surface treatment method of austenitic stainless steels, but it is difficult to expect the heat treatment effect that is required as a subsequent process during actual carburizing heat treatment. You will not have the problem.

Accordingly, an object of the present invention is to diffuse the chromium on the surface of the low-carbon alloy steel, which is widely used as a material for commercial machine parts, and then to strengthen the surface layer through the plasma carburizing heat treatment process, and to the quenching effect due to the subsequent process of carburizing Therefore, high hardness, high abrasion resistance, and high corrosion resistance of the surface are secured, and at the same time, deep fracture toughness of low alloyed steel is secured, thereby maximizing the design strength, thus eliminating the use of expensive alloy steel, thereby reducing costs, and excellent mechanical properties It is to provide a method of surface treatment of low-carbon alloy steel to improve the cost competitiveness of the product through securing.

1 is a graph showing the result of diffusion coating chromium on the surface of low carbon alloy steel according to the present invention;

2 is a graph comparing the surface hardness of the carburized specimen and the carburized specimen according to the present invention after diffusion coating with chromium according to the present invention;

Figure 3 is a graph comparing the wear test results of the carburized specimen and the carburized specimen by the prior art after the diffusion coating with chromium according to the present invention.

An object of the present invention described above is a first step of increasing the amount of alloying elements of the surface layer by diffusion coating chromium on the surface layer of low carbon alloy steel, and carburizing heat treatment of the material coated by the first step to the surface of high hardness chromium Including a second step of uniformly dispersing the carbide, provides a surface treatment method of low carbon alloy steel, characterized in that to ensure high hardness, high wear resistance, high corrosion resistance and high fracture toughness inside the low carbon alloy steel surface Is achieved by.

According to a preferred feature of the present invention, the pack cementation process is applied as the chromium diffusion coating method, and the plasma carburization process is applied as the carburization heat treatment process.

According to a preferred feature of the present invention, the weight fraction of the powder used in the pack cementation process is 5 to 25 wt% of chromium powder of the diffusion coating agent, 60 to 90 wt% of Cr ₂ O ₃ of the sintering agent and NH ₄ Cl of the active agent. A pack powder is used in which 5-15 wt% is mixed at a predetermined composition ratio.

A preferred feature of the present invention is that the pack cementation process completely charges a low-carbon treated steel, which is a material to be treated, in the pack powder mixed in a semi-closed container at a predetermined composition ratio, and then heats it up in a reducing atmosphere at 800-1200. Heat treatment at 2 ° C. for 2-8 hours causes diffusion of chromium on the surface of the workpiece.

In a preferred embodiment of the present invention, the plasma carburizing process includes a sputter cleaning step of removing a surface oxide film of a material by using a plasma oscillation property by charging a chromium diffusion coated material into a plasma carburizing chamber, and chromium diffusion coating. A carburizing step of infiltrating and diffusing carbon on the surface of the material, a diffusion step of diffusing the carburized carbon from the surface of the material to the inside, a rapid temperature change falling from the temperature of the diffusion step to room temperature, and to prevent deformation of the treated object An austenizing step and a predetermined quenching step.

According to a preferred aspect of the present invention, in the sputter cleaning of the plasma carburizing process conditions, the pressure in the furnace is 0.1-10 torr, the plasma forming voltage is 200-600 V, the temperature is 500-950 ° C, and the time is in the range of 5-30 minutes. More preferably, the furnace pressure is 1 torr, plasma formation voltage is 300 V, temperature is 500 ° C., and time is performed within 15 minutes.

According to a preferred aspect of the present invention, in the carburizing process of the plasma carburizing process, the gas pressure range is 0.5 to 30 torr, the plasma forming voltage is 300 to 800 V, the temperature is 850 to 1050 ° C., and the time is 30 to 120 minutes. The gas pressure range is more preferably 4 to 10 torr, the plasma forming voltage is 300 to 600 V, the temperature is 900 to 950 ° C., and the time is 30 to 60 minutes.

According to a preferred aspect of the present invention, in the diffusion process of the plasma carburizing process conditions, the gas pressure range is 0.5-30 torr, the plasma forming voltage is 300-600 V, the temperature is 850-1050 ° C., and the time is 10-60 minutes. The gas pressure range is 1 to 5 torr, the plasma forming voltage is 300 to 400 V, the temperature is 900 to 950 ° C., and the time is 15 to 30 minutes.

A preferred feature of the present invention, when the austenizing in the plasma carburizing process conditions, the gas pressure range is 760 torr, the temperature is treated in an inert gas atmosphere within the range of 750 to 850 ℃, more preferably the gas The pressure range is 760 torr and the temperature is treated in an inert gas atmosphere within the range 800-850 ° C.

One preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

1 is a graph showing the result of the diffusion coating of chromium on the surface of low carbon alloy steel according to the present invention, Figure 2 is a surface hardness of the carburized specimen and the carburized specimen according to the prior art after diffusion coating with chromium according to the present invention 3 is a graph comparing the wear test results of the carburized specimen and the carburized specimen according to the prior art after diffusion coating with chromium according to the present invention.

As shown in the drawings, the surface treatment method of the low carbon alloy steel according to the present invention, by impregnating chromium into the surface layer of the low carbon alloy steel, and then subjected to plasma carburization heat treatment, high hardness, high wear resistance, high corrosion resistance and It is a surface modification method used in the manufacture of products with high fracture toughness.

The pack cementation process was applied to the diffusion coating method of chromium used in the present invention.

In the above-mentioned pack cementation process, chromium powder, aluminum oxide or chromium oxide powder as a sintering agent, and ammonium halide powder as an activator are mixed in an appropriate ratio in a semi-closed container, and the object to be processed is completely mixed inside the mixed powder. It is a step of spreading chromium to the surface of the workpiece through subsequent heat treatment under reducing or inert atmosphere.

As described above, chromium penetrated into the surface of the material penetrates and diffuses to a depth of 5-300 um depending on process factors such as treatment temperature, treatment time, atmosphere, shape, etc. Depending on the process variables, such as time, it will vary within the range of 5 wt%-50 wt%.

However, as described above, the chromium that has been diffused and penetrated can only obtain a reinforcing effect by surface alloying, thereby infiltrating and diffusing carbon back to the surface in order to secure excellent fracture toughness and high surface hardness inherent in low carbon steel. The addition of a process allows subsequent heat treatment to ensure high surface hardness and high core fracture toughness.

In the case of gas carburizing, which is a carburizing method according to the related art, in most cases, when the amount of chromium is large as an alloying element, carburization is impossible, and a special pretreatment process is required for carburization.

Therefore, in the present invention, in order to overcome the limitations described above, by applying the plasma carburization process, it is possible to secure excellent carburization characteristics, and excellent core parts which can be secured in the conventional carburization process through subsequent heat treatment after diffusion coating of chromium. Mechanical properties can be secured.

Example

Hereinafter, an embodiment of the present invention will be described.

The weight fraction of the pack cementation powder for the chromium diffusion coating on the low carbon alloy steel surface layer is 5-25 wt% of the chromium powder, a diffusion coating material, 60-90 wt% of Cr ₂ O ₃ of the sintering agent, and NH ₄ Cl of the activator. 5 to 15 wt% Packed powders of mixed powders are mixed at a predetermined composition ratio. After completely charging the low-carbon treated steel as a material to be treated in the pack powder mixed at a predetermined composition ratio as described above, the mixture is heated in a reducing atmosphere or an inert atmosphere, and then heat-treated at 800-1200 ° C for 2-8 hours.

As a result, a chromium diffusion layer having a concentration of 25 wt% or more and 50-200 um can be obtained in the surface layer of the low carbon alloy steel described above. At this time, the thickness of the chromium diffusion coating layer can be adjusted to a predetermined thickness and surface chromium concentration by varying the process conditions according to the use, corrosion environment, wear conditions.

In the carburization method used in the present invention, a plasma carburization process was applied. In the above-mentioned plasma carburization process, CH ₄ or C ₃ H ₈ is used as the carbon source gas, and plasma is generated at a pressure of several torr (1 atm = 760 torr) to sputter-clean the chromium-coated surface, followed by carburization. The method of performing was used.

In general, carburizing methods include solid carburizing, gas carburizing and vacuum carburizing, and when the chromium concentration of the material is high, it becomes impossible to carburize because it forms chromium oxide on the surface of the material by combining with oxygen in the atmosphere formed during carburizing. .

On the other hand, in the plasma carburization process of the present invention, the oxide film is removed during sputter cleaning before carburization, and since no oxygen is formed in the atmosphere even during carburization, the oxide film is not formed and carburization is sufficiently performed.

The above-described plasma carburization process is a process applied to carburization of a material which is not generally carburized, that is, stainless or high chromium alloy, and thus carburization can be performed by removing a chromium oxide film that is inevitably formed on the surface of the high chromium alloy through sputtering. It has a feature to make it.

The above-described plasma carburization process is as follows.

As a first step, a chromium-diffused coating material is charged into a plasma carburizing chamber and then sputtered to remove the surface oxide film. This is essential to the carburizing process, and subsequent carburizing will not be possible unless sufficient sputter cleaning is preceded.

In the above-described sputter washing step, argon gas having excellent cleaning power is injected to maintain the furnace pressure within the range of 0.1-10 torr. More preferably, the furnace pressure is 1 torr, and the above-mentioned furnace pressure is lower than 0.1 torr. Due to the lack of the sputter cleaning of the workpiece can be made non-uniform, the surface roughness of the material may be changed by the sputter cleaning at the above-mentioned furnace pressure of 10 torr or more, and may also cause non-uniform cleaning effect. .

The plasma formation voltage for the above-mentioned sputter cleaning is required to be 200V-600V, more preferably the plasma formation voltage is 300V, the plasma is not formed or less than the above-mentioned plasma formation voltage 200V to obtain a uniform cleaning effect. It becomes impossible, and the chip | tip part is not wash | cleaned, and when the above-mentioned plasma formation voltage is more than 600V, it is feared that the surface roughness change of a material arises due to an excessive sputtering effect.

In addition, the temperature at the time of sputter washing is performed at a temperature lower than the temperature maintained in the carburizing step, that is, 500 to 950 ℃ range is efficient, more preferably 500 ℃. Sputter washing time is performed within 5-30 minutes, more preferably 15 minutes.

As a second step, plasma carburizing is performed to infiltrate carbon on the surface of the workpiece, and the gas pressure range is 0.5-30 torr, more preferably, the gas pressure range is 4-10 torr, At low pressure below 0.5 torr, the carburizing efficiency decreases, and it is not partially uniform carburizing, and the gas pressure above 30 torr causes excessive sooting and causes sooting. In this case, plasma formation becomes unstable.

The surface carbon fraction is in the range of 1.0 to 6 wt%, and when the surface carbon fraction is 1.0 wt% or less, the amount of carbon is small and the carburizing time is long, and when the surface carbon fraction is 6 wt% or more, excessive carbon Gas supply can cause sooting.

The plasma formation voltage range for carburizing the chromium diffusion coating product described above is 300V-800V, more preferably, the plasma formation voltage is 300V-600V, and the plasma formation voltage is less than 300V, the plasma formation is insufficient and uniform carburization. If the voltage is increased, the plasma sheath decreases, and the plasma seam is uniformly coated along the shape of the object, so that carburizing uniformity is formed. The carburization does not occur due to the occurrence.

The carburizing temperature is in the range of 850 ° C. to 1050 ° C., more preferably in the range of 900 ° C. to 950 ° C., and at a high temperature of 1050 ° C. or higher, it is difficult to maintain and control the vacuum, and the internal strength of the gear rapidly increases for a long time. It has a disadvantage in that it falls, and at temperatures below 850 ° C., the carburizing time becomes long and inefficient. Carburizing time is carried out within 30-120 minutes, more preferably within 30-60 minutes.

As a third step, a diffusion step of diffusing carburized carbon from the tooth surface to the inside, reducing the amount of carbon source gas in order to fix the surface carbon concentration of the final product at 2-4 wt%, and the gas pressure range is at the time of carburization. And 0.5 to 30 torr, more preferably 1 to 5 torr.

When the gas pressure is lower than 0.5 torr, the amount of carbon reaching the surface decreases gradually, thereby supplying carbon atoms that are insufficient to maintain the surface carbon concentration and ultimately decarburizing carbon atoms that have penetrated and diffused into the surface. In the above-described gas pressure of 30 torr or more, it is difficult to expect the generation of a uniform plasma, and control is difficult, thereby limiting the range.

In the diffusion step, the plasma formation voltage exhibits excellent efficiency in the range of 300 to 600V, and preferably about 300 to 400V. This is because the plasma is not uniformly formed at the plasma formation voltage of 300V or less, and the sputtering effect is increased at the plasma formation voltage of 600V or more, which may cause decarburization of surface carbon.

In addition, the higher the temperature of the diffusion step, the better, but it is difficult to control at a high temperature of 1050 ℃ or more, the appropriate temperature range is required 850 ℃-1050 ℃ the same as the carburizing process, more preferably 900 ℃-950 ℃.

The diffusion time is within 10-60 minutes, more preferably within 15-30 minutes.

When the plasma carburizing process is performed through the above-described process, the carburizing / diffusion step may be performed one or more times, preferably two to three times, thereby completing the carburizing process.

As a fourth step, the austenizing step is to control deformation, which is a problem that may occur during the subsequent quenching heat treatment, and prevents a rapid temperature change from the temperature of the diffusion step in the state of carburization to room temperature as described above, and is treated In order to prevent the deformation of water, the temperature during austenizing is preferably 750 ° C. to 850 ° C., which is about 100 ° C. lower than the temperature of the diffusion step, and more preferably 800 ° C. to 850 ° C., and carbon carburized on the surface In order to prevent decarburization, the inert gas is charged at a pressure of 1 atmosphere and maintained for 20 to 120 minutes, and then the entire process is completed by water or oil cooling. The subsequent tempering process is the same as that of conventional gas carburization. It is completed by carrying out in and around ℃.

Looking at the application examples and test results of the processed product produced by such a process are as follows.

Figure 1 shows the result obtained by the Pack Cemetation treatment (chromium) on the surface of the low carbon alloy steel for 3 hours at 1000 ℃. After chromium diffusion treatment, the surface chromium concentration was 25 wt%, and a surface diffusion layer having a thickness of 70 um was obtained. In particular, in Figure 1 it can be seen that chromium is diffused into the low carbon alloy steel, the chromium concentration is higher than the known to 200um.

After the chromium diffusion treatment as described above, the plasma carburization treatment was performed at 900 ° C for 2 hours under the aforementioned process conditions. Hardness distribution of the treated material as described above is shown in FIG. The surface hardness in FIG. 2 is 1200 Hv, which shows that the effective hardening depth is improved by more than 30%, which shows that the hardness is improved due to the diffusion of chromium.

Figure 3 shows the wear test results of the material to which the treatment process according to the present invention is applied. The material carburized in low carbon chromium molybdenum steel has a very low wear and abrasion resistance is significantly improved.

This phenomenon occurs when chromium carbide diffused coated on the surface is uniformly distributed with high hardness chromium carbide in the subsequent carburizing process, and the fraction of chromium carbide distributed on the surface can be controlled to about 5-45%. It can be seen that it can be adjusted according to various working environment such as chromium diffusion coating condition, carburizing condition and processing temperature.

As described in detail above, the method for surface treatment of low carbon alloy steel according to the present invention is to increase the amount of alloying elements of the surface layer by diffusion coating chromium on the surface layer of the low carbon alloy steel and then subject the surface to plasma carburization heat treatment. By uniformly dispersing the hard chromium carbides, it is possible to secure excellent surface properties and high internal fracture toughness, and it is not necessary to use expensive alloy steel, which reduces the overall cost cost despite the additional manufacturing process. You can improve your competitiveness.

Claims (14)

A first step of increasing the amount of alloying elements in the surface layer by diffusion coating chromium on the surface layer of the low carbon alloy steel, Carburizing heat-treating the material coated by the first step to uniformly disperse high hardness chromium carbide on the surface, the high hardness, high wear resistance, high corrosion resistance and the inside of the low carbon alloy steel on the surface of the low carbon alloy steel Surface treatment method of low carbon alloy steel, characterized in that to ensure high fracture toughness. The method of claim 1, wherein a pack cementation process is applied as the chromium diffusion coating method. The method of claim 1, wherein a plasma carburization process is applied as the carburization heat treatment process. The weight fraction of the powder used in the pack cementation process is 5 to 25 wt% of chromium powder of the diffusion coating agent, 60 to 90 wt% of Cr ₂ O ₃ of the sintering agent, NH ₄ Cl 5-of the active agent. A method of surface treatment of low carbon alloy steels, characterized in that a pack powder in which 15 wt% is mixed in a predetermined composition ratio is used. The method according to claim 2 or 4, wherein the pack cementation step, The low-carbon treated steel, which is a material to be treated, is completely charged into the pack powder mixed in a semi-closed container at a predetermined composition ratio, and then heated in a reducing atmosphere, and then heat-treated at 800-1200 ° C for 2-8 hours to surface the material. Surface treatment method of low carbon alloy steel, characterized by diffusing the chromium into. The method of claim 3, wherein the plasma carburization process, A sputter cleaning step of inserting a chromium-coated material into a plasma carburizing chamber to remove the surface oxide film of the material by using plasma oscillation characteristics; A carburizing step of infiltrating and diffusing carbon on the surface of the chromium-coated diffusion material, A diffusion step of diffusing the carburized carbon from the surface of the material to the inside, An austenizing step for preventing rapid temperature changes and deformation of the object from falling in the temperature of the diffusion step; Surface treatment method of low carbon alloy steel, characterized in that it comprises a predetermined hardening step. The method according to claim 6, wherein when the sputter cleaning during the plasma carburizing process conditions, the pressure inside the furnace is 0.1 to 10 torr, the plasma forming voltage is 200 to 600 V, the temperature is 500 to 950 ℃, the time is performed within the range of 5 to 30 minutes. Surface treatment method of low carbon alloy steel, characterized in that. The method of claim 6, wherein in the carburizing process of the plasma carburizing process conditions, the gas pressure range is 0.5 to 30 torr, the plasma forming voltage is 300 to 800V, the temperature is 850 to 1050 ℃, the time is performed in the range of 30 to 120 minutes Surface treatment method of low carbon alloy steel, characterized in that. The method of claim 6, wherein in the diffusion process of the plasma carburizing process conditions, the gas pressure range is 0.5 to 30 torr, the plasma forming voltage is 300 to 600V, the temperature is 850 to 1050 ℃, the time is performed within the range of 10 to 60 minutes. Surface treatment method of low carbon alloy steel, characterized in that. The method of claim 6, wherein, during the austenizing of the plasma carburizing process conditions, a gas pressure range is 760 torr, and a temperature is treated in an inert gas atmosphere within a range of 750 to 850 ° C. 7. The surface treatment of low-carbon alloy steel according to claim 7, wherein during the plasma carburizing process, the sputter cleaning is performed in a furnace pressure of 1 torr, plasma formation voltage of 300 V, temperature of 500 ° C., and time of 15 minutes. Way. The method of claim 8, wherein in the carburizing process of the plasma carburizing process conditions, the gas pressure range is 4 to 10 torr, the plasma forming voltage is 300 to 600V, the temperature is 900 to 950 ℃, the time is performed within the range of 30 to 60 minutes Surface treatment method of low carbon alloy steel, characterized in that. The method of claim 9, wherein in the diffusion process of the plasma carburizing process conditions, the gas pressure range is 1 to 5 torr, the plasma forming voltage is 300 to 400V, the temperature is 900 to 950 ℃, the time is performed within the range of 15 to 30 minutes. Surface treatment method of low carbon alloy steel, characterized in that. The method of claim 10, wherein the austenizing of the plasma carburizing process conditions, the gas pressure range of 760 torr, the temperature is treated in an inert gas atmosphere within the range of 800 to 850 ℃.