JPH0713294B2 - Carbonitriding method for chromium-containing steel members - Google Patents

Description

TECHNICAL FIELD The present invention relates to surface treatment of a chromium-containing steel member, and in particular,
The present invention relates to a method of hardening a surface by carburizing and nitriding a chromium-containing steel member.

(Prior Art) It has been conventionally known that a steel member is carburized and then hardened to harden the surface.

Further, as a method for hardening the surface of a steel member, a nitriding treatment in which nitrogen is infiltrated into the member is known. This nitrogen has the effect of improving hardenability and forming a stable retained austenite structure in the steel member.The retained austenite structure gives compressive stress to the steel surface by shot peening, etc. Therefore, the surface of the member can be hardened by this, and as a result, the fatigue strength of the member can be increased. Combining this nitriding treatment with carburizing treatment,
It is also known to reheat and carburize after carburizing.

Once the temperature is lowered to less than A ₁ point of the steel after the carburizing, again
A method of heating and quenching at ₁ point or more is known as grain refining treatment, and this can improve the toughness of steel parts.

(Problems to be solved) However, when the conventional method of reheating and nitriding after carburizing is applied to a chromium-containing steel member, an abnormal structure with poor hardenability occurs on the surface, and as a result, chromium The surface of the contained steel parts cannot be hardened sufficiently, therefore
There is a problem that the fatigue strength cannot be improved.

(Means for Solving the Problems) According to the findings of the present inventors, in the carbonitriding method by reheating, the occurrence of an abnormal structure is caused by chromium in the steel member during reheating after carburizing. This is due to the formation of chromium carbide by combining with carbon. This chromium carbide has the property of decomposing and re-dissolving when the heating temperature is raised. Further, the present inventors, in the nitriding treatment, in the chromium-containing steel member, similarly an abnormal structure of poor hardenability occurs, this structure is an obstacle to the improvement of the fatigue strength of the member,
It was found that the abnormal structure in this nitriding treatment increases according to the amount of nitride generated in the member. The amount of this nitride increases as the concentration of nitrogen in the nitriding treatment, the nitriding treatment temperature, and the treatment time increase, and tends to occur at a deeper position inside the member. Therefore, if the nitriding treatment temperature is increased in order to reduce chromium carbide, the amount of chromium nitride increases, and as a result, the fatigue strength cannot be improved.

The method of carbonitriding a chromium-containing steel member of the present invention is a method of carburizing a chromium-containing steel member, then performing nitriding treatment and quenching treatment. After cooling below the A ₁ transformation point of 85
Reheated to a temperature of 0 ℃ -900 ℃ to refine the crystal grains and decompose chromium carbide, then 800 ℃
It is characterized in that nitriding treatment is performed at a temperature of up to 850 ° C, and then quenching treatment is performed.

According to the present invention, the chromium-containing steel member is preferably about 900
Carburizing is performed at a temperature of ℃ to about 930 ℃ for about 2 hours to about 4 hours. In the carburizing treatment of the present invention, it is not always necessary to keep the temperature constant. Further, as the chromium-containing steel member to which the present invention can be applied, for example, SCM420, SCr420
Etc. The nitriding treatment of the present invention is performed after the above carburizing treatment. When performing the nitriding treatment, the chromium-containing steel member is reheated to 850 ° C to 900 ° C and held for a predetermined time. As a result, the crystal grains are refined and the chromium nitride generated during the temperature rising process during reheating is decomposed and re-dissolved. This holding time is preferably about 5 minutes to 20 minutes. Next, the temperature is lowered and the chromium-containing member is subjected to a nitriding treatment. The nitriding temperature is preferably 800 ° C. to 850 ° C., the nitriding time is preferably about 5 minutes to about 20 minutes, and the higher the processing temperature, the shorter the processing time.

Nitrogen for nitriding can be added from, for example, NH ₃ gas, and RX gas can be used as a carrier gas. RX gas is, for example, CO 24%, H ₂ 3.0%, CO
₂ 0.2%, CH ₄ 0.04%, H ₂ O 0.4%, balance N ₂ . The proportion of nitrogen in the nitrogen-containing atmosphere gas, that is, the nitrogen potential is preferably about 0.2% to about 0.4%. The higher the nitrogen potential and the longer the treatment time, the deeper the nitrogen penetration depth, that is, the effective nitriding depth. Therefore, the higher the nitrogen potential, the shorter the nitriding treatment time.

(Effect of the present invention) According to the present invention, the chromium-containing steel member is reheated after the carburizing treatment, and by this treatment, it is possible to promote the refinement of the crystal grains of the steel, and as a result, The toughness of the steel member can be improved. The chromium carbide generated through the reheating temperature rising process has a reheating temperature of 850 ° C.
By maintaining it at a relatively high temperature of up to 900 ° C., it is possible to re-dissolve it, and thus it is possible to suppress the generation of abnormal structure due to chromium nitride as much as possible. Further, since the nitriding treatment of the present invention is performed at a relatively low temperature such as 800 ° C. to 850 ° C. after the reheating treatment, the effective nitriding depth can be reduced. By concentrating the nitriding in the chromium oxide generation region inevitably generated through the carburizing process, the generation of chromium nitride can be suppressed as much as possible. As a result, it is possible to suppress the generation of abnormal structure due to chromium nitride. Therefore, according to the present invention, it is possible to suppress the generation of abnormal structures caused by chromium carbide and chromium nitride, respectively.
As a result, it is possible to effectively improve the toughness and fatigue strength of the chromium-containing steel member through carburization, reheating and nitriding.

(Description of Examples) Hereinafter, an example in which the present invention is applied to the case of manufacturing a secondary shaft gear made of a material SCM420 will be described. This SCM4
The composition of 20 was C 0.21% by weight, Si 0.27% by weight, Mn 0.81% by weight, P 0.016% by weight, S 0.014% by weight, Cr 1.02% by weight, Mo 0.15% by weight, and the balance Fe. The secondary shaft gear component was first carburized.

Carburizing condition 1st step Carburizing time 910 ℃ Carburizing time 3.5 hours 2nd step After the 1st step, the temperature of the parts was lowered and further carburizing was performed.

Carburizing temperature 840 ° C Carburizing time 30 minutes Next, the carburized parts were carbonitrided.

In this case, the nitriding treatment step was composed of a first step of heating again the component whose temperature has decreased after the carburizing treatment and maintaining it at a predetermined temperature, and a second step of actually performing nitriding. In addition, NH was used as the nitrogen addition gas in the second step. RX gas is used as the carrier gas and the nitrogen potential is set to 0.
It was set to 25%.

Example 1 Conditions for nitriding treatment First step Reheating temperature 860 ° C Time 20 minutes Second step After the first step, the temperature of the parts was lowered to perform nitriding.

Nitriding temperature 820 ° C Nitriding time 20 minutes Comparative example 1 Immersion nitriding condition 1st step Reheating temperature 860 ° C time 20 minutes 2nd step Nitriding temperature 860 ° C Nitriding time 20 minutes Comparative example 2 Immersion nitriding condition 1st Process Reheating temperature 820 ° C Time 20 minutes 2nd process Nitriding temperature 820 ° C Nitriding time 20 minutes The parts obtained by the above treatment are weakly etched with 5% nitric acid alcohol, and the micrograph of the cross section is first.
Shown in the figure. According to this, in Comparative Examples 1 and 2, it is observed that innumerable black lines extend inward from the surface of the component. This indicates that abnormal tissue is generated in this part. On the other hand, in the example,
The above-mentioned black streaks are slightly seen near the surface, indicating that almost no abnormal tissue is generated. The maximum depth of abnormal tissue in Example 1 was 5 μm or less, while the maximum depth in Comparative Examples 1 and 2 reached about 20 μm and about 35 μm, respectively.

Next, in the nitriding process, the process conditions in the second step were the same as the nitriding temperature: 820 ° C, the nitriding time: 20 minutes, and the nitrogen potential: 0.25%. The situation of occurrence was tested. The results are shown in FIG. According to this, when the treatment of the first step was carried out under the conditions of the shaded area in FIG. 2, there was almost no occurrence of abnormal tissue.

Next, after the carburized parts of Example 1 and Comparative Examples 1 and 2 were quenched, a fatigue strength test was conducted. In this test, the gear specifications are 2.0 modules, 21 teeth, and 26mm width.
The above parts manufactured as secondary shaft gears for
It was rotated under a torque load of kg-m and its failure cycle was measured. The results are as follows.

The material of Example 1 which has been subjected to the reheating treatment of the present invention exhibits excellent fatigue strength as compared with the materials of Comparative Examples 1 and 2.

Table 1 shows the results of fatigue strength tests similar to the above performed on the parts used in the experiment shown in FIG. As is clear from these results, the results obtained by performing the first step in the nitriding treatment under the conditions in the shaded area in FIG. 2 are all superior in fatigue strength characteristics compared to those performed under conditions other than the shaded area. showed that.

[Brief description of drawings]

FIG. 1 is a photomicrograph showing the metallographic structure of the cross-section of the carbonitrided secondary part, and FIG. 2 is the second of the nitriding treatment.
The relationship between the processing temperature and the processing time in the process is shown.

Claims (1)

[Claims] 1. A method for carburizing a chromium-containing steel member, then nitriding it, and then quenching it, wherein the chromium-containing steel member is carburized and once cooled to below the A ₁ transformation point of the steel. After that, it is heated again to a temperature of 850 ℃ to 900 ℃ to make the grains fine and to decompose the chromium carbide, then nitriding at a temperature of 800 ℃ to 850 ℃, and then quenching. A method for carbonitriding a chromium-containing steel member, which comprises performing a treatment.