JPH086169B2 - Gas carburizing method for steel parts - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to a gas carburizing method for steel parts,
Especially, the carbon potential in the atmosphere is Acm, that is,
The present invention relates to a high-concentration gas carburizing method for gas carburizing an object to be treated while maintaining the equilibrium value with the saturation concentration of carbon in austenite at the carburizing temperature.

[0002]

2. Description of the Related Art Conventionally, a gas carburizing method, particularly a high-concentration gas carburizing method, has been adopted as a means for improving the wear resistance and pitting resistance of automobile parts, machine parts and other steel parts. In this high-concentration gas carburizing method, basically, a carburizing gas is supplied into the furnace, and the object to be processed is heated at a constant temperature within the austenite temperature range while maintaining the carbon potential in the atmosphere at Acm or higher. In this way, carbon is absorbed on the surface and the second phase carbide is dispersed and precipitated in the matrix.

As this high-concentration gas carburizing method, for example,
(A) After performing a high-concentration gas carburizing process at a constant temperature, air-cooling to room temperature, and then rapidly heating it to a predetermined temperature again,
Immediately quenching and quenching (Japanese Patent Publication No. Sho 47-32182), (b) Pre-carburizing with a carbon potential of Acm or less, reheating and carburizing with a carbon potential of more than Acm, and then quenching (special Kosho 62
No. 24499 gazette) and (c) After the primary carburizing treatment under the condition that the carbon potential in the atmosphere is A cm or less, the grain boundary oxide layer on the surface is oxidized by cooling in air and the grain boundary oxide layer is shot. Various methods have been proposed, such as a method of secondary carburization under conditions where the carbon potential in the atmosphere exceeds Acm after removal by blasting (Japanese Patent Publication No. 63-60113).

[0004]

However, in all of the conventional high-concentration gas carburizing methods, carburizing is carried out at a constant carburizing temperature. For example, Cr, Si, Mn, etc. are selectively oxidized and concentrated on the surface layer and the grain boundaries in the vicinity of the surface layer, resulting in grain boundary oxidation of several tens of μ and an abnormal carburization layer, which makes it impossible to obtain sufficient quenching hardness. There's a problem. Moreover, when grain boundary oxidation occurs, Cr and Mn in the vicinity are deficient and it becomes difficult to form cementite. Further, the carbon concentration does not increase and carburizing unevenness occurs, which results in wear resistance, pitting resistance, etc. However, there is a problem in that the mechanical properties of the material deteriorate and the original characteristics of the material are impaired. Further, there is a problem that the granulation of cementite, particularly the granulation at a depth of about 0.1 mm or more from the surface layer, tends to be insufficient.

Therefore, the present invention suppresses the grain boundary oxidation and the formation of the abnormal carburization layer at the time of high-concentration gas carburizing treatment, and produces fine granular cementite at a high density in the surface layer, resulting in wear resistance, pitting resistance, etc. Its main purpose is to improve the mechanical properties of.

[0006]

[Means for Solving the Problems] As a means for solving the above problems, the present invention provides a gas carburizing process for steel parts in an atmosphere having a carbon potential of A cm or more,
The carburizing temperature is cycled in the austenite temperature range of 10 to 50 ° C., preferably 20 to 30 ° C. If necessary, the carburizing temperature is cycled in the austenite temperature range within the temperature range. In the middle of carburizing, the material is cooled to a predetermined temperature of Ar ₁ point or less.

In the high-concentration gas carburizing method according to the present invention, the carburizing temperature is cyclically changed to a predetermined temperature within the austenite temperature range and a temperature which is 10 to 50 ° C. higher or lower than the temperature, and the object to be treated is subjected to each of these. The carburizing temperature is cycled by alternately treating at a temperature for a predetermined time, and the cycle of cycling the carburizing temperature is usually set in the range of 10 minutes to 180 minutes / cycle. When the furnace is an ordinary batch furnace, it is preferable to set it to about 60 minutes / cycle in consideration of operability. The material of the steel part that is the object to be treated does not necessarily have to be carburized steel, but may be general purpose steel.

[0008] In addition, although the rapid cooling is theoretically preferable for cooling during cycling, a general furnace cooling rate in a practical furnace is 0.5-0.5.
A sufficient effect can be obtained at about 2 ° C / min. Further, theoretically, it is better that the temperature rise during cycling is slower than that during cooling, but in reality, it may be 1 ° C./min or less. This is because the carbon in the matrix of the material surface layer becomes unsaturated when it is heated too quickly.

In the embodiment of the present invention, the carburizing process is cooled at least once to a predetermined temperature of Ar ₁ point or lower. In this case, the temperature is within a temperature range of Ar ₁ point or lower and Ms point or higher. The temperature is usually 300 to 730 ° C., preferably 400 to 500 ° C. Further, the cooling rate at that time is good enough to prevent excessive ferrite from being generated or martensite to be generated, and is usually set to 3 to 60 ° C./min. The rate of temperature increase after cooling is slower than the cooling rate, usually 1 to
It is preferably set to 15 ° C / min.

[0010]

[Operation] Carburizing in the endothermic atmosphere with a carbon potential of Acm or more, that is, when performing so-called saturation value carburizing,
Carbon is absorbed and diffused in the surface layer of the object to be treated, and the surface carbon concentration is increased, but the carburizing temperature is 10 to 50 ° C during carburizing.
When the carbonization temperature is lowered, carbon in the matrix becomes supersaturated in the surface layer portion of the object to be treated, cementite is dispersed and precipitated, and carbon is released into the atmosphere when the temperature is lowered. In this way, the activated carbon generated when the solid solution carbon in austenite precipitates as cementite reacts with the oxide of the surface layer of the object to be processed, reduces the oxide and removes the oxygen content from the material. As the alloy elements such as Cr and Mn are expelled, they are diffused into the matrix again. Among alloy elements that are easily oxidized selectively, Si is not reduced once oxidized, but Cr, Mn
Etc. are reduced, the grain boundary oxidation is reduced and the generation of the abnormal carburized layer is suppressed.

The temperature range of carburizing temperature cycling is 10
The reason why the temperature is 50 ° C., preferably 20 to 30 ° C. is as follows. That is, the carburizing time (t) and carburizing depth (d) in the gas carburizing process are given by the equation: d = k√t (mm),
As shown in Table 1, the saturated carbon amount and the carbon diffusion coefficient in γFe at a certain temperature increase as the temperature increases. Therefore, the larger the temperature range of cycling is, the larger the difference in the saturated carbon amount is, which can reduce the grain boundary oxidation, but the difference in the diffusion rate is large, which is not effective in shortening the carburizing time. If the temperature of the low temperature portion is too low compared to the high temperature portion, the oxidation rate becomes higher than the reduction rate and the grain boundary oxidation cannot be reduced, so the above range was set.

[0012]

[Table 1] Saturated C amount in γFe, diffusion coefficient of C Saturated C amount Diffusion coefficient of C (k) 930 ° C 1.3% 0.63 900 ° C 1.2% 0.54 870 ° C 1.1% 0 .44

During the carburizing process, Ar ₁
When cooled to a temperature equal to or lower than the Ms point and below, a large number of fine cementite nuclei are precipitated in the crystal grains, and when the temperature is slowly raised to the Ac ₁ point or higher, stabilization and granulation of the cementite are promoted. . This intermediate cooling contributes to the granulation of cementite, especially to the granulation of the inside.

Further, the cycling of the carburizing temperature in the carburizing treatment after the intermediate cooling contributes not only to the reduction of grain boundary oxidation but also to the promotion of granulation. Particularly, the grain boundary of a portion slightly inside the surface layer It is effective for fragmentation of cementite, granulation, and prevention of the generation of abnormal structures near grain boundaries. Originally, the purpose of intermediate cooling is to promote granulation, but temperature cycling plays a role of further promoting the effect.

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

[0016]

EXAMPLES (Example 1) 0.1% C, 0.3% Si, 0.1%
6% Mn, 2.6% Cr, 0.4% Mo, balance substantially F
A carburized steel consisting of e was used as a material to be treated, and a carburizing furnace loaded with the material was put into an endothermic gas (20% CO, 40% H ₂ , 40%
A carburizing gas obtained by adding 0.8 to 1.2% of propane gas to N ₂ ) 1 m ³ / h was supplied, and as shown in FIG. 1, the ambient temperature was initially set to 850 ° C. at a rate of 30 ° C./min. After raising the temperature, 1
The temperature is raised to 880 ° C at a rate of ℃ / min, held at that temperature for 30 minutes, then raised to 900 ° C at a rate of 1 ° C / min and held at that temperature for 10 minutes, and then at 2 ° C / min. mi
The temperature was lowered to 880 ° C. at a rate of n and held for 20 minutes, and thereafter, every 30 minutes, the operation of temperature increase-holding and temperature decrease-holding was alternately repeated to perform carburizing treatment for 25 hours, and then 10 to 12
A sample was obtained by cooling to 400 to 500 ° C. at a temperature lowering rate of ° C./min and then to room temperature. During the carburizing process, the carbon potential in the atmosphere was maintained at 1.4 to 1.5 (calculated value).

(Example 2) A carburizing gas having the same composition as in Example 1 was supplied to a carburizing furnace loaded with SCR-415 as a material to be treated, and as shown in FIG. The temperature was raised to 850 ° C at a rate of 30 ° C / min, then raised to 880 ° C at a rate of 1 ° C / min, held at that temperature for 30 minutes, and then at a rate of 1 ° C / min. 900
Up to ℃ and hold at that temperature for 10 minutes, then 2 ℃
The temperature was lowered to 880 ° C. at a rate of / min and held for 20 minutes. Thereafter, every 30 minutes, a temperature raising-holding operation and a temperature lowering-holding operation were alternately repeated to perform a carburizing treatment for 15 hours.
It is once cooled to 400 to 500 ° C. at a temperature lowering rate of 12 ° C./min, and immediately heated to the original carburizing temperature of 880 ° C. at a temperature raising rate of 20 to 24 ° C./min.
Carrying treatment is repeated for 10 hours by alternately repeating temperature increasing-holding and temperature decreasing-holding every 0 minutes, then cooling to 400 to 500 ° C at a temperature lowering rate of 10 to 12 ° C / min, and then cooling to room temperature to obtain a sample. Got During the carburizing treatment, the carbon potential of the atmosphere was maintained at 1.4 to 1.55 (calculated value) by adjusting the concentration of propane gas in the carburizing gas as in Example 1.

Comparative Example The same carburized steel as the carburized steel used in Example 1 was used as a material to be treated, and a carburizing furnace loaded with the same had endothermic gas (20% CO, 40% H ₂ , 40% N _2). ) 1
A carburizing gas obtained by adding 0.5 to 0.7% of propane gas to m ³ / h was supplied, and as shown in FIG.
1 ℃ / min after heating up to 850 ℃ at a speed of ℃ / min
At a temperature rising rate of 890 ° C., carburizing at that temperature for 25 hours, and then at a temperature lowering rate of 10 to 12 ° C./min for 4 hours.
After cooling to 00 to 500 ° C., it was cooled to room temperature to obtain a sample. During the carburizing process, the carbon potential in the atmosphere was maintained at 1.4 to 1.5 (calculated value).

The samples obtained in Examples 1 and 2 and Comparative Example were reheated to 850 ° C. and then quenched, and micrographs showing macrostructures near their surfaces are shown in FIG. 4 and FIG.
5 and 6 are shown. 4 (a) shows the quenched structure of the high-concentration carburized steel obtained in Example 1, FIG. 4 (b) shows the grain boundary oxidation of the surface layer, and FIG. 5 (a) was obtained in Example 2. Fig. 6 (a) shows the quenched structure of the sample, Fig. 6 (b) shows the grain boundary oxidation of the surface layer,
Shows the quenched structure of the sample obtained in the comparative example, and FIG. 6B shows the grain boundary oxidation of the surface layer.

As is clear from FIG. 6 (a), in the comparative example, the abnormal carburization layer (black portion of the surface layer) is about 50 at maximum.
It occurs up to a depth of μm, segregation is also observed along the grain boundaries inside, and the dispersion state of cementite (white sphere) is also non-uniform. Further, from FIG. 6B, it is understood that in the comparative example, grain boundary oxidation (whisker-like portion of the surface layer) occurs up to a depth of about 50 μm.

On the other hand, in the first embodiment, as shown in FIG.
As can be seen from (a), the abnormal carburized layer is reduced to about 20 μm, and segregation along the grain boundaries is also reduced. Also, as is clear from (b) in the figure, the grain boundary oxidation depth and the occurrence distribution are also shown. Also few. In addition, in Example 2, as is clear from FIG. 5 (a), fine cementite is densely formed in the surface layer, and not only the formation of the abnormal carburized layer is not observed but also the inside is granulated. As a result, segregation along the texture network was not observed, and there was almost no grain boundary oxidation, as is clear from FIG.

[0022]

As is apparent from the above description, the present invention employs a saturation value carburizing method that causes less intergranular oxidation than ordinary carburizing, and the carburizing temperature is cycled within a predetermined range during the carburizing treatment. This makes it possible to prevent the grain boundary oxidation and the formation of the abnormal carburizing layer. By cooling the carburizing temperature to below Ar ₁ along with cycling of the carburizing temperature, it is possible to almost eliminate grain boundary oxidation and formation of an abnormal carburized layer, and further promote the internal granulation to obtain fine cementite. Can be densely formed, and therefore, an excellent effect that mechanical properties such as wear resistance and pitting resistance of the steel material component can be improved is exhibited.

[Brief description of drawings]

FIG. 1 is an explanatory view of a heat cycle showing an embodiment of the method of the present invention.

FIG. 2 is an explanatory view of a heat cycle showing another embodiment of the method of the present invention.

FIG. 3 is an explanatory diagram of a heat cycle of a conventional method.

FIG. 4 is a photograph showing a metallographic structure of a steel material subjected to gas carburizing treatment according to the first embodiment of the method of the present invention.

FIG. 5 is a photograph showing a metallographic structure of a steel material subjected to a gas carburizing treatment according to a second embodiment of the method of the present invention.

FIG. 6 is a photograph showing the metallographic structure of a steel material that has been gas carburized by a conventional method.

Claims (2)

[Claims] 1. A gas carburizing method for steel parts, which comprises cycling the carburizing temperature in the range of 10 to 50 ° C. in the austenite temperature range when performing the gas carburizing treatment in an atmosphere having a carbon potential of A cm or more. 2. When carrying out a gas carburizing treatment in an atmosphere having a carbon potential of A cm or more, the carburizing temperature is cycled within the range of 10 to 50 ° C. in the austenite temperature range, and a predetermined temperature of Ar ₁ point or less is reached during carburizing. A gas carburizing method for steel parts, characterized by cooling.