JPH03126859A - Gas carburizing method - Google Patents

Abstract

PURPOSE:To inhibit intergranular oxidation and to prevent the occurrence of microscopic unevenness of hardness and the deterioration of fatigue strength, wear and pitting resistances by modifying gaseous hydrocarbon and carrying out carburization with the resulting gas as a carburizing gas under a specified pressure. CONSTITUTION:Gaseous hydrocarbon is modified and carburization is carried out with the resulting gas as a carburizing gas under 20-600Torr pressure. Since the vol. percentage of CO2 in the modified gas can be reduced, the surface oxidation, especially intergranular oxidation of steel can be inhibited and a high-quality carburized product can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a carburizing method for steel, and more particularly to a carburizing method that prevents grain boundary oxidation during carburizing and allows high-quality carburized products to be obtained at low cost.

(Prior art) This type of conventional technology (1) uses a converted gas of propane gas or butane gas (RX gas) or a converted gas of city gas (for example, AC gas) whose main component is methane gas as a carrier gas. A carburizing method in which propane gas or butane gas is added as an enriched gas to a carrier gas and carburizing is carried out while controlling the carbon potential.
There is a vacuum carburizing method in which after carburizing for a predetermined time under a reduced pressure of 500 torr, addition of enrich gas is stopped, heating is continued for a predetermined time, and then carbon is diffused.

(Problem to be solved by the invention!!) However, in the related prior art (1), metamorphic gas (RX gas,
AX gas) contains 0.10 to 0.35% by volume of carbon dioxide gas, so at temperatures near the normal carburizing temperature of 930°C, alloying elements such as manganese and chromium contained in the steel will form at the grain boundaries. Parts are significantly oxidized, reducing the strength of grain boundaries and hardenability, resulting in significant deterioration of fatigue strength and pitting resistance. Additionally, if a large amount of propane gas or butane gas is added to reduce the proportion of carbon dioxide gas contained, sooting tends to occur, making the atmosphere in the furnace unstable and making it difficult to control the carbon potential. There are other problems. Furthermore, although conventional technology (2) is an effective carburizing method that prevents oxidation of grain boundaries, raw propane gas or butane gas is introduced directly into the furnace, which tends to cause sooting and makes it difficult to control the atmosphere inside the furnace. , Also, during carburizing, carburizing is performed under the conditions that the maximum amount of carbon IA is dissolved in the austenitic iron (γ iron), that is, the carbon activity Ac = 1.
In ordinary case-hardened steel such as M material, cementite precipitates on the surface, and in the subsequent diffusion treatment, the cementite is redissolved in solid solution to homogenize the carbon content, but in this process, microscopic segregation of alloying elements occurs. This tends to cause microscopic hardness unevenness, which causes problems in deteriorating fatigue strength, wear resistance, and pitting resistance.

(Means and Effects for Solving the Problems) In order to solve the above problems, claim (1) of the present invention provides a carburizing method in which a hydrocarbon gas is denatured and carburized as a carburizing gas, in which the pressure of the carburizing gas is set to 20 to 20. Claim (2) of the present invention is a gas carburizing method characterized in that the carbon potential n! Methane gas, propane gas, butane gas, or 2% of these gases for 1
A gas carburizing method according to claim (1), characterized in that a mixed or metamorphosed gas containing the above is added.
A claim of the present invention is (3) the gas carburizing method according to claim (1), characterized in that alcohol is added for carbon potential R11.

(Example) The present invention will be explained in detail below.

1st Example 0.0% Co was added at a dew point of ~8°C (water vapor; 0.33% by volume).
RX gas (C
When o; 23% by volume and f (!;: 3o% by volume, Nz + 47% by volume) are introduced into a furnace where carbon is present, CO!, an oxidizing component in the RX gas, is introduced. and R20 cause the reaction shown below, and when the pressure is further reduced to 229 torr, the components become as shown in Table 1, and the oxidizing components Cow and HxO are significantly reduced.

C+CO,→2CO (equilibrium constant) Kl −(PCO)” /PCOs(K,
-32, but at 900℃)C+ HtO
-G O+ H! (Equilibrium constant) Kt = PCO-PH1/PHx (K
g-29, but at 900°C) Table 1 CO in RX gas.

against HgO Effect of decompression Second example CO at point n -8°C (water vapor; 0.33% by volume).

RX gas (Co; 23 volume%, Hz; 30 volume% N!+
47% by volume) was introduced into the furnace where carbon was present, and 228
When the pressure is increased to .1 torr and a small amount (0.5% by volume) of methane gas is added, the following reaction occurs, resulting in the components shown in Table 2, and the oxidizing components Co, H□O are significantly reduced.

Co, +CH4→2 CO+2H.

(Equilibrium constant) Ks = (PCO)” ・ (PH,)
1/pcor ・P CHa (+<5=1500, however at 900℃) 11
Yo 0 + CHa → 2 CO+ 3
1+! (Equilibrium constant) K4= (PC○)!・
(P!+2)'/ P II t OP CH
a (K, = 1380, at 900°C) Table 2 Effect of methane gas addition on Co, I-1,0 in RX gas Third Example Methanol conversion gas (Co; 33 volume, II.

;65 volume%, CH4: 0.7 volume%, +(,0;1
% by volume, C0u0.3% by volume) is introduced into the furnace where carbon is present, and when the pressure is reduced to 228 Lorr, t1 shown in Table 3 is introduced.
The oxidation component is co, t+, as shown in the m1 table.
Although o is not reduced, CO□, which is still an oxidizing component, +1
20 becomes low, and methanol can be effectively used for carbon potential adjustment.

Table 3 Effect of 'Iλ pressure on COt, H, O in metamorphic gas of methanol Fourth example AC gas (Co; 22 bodies g1
%, I-1; 46% by volume, Nz+23% by volume.

When CII a+ 9% by volume) is introduced into a furnace where carbon is present, CO2 and H2O, which are oxidizing components in AC gas, are
The reaction shown below occurs, and the pressure is further reduced to 228 torr.
In the state r, the components shown in Table 4 are G, which is an oxidizing component.
O, H, and O are significantly reduced.Furthermore, it is easier to adjust the carbon potential by using AC gas than by directly adjusting the carbon potential with methane gas as in the second embodiment.

! Table 114 Effect of reduced pressure on CO□ and H2O in AC gas Fifth example RX gas modified from butane gas was carburized at a dew point of -8°C and the pressure in the furnace was varied from 10Lorr to 700Lorr for 93f)tX8 hours. and further 850℃×0
．． After heating for 5 hours, cool with oil at 60℃ and heat to 180℃
The graph in FIG. 1 shows the relationship between the variation in carburization depth and the depth of grain boundary oxidation for test pieces tempered for 2 hours.

If the pressure in the furnace is less than 20 torr, the carburizing depth will vary too much, and the pressure in the furnace will be poor.

If it is more than rr, the depth of grain boundary oxidation becomes deep and no improvement in grain boundary oxidation can be expected.

Next, the results of rotary bending fatigue testing of test pieces carburized by the carburizing method according to the present invention and the conventional carburizing method will be explained.

The rotary bending fatigue test piece shown in FIG.
Co: 23% by volume, Hz: 30%, 1% N: 47%,
! Carburizing gas (05% by volume of methane gas added to 'i point knee 8℃) was reduced to 228Lorr, carburized at 930℃ for 8 hours, further heated at 850℃ for 0.5 hours, cooled with 60℃ oil, and then heated at 180℃. A specimen tempered for 2 hours, a specimen carburized by the conventional carburizing method using RX gas, a specimen carburized by the vacuum carburizing method, a specimen carburized by the conventional carburizing method using RX gas, and the surface ground to 0825. Table 5 shows the test pieces in which grain boundary oxidation was completely removed, and the rotating bending fatigue strength of these test pieces was measured to determine the fatigue limit.

As shown in this table, the fatigue limit of the test piece carburized by the carburizing method according to the present invention is significantly higher than that of the test piece carburized by the conventional RX gas.

Table 5 Fatigue due to differences in carburizing, grain boundary oxygen depth (effects of the invention) As explained above, the present invention
O! This reduces the volume percent of CO2, which suppresses oxidation on the steel surface and grain boundary oxidation, and promotes decomposition reactions of propane gas, methane gas, butane gas, etc., which are added in small amounts as enrichment gases, reducing CO2 and significantly reducing the volume percent of CO□. suppressed,
Further, since only a small amount of propane gas, methane gas, butane gas, etc. can be added, sooting does not occur, and since it is under reduced pressure, no seasoning trouble occurs.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the variation in carburization depth of test specimens and the depth of grain boundary oxidation, and FIG. 2 is a diagram showing rotating bending fatigue test specimens.

Claims (3)

[Claims] (1). In a carburizing method in which hydrocarbon gas is denatured and carburized as a carburizing gas, the pressure of the carburizing gas is set to 20 to 60
A gas carburizing method characterized by using 0 torr. (2). The gas carburizing method according to claim (1), characterized in that methane gas, propane gas, butane gas, or a mixed or converted gas containing 2% or more of these gases is added for carbon potential adjustment. (3). The gas carburizing method according to claim (1), characterized in that alcohol is added for carbon potential adjustment.