JPS6147902B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for gas carburizing and quenching of steel parts containing Mn, Cr, etc.

<Prior Art> Methods for gas carburizing steel materials containing Mn, Cr, etc., such as case hardening steel, are roughly divided into the following two types depending on the carburizing gas used during gas carburizing. That is,
A modified gas method that transforms hydrocarbon gas such as propane or methane and uses this modified gas, or a drop injection method that uses this modified gas by thermally decomposing a liquid organic agent such as lower alcohol or lower fatty acid ester. There are methods that use a carburizing gas such as the carburizing method, and methods that supply a single hydrocarbon gas such as propane or methane or a mixture of _N2 gas and hydrocarbon gas into the furnace, and then thermally decompose the hydrocarbon gas. This is how to use it.

<Problems to be solved by the invention> However, gas carburizing in the method using carburizing gas mainly occurs due to a thermal decomposition reaction on the steel surface consisting of 2CO→(C)+CO ₂ ...(1) but,
At this time, since alloying elements [M] such as Cr, Mn, and Si contained in steel parts have a strong affinity for oxygen, CO + [M] → [M] O + C ... (2) CO ₂ + [M] → [M]O+CO is selectively oxidized by one of the following reactions (3). This selective oxidation occurs along the steel surface and grain boundaries, and as a result, Cr and
As alloying elements such as Mn become concentrated, their concentration in the matrix decreases. Therefore, if quenching is performed after carburizing, an abnormal carburized layer with an incompletely quenched structure (troostite) will occur.
There is a problem of decreasing surface hardness, fatigue strength, and impact value.

On the other hand, in methods that utilize thermal decomposition of hydrocarbon gases such as vacuum carburizing, the problem of abnormal carburized layers or selective oxidation (grain boundary oxides) is almost eliminated because the carburizing gas does not contain CO, _CO2, etc. However, the amount of surface carbon can be adjusted by performing excessive carburization and then diffusion.

Therefore, the diffusion situation of the carburized layer differs depending on the surface shape of the treated material, and the surface carbon content of convex parts tends to be high and that of concave parts tends to be low. There are problems with low performance and high equipment costs.

<Means for Solving the Problems> The present invention provides that the selective oxide (grain boundary oxidation layer) in the gas carburizing method using a carburizing gas such as an endothermic metamorphic gas satisfies the above formula (2) and (3). ), and decomposing the above formulas (2) and (3), CO=(C)+1/2O ₂ ...(4) CO ₂ =CO+1/2O ₂ ...(5) [ M] + 1/2O ₂ = [M]O ... (6) By focusing on the fact that the selective oxide is caused by a trace amount of oxygen in the carburizing atmosphere, Mn, Cr
After gas carburizing treatment of steel parts containing the like in an atmosphere, the grain boundary oxides generated during the treatment are held in a high temperature atmosphere in a reducing region for a predetermined time,
The grain boundary oxide was subjected to a reduction treatment, and then a quenching treatment was performed.

<Example> Next, the present invention will be explained with reference to the drawings.

The present invention eliminates grain boundary oxides that are inevitably formed when steel parts are carburized in a carburizing gas by controlling the oxides by controlling O ₂ partial pressure, etc., before quenching. By holding the steel parts in a high-temperature atmosphere in a reducing region for a predetermined period of time, it is possible to obtain carburized and hardened steel parts with less grain boundary oxides, despite the carburizing treatment in a carburizing gas.

First, the basic concept of the present invention will be explained with reference to FIG. Figure 1 shows the calculated equilibrium O ₂ partial pressure of the main alloy components of steel as a pure metal. For example, when the temperature is 850°C and the O ₂ partial pressure is
At 10 ^-25 atm, Cr, Mn, and Si are in the oxidation region, while Mo and Fe are in the reduction region. Note that the solid line A in the figure indicates the O ₂ partial pressure in the furnace when the carbon potential value during carburizing is set to 0.9.

As shown in Figure 1, the carbon potential value is
When carburized as 0.9, the alloying element Cr,
Mn, Si will be exposed to the oxidized region,
They are formed as oxides of Cr ₂ O ₃ , MnO, SiO ₂ and the like on the steel surface and grain boundaries.

Therefore, in order to reduce the grain boundary oxidation layer caused by the above-mentioned oxides, since the alloying elements that affect the hardenability after carburizing are Cr and Mn, the above-mentioned grain boundary oxides are removed before the quenching process. All that is required is reduction treatment, and Mn
Since the reduction region of is the reduction region of Cr, Mn
It can be seen that if the oxide is reduced, the Cr oxide is also reduced.

Therefore, considering Mn in steel, Mn
is oxidized by a reaction consisting of oxygen in the furnace and 2Mn+O ₂ = 2MnO (7). In other words, the Mn oxide will be reduced by the amount of O ₂ as shown in FIG. The amount of O ₂ exhibiting this reduction region can be theoretically determined from the following equation (8), which shows the relationship between the equilibrium O ₂ partial pressure of equation (7).

Po ₂ = e〓 ^FT/1 . ^987T ……(8) However, ΔFT: free energy (c
al/mol) T: Absolute temperature (°K) In other words, grain boundary oxides formed in steel parts treated in a carburizing gas are lower than the grain boundary oxides in the atmosphere of the reducing region after carburizing treatment. If maintained for a predetermined period of time, the grain boundary oxide is reduced, and as a result, the grain boundary oxidation layer is reduced.

Also, based on the value obtained by the above formula (8),
The O ₂ partial pressure value, which is in the reduction range for Mn oxide, can be determined by evacuating the inside of the furnace using a vacuum exhaust device, or by continuously supplying high-purity N ₂ gas or inert gas such as Ar gas. It can be easily obtained by feeding it into a furnace.

Figure 2 is a typical heat treatment cycle diagram of the gas carburizing and quenching method of the present invention, in which steel parts are heated to the carburizing temperature and the carbon potential value is controlled to correspond to the desired carburized layer. It consists of gas carburizing treatment, subsequent holding in an atmosphere maintained in a reducing region for grain boundary oxides, and then quenching treatment.

Next, a specific embodiment of the present invention in a method of evacuating the inside of the furnace as a means for forming an atmosphere in the reduction region will be described.

The broken line in Figure 1 shows the relationship between the degree of vacuum in the furnace and the O ₂ partial pressure in the furnace calculated using equations (1) and (3). 5×10 ^-2 Torr
If the temperature inside the furnace is 860°C, the atmosphere inside the furnace becomes a reduction region for the alloying elements Cr and Mn.

The reason for using equation (3) when calculating the required O ₂ partial pressure value in the furnace is that the oxidizing power due to the reaction in equation (3) is stronger than the oxidizing power due to the reaction in equation (2). .

Example 1 Treated material: SCr415 round bar, SCr420 gear Carburizing treatment: Carburizing temperature: 930°C Carburizing period: cp value = 1.2, time = 105 minutes Diffusion period: cp value = 0.9, time = 45 minutes Reduction treatment: After diffusion, Holding under vacuum at 10 ^-2 Torr for 30 minutes Quenching treatment: Direct oil quenching from carburizing temperature (930°C) (Results) Both the SCr415 round bar and the SCr420 gear had a grain boundary oxidation layer of 5 to 10μ.

Comparative Example 1 When the reduction treatment is not performed under the same conditions as Example 1,
The grain boundary oxidation layer was 15 to 20μ in the SCr415 round bar and 15 to 20μ in the SCr420 gear.

Example 2 The treated materials and carburizing treatment conditions were the same as in Example 1,
Reduction treatment was performed under the same conditions (10 ^-2 Torr x 30 minutes) during the cooling period up to the quenching temperature (850°C), and then oil quenching was performed. As a result, SCr715 round bar, SCr420
The grain boundary oxidation layer in both gears was 7 to 12μ.

Comparative Example 2 When the reduction treatment is not performed under the same conditions as Example 2,
The grain boundary oxidation layer was 15 to 20μ in the SCr415 round bar and 15 to 25μ in the SCr420 gear.

Example 3 Treated material: SCM420H round bar Carburizing treatment: Carburizing temperature: 930°C Carburizing period: cp value = 1.1, time = 120 minutes Reduction treatment: After the carburizing period, under a vacuum of 5 × 10 ^-2 Torr
Hold for 30 minutes (diffusion phase), then increase the quenching temperature (850
℃) for 20 minutes. After cooling, the quenching temperature was oil-quenched. (Result) The grain boundary oxidation layer was 5μ or less.

Comparative example 3 Treated material: SCM420 round bar carburizing treatment: Carburizing temperature: 930℃ Carburizing period: cp value = 1.1, time = 105 minutes Diffusion period: cp value = 0.9, time = 45 minutes Quenching treatment: After diffusion, quenching The temperature was lowered to (850°C), and the grain boundary oxidation layer was 15 to 20 μm in oil quenching (result).

As shown in the above examples, if the carburized material is held in an atmosphere that is a reducing region for the grain boundary oxides of the carburized material for a predetermined period of time before the quenching treatment, the grain boundary oxidation layer is reduced. This was confirmed.

In addition, the degree of vacuum inside the furnace was determined from the theoretical value.
Similar effects were obtained in tests conducted at slightly lower O ₂ partial pressures. This is considered to be due to the fact that Mn, which is contained in steel at about 1%, is balanced at an oxygen partial pressure higher than the theoretical value.

Then, the reduction treatment conditions were maintained at 850°C x 1 hour, 850°C x 2 hours, 930°C x 1 hour, and 930°C x 2 hours.
When the experiment was carried out by changing the time, almost the same tendency as in the above example was observed, but at the same degree of vacuum, the grain boundary oxidation layer was more reduced at higher temperatures.

Furthermore, it has been found that the same effect can be obtained even if the grain boundary oxide reduction treatment is performed in the latter half of the diffusion step or after the temperature is lowered and maintained.

That is, when the temperature is lowered to the quenching temperature after the carburizing process, the reduction process is performed during the temperature lowering and holding step, and when the quenching process is performed immediately after the carburizing process, the reduction process is performed in the latter half of the diffusion period. Furthermore, the quenching treatment is not limited to oil quenching treatment, but may also be gas quenching treatment. Furthermore, as a means of forming an atmosphere in the reduction region, it is possible to supply an atmosphere gas that does not contain O ₂ such as high-purity N ₂ gas or Ar gas, but if a vacuum exhaust method is adopted, it is possible to The atmosphere inside the furnace can be easily controlled based on the relationship between O ₂ partial pressure and degree of vacuum.

<Effects of the Invention> As is clear from the above explanation, according to the present invention,
After the carburizing process using a carburizing gas, which is an endothermic gas, the carburizing process is held for a predetermined period of time in a high-temperature atmosphere that serves as a reducing region for grain boundary oxides, and then the quenching process is performed. The grain boundary oxidation layer can be reduced, and the fatigue strength can be improved accordingly, and the reduction of the grain boundary oxidation layer also makes it possible to omit the shot treatment as a post-process.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the oxidation and reduction characteristics of alloying elements during carburizing, and FIG. 2 is a heat treatment cycle diagram of the gas carburizing and quenching method of the present invention.

Claims (1)

[Claims] 1. After carburizing steel parts containing Mn, Cr, etc. in a carburizing gas atmosphere, the grain boundary oxides formed during carburization are kept in a high temperature atmosphere in a reducing region for a predetermined period of time to remove the grain boundary oxides. 1. A method for gas carburizing and quenching of steel parts containing Mn, Cr, etc., which comprises reducing and then quenching.