JPS61127812A - Heat treatment of steel - Google Patents

Abstract

PURPOSE:To obtain a sufficient depth of hardening and high fatigue strength by specifying the composition of a steel and applying austempering as pretreatment and high frequency treatment as surface hardening treatment. CONSTITUTION:The composition of a steel is composed of, by weight, 0.45-0.65% C, 0.9-1.2% Mn, 0.7-1.2% Ni, 0.5-1.2% Cr, 0.19-0.12% Mo, Si, P, S and Cu by amounts as ordinary impurities and the balance Fe. The steel is heated and held at a conventional hardening temp. such as 800-850 deg.C, and it is isothermally hardened by quenching from the hardening temp. in an isothermal bath at 200-400 deg.C, holding for a fixed time and air cooling. The steel is then subjected to induction hardening.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a heat treatment method for hardening the surface of steel.

Conventional technology Conventional methods for hardening the surface of steel include (a) carburizing method;
(b) Nitriding method, (c) induction hardening method, etc.

In each of these methods, the hardening depth is approximately 0.5 to 3 strokes in the case of (a) carburizing method, and 0.1 in the case of (b) nitriding method.
In the case of the induction hardening method of ~0.5 tone, H→, it was 1~5-.

Problems to be Solved by the Invention In recent years, there has been a strong demand for machines to be larger and faster, and there is a desire for machines that have a deeper hardening depth and higher fatigue strength than those obtained by conventional methods.

However, if the hardening depth is made deeper,
In the case of the carburizing method (2) and the nitriding method (2), the processing time is longer and a large amount of energy is consumed, resulting in increased costs.

On the other hand, in the case of induction hardening, the hardening depth largely depends on the frequency, and it is technically very difficult to harden the material deeper than the current state.

Accordingly, an object of the present invention is to provide a method for heat treating steel, which improves the drawbacks of the conventional methods as described above and provides a sufficient hardening depth and high fatigue strength in an extremely short treatment time.

Means for Solving the Problems The present invention uses a suitable steel to obtain the required hardness,
The above objective is achieved by combining austempering treatment and induction hardening.

Namely, the method for heat treatment of steel according to the present invention is as follows.

C0.45~0.65%, Mn0.90~1.20
%, Ni0.70~1.2%, (:'rQ, 5Q
~1.2%, MO0. Il ~ 0.211 Si, p, S or Cu in the range mixed as normal impurities, balance F-
1. Constant temperature quenching is carried out from the normal quenching temperature to 250 to 350°C.

It is characterized in that it is air-cooled and then hardened by induction heating on one surface.

Functions and Modes of the Invention As mentioned above, the present invention is characterized by the selection of an appropriate steel to obtain the required hardness, the adoption of austempering treatment as a pretreatment, and the adoption of induction hardening as a surface hardening treatment, With these combinations, the internal hardness is high and the hardening depth is 1. Steel that is surface hardened to a depth of o+o+ or more and has high fatigue strength can be obtained in a short time. A summary of the features of the present invention in comparison with conventional methods is shown in Table 1-IK.

Table-1 Next, the processing process will be explained.

Ordinary steel for surface hardening is subjected to heat treatment or normalizing treatment before surface hardening treatment, and then to the surface hardening treatment. That is, in normal refining or normalizing, is the material heated to the quenching temperature or above the quenching temperature and then quenched in water or oil?

For those that have been air-cooled from that temperature, they are further tempered.

On the other hand, in the method of the present invention, after heating and holding at a normal sintering temperature, for example, 800 to 850°C.

Subsequently, constant temperature quenching is performed. In other words, 2 from the quenching temperature
It is rapidly cooled in a constant temperature bath of 00 to 400°C, for example, in a salt bath, and after being kept for a certain period of time, it is taken out.

It is air cooled. After that, induction hardening is applied. An example of a thermal cycle for pretreatment in the method of the present invention is shown in FIG. Note that the constant temperature is maintained at 250 to 3
Q Examples where the temperature is preferably in the range of 50° C. The effects of the present invention will be specifically explained below with reference to Examples and various test examples.

The chemical composition of the steel used in the test is shown in Table 2.

Figure 2 shows the relationship between the internal hardness of steel treated in the cycle shown in Figure 1 using the samples shown in Table 2 above. In addition,
Although the constant temperature holding time was varied within the range of 30 minutes to 60 minutes, the change in internal hardness depending on the constant temperature K was almost the same as the results shown in FIG. 2.

In the case of carburizing, the surface hardening depth is expressed as a distance from the surface showing a value of HxC50-55. Therefore, from Figure 2,
The conditions for internal hardness to satisfy the definition of surface hardening depth are steel with a carbon content of 0.45% or more and 0.70 t, and a constant temperature of 2.
This is the case in the range of 50°C to 350°C.

However, sample/I67 steel with a carbon content of 0.7 oz is not suitable because it tends to cause quench cracking during induction hardening and generates tensile residual stress, as will be described later. Therefore, the upper limit of the carbon content is set at 0°65%.

Alloying elements other than carbon were determined in consideration of workability in maintaining constant temperature. That is, when rapidly cooling from 850° C. to 250° C., it is necessary to move the S curve to the right in order to prevent the material to be processed from undergoing change (diffusion transformation) during the rapid cooling. When the processing amount is 100 kP per cycle and the weight floatation of - pieces of processed material is 8 Owaa, an Ihf of ~9 inches is required, and a composition that satisfies this value was determined through experiments. As a result, Mn was 0.90 to 1.20%, Ni was 0.70 to 1.2%, Cr was 0.50 to 1.2%,
No. 0.19 to 0.21%, and Si, p, S, and CtL to be mixed as normal impurities.

Following the process shown in FIG. 1, induction hardening was performed.

The samples used were Samples/162-7, which were selected based on the results shown in FIG. The diameter of each sample is 60 m, and the length is 300 m.
It was m.

The frequencies used were l OKHz, 100 KHz (0
In class 2aI.

The quenching heating depth is 1. I aimed for oo+m. The hardness distribution (from the surface to the inside) after quenching was as shown in FIG.

It was found that even if the frequency was changed, the distribution of the hardening depth remained within the shaded range in FIG. 3, and that the influence of one frequency did not pose a practical problem.

In addition, for the steel sample /i67, quench cracking occurred in three of the ten steels treated.

3-point bending fatigue test: A fatigue test by 3-point bending was conducted using the steel samples/162-7 obtained by processing as described above. The results are shown in Figure 4. Note that the results for ordinary carburized and quenched materials and ordinary induction quenched materials are also shown.

As is clear from Fig. 4, the treated material of the present invention (sample/1
62-6) showed high fatigue strength.

On the other hand, steel sample /167 showed an abnormally low value. When we measured the residual stress that contributes to fatigue strength, we found that for samples /162-6, the compressive residual stress was 70-1o.
ok)/-There was. However, sample /167 exhibited a tensile residual stress of 10 kp/-. That is, sample /167 had a high carbon content and high internal hardness, so it generated tensile residual stress. Therefore, it was found that steel with a high carbon content, such as sample 167, cannot be used in the method of the present invention.

0-Rabinte/G Test: The surface fatigue strength (pitting resistance) of the steels of sample layers 2 to 6 treated according to the method of the present invention was investigated. The results are shown in Figure 5. In addition, the test surface pressure is 200kIP
It was /-.

All of them were found to be superior to the conventional carburized version.

Effects of the Invention As described above, 1. The method for heat treatment of steel according to the present invention involves selecting a specific steel suitable for obtaining the required hardness, and employing austempering treatment as a pretreatment and induction hardening as a surface hardening treatment. Therefore, there is no need for thermal refining or normalizing as a pre-treatment as in conventional methods, and there is no need for tempering treatment.The combination of the above treatments results in high internal hardness and hardening. This steel can be surface hardened to a considerable depth of 1.0 m or more, and has higher fatigue strength than conventional carburized materials or induction hardened materials.

It has the unique effect of being obtained through extremely short processing time.

[Brief explanation of the drawing]

Figure 1 is a thermal cycle diagram of the constant temperature quenching treatment of the method of the present invention, Figure 2 is a graph showing the relationship between the internal hardness (ronfuranil hardness) and constant temperature holding temperature of the steel treated with the cycle of Figure 1; Figure 3 shows sample 42~ which was induction hardened after constant temperature hardening.
FIG. 4 is a graph showing the results of a three-point bending fatigue test, and FIG. 5 is a graph showing the results of a roller pitting test.

Claims (1)

[Claims] C0.45-0.65%, Mn0.90-1.20%,
Ni0.70-1.2%, Cr0.50-1.2%, M
o 0.19 to 0.21%, Si, P, S or Cu within the range of normal impurities, and the balance Fe.
Isothermal quenching from normal quenching temperature to 250-350℃,
A method for heat treating steel, which comprises air cooling and then hardening the surface by induction heating.