The workpiece induction heat treatment by medium frequency induction heating equipment. What are the factors that affect the non-uniformity of its austenite composition?

When the workpiece is subjected to induction heating heat treatment using medium frequency induction heating equipment, affected by many factors, the austenite composition may be uneven, which will greatly affect the service life of the workpiece. Therefore, it is very important to understand the factors affecting austenite composition heterogeneity.

Under rapid heating conditions, the inhomogeneity of the austenite composition is more prominent than during ordinary heating. The reason is that as the heating rate increases, the austenite formation time is shortened, and the holding time is shortened, so that the carbon and alloy element compounds in the steel are too late to fully dissolve and diffuse uniformly. As a result, carbon and alloying element compounds are unevenly distributed in austenite. This composition inhomogeneity intensifies with the increase of heating rate, weakens with the increase of quenching heating temperature, and alleviates with the extension of austenitization holding time. The inhomogeneity of austenite composition will affect the later heat treatment. Properties of steel. For this reason, corresponding heat treatment process measures must be taken to reduce the inhomogeneity of austenite composition and minimize its impact.

The factors affecting the heterogeneity of austenite composition are as follows.

(1) The effect of induction heating on the uniformity of carbon distribution in austenite. The following focuses on the carbon element in carbon steel, under continuous heating conditions, the influence of heating speed and quenching heating temperature on the uniformity of carbon distribution in austenite.

When the quenching heating temperature is constant, the heterogeneity of carbon element in the austenite increases with the increase of the heating rate. For example, the original annealed structure steel is heated to 1000°C for quenching, and when the heating rate is 150°C.S-1, the maximum carbon content in the austenite is about 0.90%. Under the same conditions, the heating rate is 200°C.S-1 , the highest carbon content in austenite is about 0.60%.

As the heating rate increases, the maximum carbon content in austenite increases and moves to high quenching temperature, thereby expanding the carbon inhomogeneity in austenite.

When the quenching heating temperature is constant, with the increase of the heating rate, the transformation speed is accelerated and the transformation time is shortened. In the area of original pearlite and ferrite in the original structure, the difference in the carbon content of the formed austenite increases, and does not change. The dissolved residual carbides increase, causing the carbon content of the austenite to be lower than the average. When the heating rate is constant, the difference in carbon content in austenite decreases as the quenching heating temperature increases.

In short, the uniformity of carbon distribution in the austenite formed during the quenching heating process mainly depends on the heating rate and the quenching heating temperature (ie austenitizing temperature) under induction heating conditions. The faster the heating rate, the worse the uniformity of carbon in the austenite. The higher the austenitization temperature, the more uniform the distribution of carbon in the austenite. However, the austenitizing temperature should not be too high, as it will be constrained by the growth of austenite grains.

(2) Effect of induction heating on the uniformity of distribution of alloy elements in austenite. Under rapid heating conditions, it is more difficult for alloy elements to dissolve into austenite than carbon elements during the austenitization process. Because some alloying elements such as A1, Ti, V, Nb, Ta, Zr, etc. have a strong affinity for carbon and nitrogen in steel, they can form high melting point carbides or nitrides with carbon and nitrogen. These compounds are difficult to dissolve into austenite during heating, and the diffusion coefficient of these compounds at high temperatures is very small, 104-105 times smaller than the diffusion number of carbon atoms. Therefore, the inhomogeneity of austenite is much greater than that of carbon.

When formulating the induction heating austenitizing process, the heating speed must be slowed down as much as possible, the austenitizing temperature should be increased appropriately, and measures such as increasing the holding time should be taken if necessary, in order to improve the uniformity of the distribution of the above alloy elements in the austenite.

(3) Steel with fine grain structure is conducive to the homogenization of austenite components. Steel pretreated by induction heating has a fine crystal structure, which can promote the homogenization of austenite components. It can be seen from the test that when the steel with an original carbon content of 0.86% is heated at the same heating rate of 150°C.S-1 and the quenching heating temperature is 900°C, the original structure is quenched. It is 0.86%. Under the same conditions, in the original structure of annealed coarse-grained steel, the carbon content of austenite is 0.70%, and the carbon content of the two austenites differs by 0.16%. It can be seen that the finer the steel structure, the more conducive to the uniform distribution of carbon and alloying elements in austenite. Compared with ordinary heating, the structure pretreated by induction heating has a finer structure, so it is more conducive to the uniform distribution of austenite components.

Based on the above factors that influence the uniformity of austenite composition by rapid heating, it can be considered that induction heating has more adverse effects on the uniformity of austenite composition than ordinary heating. However, as long as the heating rate and austenitization temperature are reasonably selected, and the holding time is increased if necessary, good results can be achieved.