Induction heating is a heated steel bar for transmissive quenched fire , induction heating and subsequently heating the high-temperature tempering, the job can be done in a continuous production line quenching process, it has been increasingly applied to all walks of life. This method is applicable to steel rods , pipes and shaft parts of medium and small section sizes of various medium carbon steels (including low alloy steels) . It has high production efficiency, low energy consumption, no decarburization and oxidation, no pollution, and production. The process is extremely easy to automate and is especially suitable for mass production. To this end, our company introduced a new continuous induction heating and quenching and tempering production line to meet the product structure adjustment needs and expand product coverage. Since there is no relevant induction heating and quenching process data in the early stage, the induction heating and quenching process of hot rolled steel bars is determined by the quenching process test and microstructure performance analysis of 42CrMoS4 steel bar .

1. Induction heating and tempering equipment

Steel bar induction heating and quenching production line , can handle specifications Φ20 ~ Φ160mm . There are five sets of coils in the whole line, which are controlled by multiple zones by five independently independent variable frequency modules. The last group in each zone is used as a holding zone for better austenitizing effects. The steel cooling system is water-cooled and consists of five quenching water rings and a linear cutting nozzle. The whole process of quenching and tempering of steel is carried out continuously by continuous bevel roller. The self-rotating advancement of steel can ensure the uniformity of heating and cooling of steel and obtain better flatness.

2. Test process

The 42CrMoS4 hot-rolled steel rods are available in two sizes: Ф32mm and Ф65mm . The production process is: continuous casting billet → heating → rolling → straightening → induction heating and quenching → inspection → flaw detection → finished product. The steel composition is shown in Table 1 .

Table 1 Chemical composition of 42CrMoS4 (mass fraction) ( % )     

State before hot rolled steel is quenched, 42CrMoS4 alkylene eutectoid steel, 42CrMoS4 steel Ac3 temperature of about 780 deg.] C , the conventional heat treatment temperature is generally selected hardening 850 ~ 860 deg.] C . However, in induction heating, the heating rate is very fast, and the critical point Ac3 increases with the increase of the heating rate, and we must consider the adequacy of austenitization, and the heating temperature should be appropriately increased. According to the mechanical properties of the steel standard, it is necessary to have a certain strength and hardness, but also have good plasticity and impact toughness. It is determined that the tempering temperature of the steel should be high temperature tempering, and it is hoped to obtain more tempered sorbite. Therefore, we test the setting of induction heating and quenching and tempering process as shown in Table 2. We sampled and tested the longitudinal mechanical properties on the quenched and tempered steel. The intercepting position is 12.5mm from the surface , and two samples are taken. The test results and standard performance index requirements are shown in the table. 3 .

Table 2 42CrMoS4 induction heating and quenching process 

Table 3 mechanical performance indicators and test results 

3. Organizational performance analysis

From the test results, the mechanical properties meet the standard requirements, Ф35mm due to the small size, the full section can achieve a very fast cooling rate during the quenching process, we take the metallographic structure of the radius 1/2 position after quenching and tempering (such as Figure 1 shows that we can get a relatively uniform tempered sorbite structure, no bainite structure and massive ferrite. For the Ф65mm specification, we sampled the metallographic structure at different positions on the cross section of the steel, and the tempered sorbite near the surface is relatively uniform (within the surface distance ≤15mm ), as shown in Figure 2 .

As the distance from the surface increases, bainite gradually appears. Figure 3 shows the structure at 16 mm from the surface , tempered sorbite + a small amount of bainite structure, and Figure 4 shows the structure at 25 mm from the surface , tempered sorbite + Bainite + a small amount of ferrite. As the steel cross-section depth increases during the quenching and cooling process, the cooling strength gradually decreases, and the slower the cooling rate toward the core, the bainite precipitates when the critical cooling rate is lower than the martensite transformation.

As shown in Fig. 5 , the austenite continuous cooling transition curve of 42CrMo steel shows that the supercooled austenite transformation has both pearlite and bainite regions, but the bainite region leads the pearlite transformation region. Under the existing cooling conditions of the equipment, there will be no pro-eutectoid ferrite precipitation in the central part of the continuous cooling process of the small-diameter steel rod . Therefore , the ferrite phase in Figure 4 is caused by insufficient heating and insulation at the center of the heating process. A clear judgment can also be made from the distribution of ferrite. However, in general, both the mechanical properties and the metallographic structure can meet the performance requirements.

4. Conclusion

( 1 ) For the induction heating quenching + tempering continuous heat treatment line, due to the physical properties of the heating can achieve a very fast heating rate, we need to use a higher quenching and tempering temperature than the traditional heating method, when using different The heating rate should also be adjusted accordingly.

( 2 ) For the hot-rolled Φ65mm hot-rolled material, bainite gradually appears from the surface to the core, and a small amount of massive ferrite is caused by insufficient cooling strength and insufficient heating and insulation, but the steel quenching and tempering performance fully meets the standard. It is required that when the material proposes higher mechanical properties, we can extend the steel holding time to obtain better metallographic structure and comprehensive mechanical properties by appropriately increasing the cooling strength or appropriately reducing the steel passing speed.

( 3 ) Our current induction heating and quenching process can fully meet the mechanical properties of the product.