Large 5CrMnMo steel hot forging dies use high-frequency induction heating power supply for quenching, the causes of cracks and their preventive measures

5CrMnMo hot work die steel is often used in the manufacture of small and medium-sized forging dies, while large and extra-large forging dies are usually made of 5CrNiMo steel, 5CrNiW steel or 5CrNiTi steel to obtain good hardenability, strength and toughness and long fatigue life. The size of a certain hot forging die is 600mmx600mmcx350mm，It is a large forging die, but due to material limitations, it is produced with 5CrMnMo steel. Then a high-frequency induction heating power supply is used for 850°C heating oil cold quenching treatment. During production, it was found that in large forging dies, arc-shaped cracks are often found at the four corners of the forging die, causing the forging die to fail and be scrapped. Today, we will take a look at the causes of crack defects and their preventive measures.

The inspection found that the quenching cracks of large forging dies mostly appeared in the four corners of the forging die and were in the shape of arc-shaped cracks. Analysis believes that this is due to insufficient hardenability of 5CrMnMo steel, which makes large forging die parts impervious to quenching. During the quenching of the workpiece, the oil cooling time is too long and the tempering process is improper, which causes the quenching stress of the forging die to be too high, resulting in quenching cracks and failure of the workpiece. 

The key points for improving the heat treatment process of 5CrMnMo steel forging die are as follows:

(1) When using high-frequency induction heating power for quenching, we need to increase the quenching temperature. The effects of increasing the quenching temperature of the workpiece are: First, the quenching temperature increases, making the austenite more uniform, reducing the possibility of needle-like martensite forming in local high-carbon areas, and the workpiece obtains more lath-like martensite. Strong toughness, good comprehensive strength, and reduced brittleness of the workpiece. The second is to dissolve more Cr, Mo and other alloying elements in the workpiece into austenite, which enhances the solid solution strengthening effect and improves the thermal strength of the workpiece. The third is to increase the hardenability of the die and improve the fatigue resistance of the forging die. 

(2) During oil cold quenching of 5CrMnMo steel, quenching cracks are likely to occur if the oil outlet time is late. Therefore, the cooling time of oil cold quenching must be controlled.

(3) When the mold is quenched and taken out of the oil, the surface temperature is about 200°C, while the core is about 400°C. If it is tempered immediately at 180°C, the martensite transformation will continue in the core, but the martensite transformation on the surface will have stopped. After completion, the surface is hardened, and the martensite transformation in the core causes the volume to expand and generate tensile stress, causing cracks in the workpiece. At this time, if it is placed in a 490°C tempering furnace for tempering, an upper bainite structure with low strength and toughness will be formed in the center of the workpiece, and the mold performance will deteriorate. Therefore, neither of the above two treatments should be taken. To improve the process, the forging die is cooled to the Ms point (≈220°C), and then isothermally treated (280°C Temperature composite structure, the workpiece has high toughness and thermal fatigue resistance, and the service life of the mold is greatly extended.

After the large-scale 5CrMnMo steel forging die adopts improved technology, quenching crack accidents are eliminated, the die performance is excellent, and the service life is significantly improved. Production applications show that the working life of the forging die has been increased from the previous 2,000 pieces to about 6,000 pieces. The service life of the workpiece is three times that of the original process, and the technical and economic benefits are significant.

Cracks, as one of the common defects in heat treatment, may affect the service life of the workpiece at least, or cause the workpiece to be scrapped at worst. Therefore, it is necessary to understand the causes of cracks and master their preventive measures. Today, this article talks about some measures to prevent cracks. I hope you will like it. However, I hope you can apply it to your work.