Analysis and measures of 7 kinds of cracks in die steel quenching

6 cold treatment cracks

Die steels are mostly medium and high-carbon alloy steels. After quenching, some of the subcooled austenite has not been transformed into martensite, and it remains in use as retained austenite, which affects the performance. If it is placed below zero to continue cooling, it can promote the martensite transformation of retained austenite. Therefore, the essence of cold treatment is to continue quenching. The quenching stress at room temperature and the quenching stress at zero are superimposed, and cold treatment cracks are formed when the stack response force exceeds the strength limit of the material.

Precaution:

(1) After quenching, place the mold in boiling water for 30-60 minutes before cold treatment, which can eliminate 15%-25% of the internal quenching stress and stabilize the retained austenite, and then perform conventional cold treatment at -60℃, or -120℃ Cryogenic treatment, the lower the temperature, the greater the amount of retained austenite transformed into martensite, but it is impossible to complete the transformation. Experiments show that about 2%-5% of retained austenite remains, and a small amount of retained austenite is retained as needed. The tensite can relax the stress and act as a buffer. Because the retained austenite is soft and tough, it can partially absorb the energy of the rapid expansion of martensitization and ease the transformation stress;

(2) After the cold treatment is completed, take out the mold and put it in hot water to heat up, which can eliminate 40%-60% of the cold treatment stress. After heating to room temperature, it should be tempered in time, and the cold treatment stress will be further eliminated to avoid the formation of cold treatment cracks, obtain stable tissue performance, and ensure the mold No distortion occurs during product storage and use.

7 Grinding cracks

It often occurs in the grinding and cold working process of the finished mold product after quenching and tempering. Most of the micro cracks formed are perpendicular to the grinding direction, and the depth is about 0.05-1.0mm.

(1) Improper pretreatment of raw materials, failure to fully eliminate lumps, nets, and bands of raw materials, and serious decarburization;

(2) The final quenching heating temperature is too high, overheating occurs, the grains are coarse, and more retained austenite is generated;

(3) Stress-induced phase transformation occurs during grinding, which transforms the retained austenite into martensite, and the structure stress is large. In addition, due to insufficient tempering, there is more residual tensile stress, which is superimposed with the grinding structure stress , Or due to the grinding speed, large feed rate and improper cooling, the metal surface grinding heat will rise sharply to the quenching heating temperature, and then the grinding fluid will be cooled, resulting in secondary quenching of the grinding surface. Various stresses are integrated, exceeding this The material strength limit causes grinding cracks on the surface metal.

Precaution:

(1) The raw material is reformed for forging, and the double cross shape is changed to upsetting for multiple times. After four upsetting and four pulling, the forged fiber structure is symmetrically distributed around the cavity or axis, and the final high temperature residual heat is used for quenching. Subsequent high-temperature tempering can fully eliminate lump, mesh, band and chain carbides, and refine the carbides to 2-3 levels;

(2) Formulate advanced heat treatment technology to control the final quenched retained austenite content not to exceed the standard;

(3) Tempering in time after quenching to eliminate quenching stress;

(4) Appropriately reducing the grinding speed, grinding amount, and grinding cooling rate can effectively prevent and avoid the formation of grinding cracks.

8-wire cutting crack

Precaution:

(1) After quenching, the die steel should be tempered in time, fully tempered, and repeatedly tempered to eliminate the internal stress of quenching;

(2) After quenching, the mold steel is generally not suitable for tempering at 350-400~C, because T structure often appears at this temperature, the mold with T structure should be reprocessed, and the mold should be rust-proofed to improve corrosion resistance;

(3) Low-temperature preheating of hot-worked molds before service, and a low-temperature tempering after a period of service of cold-worked molds to eliminate stress, not only can prevent and avoid stress corrosion cracking, but also greatly increase the service life of molds. Significant technical and economic benefits.