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Cold Work Die Steel Heat Treatment

There are many factors that cause damage to the cold work mold. The quality of the mold heat treatment process is a key factor to improve the service life of the mold, and sufficient attention must be paid. Inappropriate heat treatment process can easily change the size of the mold; the cold work mold produces deformation, cracks, excessively high or low hardness, uneven hardness, etc. The root cause is improper quenching or tempering process.

The cold-worked convex and concave molds are processed by wire cutting, and deformation or even cracking occurs after processing, especially for carbon tool steel. To solve this problem, in addition to the selection of alloy steel, the correct choice of steel grades and the mastering of the forging process are very important measures to formulate the correct heat treatment process of the die.

Some cold-worked dies for fasteners with complex cavities require not only high hardness, good strength and toughness, but more importantly, to ensure hardenability before on-line cutting processing. On the premise of ensuring the mechanical properties, it is more important to take some necessary measures to keep the quenching stress of the mold at the lowest state:

1) The large and medium-sized mold blanks must be preheated at 650°C for 0.5~1.5h before quenching and heating to eliminate machining stress, reduce the chance of the material from elastic distortion to plastic distortion, and increase the dimensional stability of the mold.

2) Try to use hierarchical quenching or austempering process to prevent thermal stress and structural stress from occurring at the same time.

3) Strictly control the tempering temperature and require accurate temperature measurement. It is better to use an oil bath furnace with uniform temperature or a circulating air resistance furnace. After quenching, the die blank needs to be tempered while it is hot, and the tempering temperature should be increased as much as possible under the premise of ensuring the hardness. Tempering while hot is essentially austempering to form unstable austempered martensite structure inside the mold, and then perform secondary tempering to obtain tempered martensite. For large mold blanks and molds with thick cross-sections, a third tempering is sometimes necessary, and artificial aging is added to ensure the stability of the mold.

4) In order to reduce the deformation of various mold blanks, the unprocessed mold material can be quenched and tempered once. Generally, rough machining should be carried out before wire cutting, and then quenched to reduce the machining allowance of wire cutting and the deformation after wire cutting.

5) After forging, large die blanks need high temperature diffusion annealing or normalizing treatment once, and then isothermal spheroidizing annealing to eliminate network carbides, and then pre-quenched and tempered heat treatment to refine the structure and reduce stress.

6) For alloy tool steels, cold work die steels with thicker sections and H13, 65Nb, and GD steels, the quenching temperature should be increased by 20~30℃, which can greatly improve the hardenability, hardenability and impact toughness.

7) For molds with complex cavities and high precision requirements, the use of LD, Cr12MoV steel for high temperature quenching, high temperature tempering, secondary hardening treatment and medium temperature tempering again, can well eliminate the quenching stress.

8) In order to improve the life of the mold, chemical heat treatment such as nitrocarburizing, ion nitriding or surface electric spark infiltration should be carried out after online cutting. After chemical heat treatment, the tempering stability of the surface layer is obviously improved, which creates conditions for improving the service life of the mold.

9) For CrWMn steel and GCr15, 9Mn2V steel molds with poor hardenability and large and thick dimensions, due to incapability of hardening, the mold will have a large internal stress, which is easy to deform or even crack after online cutting. Therefore, it is necessary to adopt the method of opening the cavity in advance, that is, reserve a 1~2mm wire cutting allowance at the place where the wire cutting is required before quenching, and then cut after quenching. Pre-cavity quenching has the advantage that the cutting edge is easy to harden, and it changes the distribution of internal stress. Therefore, no deformation or cracking occurs.

Shanghai Geheng Vacuum Technology Co., Ltd. is a vacuum heat treatment furnace manufacturer with many years of experience in design, manufacturing and production. It mainly produces true air quenching furnaces, double chamber vacuum oil quenching furnaces, vacuum annealing furnaces, vacuum tempering furnaces, and other vacuum furnaces.