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2022.5.25
Today we are going to learn about cold work tool steel with its performance requirements and grade classification.
Cold work tool steel is mainly used to manufacture the die for pressing the workpiece in a cold state (room temperature). Such as cold stamping die, cold drawing mold, cold extrusion die, etc. When the die is cold. Because the deformation resistance of the processed material of cold work tool steel is relatively large, the working part of the die bears great pressure, bending force, impact force, and friction force. Therefore, the normal scrapping reason for cold work tool steel is generally worn. There are also early failures due to fracture, collapse force, and out-of-tolerance deformation. The common failure modes of cold work tool steel are wear, angle collapse, plastic deformation, fracture, and adhesion.
– Good wear resistance, keep sharp edges when working When the cold working die is working, there are many times of friction between the surface and the blank. In this case, the die must still maintain a low surface roughness value and high dimensional accuracy to prevent early failure. Because the hardness and structure of die material are important factors affecting the wear resistance of die, in order to improve the wear resistance of cold working die, the hardness of mold is usually required to be 30% – 50% higher than that of the machined part. The structure of the material is tempered martensite or lower bainite, on which evenly distributed and fine granular carbides. To achieve this goal, the mass fraction of carbon in steel is generally more than 0.60%.
– The quenched state has high hardness and certain quenching depth
– Heat treatment deformation is small, complex shape is not easy to crack
– Certain strength and toughness Die strength refer to the ability of die parts to resist deformation and fracture in the working process. The strength index is an important basis for cold working die design and material selection, mainly including tensile yield point, compressive yield point, and so on. The yield point is an index to measure the plastic deformation resistance of die parts, and it is also a commonly used strength index. In order to obtain high strength, in the process of die manufacturing, the toughness of die materials should be determined according to the working conditions of the die. For dies with strong impact load, such as punch and cold heading die of cold working die, due to large impact load, high toughness is required. The cold working mold under general working conditions is usually affected by small energy multiple impact load. The failure form of the die is fatigue fracture, so the die does not have to have too high impact toughness.
– Good red hardness & heat softening ability The heat softening ability reflects the influence of temperature rise of cold work tool steel on hardness, deformation resistance, and wear resistance. The main indicators to characterize the thermal softening resistance of cold work tool steel are softening temperature (℃) and secondary hardening hardness (HRC).
– Strong bite resistance Occlusal resistance is actually the resistance to “cold welding”. Generally, under dry friction conditions, the tested die steel sample and the material with bite tendency (such as austenitic steel) are subjected to constant speed dual friction movement, and the load is gradually increased at a certain speed, and the torque is also increased accordingly. When the load increases to a certain critical value, the torque suddenly increases sharply, which means bite. This load is called “bite critical load”. The higher the critical load, the stronger the bite resistance.
Process performance requirements of cold work tool steel
The process performance of cold work tool steel is directly related to the manufacturing cycle and manufacturing cost of the die. The technological performance requirements of cold work tool steel mainly include forging technology, cutting technology, heat treatment technology, and so on.
(1) Forging processability
Forging not only reduces the machining allowance of die materials and saves steel, but also improves the internal defects of die materials, such as carbide segregation, reducing harmful impurities, and improving the microstructure of the steel. In order to obtain good forging quality, the requirements for malleability are low hot forging deformation resistance, good plasticity, wide forging temperature range, and the small tendency of forging crack, cold crack, and precipitation of network carbide.
(2) Cutting technology
The wear is small and the surface of the die is smooth after processing. cold work tool steel mainly belongs to hypereutectoid steel and ledeburite steel, and most cutting processing is difficult. In order to obtain good cutting machinability, it is necessary to carry out the correct heat treatment. For dies with high surface quality requirements, free-cutting die steel containing s, Ca and other elements can be selected.
(3) Heat treatment processability
The heat treatment process mainly includes hardenability, hardenability, resistance to re-ignition, overheating sensitivity, oxidation decarburization tendency, quenching deformation and cracking tendency, etc.
(1) Low alloy cold work tool steel
SKS3, YK30, DF-2, and more are often used to make short-life molds, tool supports, etc. The hardenability is better than that of nonalloy die steel, with high surface hardness and certain hardenability depth.
(2) Medium alloy cold work tool steel
Represented by Cr5Mo1V (A2), including Cr4W2MoV, Cr5Mo1V, etc. Due to the relatively low carbon content and low chromium content, it belongs to hypereutectoid steel. The carbides in this kind of steel are mainly Cr7C3 type and a small amount of M6C and MC type. It has the characteristics of good wear resistance and small heat treatment deformation.
(3) High alloy cold work tool steel
Represented by Cr12Mo1V1 (D2), including Cr12, Cr12W, Cr12MoV, Cr12MoV1, W6Mo5Cr4V2, 5Cr4W3Mo2VNb, 6W6Mo5Cr4V, etc.
– SLD, Cr12MoV, Cr12Mo1V1 (SKD11), XW – 41, and DC53 are often used to make medium life die steel. It has high compressive strength, high quenching hardness, good quenching toughness, and high wear resistance. Among them, Cr12MoV and Cr12Mo1V1 have higher hardenability, hardness, strength, and toughness after quenching and tempering than Cr12. Workpieces with a section of less than 300 – 400mm can be completely quenched, with good wear resistance and plasticity, and small deformation but poor high-temperature plasticity. They are used in various casting, forging and dies, such as various punching punch dies, trimming dies, edging dies, seam dies, wire drawing dies, and standard tools, and dies. Cr12Mo1V1 has better hardenability and toughness than Cr12MoV, and the rest are the same.
– ASP23, ASP60SKH51, V4, and V10 are often used to make long-life die steel. It has high compressive strength, high quenching hardness, good wear resistance, and stability.