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Vacuum heat treatment process of Cr12 steel for tools and dies

Cr12 steel is a high carbon, high chromium cold work die steel. The carbon content of this steel is very high (mass fraction is 2%-2.3%), which is the highest carbon content in the cold work die steel currently used, and the chromium content is also very high (mass fraction is 11.5%-13.0%), which belongs to Leys body steel. Therefore, the steel has high hardenability, hardenability and wear resistance, and the heat treatment distortion is small. But when the carbide is not uniform, the deformation is multidirectional and irregular. Its poor structure is the main shortcoming, and the uneven carbide is difficult to improve by heat treatment unless it is manufactured by powder metallurgy. Cr12 steel has poor impact toughness, poor thermal conductivity and high temperature plasticity.

The steel forms a large number of eutectic network carbides during the crystallization process, of which the carbide content accounts for about 20% (volume fraction), and the eutectic temperature is about 1150°C. These carbides are very hard and brittle. During rolling and forging, the carbides are broken to a certain extent, but the carbides are distributed along the rolling direction in the form of bands, nets, blocks, and piles, and the degree of segregation becomes more serious as the diameter of the steel increases.

Vacuum heat treatment process:

During vacuum quenching, the higher the heating temperature, the greater the amount of retained austenite. Vacuum quenching at 975°C, the amount of retained austenite is 29% (volume fraction); vacuum quenching at 1100°C, the amount of retained austenite reaches about 60%.

When the vacuum tempering temperature is below 450°C, the amount of retained austenite is not much; when the vacuum tempering temperature is above 450°C, the amount of retained austenite is greatly reduced; when the vacuum tempering temperature is 550°C, the amount of retained austenite is reduced by 80%. to below 20%. In actual production, heating at 980°C and isothermal treatment with nitrate salt at 240-260°C are often used to obtain a mixed structure of lower bainite + martensite + retained austenite + carbide, and then tempered at an appropriate temperature. obtain satisfactory mechanical properties. The biggest advantage of austempering of Cr12 steel tools is that the toughness of the tool can be greatly improved without reducing the strength of the workpiece, and the life of the tool and mold can generally be increased by 30%-50%.

The solid solution double refinement treatment fully utilizes the vacuum heat treatment method to refine carbides, round edges and corners, and refine austenite grains at the same time. Its main measures are high temperature solid solution and cycle refinement. High-temperature solid solution can improve the morphology and particle size of carbides. The purpose of cyclic refinement is to make the austenite grain ultrafine. After solid solution double refinement treatment, the impact toughness of Cr12 steel can be doubled, and the life of the mold can be doubled several times. Although the process is more complicated and the production cost will increase, the overall economic benefits are still considerable.

Application:

1) It is used for drawing and punching dies requiring high wear resistance and simple shape under the condition of small dynamic load of silicon steel sheet dies.

2) It is widely used in the manufacture of cold work dies with large cross-section, complex shape, high impact force and wear resistance, such as silicon steel sheet punching die, thread rolling die, wire drawing die, etc.

3) To manufacture the concave die that requires wear resistance in the drawing die, the recommended hardness is 62-64HRC.

4) For cold extrusion punches of aluminum parts, the recommended hardness is 60-62HRC.

5) It is used in the manufacture of punches, dies, and inserts that require high wear resistance and complex shapes in bending dies. The recommended hardness for convex and dies is 60-64HRC.

6) Spring steel plates with a mass fraction of 0.65%-0.80% carbon for bending, a mold hardness of 37-42HRC, and a service life of up to 220,000 times.

7) Boronizing treatment of the blanking die: the original C14-28 type integrated circuit blanking die is made of Cr12 steel, and the punched material is 95 tiles. Boronizing with paste: 950-970°C*4h, furnace cooling to 500°C and air cooling; vacuum annealing after boronizing: 850-870°C*3h, furnace cooling to 500°C and air cooling; 980°C vacuum oil quenching, 180°C Vacuum tempering, 150℃*24h artificial aging. The life of the mold treated with boronizing is more than 20 times longer than that of the uninfiltrated one.

8) Vanadium infiltration treatment of Cr12 steel thread rolling plate: preheating at 800-820°C, heating at 1040-1060°C and vacuum oil quenching; vanadium infiltration in salt bath at 940-960°C*5h, oil cooling or quenching at 220°C with nitrate salt after vanadium infiltration , 200-220 ℃ vacuum tempering.

As a traditional cold die steel, Cr12 has a wide range of applications; however, due to its large inhomogeneity of carbides, it leads to serious anisotropy of mechanical properties and relatively high brittleness. Based on this situation, many new varieties of cold die steel have appeared to meet the needs of the market.