Die steel vacuum quenching process

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Cr12mov steel pipe is a cold work die steel. There are two heat treatment processes: the first process is called low quenching and low recovery. It is used in an environment where the operating temperature is not high and has good toughness. With the increase of tempering temperature, the hardness decreases. The two are called high quenching and high back, secondary hardening, with strong red hardness, and nitriding can be used to further improve the surface hardness, can be 68-72 °

Cr12Mov vacuum heat treatment HRC60 cracking solution:

1) The vacuum heat treatment process is not suitable due to material errors.

2) Improper cooling, rapid cooling below Ms temperature, and excessive stress.

3) The cross-sectional size of the workpiece is too different, or there are many holes, or there are places where stress is concentrated.

4) The heating temperature of vacuum quenching is too high, the grains are coarse and the brittleness is large. Grain boundaries oxidize or melt during overfire.

5) The workpiece is not preheated, and the heating speed is too large to cause heating cracking.

6) The original structure is poor, the raw material has network eutectic carbide or poor spheroidizing annealing, or the raw material has microcracks, and the cracks expand and crack during quenching.

7) Intermediate annealing is not performed before repeated vacuum quenching.

8) Not tempered in time after vacuum quenching.

When Cr12MoV steel is heated at 980 ℃ vacuum quenching (ie low quenching), the melting of carbides is less, the carbon content of the matrix is ​​about 0.5%, Cr is 6%, molybdenum is only 0.5%, and the carbides of vanadium are less melted and distributed. The amount of carbides in the matrix is ​​about 15%, while the residual austenite is only less than 20%, and the hardness after vacuum quenching is HRC60~62. Due to the low carbon content of the matrix, the toughness of the matrix after vacuum quenching is high. However, the compressive yield strength does not reach a high level, and the low quenching temperature can generally only be tempered at low temperature (180-200 °C), which is suitable for cold die with low load and high speed. Second-rate.

Cr12MoV steel adopts medium quenching temperature (1025-1030℃) (that is, normal quenching) to increase the carbon concentration of the matrix, the alloy carbides are further melted, the hardness reaches the highest value, and the amount of retained austenite also rises to nearly 40%. Quenching hardness is around HRC62-63. Medium quenching temperature After quenching, it can be tempered at a low temperature of 180-200 ° C to obtain the highest hardness and best wear resistance, but the toughness is slightly lower and cannot be used in heavy-duty cold punching dies. It can also be tempered at 380-400 ℃ after quenching at medium quenching temperature, but the hardness will drop to about HRC58, which can obtain the best combination of strength and toughness and significantly improve the fracture resistance of cold forging die, but the wear resistance will decrease.

Vacuum quenching (ie high quenching) of Cr12MoV steel exceeding 1050 ℃ temperature belongs to high quenching temperature quenching. With the increase of vacuum quenching temperature, carbon, alloying elements and carbides are further melted into austenite, the carbon and alloying elements increase, the stability of austenite increases, and the amount of retained austenite after vacuum quenching also increases. It rises sharply to 40-60% or even higher, and the quenching hardness decreases to around HRC55 due to the increase in the amount of retained austenite. Due to the increase of retained austenite and the increase of austenite stability, low temperature tempering cannot transform austenite, and only high temperature tempering can promote a large amount of austenite transformation. When the hardness is increased (1150 ℃ quenching, the quenching hardness may drop to below HRC45-50), secondary hardening phenomenon occurs. High temperature tempering (520-540 ℃), must be tempered three times. In addition, with the increase of quenching temperature, the grain size in the steel grows rapidly, and the grain size grows to grade 9 at 1080℃, and even grows to grade 7-8 at 1150℃. As the quenching temperature increases, the number of carbides (volume fraction) decreases, so the toughness and strength decrease, and the wear resistance also decreases, but the red hardness increases. Therefore, only when high stroke times, low load and red hardness are required, the high-quenching and high-return process is used. The hardness after high temperature tempering can also reach above HRC62. It should be pointed out that the high temperature tempering temperature range of secondary hardening is very narrow. 5°C), the hardness fluctuates greatly.

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