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Cr12MoV heat treatment knowledge
Cr12MoV steel is a high carbon high chromium Leysite steel, which is commonly used in cold working molds and has a lower carbon content than Cr12 steel. The steel has high hardenability, and can be completely hardened through a section of 300 mm or less. The volume change during quenching is also smaller than that of Cr12 steel.
The heat treatment system is: steel rod and forging 960 °C air cooling + 700 ~ 720 ° C tempering, air cooling.
Final heat treatment process:
1. Quenching:
First warm-up: 300 ~ 500 ° C,
The second preheating is 840 ~ 860 ° C;
Quenching temperature: 1020 ~ 1040 ° C;
Cooling medium: oil, medium temperature: 20 ~ 60 ° C,
Cool to oil temperature; then, air-cooled, HRC = 60-63.
2. Tempering:
Through the following quenching process, the effect of reducing the hardness can be achieved. The specific tempering process is as follows:
The heating temperature was 400 to 425 ° C, and HRC = 57 to 59 was obtained.
Note: Tempering between 480--520 degrees is just the brittle tempering zone of this steel. In this interval, tempering can easily cause the mold to collapse. The most ideal tempering range is 380-400 ° C. This zone is tempered, has the best toughness and has good wear resistance. If quenched, combined with cryogenic treatment (ideal temperature is minus 120) combined with medium temperature tempering, good results and high life are obtained. The temper brittleness temperature of Cr12MoV ranges from 325 to 375 °C. CR12MoV380-400 has a hardness of 56-58HRC after tempering. It has the advantage of being hard to crack, and especially when the quality of raw materials is not very good, it is economical.
Cr12MoV grade hardening process:
850 degrees preheating - 1050 degrees heating - 620 degrees grading, time is generally 2-3 minutes - oil cooling to about 200 degrees - (also 260 degrees bainite isothermal) - 520 tempering 2-3 times, each time 2 hours.
The hardness is about 56-61HRC.
Solution for Cr12Mov heat treatment HRC60 cracking:
Analysis process: (complete)
1 Material composition
2 material original organization
3 workpiece process
4 heat treatment process
5 cracking photos
6 workpiece size
It cannot be said that the hardness of 60HRC must be cracked. There are many reasons for cracking. You can check them one by one with reference to the instructions for completion. If it is quenched, it may crack directly without the following reasons:
1) The material mis-heat treatment process is not suitable.
2) Improper cooling, rapid cooling below Ms temperature, excessive stress.
3) The cross-section dimensions of the workpiece are too different, or there are many holes, or where stress is concentrated.
4) The quenching heating temperature is too high, the crystal grains are coarse, and the brittleness is large. The grain boundary is oxidized or melted during overburning.
5) The workpiece is not preheated, and the heating rate is too high to cause heating cracking.
6) The original structure is poor, the raw materials are in the form of reticulated eutectic carbides or spheroidal annealing, or the raw materials have microcracks, and the cracks expand and crack during quenching.
7) No intermediate annealing is performed before repeated quenching.
8) Temporarily tempered after quenching.
1050 quenching, with 260-280 bainite isothermal, the toughness of the mold will be greatly improved, and will not crack.
Cr12MoV
When Cr12MoV steel is quenched at 980 °C (ie, low quenching), the melting of carbides is less, and the carbon content of the matrix is ​​about 0.5%.
Cr is 6%, molybdenum is only 0.5%, vanadium carbides are less melted, the amount of carbides distributed in the matrix is ​​about 15%, and the retained austenite is only 20% or less, and the hardness after quenching is HRC60-62. Since the carbon content of the matrix is ​​low, the matrix after quenching has high toughness. However, the compressive yield strength does not reach a high level, and the low quenching temperature can only be tempered at a low temperature (180-200 ° C). It is suitable for cold dies with low load and high speed. After low temperature tempering, the hardness is HRC>60, and the number of tempering is two. Times.
Cr12MoV steel uses medium quenching temperature (1025-1030 °C) (ie normal quenching) to increase the carbon concentration of the matrix, alloy carbide is further melted, the hardness reaches the highest value, and the retained austenite amount also rises to nearly 40%. The quenching hardness is around HRC62-63. After quenching at medium quenching temperature, it can be tempered at 180-200 °C for the highest hardness and best wear resistance, but the toughness is slightly lower and can not be used in heavy-duty cold die. After quenching at medium quenching temperature, it can also be tempered at 380-400 °C, but the hardness will drop to about HRC58, and the best toughness fit can be obtained and the fracture resistance of the cold forging die can be obviously improved, but the wear resistance is reduced.
Cr12MoV steel quenching at temperatures above 1050 °C (ie high quenching) belongs to high quenching temperature quenching. As the quenching temperature increases, carbon and alloying elements and carbides are further melted into austenite, the carbon and alloying elements increase, the austenite stability increases, and the amount of retained austenite after quenching also rises sharply. To 40-60% or even higher, the quenching hardness is reduced to about HRC55 due to an increase in the amount of retained austenite. Due to the increase in retained austenite and the increase in austenite stability, low temperature tempering cannot transform austenite, and only a high temperature tempering can promote a large amount of austenite transformation. Increase the hardness (1150 ° C quenching, its quenching hardness may drop below HRC 45-50) secondary hardening phenomenon. High temperature tempering (520-540 ° C), must be tempered three times. In addition, as the quenching temperature increases, the grain size in the steel rapidly grows, and the grain size at 1080 ° C is as long as 9 grades, and 1150 ° C is even as long as 7-8 grades. The quenching temperature increases the number of carbides (volume fraction), so the toughness strength is lowered, and the wear resistance is also lowered, but the red hardness is improved. Therefore, the high quenching and high-return process is only used when high punching, low load and red hardness are required. The hardness after high temperature tempering can also reach above HRC62. It should be noted that the high temperature tempering temperature range of secondary hardening is very narrow, at 520 °C for Cr12MoV steel and 540 °C for Cr12Mo1V1, beyond this temperature zone (often only ± 5 ° C), the hardness fluctuations are large.
If it is Cr12 steel, the heating temperature can be higher and no more than 1000 degrees. Insulation 30-40 minutes is enough. Quenching to above 60HRC is convenient. The quality difference of Cr12 steel is very large, and the requirements for forging are very high. Forging requires "two light and one heavy six-face forging" to make the distribution of carbides as uniform as possible, but in reality it does not meet the requirements.
Cr12MoV, which is a widely used high carbon and high chromium cold work die steel in the world, is a Leysite steel with high hardenability, hardenability and high wear resistance; high temperature oxidation resistance, after quenching and polishing Good rust resistance and small heat treatment deformation; suitable for manufacturing high precision, long life cold working dies, cutting tools and measuring tools, such as complex punching die, cold extrusion die, rolling wheel, crepe plate, cold shear Knives and precision measuring tools. See GB/T1299-2000 for chemical composition.
C Si Mn P S Cr Mo V Co
1.40-1.60 ≤0.60 ≤0.60 ≤0.03 ≤0.03 11.00-13.00 0.70-1.20 ≤1.10 ≤1.00
The steel can be used in two quenching temperatures:
(1) Use a lower quenching temperature of 950-1040 ° C 180-230 ° C low temperature tempering hardness of 60-64HRC (ie: low quenching low back)
(2) Using a high quenching temperature of 1050-1100 ° C 510-540 ° C high temperature tempering 2 times hardness 60-64HRC (ie: high quenching high back)
Cr12MoV, the relationship between the mechanical properties of steel and the quenching temperature is: after 1000~1075 degrees quenching, good mechanical properties can be obtained, and the quenching temperature is increased, which will significantly reduce the strength and plasticity.
Cr12MoV is quenched at 1130 degrees, ie secondary hardening. The secondary hardening method has the advantage that a certain degree of red hardness can be obtained, and the disadvantage is that the grain size is large due to the high quenching temperature. Does not meet the requirements of the landlord.
The higher the toughness requirement, the better the hardness reaches HRC 52~64, which means that the hardness, strength and toughness are better. According to the requirements of the landlord, it is recommended to use about 1030 degrees of quenching. The tempering temperature is 200~450 degrees (the temper brittle zone avoiding 300~375 degrees). The hardness after tempering is HRC 55~60.
Three times of tempering for the first high temperature, the second time based on the first tempering hardness to determine the tempering temperature, the third time low temperature
Cr12mov low temperature tempering is only used after low temperature quenching, can maintain high hardness, you use normal temperature quenching, tempering temperature is best to use 230-250 °, if the drawing hardness requirements are not very high, it is recommended that you use 390-400 ° back Fire, so the mold's toughness and fit will be ideal, not easy to chip.
Solid solution double refining process of Cr12MoV: (The purpose is to refine the rounding and cornering of carbides while making the austenite grains ultra-fine.)
Oil quenching after Cr12MoV is 1000 degree heat preservation; before 220 degree tempering: first do 1100 degree heat preservation, oil quenching, 700 degree tempering. The life of the material will increase a lot.
Since materials such as Cr12 are high-carbon high-alloy steels, severe C segregation, so-called reticulated cementite, occurs during the rolling process. As we all know, cementite is high in hardness and brittle, and has no toughness. There has been a strict level of control over C segregation of such materials. Due to the presence of reticulated cementite in the material, it will be cracked along the cementite in the subsequent heat treatment. Therefore, the material must be repeatedly forged before the mold is made (not just simply changing the shape) to break the mesh cementite and improve the material properties.
Pre-preheating of the die steel before quenching---spheroidizing annealing to refine the structure, reduce quenching deformation and prevent quenching cracking!
General stamping dies are required to have high hardness, good wear resistance, corrosion resistance, good impact toughness and other basic requirements, and H13 is a common steel for hot working, its high strength, high toughness, certain resistance Grindability; high red hardness, high thermal stability, high resistance to heat fatigue, high hardenability and high thermal conductivity.
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