Basic properties of AISI A2 cold working die steel

A2 is a cold-working die steel improved on the basis of SKD11 (Cr12Mo1V1). Under the conventional heat treatment condition, the residual austenite is almost completely decomposed. Generally, cryogenic treatment can be omitted, and high toughness can still be maintained under strong hardness.
I. Experiment design
After being quenched at 1040 ℃ and tempered at 520~530 ℃, A2 has a hardness HRC of 60~62 and a toughness of twice that of Cr12Mo1V1, which is the highest among the commonly used cold-worked die steels at present. It has good machinability and grindability, low residual stress in the modified layer of electric machining, few residual austenite, and fine and evenly distributed carbides.
Due to the complex stress situation of the die, some working parts of the die need to have some special mechanical properties. If the standard heat treatment process often cannot meet the ideal working performance requirements, the basic characteristics such as hardness, toughness and wear resistance need to be properly adjusted through heat treatment to achieve the best working state of the die. Quenching temperature and tempering temperature are the main process parameters of heat treatment. This paper focuses on the tempering characteristics of A2.
II. Experiment Design
In the experiment, the A2 heat treatment specification was slightly changed, and the quenching temperature was appropriately adjusted, and the tempering temperature was taken as six grades, namely 100 ℃, 200 ℃, 300 ℃, 400 ℃, 500 ℃, and 600 ℃. For 100 ℃ tempering, 101-2 drying oven is used for heating, and SX-25-12 box-type resistance furnace is used for heating. Two samples are taken for each tempering temperature.
Metallic Rockwell hardness test is selected for hardness test, which is conducted at normal temperature, and HBRVU-187.5 Brinell optical hardness tester is used.
10mm for impact test × 10mm × 55mm non-notched specimen is tested on JB30B impact testing machine with impact energy of 0.3 KN. m or 0.15 KN. m.
Experimental results and analysis
1. Hardness value
Take three different positions for each sample to measure the hardness, and get the hardness value at each tempering temperature. According to the hardness value of each sample, A2 has little change when tempered at 100~500 ℃; The hardness is slightly higher when tempered at medium temperature of 400 ℃, and the peak hardness after standard heat treatment and tempering is generally around 520 ℃; After high-temperature tempering at 600 ℃, the hardness decreases significantly, and the average HRC hardness value is only 52.4, so the tempering temperature should not be too high.
2. Impact toughness
After tempering, the oxidation decarburization layer on the surface of the sample is removed, and the impact value of each sample at different tempering temperatures is measured. According to the impact value of each sample, when DC53 is tempered at 200 ℃, the average impact value reaches more than 60 J/cm2. When tempered at 500 ℃, the impact toughness is poor, showing a certain high temperature tempering brittleness. Tempering impact toughness above 600 ℃ is very good, but the hardness is greatly reduced, which can not meet the use requirements
The experimental results show that A2 has good overall tempering stability, and the hardness and impact value change little within a certain tempering temperature range; When tempering at 400~500 ℃, the toughness decreases greatly and temper brittleness occurs; When tempering at 600 ℃, the toughness of the sample is very high, and the impact value reaches 85 J/cm2, but the hardness is greatly reduced. In production, for some cold working dies with low requirements for hardness and wear resistance and high requirements for toughness, high-temperature tempering can be used; For cold working dies with high hardness requirements and high toughness, low temperature tempering at about 200 ℃ should be adopted. The hardness and impact value at other tempering temperatures can be predicted by appropriate calculation methods (such as interpolation method, function approximation, etc.), and then verified by experiments. The carbide in the quenched sample is distributed in discontinuous thin bands, and the carbide is uniformly distributed after tempering at 200 ℃, and there is almost no massive carbide in the structure, Therefore, the toughness is good. From the fracture morphology, the cleavage step of the fracture of the tempered structure at 200 ℃ is far less than that of the quenched sample, and there are some small and shallow dimples in the fracture of the metallographic structure at 5000 times, indicating that it has a certain toughness. After tempering, the residual austenite transforms fully, and the carbide is fine and evenly distributed, which increases the toughness
conclusion
1. After properly adjusting the quenching temperature, A2 has higher hardness and impact toughness when tempered at 200 ℃; When tempering at 400~500 ℃, the hardness is higher and the toughness decreases greatly; When tempered at 600 ℃, the impact toughness is very high and the hardness decreases significantly
2. Low-temperature tempering process should be adopted for precision die, trimming die, cold roll wheel and other tools and dies with complex shapes to make the working parts of the die obtain high hardness, high toughness, good wear resistance and high strength, which can effectively extend the life of the die and prevent excessive wear, deformation, cracking and other early failure phenomena
3. Low quenching and high recovery process can be adopted for complex molds with large impact load to obtain high impact toughness and prevent brittle fracture of the mold.