After quenching D2 cold working die steel and then ultra-low temperature treatment in an industrial refrigerator at 196 ℃, the retained austenite can be eliminated and all martensite can be obtained, and the microstructure can be further refined. After ultra-low temperature treatment, the Rockwell hardness (HRC) of D2 steel is increased by 4 units, the AK value after tempering is increased nearly 1 times, and the wear resistance is also improved. The service life of the die made of D2 steel after ultra-low temperature treatment is 2.3 times longer than that of ordinary treatment.
The conventional heat treatment method of cold working die made of Cr12 die steel is low temperature tempering after quenching. Although the steel matrix can be transformed into martensite by this treatment method, there is more retained austenite in it, which is an important factor for die failure. If the retained austenite can be transformed into martensite, it will not only eliminate a hidden danger of die failure, but also improve the hardness, wear resistance and dimensional stability of the die.
The hardness of D2 steel after ultra-low temperature treatment reaches HRC63, which is 4 units higher than that of ordinary treatment. The impact value AK of D2 steel is 14.7 J / cm², which is about one time higher than that of ordinary treatment.
Although the amount of retained austenite in D2 steel after ordinary treatment and ultra-low temperature treatment is different, the difference can not be identified by optical microscope. This is due to the fact that the grain size of D2 steel quenched at 1050 ℃ is relatively fine, about ASTM grade 10, and it is difficult to observe the retained austenite. However, when the quenching temperature is increased to 1250 ℃, the difference of retained austenite content between the two treatment methods is obvious. The white particles are carbides, the black matrix is martensite, and the white is retained austenite. The retained austenite content of D2 steel after ultra-low temperature treatment is obviously lower than that of ordinary treatment, which fully indicates that ultra-low temperature treatment at – 196 ℃ can effectively promote the transformation of retained austenite to martensite.
In addition, there is retained austenite in the microstructure after ultra-low temperature treatment, which is different from the results shown in the paper. This is due to the different quenching temperature. Increasing the quenching temperature will make a large amount of carbides such as cr2c3, Mo2C and VC dissolve into austenite in D2 steel, and decrease the Ma and Mt points, and increase the residual austenite content after quenching. The microstructure of D2 steel is fine after ultra-low temperature treatment. This is because the transformation of austenite to martensite is continuous in the process of cooling, and the temperature is very low, so the martensite formed is fine. The hardness (HRC) of D2 steel after ultra-low temperature treatment is 4 units higher than that of ordinary treatment, which is obviously caused by the transformation of retained austenite into martensite. The increase of hardness can greatly improve the wear resistance of steel, thus greatly improving the service life of die. A
fter ultra-low temperature treatment, the AK value of D2 steel is about 2.5% of that of ordinary treatment This may be related to the amount of retained austenite and the refinement of martensite. The improvement of toughness is very important for cold working dies, because brittle fracture is the most common failure mode of cold working dies. It is precisely because of the above reasons that the service life of dies is increased by 1.3 times.