Research on properties of several hot work die steels
The mechanical properties and thermal stability of GMH20, PH25, DIEVAR and H13 hot work die steels were tested. The results show that the plasticity and hardness of GMH20 steel, PH25 steel and DIEVAR steel are obviously better than H13 steel after vacuum heat treatment with the same process. Overall, GMH20 and DIEVAR steel have the best mechanical properties. In addition, with the extension of the holding time at 610 ℃, the hardness of these four kinds of steel is getting closer and closer, about 35 HRC.
H13 steel is a very widely used hot work die steel in China. At present, the alloying method of low Si and high Mo is a development trend of improved H13 steel. The advantages of reducing Si are: (1) reducing ∧ shape or ∨ shape segregation; (2) homogenize the macro organization; (3) thinning dendrites of microsolidified structure; (4) Reduce the supercooling of the components on the solidification interface during solidification; (5) Reduce eutectic carbides; (6) Refining austenite crystals; (7) Improve plasticity and toughness; (8) Reduce the high temperature fatigue crack propagation rate; (9) Reduce the creep crack propagation rate; (10) Inhibit bainite transformation during quenching cooling; (11) Improve thermal cracking resistance. The advantages of increasing Mo are: (1) improving the hardenability, inhibiting the precipitation of grain boundary carbide and bainite transformation; (2) Improve tempering resistance; (3) Increase high temperature strength and high temperature creep strength; (4) Improve thermal cracking resistance; (5) Improve toughness; (6) eutectic carbide refinement and uniform carbide distribution. When studying the high temperature properties of hot work die steel, many scholars use thermal stability tests to characterize the material's ability to maintain organizational stability and the changes in mechanical properties such as hardness and strength when working at high temperatures.
DIEVAR steel, GMH20 steel and PH25 steel belong to the low Si and high Mo improved H13 steel. Now the three kinds of steel and H13 steel are heat treated together, using the same heat treatment process to compare the hardness, mechanical properties, impact toughness and thermal stability of the four kinds of steel.
Test materials and methods
1. Test the composition of the material
The test materials were GMH20 steel, PH25 steel, DIEVAR steel and H13 steel. The four materials were processed into tensile, impact and thermal stability hardness samples respectively. The chemical composition of the test materials was shown in Table 1.
2. Test method
V-shaped notch rectangular sample was used for impact at room temperature, and the impact sample was 10 mm × 10 mm × 55 mm, which was measured according to GB/T 229-2007 "Metal Charpy Notch impact test Method", and the impact test was carried out on ZBC230Z-B type impact test machine. The tensile test was carried out on the CMT5305 universal test machine, and 10 tensile test rods were selected. The microstructure was observed with Neophot30 metallographic microscope. The hardness was determined by HRS-150 digital display Rockwell hardness tester. The heat treatment process of the mechanical properties of the four materials is: direct air cooling quenching at 1020 ℃, and tempering at 560 ℃ and 595 ℃ for 3 h.
Thermal stability is one of the main properties of hot work die steel, which reflects the ability of die steel to resist softening when working at high temperature, and is related to the high temperature performance of steel. 4 kinds of test steels belong to H13 class hot work die steels, mainly used as aluminum alloy die casting materials. The melting point of aluminum alloy is generally about 600 ° C, and if the billet flows in the mold cavity, the temperature will be higher. Therefore, the thermal stability test temperature was selected at 610 ℃. The heating furnace of this test is YFX10/13Q-GC high temperature box resistance furnace, and the sample size is 15 mm × 15 mm × 10 mm. The samples were pre-cooled at 1020 ℃ and then air-cooled, tempered twice at 560 ℃ and 590 ℃, and then placed in a high-temperature box furnace for 610 ℃ tempering time of 2 h, 4 h, 6 h, 8 h, 10 h, 15 h and 20 h, respectively. The hardness of the sample after different holding time was measured and the curve was drawn.
3. Conclusion
(1) The mechanical properties of GMH20 steel and PH25 steel are better than H13 steel and close to DIEVAR steel, especially GMH20 steel, and the mechanical properties are very close to DIEVAR steel. The plasticity of GMH20, PH25 and DIEVAR steel is significantly better than that of H13 steel, mainly because the content of Si is greatly reduced compared with the chemical composition of H13 steel, and the content of Mo is increased, and this change in composition can improve the plasticity and toughness of steel. The reduction of Si content can improve the plasticity of steel, and DIEVAR steel has the lowest Si content, so its plasticity is the best. Overall, DIEVAR steel and GMH20 steel have the best mechanical properties.
(2) At 610 ℃ for a long time, the thermal stability change trend of GMH20 steel and PH25 steel is very close, and the curves almost coincide; With the extension of holding time, the hardness of the four kinds of steel tends to be the same, about 35 HRC.
