The hot heading die is a die used to make larger specifications of fasteners. It has to withstand greater loads (such as pressure, expansion force, etc.) during work, and should meet the following performance requirements, high thermal hardness and high temperature wear resistance , Sufficient strength and toughness, high thermal fatigue resistance, under the action of repeated thermal stress, cracking and oxidation will not occur.
H13 (4Cr5MoSiV1) steel is currently widely used in fastener companies, mainly containing Cr, Mo, V and other alloying elements. It has good manufacturability and performance. H13 has gradually replaced due to insufficient toughness and insufficient thermal fatigue resistance. And cause the failure of 3Cr2W8V steel. In terms of balance of strength and toughness, process stability, production cost, and scope of application, H13 steel is still the first choice in fastener companies.
In the past 10 years, a lot of work has been done in the development of high-quality die steel at home and abroad, and a variety of H13 improved steel grades have been introduced. The development of high-speed, high-efficiency, environmental protection, and energy-saving modern fastener industry has continuously put forward higher requirements on the performance of die steel. For this reason, domestic and foreign steel companies have successfully developed a series of steels that are superior to H13 steel in certain properties. This should be mainly attributed to the application of new technology and new equipment in the metallurgical industry and hot work die steel quality control.
1. HD2 hot work die steel
HD2 steel is an alloying scheme steel with a variety of chemical compositions, and the working temperature can be in the high temperature zone of 870-930℃. Through a reasonable component distribution ratio, it can achieve high thermal strength while maintaining good toughness, thermal conductivity and process performance. The thermal fatigue performance is significantly better than 3Cr2W8V steel. In addition, the addition of a trace amount of Nb to the steel improves the stability of M6C23 and MC carbides, and makes the grain size of the microstructure more refined, thereby reducing the overheating sensitivity of the steel during service. HD2 steel material is not only The ideal material for hot heading dies is also suitable for hot extrusion dies.
HD2 and 3Cr2W8V steels are quenched and tempered under the same process. HD2 steel has good tempering stability, and the Rockwell hardness is 3-5HRc higher respectively.
2.4Cr3Mo3W4VNb (GR steel)
GR steel is a kind of die steel developed for high-speed hot heading dies and hot extrusion dies. It has high-temperature strength and good thermal stability, appropriate plasticity, toughness, good hardenability, and thermal conductivity. The addition of trace elements such as Nb and carbon and nitrogen elements form a dispersion of carbides, which further improves the high-temperature strength and Tempering stability, wear resistance of steel.
Practice has proved that GR steel works at a temperature of 630℃ compared with 3Cr2W8V steel. The hardness of the former is 51.5HRc and the latter is 47.3HRc. GR steel has more thermal fatigue cracks than 3Cr2W8V steel, but has many cracks and shallow depth. 3Cr2W8V steel has fewer cracks and obviously deep cracks, which is more likely to cause thermal fatigue failure and reduce the life of the mold when it is used. Therefore, GR steel has obvious resistance to thermal fatigue.
3. DIEVAR hot work die steel
DIEVAR hot work die steel is optimized on the basis of H13 steel, the main chemical composition is adjusted, the molybdenum content is increased and the carbon content is reduced, and it is manufactured by advanced metallurgical production technology. Generally speaking, the following arrangement of the failure modes of the hot upsetting die is thermal wear, severe cracking, plastic deformation and thermal cracking. Compared with DIEVAR steel, ASSAB8407 steel is quenched at 1030℃ and tempered at 600℃. The former is kept for 44h and the latter is kept for 55h. The hardness of DIEVAR hot work die steel is reduced from 45HRc to 35HRc. This shows that the softening resistance of DIEVAR steel is increased by 25%, which is one of the reasons for the long service life.
4.HOTVAR hot work die steel
HOTVAR steel has a hardness of 54-58 HRc, which has excellent thermal stability, and also has quite good anti-tempering stability and higher high temperature strength. The hardness is still 48-56 HRc at the tempering temperature of 550-650℃, which is a very important service condition for the application of hot upsetting molds. The change in hardness shows the basic data of the tempering resistance of the material.
SWPH13 steel is an upgraded product of H13 steel, and all technical indicators are higher than H13 steel. Adopt advanced smelting technology, DC DC electric arc furnace, LF ladle refining furnace, VD vacuum degassing device, AC three-phase AC electric arc furnace, LFV ladle refining, high temperature diffusion annealing process, eliminate primary eutectic carbide, improve composition segregation, and obtain uniformity Organization. The crack depth morphology comparison of the samples after 3000 cooling and heating cycles shows that the SWPH13 steel sample has the least number of cracks, and the average depth of the five cracks is also the smallest. The H13 steel and ASSAB8407 steel not only have a large number of cracks, but the depth exceeds 60%. Therefore, the thermal fatigue performance of SWPH13 steel is improved after high temperature homogenization. Therefore, the fatigue performance is better, and today’s mainstream product of hot work die steel.