Research On Economic And Efficient Pretreatment Process Of H13Steel

H13steel is one of the most widely used hot die steel in the world and thespheroidizing annealing organization and performance quality directly affect the finalperformance. Due to the higher content of carbon and other alloy elements in hot diesteel, the secondary carbides easily precipitate on the grain boundary during the slowcooling process after forging rolling and then influence the performance of H13module. Therefore, the solid solution pretreatment and the spheroidizing annealingtreatment must be applied to obtain the uniform microstructure and the lower hardnessfor machining. Currently, the high temperature solid solution at1100℃and two stagesspheroidizing annealing process are widely used to prevent the secondary carbideprecipitate on the grain boundary and obtain the uniform microstructure. However, thewhole process is very complicated, time-consuming and difficult to implement inmanufacture.In this study, a plenty of experiments are designed to research the effect of theprior microstructures, solution temperature and spheroidizing annealing temperatureon the spheroidizing microstructure. In addition, the microstructure and propertiesanalysis and carbide statistics are performed by OM, SEM, impact testing machine,and Image Pro Plus software. An economic and efficient pretreatment process hasbeen developed to satisfy the actual production conditions in our country, simplify thetechnological process, shorten the production cycle and product H13module withhigh quality and low cost. The performance of pretreatment module can reach thestandard North America Die Casting Association (NADCA). The main researchresults are as follows:The original microstructure has obvious influence on the spheroidizingmicrostructure. According to the CCT diagram of H13, three cooling speed, namely0.03℃/s,0.3℃/s and10℃/s, are selected to obtain pearlite, bainite and martensitequenching microstructure, respectively. When the initial microstructure is pearlite, the carbide after spheroidizing annealing is coarser with the lower hardness. It is worthnote that, when the initial microstructure is martensite, the spheroidizing is faster, thesize and distribution of carbide are very uniform and the hardness is higher. When theinitial microstructure is bainite, the spheroidizing effect is in between. The finalresults show that when the original organization is martensite, the spheroidizing effectis the best.When the initial microstructure is martensite, solid solution temperature hasobvious influence on the microstructure after spheroidizing annealing. By comparingto the microstructure after the same long time heat preservation following differenttemperature of solid solution found: after1060℃quenching, austenitic grain size、remaining carbon number and size are all moderate, meanwhile the microstructureand hardness after long time heat preservation can reach the standardNADCA#207-2006of high-quality H13steel, and strong toughness ratio is higherafter quenching and tempering.Through the comparative analysis of the microstructure and performance afterquenching and tempering of the H13treated by different pretreatment process,considering the factors such as economic, efficient and practical at the same time,show that1060℃×30min+840℃×12h is the most reasonable pretreatment technologyof H13steel, whose spheroidizing microstructure and impact toughness are all betterthan the pretreatment process of1100℃solution+two stages isothermalspheroidizing which is widely used at present.Combining the SEM images with Image Pro Plus6.0software, a new method ofcarbide statistics of the H13steel treated by spheroidizing annealing is proposed,which can provide a more accurate and scientific analysis method for thespheroidizing microstructure of hot die steel. Contrasting the carbide statistical resultsof imported H13steel, SWPH13steel and the domestic H13steel treated by economicand efficient pretreatment process, it showed that: the parameters of the domestic H13steel treated by economic and efficient pretreatment process such as the number of carbide particles, long axis, short axis, the ratio of the long axis and short axis,average diameter and the area are all between imported H13steel and SWPH13steel,the ratio of the long axis and short axis can reflect the degree of roundness of thespheroidal carbide. That is to say, the number of carbide particles, distribution and thedegree of roundness of the domestic H13steel treated by economic and efficientpretreatment process are all close to the imported H13steel.