MICROSTRUCTURES AND MECHANICAL PROPERTIES OF SPRAY FORMED H13 TOOL STEEL

Abstract

H13 steel is well-established for hot-work applications in different parts of the world, combining good hot strength, plasticity, toughness, oxidation resistance and thermal fatigue resistance due to its reasonable design of alloying elements. However, the alloying elements segregation and coarse primary carbides are serious and unavoidable in traditional cast H13 steel, which develops into band structure after the subsequent hot-working process, causing a poor isotropy, and finally reduces the service life of tool. Spray forming, as one of the rapid solidification technologies, fills the gap between casting and powder metallurgy, combining advantages of rapid solidification, and gaining a fine grained microstructure without macro-segregations. H13 steel was prepared by traditional casting and spray forming respectively, subsequently forged and conventional heat treated. The microstructures of as-cast and spray-formed H13 steels at various stages of processing were studied by OM, SEM and XRD. Mechanical properties were tested at ambient and elevated temperatures for both steels under the same heat treatment processes. The microstructure of as-cast H13 steel is characterized by coarse dendritic structure and primary carbides,while spray forming resulted in a much refined equiaxed grain structure without segregation of carbides. Meanwhile, the as-deposited H13 steel contains higher volume of austenite than that of the as-cast H13 steel, induced by the high cooling rate during the atomization process in spray forming. The density of the spray-formed H13 billet before and after hot-forging process are 98.2% and 99.7% respectively, indicating pores are completely eliminated.Compared to the as-cast H13, the spray-formed H13 has better temper resistance, higher room temperature tensile strength and hot strength. Moreover, the spray-formed H13 increases the room temperature impact toughness two times than that of the as-cast H13. The band structure which is obvious in the cast H13 steel can hardly be seen in the spray-formed H13 steel, thereby better isotropy can be expected in the spray-formed H13 steel. The improvement of mechanical properties of the spray-formed H13 can be attributed, to a large extent, to the refined structures without coarse primary carbides and macro-segregation, leading more alloying elements dissolved into the matrix,distributed more uniformly. Consequently, the secondary precipitates during tempering are more uniform and the grains are finer.