A Study of Carbide Precipitation in a H21 Tool Steel

Abstract

Carbide precipitation in a H21 tool steel during conventional heat treatment was studied. The aim of this work was to study the exact microstructure and for better understanding of carbide formation during double tempering process of the H21 tool steel. The steel was austenised either at 1 100°C or 1 250°C for 1 hour, and water quenched. Double tempering was performed at 650, 750 and 800°C for 1 hour with air cooling in the first and second temper for each austenising temperature. The results showed that the double tempered microstructure consisted of tempered martensite, lower bainite and carbides. The current study confirmed previous findings and contributed to existing knowledge that depending on the tempering temperature, the types of carbide formed during double tempering were M2C, Fe3C, M6C and M23C6 carbides. The present study findings add substantially to our understanding of the carbide formation sequence in the H21 tool steel during double tempering. No secondary peak hardening was observed, and the highest hardness (505 HV) was obtained after austenising at 1 250°C and double tempering at 650°C, which implies that the double tempering of the H21 tool steel should be carried out below 650°C.

Microstructural evolution of a heat-treated H23 tool steel

“…The chromium content in the M 7 C 3 carbides was lower at an austenizing temperature of 1250°C than at an austenizing temperature of 1100°C when the same aging temperature was used, which indicated that the M 7 C 3 carbides in the sample austenized at the higher temperature were not stable at an aging temperature of 650°C. This finding is consistent with the results of a previous study [6] in which the authors reported that a higher austenizing temperature accelerated the carbides' approach to equilibrium. The evolution of the chemical composition of the M 6 C carbides was different than that of the M 7 C 3 carbides.…”

“…(5) The approach of carbides to their equilibrium composition during double-aging was strongly affected by the austenizing temperature. (6) The maximum hardness of the double-aged H23 tool steel (448 HV) was achieved via austenization at 1250°C followed by double-aging at 650°C.…”

“…The volume fraction of secondary carbides was higher in the samples austenized at the higher temperature than in the samples austenized at the lower temperature when the same double-aging temperature was used. This greater volume fraction was due to the increased dissolution of carbides at the higher austenizing temperature causing more carbon to be dissolved in the matrix, which provided the driving force for carbide precipitation during the double-aging process [6]. The presence of retained austenite after water quenching also affected the secondary carbide precipitation process during aging.…”

“…The highest hardness (448 HV) was achieved in the case of the sample austenized at 1250°C and double aged at 650°C. This result is attributable to the carbides dissolving more extensively into the matrix with increasing austenizing temperature, thereby providing a high nucleation rate of carbide precipitation during the double-aging process [6]. Double- aging at temperatures of 650°C and 800°C did not reveal a secondary hardening peak, which has been correlated to fine M 2 C carbides [1].…”

Effect of Electroslag Remelting Parameters on Primary Carbides in Stainless Steel 8Cr13MoV

“…So far, reports about carbides mainly focused on the heat treatment stage. [7][8][9] However, during heat treatment stage, it was dif cult to eliminate defects caused by primary carbides. 10) Primary carbides can also precipitate during ESR process.…”