STD11 ( AISI D2, JIS SKD11) tool steel is one of the most popular cold work die steels widely being applied in many manufacturing fields such as automotive, electronic, home appliance and other industries especially for punching and forming operation, because it has high wear resistance, high compre...
STD11 ( AISI D2, JIS SKD11) tool steel is one of the most popular cold work die steels widely being applied in many manufacturing fields such as automotive, electronic, home appliance and other industries especially for punching and forming operation, because it has high wear resistance, high compressive strength and good through-hardening properties with moderate toughness. However, STD11 has been known that forged and flat-bar rolled STD11 tool steel shows anisotropic dimensional change during heat treatment. That is, the dimensional change in the rolling direction is larger than that in the transverse direction. The cause of the anisotropic dimensional change has been supposed to be that the steel is anisotropic in composition, microstructure and other properties. However, a conclusive explanation for the mechanism of anisotropic dimensional change has not been given yet. In order for the STD11 to be applied to better precision cold working dies, it is important to improve or minimize the anisotropic distortion property of STD11 flat bar products. The aim of this study was to investigate the effect of manufacturing and heat treatment conditions of STD11 flat bar on the anisotropic dimensional change during hardening heat. Firstly, the effect of manufacturing conditions, such as ingot weight and hot rolling reduction ratio, on the anisotropic dimensional change during heat treatment of STD11 has been studied. Dimensional changes during high pressure gas quenching and tempering were measured in three direction by a coordinate measuring machine for the rectangular shaped flat bar specimens with different ingot weight and hot rolling reduction history. Experimental results revealed that all the rolled flat bar products showed anisotropic distortion to some degree, but the anisotropic dimensional change was reduced quadratically as hot rolling reduction ratio increased in the range of 40 ~ 80 % . Ingot weight was found to have a little effect on the anisotropic dimensional change. Microstructural observation showed that the anisotropic dimensional change of STD11 was closely related to the amount and distribution of the coarse eutectic carbides remaining after hot forging and rolling. Texture analysis by EBSD showed that both as-spheroidized ferrite matrix before heat treatment and quenched martensitic matrix after heat treatment were found to be nearly randomly oriented. On the other hand, the morphological texture observed for the carbides by optical and SEM inspection is accompanied by a significant preferential orientation of the (0001) basal planes of the hexagonal crystal lattice of M7C3 perpendicular to the rolling direction. This mechanical alignment of M7C3 carbides in longitudinal direction is thought to cause anisotropic dimensional change. Effect of as-cast microstructure on the anisotropic dimensional change was investigated by comparing two simultaneously casted ingots. One ingot was used for as-cast microstructure analysis and the other one was used for manufacturing flat bars as usual and sampled for measuring dimensional change at 5 equal interval positions on full length of the flat bar, where the top and bottom had been monitored continuously during forging and rolling in order for the position not to be changed. The anisotropic dimensional change is larger as the sampling position is close to the top of ingot in 6 ton ingot, and it is well correlated to the wall thickness of primary eutectic carbide networks. Secondly, the continuous dimensional change and the anisotropic behavior of STD11 during austenitizing and tempering were investigated by quenching dilatometry. It is found that anisotropic dimensional change in STD11 occurs not during heating but mostly during cooling, and it increases with increasing austenitizing temperature for hardening. The distribution of elongated primary carbides and their dissolution behavior with austenitizing temperature were considered to explain schematically the anisotropic dimensional change of SKD11 tool steel during heat treatment. Thirdly, carbide precipitation and dissolution behavior in the range of temperatures used for flat-bar manufacturing or hardening heat treatment have been studied by using confocal scanning laser microscopy, dilatometry, and X-ray diffraction analysis. The equilibrium phase diagram and equilibrium phase fractions with temperature were calculated using a FactSage program. Confocal laser microscopic observation revealed that α to γ transformation temperature of STD11 is near 800 °C, primary eutectic M7C3 carbides melt at 1245 °C, and the melting temperature of STD11 is near 1370 °C. XRD results indicated that the M23C6 carbides dissolve in the matrix if austenitized at over 1030 °C, while the M7C3 carbides remain up to 1200 °C although their amount decreases. The calculated equilibrium phase diagram showed good agreement with experimental results on carbide dissolution and phase transformation temperatures.
STD11 ( AISI D2, JIS SKD11) tool steel is one of the most popular cold work die steels widely being applied in many manufacturing fields such as automotive, electronic, home appliance and other industries especially for punching and forming operation, because it has high wear resistance, high compressive strength and good through-hardening properties with moderate toughness. However, STD11 has been known that forged and flat-bar rolled STD11 tool steel shows anisotropic dimensional change during heat treatment. That is, the dimensional change in the rolling direction is larger than that in the transverse direction. The cause of the anisotropic dimensional change has been supposed to be that the steel is anisotropic in composition, microstructure and other properties. However, a conclusive explanation for the mechanism of anisotropic dimensional change has not been given yet. In order for the STD11 to be applied to better precision cold working dies, it is important to improve or minimize the anisotropic distortion property of STD11 flat bar products. The aim of this study was to investigate the effect of manufacturing and heat treatment conditions of STD11 flat bar on the anisotropic dimensional change during hardening heat. Firstly, the effect of manufacturing conditions, such as ingot weight and hot rolling reduction ratio, on the anisotropic dimensional change during heat treatment of STD11 has been studied. Dimensional changes during high pressure gas quenching and tempering were measured in three direction by a coordinate measuring machine for the rectangular shaped flat bar specimens with different ingot weight and hot rolling reduction history. Experimental results revealed that all the rolled flat bar products showed anisotropic distortion to some degree, but the anisotropic dimensional change was reduced quadratically as hot rolling reduction ratio increased in the range of 40 ~ 80 % . Ingot weight was found to have a little effect on the anisotropic dimensional change. Microstructural observation showed that the anisotropic dimensional change of STD11 was closely related to the amount and distribution of the coarse eutectic carbides remaining after hot forging and rolling. Texture analysis by EBSD showed that both as-spheroidized ferrite matrix before heat treatment and quenched martensitic matrix after heat treatment were found to be nearly randomly oriented. On the other hand, the morphological texture observed for the carbides by optical and SEM inspection is accompanied by a significant preferential orientation of the (0001) basal planes of the hexagonal crystal lattice of M7C3 perpendicular to the rolling direction. This mechanical alignment of M7C3 carbides in longitudinal direction is thought to cause anisotropic dimensional change. Effect of as-cast microstructure on the anisotropic dimensional change was investigated by comparing two simultaneously casted ingots. One ingot was used for as-cast microstructure analysis and the other one was used for manufacturing flat bars as usual and sampled for measuring dimensional change at 5 equal interval positions on full length of the flat bar, where the top and bottom had been monitored continuously during forging and rolling in order for the position not to be changed. The anisotropic dimensional change is larger as the sampling position is close to the top of ingot in 6 ton ingot, and it is well correlated to the wall thickness of primary eutectic carbide networks. Secondly, the continuous dimensional change and the anisotropic behavior of STD11 during austenitizing and tempering were investigated by quenching dilatometry. It is found that anisotropic dimensional change in STD11 occurs not during heating but mostly during cooling, and it increases with increasing austenitizing temperature for hardening. The distribution of elongated primary carbides and their dissolution behavior with austenitizing temperature were considered to explain schematically the anisotropic dimensional change of SKD11 tool steel during heat treatment. Thirdly, carbide precipitation and dissolution behavior in the range of temperatures used for flat-bar manufacturing or hardening heat treatment have been studied by using confocal scanning laser microscopy, dilatometry, and X-ray diffraction analysis. The equilibrium phase diagram and equilibrium phase fractions with temperature were calculated using a FactSage program. Confocal laser microscopic observation revealed that α to γ transformation temperature of STD11 is near 800 °C, primary eutectic M7C3 carbides melt at 1245 °C, and the melting temperature of STD11 is near 1370 °C. XRD results indicated that the M23C6 carbides dissolve in the matrix if austenitized at over 1030 °C, while the M7C3 carbides remain up to 1200 °C although their amount decreases. The calculated equilibrium phase diagram showed good agreement with experimental results on carbide dissolution and phase transformation temperatures.
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#tool steel STD11 dimensional change anisotropy distortion FactSage
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