Micrometer-scale synchrotron diffraction mapping analysis of carbide precipitation in deep cryogenically treated low car
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Carbide precipitation within low carbon AISI H13 hot work tool steel that had either been quenched at 80 °C or cryogenically treated at 196 °C prior to tempering has been examined using micrometer-scale synchrotron diffraction mapping. Vanadium-rich cubic M8C7 carbide, lattice parameter of approximately 0.8610 nm, not identifiable using laboratory powder x-ray diffraction (PXRD), was found to be present in all samples. The concentration of this phase was greatest in the rapidly cooled cryogenically treated sample. However, all cryogenic treatments resulted in greater carbide concentrations than in the quenched sample. In addition rapid cryogenic cooling by immersion in liquid nitrogen (N2), as compared with slow cooling to 196 °C over a 3 h duration, results in greater order within the carbide phase subsequent to tempering, as interpreted by analysis of unit cell size variation, and the smallest stress, as interpreted by diffraction peak full width half maximum height distributions.
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.135
Stratton and Graf13 investigated the effect of deep cryogenic treatment on wear resistance and tool life of AISI M2 tool steel, AISI H13 hot work tool steel, M2 HSS drill, AISI D6 tool steel, case carburized steel En353, AISI D2 tool steel and carburizing steel 20MnCr5 respectively. They all observed that a finer and more homogeneous carbide precipitate was found after tempering of the deep cryogenically treated steels and concluded that this is responsible for the observed improvement in wear resistance. AISI H13 is a chromium hot work tool steel which is designed to withstand the temperature, pressure, abrasion, and thermal cycling associated with various hot working operations, including plastic injection moulding, die casting, forging, and extrusion.14 AISI H13 has a small carbon content (0.4 wt.%) to promote toughness, medium chromium content (5 wt.%) to provide good resistance to high temperature softening, 1 wt.% Si to improve high temperature oxidation resistance, and molybdenum and vanadium additions (about 1 wt.%), that form stable carbides, to increase resistance to erosive wear.14–16 Using synchrotron microdiffraction analysis we have found, that on tempering, the martensite phase became crystallographically more homogeneous, as evaluated by variation on the micrometer spatial scale of the unit cell parameter and diffraction peaks full width half maximum height values, compared with the same samples prior to tempering.17 For cryogenically treated samples, the effect was most pronounced for the rapidly cooled sample, which was the least homogeneous sample prior to tempering but was the most homogeneous sample after tempering. This suggests that the considerable degree of disorder resulting from rapid cryogenic cooling results in
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Ó Materials Research Society 2012
I. INTRODUCTION
Cryogenic treatment has been used for several decades to improve the mechanical properties of tool steels. Collins1 describes two categor
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