Heat Treatment of Thixo-Formed Hypereutectic X210CrW12 Tool Steel
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SEMISOLID metal (SSM) forming was developed at MIT in the early 1970s.[1] SSM technique is based on a thixotropic flow of semisolid slurry.[2] The system is described as thixotropic, when a reduction in magnitude of its rheological properties, such as elastic modulus, yield stress, and viscosity occur reversibly and isothermally with a distinct time dependence on shear strain applied.[3,4] A metal alloy can behave in a thixotropic way if it has a wide temperature range of solidus– liquidus and a globular microstructure within this range. ŁUKASZ ROGAL, Ph.D. Student, and JAN DUTKIEWICZ, Professor, are with the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, 30-059 Krakow, Poland. Contact e-mail: [email protected] Manuscript submitted February 11, 2012. Article published online September 29, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
The applied amount of liquid phase is usually from 20 pct to 60 pct of volume and depends on the type of process.[4–6] The globular microstructure, which appears during the semisolid process, depends on the process type: the thixo-process using reheating of a previously produced billet and the rheo-process, which starts directly from the melt. Semisolid metal forming is an alternative method of forming metal alloys for casting and forging.[6] Nowadays, rheoforming and thixoforming of Al-based, Zn-based, and Mg-based alloys have commonly been applied in the industry.[7–9] In the case of high-melting alloys, such as steel and titanium, a lot of restrictions referring to a die material, strong oxidation of feedstock, and temperature control[10] should be considered. In spite of these difficulties, a prototype device for a serial production of steel thixo-casts was built at the Institute of Metallurgy and Material Science, PAS in Krakow (described in Reference 11). A high cost VOLUME 43A, DECEMBER 2012—5009
of feedstock manufacture is also an obstacle to its widespread application. As a result, steel thixoforming can be applied only in the case when forging and casting cannot be used.[12] Several steels, e.g., D2, M2, HP/9/4/ 30, 100Cr6, C45, C38, 34CrNiMo4, 16MnCr5, and 304 stainless steel, predicted for the SSM applications have been investigated so far.[13–19] The hypoeutectic chromium X210CrW12 steel is very interesting from the point of view of SSM forming as it reveals a wide range of solidus-liquidus, low sensitivity to temperature changes in the semisolid state, and a relatively low process temperature.[20–22] The globular microstructure is produced by the strain-induced and melt-activated technique and recrystallization partial melting for the thixo-process, while the slope induced flow is applied for the rheo-process.[20–23] The presence of a high amount of carbides is responsible for a decrease of the grain growth during heating to a semisolid state.[23,24] The microstructure after semisolid processing consists of austenitic globular grains (average size 46 lm) surrounded by the eutectic mixture.[25] The X210CrW12 thixo-cast direct after
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