Thermal Fatigue Testing of Plasma Transfer Arc Stellite Coatings on Hot Work Tool Steels under Steel Thixoforming Condit
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INTRODUCTION
FORMING in the semisolid state today is a wellestablished near-net-shape manufacturing route for magnesium and aluminum alloys.[1,2] This innovative process enjoys an abundance of tooling materials that perform even better than in conventional casting owing to relatively lower feedstock temperatures. However, these tool materials fail to sustain the severe conditions encountered when thixoforming higher melting point alloys such as steels.[3–7] Thermal fatigue, combined with the erosive wear due to abrasion and impact of the already solid particles in slurries and high-temperature oxidation, renders the conventional hot work tool steels entirely inadequate.[7–11] While a number of suitable replacements were identified in recent years,[10,12–24] cost considerations favor coating hot work tool steels over employing high-temperature alloys for tooling applications. Such coatings must offer thermal and chemical stability as well as adequate resistance to thermal fatigue, oxidation, and abrasion by the slurries. Potential of PVD coatings, employed to reduce friction and wear of tools for cutting, hot- and cold-working, and die-casting processes was investigated.[25–38] Co-based coatings, employed to reduce wear of cold-working and die-casting tools, also deserve attention.[39–45] Co-based alloys retain their hardness and offer excellent thermal YUCEL BIROL, Senior Scientist and AGCA B. KAYIHAN, Researcher, are with the Materials Institute, Marmara Research Center, TUBITAK, Kocaeli, Turkey. Contact e-mail: yucel.birol@ mam.gov.tr Manuscript submitted March 11, 2010. Article published online December 9, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
fatigue, wear, and oxidation resistance at elevated temperatures and are frequently used in applications where high-temperature wear and chemical attacks occur.[46–49] Their performance as coatings on casting and forging dies was exceptional. Among several advanced deposition techniques employed in cladding wear-resistant layers on tool materials, the plasma transfer arc (PTA) process stands out owing to its higher deposition rate and lower heat input.[49] The PTA process is capable of producing sound metallurgical bonds between the coating and the substrate material with very low dilution and distortion at competitive cost.[50] Thus, it was applied extensively in the deposition of coatings for wear and hightemperature applications.[46,51–53] The present work was undertaken to investigate the performance of Stellite 12 coating deposited on X32CrMoV33 hot work tool steel via the PTA process under steel thixoforming conditions. II.
EXPERIMENTAL PROCEDURE
Stellite 12 alloy powder from Deloro Stellite Inc. (Swindon, Wiltshire, UK) was deposited by the PTA process on 30-mm-thick X32CrMoV33 steel plates using a Castolin Eutronic GAP400 PTA unit (Castolin, Istanbul, Turkey). The chemical compositions of the 3-mm-thick coating thus obtained and the tool steel substrate are listed in Table I. The coated X32CrMoV33 samples were subsequently austenized at 1025 °C for 30 m
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