Effects of Isothermal Aging Process on Microstructure and Mechanical Properties of Stellite 6 Coatings by Plasma Arc Cla
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UCTION
STELLITE alloys are typical cobalt-based superalloys, which are widely used as hardfacings in high-temperature key parts, such as burners, turbines, and hot-working dies, owing to their excellent properties at service temperature.[1–5] The main elements of Stellite alloys are cobalt (Co); chromium (Cr); tungsten (W); other replacement elements, which form the substitutional solid solution; and carbon (C), which constitutes the carbides and interstitial solid solution. Typically, the stable phases of the as-cast Stellite alloys at room temperature (RT) are c-Co
DEZHI YANG, QINGZHAO WANG, XIAO WEI, CHEN HUA, JIJIN XU, JUNMEI CHEN, CHUN YU, and HAO LU are with the Key Lab of Shanghai Laser Manufacturing and Materials Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mails: [email protected]; [email protected] SHENGZHI QU is with Shanghai Boiler Works, Ltd., Shanghai 200245, P.R. China. Manuscript submitted December 5, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
(face-centered-cubic structure) matrix and primary carbides (mainly M7C3).[6–8] However, there will be an allotropic transformation, from c-Co to e-Co (hexagonal-close-packed crystal structure), under conditions such as rapid cooling, isothermal aging, and stress.[9,10] Since the e-Co possesses less slip systems compared to the c-Co, the dislocation movement in the e-Co is impeded; further, the plastic deformation is resisted. Therefore, the mechanical properties, mainly the hardness and wear resistance, will be enhanced due to transformation.[11] Solid solution strengthening and carbide strengthening are the main strengthening mechanisms in Co-based alloys. Therefore, many works have been performed to modify the microstructure and further improve the mechanical performances through the heat treatment process. The main heat treatment methods are solid solution and isothermal aging.[12] It was found that the isothermal aging treatment at 850 °C and 950 °C up to 24 hours promoted the c-Co to e-Co transformation (martensite transformation) and the carbide precipitation on the stacking faults of e-Co.[9,13] Gdoutos[14] found that the wear resistance of Stellite alloy was enhanced owing to the isothermal transformation from
c-Co to e-Co as aged at 600 °C for 116 hours. Ren et al.[12] indicated that the solid solution aging plus thermal aging (950 °C, 12 hours) treatment could greatly enhance the hardness and wear resistance of Stellite 12 alloys. Carbide precipitations also improved the mechanical or thermal plastic deformation at high temperature.[15] On other hand, the eutectic M7C3-type carbides had been reported to be easily decomposed to M23C6 and M6C/MC during the isothermal aging treatment above 650 °C for 4 hours, and transformation between different carbides was attributed to the different mechanical performance.[16,17] In addition, Desai et al.[18] indicated that the wear rate decreased with the increase of carbide size in Co-based alloys, because the relative
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