Influence of Microstructure and Process Conditions on Simultaneous Low-Temperature Surface Hardening and Bulk Precipitat
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LOW-TEMPERATURE thermochemical surface engineering of austenitic stainless steels by nitriding and/or carburizing has been widely studied in the past 30 years, because it provides the possibility to improve wear and fatigue performance without impairing the corrosion performance.[1–3] Through the years, plasma-based technologies have been applied for low-temperature nitriding processes and in the last 15 years gaseous processes for nitriding and carburizing of stainless steel were developed, matured, and commercialized. Gaseous processes provide a substantial advantage over plasma and implantation processes, in particular with respect to process control, materials handling, and geometrical constraints.[4] So far, mainly low-temperature nitriding and carburizing of austenitic stainless steels have been studied, while thermochemical treatment of ferritic, duplex, martensitic, and precipitation hardening stainless steels has not received the same attention. However, a growing interest is recognized in low-temperature surface engineering processes for martensitic and precipitation FEDERICO BOTTOLI, Ph.D. Student, GRETHE WINTHER, Associate Professor, THOMAS L. CHRISTIANSEN, Senior Researcher, and MARCEL A.J. SOMERS, Section Head, Professor, Dr.Ir., are with the Department of Mechanical Engineering, Technical University of Denmark, Produktionstorvet b.425, 2800 Kgs, Lyngby, Denmark. Contact e-mail: [email protected] Manuscript submitted February 24, 2015. Article published online August 18, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
hardening stainless steels, which have favorable bulk properties as high strength in combination with good corrosion resistance.[5,6] In this respect, it has been demonstrated that precipitation hardening steels can be simultaneously surface and bulk hardened in a single treatment.[7–11] Among the precipitation hardening stainless steels, Nanoflex* has been widely investigated and it *Nanoflex is a registered trademark of Sandvik Materials Technology.
is applied in several industrial applications because of its interesting mechanical properties.[12–15] Nanoflex’s Ms temperature, defined as the temperature at which martensite forms instantaneously during cooling, was reported to be 83 K ( 109 C).[16] However, since Nanoflex is a metastable precipitation hardening steel, martensite can form isothermally at a temperature appreciably higher than Ms. Depending on the (tailorable) stability of the alloy, the formation of isothermal martensite is observed even at room temperature.[16–20] In general, the stability of Nanoflex at room temperature is a function of various processing parameters, including the annealing conditions, temperature, and the applied mechanical load.[20–22] It is well documented by transmission electron microscopy that the precipitation hardening effect in Nanoflex derives from the precipitation of Cu-clusters, VOLUME 46A, NOVEMBER 2015—5201
as well as the ordered intermetallic phases g-Ni3(Ti,Al) and Ni3Mo and Fe2Mo (Laves phase) from the martensite matrix.[23–28] Th
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