Low-Temperature Nitriding of Deformed Austenitic Stainless Steels with Various Nitrogen Contents Obtained by Prior High-
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INTRODUCTION
THE increasing demand for high-strength and corrosion-resistant alloys has resulted in the advent of high nitrogen steels (HNS) as a potential substitute for conventional stainless steels in various engineering applications.[1] Interstitial alloying with nitrogen offers unique advantages over dissolving interstitial elements as boron and carbon, because of the high solid solubility of nitrogen in austenite combined with an effective solid-solution strengthening effect.[2–4] Nitrogen provides a remarkable increase in yield and tensile strength, without sacrificing the toughness.[4–7] Furthermore N dissolved in steel is a strong austenite stabilizer, which means that lower amounts of expensive austenite formers such as nickel are required and the formation of ferrite and/or martensite upon solidification or deformation can be prevented.[8] In addition to a considerable improvement of the mechanical properties, the interstitial dissolution of nitrogen in stainless steel can lead to a significant improvement of the resistance against localized corrosion as crevice and pitting corrosion as it enhances the PREN and MARC numbers importantly.[9–12] Due to the FEDERICO BOTTOLI, Former Ph.D. Student, GRETHE WINTHER, Associate Professor, THOMAS L. CHRISTIANSEN and KRISTIAN VINTER DAHL, Senior Researchers, and MARCEL A.J. SOMERS, Section Head and Professor, are with the Department of Mechanical Engineering, Technical University of Denmark, Produktionstorvet b.425, 2800 Kongens Lyngby, Denmark. Contact e-mail: [email protected] Manuscript submitted November 26, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
favorable combination of improved mechanical properties and corrosion performance, in the past decades HNS have become an important class of engineering materials. However, despite several advantages, the major obstacle to a wider application of this class of stainless steels is related to their production. The solubility of nitrogen in liquid Fe is only 0.045 wt pct at 1873 K (1600 C).[13,14] Consequently, the production of HNS requires high-pressure melting technologies or the utilization of powder metallurgy techniques.[9,10] In contrast to the liquid state, the solubility of nitrogen is higher in the solid state.[2] Hence, rather than dissolving nitrogen in the liquid state or the synthesis of high nitrogen steel powder, gas treatment of the steel component, such as solution nitriding, followed by rapid (gas) quenching to prevent nitride formation,[15,16] can be applied to dissolve nitrogen in the matrix. This process can be applied to austenitic stainless steel grades to enhance the austenite stability or to duplex and ferritic stainless steel in order to stabilize the austenitic phase at high temperature. The excellent bulk properties that can be obtained by solution strengthening can be coupled with outstanding surface properties if the material is subsequently subjected to a low-temperature thermochemical treatment.[17–19] Low-temperature nitriding for instance allows the dissolution of a high amount of n
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