Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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(a) (b) (c) HTPRO 17 For the case of heavy surface loading, austenite load bearing capacity should be enhanced further. The simplest solution is to prolong the duration of the nitriding/ nitrocarburizing treatment. However, this enhances the risk for precipitation of chromium nitrides and associated loss of corrosion resistance. In such demanding applications, the low-temperature surface hardening treatment can be preceded by a high-temperature solution nitriding treatment[16], which dissolves a relatively low amount of nitrogen into austenite up to a depth of several millimeters. Cooling from the solution nitriding temperature should be done carefully to prevent development of chromium nitride (Cr2N or CrN) precipitation (see Fig.1). Alloy grades other than austenitic stainless steels can be treated. Most stainless steel types including austenitic, ferritic, duplex, martensitic, and precipitation-hardening (PH) grades can develop a surface case of nitrogen and/or carbon-expanded austenite by undergoing gaseous nitriding, carburizing, and nitrocarburizing treatments. Expanded austenite can also be formed in other types of (similar) alloy systems, such as many Ni-base alloys. For example, Ni- Summary and outlook Surface hardening of stainless steel can be achieved by low temperature nitriding, carburizing, and nitrocarburizing by transformation of the surface into nitrogen and/or carbon-expanded austenite. Gaseous processing provides a high degree of tailorability of the hard surface case enabling tailoring of materials properties, and therefore, performance. Most stainless steels and similar alloy systems can be surface hardened by means of gaseous processing. Today's stainless steel alloys treated using LTSH are designed for purposes other than surface hardening. New stainless steel alloys with compositions tailored for optimal LTSH will further expand the applicability of low-temperature surface hardening. HTPRO References 1. B.H. Kolster, VDI-Berichte, 506, p 107–113, 1983. 2. B.H. Kolster and A.J. Rogers, Corrosion and Mass Transfer, AIME, p 252–264, 1973. 3. F.B. Litton and A. E Morris, J. Less-Common Metals, 22, p 71–82, 1970. 4. Z.L. Zhang and T. Bell, Surf. Engrg., 1(2), p 131–136, 1985. 5. T. Christiansen and M.A.J. Somers, Scripta Materialia, 50, p 35–37, 2004. 6. J. Oddershede, et al., Scripta Materialia, 62, p 290–293, 2010. 7. J. Oddershede, et al., Steel Research Intl., 82(10), p 1248–1254, 2011. 8. M. Tahara, et al., European patents EP 0 588 458 B1 and EP 0 787 817 A2. 9. S. Collins and P. Williams, Adv. Matls. & Proc., 164, p 32–33, 2006. 10. S.V. Marx and P.C. Williams, European patent EP 1 095 170 B1. 11. M.A.J. Somers, T. Christiansen, and P. 10 1400 8 1200 Nitrogen content, wt% base superalloys such as the Nimonic series can be nitrided, but low temperatures (360–400°C) are required to suppress formation of unwanted CrN[17]. Also, martensitic and austentic PH steels can be nitrided and simultaneously bulk hardened[18, 19]. 1000 6 800 4 600 2 400 0 0 200 -2 Stress, MPa tages. For example, dissolved nitrogen has a positive effect on corrosion resistance (e.g., pitting). Carburizing produces an advantageous shallow case-core transition because the affinity of chromium for carbon is not as high as for nitrogen. By comparison, nitriding yields a relatively sharp case-core transition. A gradual transition in hardness/composition can be tailored by adopting gaseous nitrocarburizing or the two-stage process of carburizing followed by nitriding[15]. These processes produce a hardened case consisting of a hard zone of nitrogen-expanded austenite and a zone of carbon-expanded austenite underneath (Fig. 2c). Hardness HV10, (MPa) Fig. 2 — Cross sections of (a) AISI 316 after nitriding at 445°C for 22 h in a gas mixture containing 60% NH3 and 40% H2, (b) AISI 316 carburized in acetylene at 520°C for 3 h (the transition from core to hardened case is more diffuse than for nitriding), and (c) cold-worked AISI 304 nitrocarburized at 420°C for 19 h (the nitrocarburized case is subdivided in a zone of nitrogen-expanded austenite and a zone of carbon-expanded austenite below). -4 -6 -8103 0 5 10 15 Depth, m 20 25 Fig. 3 — Nitrogen content, hardness, and residual stress in AISI 316 after nitriding at 445°C for 22 h in a gas mixture containing 60% NH3 and 40% H2. Møller, European patent EP 1 521 861 B1. 12. M.A.J. Somers, T.L. Christiansen, European patent EP 1 910 584 A1, 2005. 13. T.L. Christiansen, T.S. Hummelshøj, and M.A.J. Somers, Surf. Engrg.,27, p 602–608, 2011. 14. T.L. Christiansen, T.S. Hummelshøj, and M.A.J. Somers, WO 2011 009463 A1. 15. T. Christiansen and M.A.J. Somers, Surf. Engrg., 27, p 445–455, 2005. 16. T.L. Christiansen, T.S. Hummelshøj, and M.A.J. Somers: PCT application PCT/ DK2012/050139, 2011. 17. K.M. Eliasen, T.L. Christiansen, and M.A.J. Somers, Surf. Engrg., 26, Vol 4, p 248–255, 2010. 18. R.B. Frandsen, T. Christiansen, and M.A.J. Somers, Surf. & Coat. Tech., 200 (16/17), p 5160–5169, 2006. 19. F.A.P. Fernandes, T. L. Christiansen, and M.A.J. Somers, Heat Treat & Surf. Engrg. Conf. & Expo, Chennai Trade Centre, Chennai, India, May 2013. For more information: Marcel A.J. Somers, Prof. and Section Head – DTU Mechanical Engineering, Technical University of Denmark, Produktionstorvet Bldg. 425, 2800 Kgs. Lyngby, Denmark;; ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 53

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