Project Idea • Application of austempering to martensitic stainless steels should bring some advantages compared to standard heat treatments. • Comparison of corrosion behavior of hardened, tempered and austempered samples. • Application could be interesting, for example, for knives, blades, cutter, … Austempering of Martensitic Stainless Steels and the Influence to Corrosion Resistance Matthias SORG , Paul GÜMPEL, Arnulf HÖRTNAGL 1 Institute for Materials System Technology Thurgau, Tägerwilen, Switzerland, [email protected] Introduction • Martensitic stainless steels has a wide use, for example for blades, knifes or cutter and also for valves or shafts, etc. • Corrosion resistance is an important property of this kind of steels. • The best corrosion resistance of these materials is in hardened condition, because all important alloying elements are in solution and the microstructure shows the most homogeneous condition [1]. • Hardened condition results in a brittle mechanical behavior, so a tempering is normally used to increase the ductility. • Tempering precipitate fine carbides, which reduces the corrosion resistance. • Austempering process can produce excellent mechanical properties like strength, ductility and better capacity to dynamical stress [2]. • The resulting microstructure after austempering process should also have a better corrosion resistance like a tempered microstructure, because it is more homogeneous [3]. Experimental Results • Hardness measurements: • Metallographic investigation: • Potentiodynamic scans – 0,1 mol/l NaCl, 20°C • Potentiodynamic scans – 0,1% NaCl, 20°C Literature [1] P. Gümpel et al., Rostfreie Stähle, expert Verlag, 1996 [2] H.-J. Bargel, G. Schulze, Werkstoffkunde, Springer-Verlag, 2008 [3] H. K. D. H. Bhadeshia, BAINITE IN STEELS - Theory and Practice, Maney Publishing, 2015 Conclusions & Future Work • First experimental work shows a reduction of hardness after the austempering process. → The ductility should still be tested by means of notched bar impact bending test. • The metallographic investigation shows a slight reduction of the needle-shaped microstructure. → The bainitic structure should be confirmed by other methods (XRD, EBSD,…). • The corrosion measurements show a reduction of the CPP for the quenched & tempered specimens. The CPP of the austempered specimens is similar to the CPP of the hardened specimens. → Microscopic investigation of the pitting attack; Immersion corrosion tests. • Detailed investigation of austempering requirements for martensitic stainless steel. Experimental • Two similar martensitic stainless steels: 1.4110 (X55CrMo14) & 1.4116 (X50CrMoV15) • Three kinds of heat treatment: • Hardness measurements according to Vickers HV1. • Metallographic investigation of microstructure. • Potentiodynamic corrosion scans for determination of critical pitting potential (CPP): • 0,01 mol/l NaCl, 20°C • 0,01 % NaCl, 20°C Austenitizing Fast cooling and tempering → Martensite Ductility ↑ Hardness ↓ Strength ↓ Corrosion resistance ↓ Cooling and holding above M S → Bainite ? Ductility ↑ Hardness ↓ Strength ↓ Corrosion resistance ↔ ? C Si Mn P S Cr Mo V 1.4110 0,583 0,300 0,342 0,030 0,001 14,250 0,640 0,054 1.4116 0,490 0,600 0,410 0,034 0,002 14,400 0,560 0,110 Austenitizing (1050°C) Hardened Cooling in water Austenitizing (1050°C) Tempering (300°C) Cooling in water Cooling in air Quenched and tempered Austenitizing (1050°C) Holding (320°C) Cooling in salt Cooling in air Austempered Hardened Austempered Quenched & Tempered