austempering of martensitic stainless steels and the

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 HÖRTNAGL1Institute for Materials System Technology Thurgau, Tägerwilen, Switzerland, m.sorg@witg.ch

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 ↓

Corrosionresistance ↓

Cooling and holding above MS

→ Bainite ?

Ductility ↑ Hardness ↓Strength ↓

Corrosionresistance ↔ ?

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) HardenedCooling 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