The heat treatment of austenitic stainless steel wire with different work hardening degree

Transcription

1 The heat treatment of austenitic stainless steel wire with different work hardening degree R. Gerosa a, B. Rivolta a, G. Poli b, M. Valsecchi c a Politecnico di Milano Milano Italy b Università degli studi di Modena e Reggio Emilia Modena Italy c Rodacciai Spa Bosisio Parini (LC) - Italy Berlin 2013, September, 20 th

2 Introduction Some theoretical notes 2 Austenitic stainless steels (SS) are charaterized by the presence of deformation induced or strain induced martensite, i.e. martensite induced by cold working processes such as drawing Two types of martensites can form in austenitic SS. These are the body centered cubic, α martensite and the hexagonal close packed, ε, martensite. The α martensite is ferromagnetic, hence the paramagnetic austenitic SS become ferromagnetic after deformation

3 Introduction Some theoretical notes steel (AISI 302), as drawn % martensite % deformation the martensite amount depends on the type of deformation (rolling, drawing, ) the stability of austenite (chemical composition of the steel) temperature strain rate stress state grain size.

4 Introduction Some theoretical notes 4 Results of XRD analysis showing the effects of strain on the formation of α - martensite. (a) ε=0, (AISI 304), (b) ε=0,4, (AISI 304), (c) ε=0.8, (AISI 304) ; (d) ε=0, AISI 304/Cu; (e) ε=0,4, AISI 304/Cu, (f) ε=0.8, AISI 304/Cu [J.Y.Choi, W.Jin, Strain induced martensite formation and its effect on strain hardening behavior in the cold drawn 304 austenitic stainless steels, Scripta Materialia, no.1, Vol 36, , 1997]

5 Introduction Some theoretical notes 5 It is known that strain aging at the temperature of C results in a further increase of strength in cold worked metastable austenitic stainless steels Proposed mechanisms are the formation of additional α -martensite after aging due to a local increase of the Ms temperature caused by a depletion of chromium and carbon in the surrounding matrix due to the precipitation of fine carbides [P.L. Mangonon and G.Thomas Jr.: Metall. Trans. 1 (1970); C.K. Mukhopadhyay, T.Jayakumar, K.V. Kasiviswanathan and B. Raj: J. Mater. Sci. 30 (1995) ] hardening of α -martensite by diffusion of interstitial solute atoms and their interaction with dislocations [R. W. Rathbun, D. K. Matlock and J. G. Speer: Scr. Mater. 42 (2000); S. Okamoto, D. K. Matlock and G. Krauss: Scr. Metall. 25 (1991)]

6 Introduction 6 Aim of the work: to evaluate the influence of the heat treatment (strain aging) on the mechanical properties of austenitic SS as drawn wires with different drawing ratios different chemical compositions

7 Materials 7 Steel %C %N %Si %Mn %Cr %Ni %Mo Md30 [ C] %C %N %Si %Mn %Cr %Ni %Mo Md30 [ C] M d30 [ C] = (C+N) - 9.2(Si) - 8.1(Mn) (Cr) - 9.5(Ni) (Mo) Angel formula M d30 is the temperature at which 50% of α martensite is produced 30% true deformation under tensile condition after

8 Materials Ø [mm] % R (vs D0) % R (vs Dn-1) Ø [mm] % R (vs D0) % R (vs Dn-1)

9 Methods 9 tensile tests and X-rays analysis were performed on as drawn wires with different drawing ratios a model was developed for estimating the martensite amount vs the drawing ratio heat treatments were performed on as drawn wires with the following parameters: Time [h] 0,5 2,0 4,0 Temperature [ C] Cooling medium Air Temperature and time in the range of UNI EN standard Tensile tests on the wires after annealing

10 Results Tensile tests as drawn 10

11 Results Tensile tests as drawn 11

12 Results Estimation of martensite amount on as-drawn wires 12 Development of a phenomenological model allowing the estimation of martensite amount, starting from the knowledge of the mechanical properties. It is based on a mechanical approach, with the these main hypothesis: Ys = X γ Ys γ + X m Ys m Ys = yield strength X = volume fraction γ = austenite m = martensite X γ = 1 - X m

13 Results Estimation of martensite amount on as-drawn wires 13 Martensite (%) R%

14 Results Mechanical properties after strain ageing 14

15 Results Mechanical properties after strain ageing 15

16 Results 16 Influence of the reduction ratio and heat treatment on Ys

17 Concluding Remarks 17 the effect of heat treatment was studied on the mechanical properties of austenitic stainless steel as drawn wires the heat treatment acts as a strain ageing in increasing the tensile properties of the investigated steels, especially if increasing the work hardening degree the obtained data allow to study the kinetics of the ageing so obtaining an optimization of the heat treatment parameters, useful for the industrial optimization of the whole process

18 Contacts 18 Contacts: Prof. Barbara Rivolta Politecnico di Milano Dipartimento di Meccanica Via La Masa, MILANO ITALY Tel