Technical Paper: PIM International, Vol.4 No.2 June 2010, pages 66-70, 2193 words

Authors: P. V. Muterlle[1] , M. Perina[2] , A. Molinari[1]

[1] Dipartimento di Ingegneria dei Materiali e Tecnologia Industriali, Università di Trento, Mesiano 77, 38100, Trento, Italy.
[2] MIMEST, Viale Dante 300, 38057, Pergine, Valsugana, Trento, Italy.

Abstract

A decrease in tensile ductility and axial fatigue strength was measured on increasing the content of delta ferrite in the metal injection moulding processed AISI 316L stainless steel. However, since delta ferrite activates densification, these effects can be compensated by an increase in the sintered density.

No features attributable to delta ferrite were observed on the fracture surfaces in both tensile and fatigue loading. Only a slight increase in the passivation current density due to delta ferrite was observed in potentiodynamic tests in sulphuric acid solution, correlated to a selective corrosion of this secondary constituent.

Despite these effects, all these properties remain comparable up to 8% of delta ferrite.

Introduction

AISI 316L stainless steel is one of the most widely used materials in high added value applications of Metal Injection Moulding [1] and is nowadays attracting a growing interest from new market sectors, such as that of the components for hardware [2].

AISI 316L is a typical austenitic stainless steel but, when processed by MIM, its microstructure may contain some delta ferrite [3] because of the extremely high sintering temperature (above 1300°C). The amount of delta ferrite increases with the sintering temperature and decreases with the carbon content which, as well known, stabilises the austenite phase. The effect of carbon, in particular, may be quite critical. The carbon content of the sintered austenitic steels depends on several parameters:

a) the carbon content of the starting powder;

b) the effectiveness of the debinding process, which may leave residual carbon in the brown parts that is subsequently dissolved in austenite on sintering; this effect has been investigated in the 17-4 PH stainless steel [4, 5];

c) the oxygen content of the powder, since carbon reduces the surface oxides, resulting in an activation of the sintering mechanisms but at the same time in decarburisation [6].

All these effects are active on sintering, irrespective of the sintering atmosphere: reductive (H2), inert (N2) or combined (N2/H2). Sintering of austenitic stainless steel at high temperature can be conveniently carried out in vacuum furnaces [7-10].  In these furnaces the graphite refractories, differently from the metallic ones, may produce a partial pressure of carbon which slightly increases the as sintered carbon content of the material. This effect, in turn, depends on the pressure of the backfilling gas which is added to prevent evaporation of Cr and Mn at high temperature.......

Further sections of this paper include:

- Experimental Procedure

- Results And Discussion

- Microstructure

- Microhardness and hardness

- Mechanical properties

- Corrosion resistance

- Conclusions

- References

Figures and Tables:

Fig. 1 Optical micrographs of 316L materials: NF (a), F (b), HF (c) and after specific etching for the quantitative determination of delta ferrite by Image Analysis (d)

Fig. 2 Tensile test curves of NF and F materials

Fig. 3 Tensile fracture surface of 316L HF material

Fig. 4 Nucleation site of the fatigue crack (a) and fatigue fracture surface (b) of 316L NF material

Fig. 5 Ductile morphology of the fast fatigue crack propagation (a) and path of crack propagation (b)

Fig. 6 Potentiodynamic curves

Fig. 7 Corrosion behaviour for NF (a), F (b) and HF (c) materials after breakdown.

Table 1 Sintering conditions

Table 2 Density, delta ferrite and carbon contents

Table 3 Microhardness and hardness of the investigated materials

Table 4 Tensile properties of the investigated materials

Table 5 Fatigue tests

Table 6 Electrochemical parameters