Technical Paper: PIM International, Vol.2 No. 4 December 2008, pages 56-59, 2019 words

Authors: P. V. Muterlle [1], M. Zendron [2], M. Perina [2], R. Bardini [2] & A. Molinari [1]

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

Abstract

The effect of the carbon content on microstructure and tensile properties of a MIM 17-4PH stainless steels has been investigated. Carbon content influences the formation of delta ferrite at high temperature, which increases sintered density. However, delta ferrite causes a decrease of tensile strength, without a corresponding increase in ductility. In the presence of large amounts of delta ferrite, ductility decreases significantly.

Introduction

17-4PH stainless steel has an excellent combination of mechanical properties, provided by the precipitation hardened low carbon martensitic matrix, and corrosion resistance. Mechanical properties are optimised by a precipitation hardening treatment, and depend on the aging temperature. The maximum hardness and strength are obtained by aging between 450°C and 510°C due to the precipitation of coherent copper particles [1,2,3]. The maximum ductility and toughness are instead obtained by aging at higher temperatures (> 540°C), when the copper particles become incoherent with the martensitic matrix [1,2,3].

In 17-4PH stainless steel produced by Metal Injection Moulding (MIM), one of the most important parameters is carbon content, since it has a noticeable effect on density, microstructure and corrosion resistance [4,5].

The sintering temperature varies between 1250°C [6] and 1390°C [7]. At these temperatures, the microstructure comprises a certain amount of delta ferrite in the austenitic matrix. It is well known that volume diffusion is faster in the b.c.c. ferrite than in the f.c.c. austenite, then ferrite enhances sintering and densification. Since carbon stabilises austenite, at a given sintering temperature densification is inversely proportional to its amount. As a consequence, an increase in carbon content requires a corresponding increase in the sintering temperature to get full densification.

In addition, delta ferrite itself decreases mechanical strength. Therefore, the effect of carbon on the mechanical properties results from two opposite trends, both related to the amount of delta ferrite.

In this paper, the results of an experimental study aimed at the optimisation of the properties of the 17-4PH stainless steel produced by MIM are presented. Two prealloyed powders were used, differing in the carbon content, and sintering was carried out at two temperatures. Density, microstructure and mechanical properties were investigated after aging in the two typical conditions of the industrial practice: H900 (aging at 482°C) and H1100 (aging at 560°C).....

Further sections of this article include:

- Experimental Procedure
- Results and Discussion
- Conclusion
- Acknowledgment
- References

Figures and Tables:

Fig. 1 LC steel sintered at 1280°C (a) and 1330°C (b)
Fig. 2 HC steel sintered at 1280°C (a) and 1330°C (b)
Fig. 3 DSC curves (high temperature) of the two materials investigated
Fig. 4 DSC of the as sintered materials
Fig. 5 Microstructure of H900 aged LC (a) and HC (b) sintered at 1280°C
Fig. 6 Microstructure of H1100 aged LC (a) and HC (b) sintered at 1330°C
Fig. 7 Results of tensile tests on LC sintered at 1280°C
Fig. 8 Results of tensile tests on LC sintered at 1330°C
Fig. 9 Results of tensile tests on HC sintered at 1280°C
Fig. 10 Results of tensile tests on HC sintered at 1330°C
Fig. 11 Synthesis of the tensile properties of materials aged H900
Fig. 12 Synthesis of the tensile properties of materials aged H1100
Fig. 13 Fracture surfaces of H900 materials: a) LC sintered at 1280°C, b) LC sintered at 1330°C, c) HC sintered at 1280°C and d) HC sintered at 1330°C
Fig. 14 Fracture surface of H1100 materials: a) LC sintered at 1280°C, b) LC sintered at 1330°C, c) HC sintered at 1280°C and d) HC sintered at 1330°C

Table 1 Carbon and oxygen contents and density of the investigated materials
Table 2 Hardness and microhardness