Sintered duplex stainless steels from premixes of 316L and 434L powders
Introduction
Duplex ferritic–austenitic stainless steels are characterized by an outstanding combination of strength, toughness and corrosion resistance. Ferritic steels have lower toughness and austenite steels have lower stress corrosion resistance, whereas duplex stainless steels have properties which are compromise between those two. Furthermore, due to lower nickel content duplex stainless steels are much cheaper than comparable austenite grades. They have therefore, found applications in off-shore constructions, heat-exchangers, gas and oil pipe lines as well as in the chemical industry. However, these steels posses considerable difficulties from the manufacturing point of view. This serves powder metallurgy an excellent alternative to produce this steel. Attempts have been made to introduce ferrite phase through the introduction of silicon powder in austenitic stainless steel [1], [2]. Another reported work is of Ruiz-Prieto et al. [3] who reported on such materials produced through powder metallurgy route based on straight stainless steel powder premixes.
In the present work powder metallurgical duplex stainless steels obtained from premixes of 316L and 434L powders are studied. The study includes mechanical, magnetic, microstructural and corrosion behaviors.
Section snippets
Experimental procedure
The investigation was carried out using water atomized and annealed 316L austenitic and 434L ferritic stainless steel powders supplied by AMETEK (Eighty Four, PA, USA). The chemical compositions and properties of two powders are shown in Table 1. This confirms that, apart from their compositions, all other characteristics of the steel powders are same.
Premixes of 316L and 434L were prepared in Turbula mixer, type 12C Nr921266 for half an hour. 434L content in different premixes were selected as
Results
Sintered density increases with increase in sintering temperature and the variation of sintered density with respect to temperature is shown in Fig. 1. With increase in sintering temperature porosity decreases leading to higher densification. However, the effect is not so visible after 1150 and 1250°C sintering.
Fig. 1 also shows variation of microhardness of steels with respect to composition for various sintering temperatures. In general, hardness decreases with increase in sintering
Discussion
It is well known fact that ferritic stainless steel increases yield strength and provides resistance to stress corrosion cracking, whereas austenitic steel imparts toughness along with good corrosion resistance. Keeping this in view, a mixed structure of ferrite and austenite would offer good corrosion resistance along with high-strengths and reasonable toughness. So far, the attention of stainless steel powder manufacturers has not yet turned to produce ferrite–austenite duplex stainless steel
Conclusions
- 1.
Densification increases with increasing sintering temperature for duplex stainless steels. Percentage porosity decreases with increase in 434L content only after 1350°C sintering.
- 2.
Ferrite–austenite two phase structure is observed for 40 and 60w/o 434L containing compositions after 1350°C sintering. Divergence of microhardness in 80w/o 434L containing alloy composition also suggests possibility of two phases.
- 3.
316L-60w/o 434L compositions exhibits maximum TRS value.
- 4.
Wear loss during sliding. Wear is
Acknowledgements
The authors are grateful to Ametek, Inc., Pennsylvania, USA for supplying the austenite and ferrite stainless steel powders.
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