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2 2007

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Sinter-hardening process applicable to stainless steels

Dobrzański L. A., Brytan Z., Rosso M.

Journal of Achievements in Materials and Manufacturing Engineering

2007

Vol. 24, nr 2

11-18

Purpose: of this paper was to describe sintered duplex stainless steels manufactured in sinter-hardening process and its usability in field of stainless steels. Design/methodology/approach: In presented study duplex stainless steels were obtained through powder metallurgy starting from austenitic, ferritic base powders by controlled addition of alloying elements, such as Cr, Ni, Mo and Cu. In the studies apart from the preparation of mixes, Schaeffler's diagram was taken into consideration. Prepared mixes have been compacted at 800 MPa and sintered in a vacuum furnace with argon backfilling at temperatures from 1200 to 1285 degrees centigrade for 0.5, 1 and 2 h. After sintering different cooling cycles were applied using nitrogen under pressure from 0.6 MPa to 0.002 MPa in argon atmosphere. Produced duplex stainless steels have been studied by scanning and optical microscopy and EDS chemical analysis of microstructure components. Findings: Obtained microstructure and mechanical properties of sintered duplex stainless steel strictly depend on the density and the pore morphology present in the microstructure and especially on cooling rate directly from sintering temperature in sinter-hardening process. The lowest cooling rate - applied gas pressure, the mechanical properties and corrosion resistance decrease due to precipitation of sigma phase. Proper bi-physic microstructure was obtained using nitrogen under pressure of 0.6 and 0.2 MPa. Research limitations/implications: Applied fast cooling rate seems to be a good compromise for mechanical properties and obtained microstructures, nevertheless further tests should be carried out in order to examine its influence on corrosion properties. Originality/value: The utilization of sinter-hardening process combined with use of elemental powders added to a stainless steel base powder shows its potentialities in terms of good microstructural homogeneity and especially working with cycles possible to introduce in industrial practice.

Sintering of Ni-Mo-W steels and their properties

Dobrzański L. A., Otręba J., Actis Grande M., Rosso M.

Archives of Materials Science and Engineering

2007

Vol. 28, nr 2

77-82

Purpose: The paper focuses on microstructural and mechanical characteristics of Ni-Mo and Ni-Mo-W sintered steels with increasing amount of tungsten, low and high addition of carbon. Design/methodology/approach: Prepared mixes of powders have been compacted at 700 MPa and sintered in a vacuum furnace with argon backfilling at 1120°C for 60 minutes; after sintering rapid cooling has been applied with an average cooling rate of 2,5°C/s. Produced sets of samples have been studied by scanning electron microscopy (SEM) with EDS for phase distribution and mapping and light optical microscopy (LOM) for microstructure observations. Mechanical properties such as impact energy, microhardness and wear rates were evaluated depending on chemical composition and the effect of applied sintering process with rapid cooling. Wear resistance was investigated using both pin on disk and disk on disk tests. Findings: The highest value of impact energy was achieved for set of steels with smaller amount of carbon. It was noticed that the presence of tungsten didn't differ much the results. The situation was opposite in the case of microhardness, where the best results were obtained for set with addition of 0.6%C and reached 452 HV0.1. The microstructure of all investigated alloys was mainly martensitic with minor presence of bainite in the set of steels containing low addition of carbon. Research limitations/implications: According to the powders characteristic, the applied cooling rate seems to be a good compromise for mechanical properties and microstructures, nevertheless further tests should be carried out in order to examine different cooling rates and parameters of tempering process. Originality/value: The effect of small additions of W and WC to low alloyed steels, especially in terms of hardenability and wear resistance, was investigated.