Microstructure formation and abrasive wear resistance of a boron-modified superduplex stainless steel produced by spray
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microstructure formation and wear resistance of a superduplex stainless steel modified with the addition of 3 wt% boron produced by spray forming were investigated. Thermodynamic simulations were used as comparison basis and to explain the experimentally observed microstructure, which was composed by primary M2B-type borides, an austenitic-ferritic matrix, and eutectic M3B2-type borides. The predicted solidification sequence started with the precipitation of primary M2B boride, followed by ferrite/austenite formation and a final eutectic reaction resulting in M3B2 borides. A good correlation with the simulations and final microstructure was found. The abrasive wear resistance was investigated with the dry sand/rubber wheel test and the results indicated an outstanding performance, similar to the cobalt-based Stellite 1016 alloy. The excellent wear resistance resulted from the presence of a significant amount (about 35 vol%) of hard borides homogeneously dispersed in the microstructure, which was effective at increasing hardness and protecting the duplex matrix against abrasion.
I. INTRODUCTION
Stainless steels are the material of choice for structural applications that require a good balance of mechanical strength and corrosion resistance.1,2 For some specific demanding conditions, such as in the offshore industry, there are grades available with exceptional resistance to corrosion, for instance, the superduplex steels.3 The duplex microstructure is based on balanced phase fractions of ferrite and austenite, whereas the term superduplex is defined based on the pitting resistance equivalent number (PREN: wt%Cr 1 3.3 wt%Mo 1 16 wt%N) larger than 40.4 When compared to regular ferritic or austenitic stainless steels, the superduplex series show higher strength, improved local pitting and stress corrosion cracking resistance.5 Despite the exceptional corrosion resistance of these stainless steel grades, depending on the application conditions, the wear resistance might be rather unsatisfactory.6 For instance, in components from offshore or chemical process industries where superduplex steels are used due to the corrosion resistance, but are simultaneously subjected to metal to metal moving interfaces (pumps, valves, bearings, etc.). Therefore, methods to improve the wear behavior have been extensively investigated.7–9 The most common
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.323
approach is the application of coatings or surface modifications, in which the wear resistance originates from a wear resistant material deposited at the surface.10–12 From this point of view, developing coating materials which increase wear resistance of the stainless steel parts while keeping their corrosion properties is an interesting strategy. Recently, Zepon et al.13 reported the development of the spray-formed boron-modified supermartensitic stainless steel grades with different boron contents. The authors have shown that the addition of boron contents
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