Significant improvement in elevated temperature galling resistance of austenitic iron-based hard-facings through heat treatment

This paper describes the development of a novel heat treatment to significantly improve the high temperature galling behaviour of austenitic iron-based hardfacings. The alloy examined in this work is hot isostatically pressed (HIPed) Tristelle 5183 (T-5183) which is comprised of an austenitic matrix with (Cr, Fe) 7 C 3 and NbC hard phases. In the HIPed condition, the galling pressure, σ g was bounded as 211 MPa < σ g < 317 MPa at room temperature, but this fell to σ g < 53 MPa at 100 °C. In the as-HIPed state, the (Cr, Fe) 7 C 3 fraction was significantly smaller than its equilibrium value due to its sluggish nucleation and growth kinetics during HIPing, with this resulting in a commensurate supersaturation of carbon in solution in the austenitic matrix. This carbon supersaturation results in an increase in the austenite stacking fault energy (SFE) and it is suggested that this in turn results in reduced resistance to galling at elevated temperatures. An extended high temperature heat treatment of the alloy has resulted in the following: (i) an increase in the carbide particle size; (ii) an increase in the volume fraction of carbide; (iii) a commensurate reduction in the carbon content of the austenite and thus a reduction in its SFE. Following heat treatment, the room temperature galling pressure remained bounded as 211 MPa < σ g < 317 MPa (i.e. no significant improvement in galling resistance was observed); however, the heat treatment resulted in a significant increase in the elevated temperature capability of the alloy with the galling pressure being again bounded as 211 MPa < σ g < 317 MPa at temperatures as high as 300 °C. Given the temperature-sensitivity of the observed improvements in galling behaviour and the temperature-sensitivity of the SFE of the austenitic matrix, it is argued that it is the change in austenite SFE associated with the heat treatment that is the dominant influence in the observed improvement in the galling resistance of this alloy. In light of a review of relevant adjacent literature related to the destabilisation of high chromium cast irons, it is argued that the heat treatment method proposed here for improving high temperature galling capability has general applicability to any ferrous system composed of austenite and complex carbides (including hardfacings) and is generalisable to a variety of methods of manufacture (e.g. HIPing, weld and laser cladding, casting etc). • Iron-based austenitic hardfacings are prone to gall at elevated temperatures. • The galling pressure of such a HIPed hardfacing was <53 MPa at 100 °C. • A heat treatment was developed to reduce the austenite carbon concentration. • The galling pressure of the heat treated hardfacing was >211 MPa at 300 °C. • The change was mediated via a reduction in stacking fault energy.