Wear resistant steels

Everyone knows, approximately, what wear is; however, wear is not an exact science.¹ Its minimisation in innumerable industrial operations and products would present substantial economic benefits. A useful, general definition is the progressive loss of material from the surface of a solid body due to mechanical action, i.e. the contact and relative motion against a solid, liquid or gaseous counterbody.² Various modes of wear have been defined^2,3: adhesive, abrasive, surface fatigue and tribochemical reaction, each with its own sub-divisions, whilst noting that they often occur in combination. Other authors have added extra sub-divisions, or even extra categories which it could be argued are cases (albeit conveniently) combining elements of the existing definitions. A whole paper could be spent, or maybe wasted, trying to define and argue these, but that is not the purpose here. Suffice it to say that wear is a property: ‘characteristic of the engineering system’,³ rather than purely a material property, which itself raises significant issues as to how to measure it most relevantly for the application in question.^2,4 Hardness has long been known as a key material property for wear resistance, but by no means a sufficient one,⁵ but which is often achieved with a reduction in other desirable properties, notably ductility. Although wear is a mature subject and steel a mature material, both continue to benefit from further understanding and development, with major economic implications.

The declared target for the contributions to this thematic issue was new steel developments from the laboratory stage up to and including trial applications, and the state of current research on the material aspects of obtaining good wear resistance in combination with improvement of other mechanical properties. Surface treatment of the steel was within scope but not coatings as such.

Clearly, this target could not be met exhaustively for such a wide, active and important field. However, interesting contributions were received on a range of different aspects.

The papers could be split between those concerned with the steel itself, and those concerning surface modification of the steel. The former is dominated by submissions which are based upon a largely bainitic microstructure.^6–9 Caballero et al.⁶ review the work examining the wear resistance of the recently developed bulk steels with a nano-crystalline bainitic microstructure. These exhibit improved wear resistance in a wide range of systems although different systems may require different optimisation of the detailed microstructure. Bo Lv et al.⁷ consider bainitic steels containing 0–1.8% Al, where oxide formation as well as phase balance affects the performance. Sharma et al.⁸ review developments regarding bainitic rail and wheel steels. Low carbon bainitic rails are already widely in service but there are good prospects for higher carbon variants, too. In addition to the improved wear performance, the relevant mechanical properties can also be improved. This group is tied up by one addressing how to weld a bainitic/tempered martensitic wear-resistant steel⁹; for many bulk applications, there is little point developing high wear resistance at the expense of the ability to weld the steel successfully.

Two other contributions concern the steel itself. Wang et al.¹⁰ investigate the influence of epsilon carbide shape and size in a tempered low alloy steel on the three-body impact wear resistance and mechanical properties. Very different carbides are involved in Seifert’s work,¹¹ developing a powder-route bearing steel containing large NbC particles which has sufficient corrosion resistance for use in seawater.

Amongst the papers addressing surface modification of the steel, we have the embedding of TiC particles,¹² plasma nitriding,¹³ and improving the overall wear resistance through irregularities in shape and composition of the steel surface.^14,15 Addition of TiC particles to the wear surface dramatically reduced the wear rate against an alumina ball,¹² and as with⁷ oxide generation at the surface played a key role. Ma et al.¹³ conduct ball-on-disc tests in vacuum on a nitrided low alloy steel, finding different wear modes at different speeds. Tian et al.¹⁴ introduced biologically-inspired holes on the wear surface, which can serve as local repositories for wear debris, or lubricating oil. Finally, we return to steel rails, but this time with laser-carburised stripes across the width.¹⁵ The alternating microstructure and associated hardness showed a reduced wear resistance in laboratory tests.