Effects of modification on microstructure and properties of ultrahigh carbon (1.9 wt.% C) steel

► We employed a new type of modifier to the UHCS and discussed its mechanism. ► We achieved the refinement of structure and remarkable improvement in properties. ► The phase transformation temperatures were observed to be changed. ► Friction coefficient decreased with increasing loads associating with the oxide film. The effects of a modifier that contains Rare Earths (RE), low melting point alloy (Al–Bi–Sb) and Ca–Si alloy on an ultrahigh carbon steel containing 1.9 wt.% C were studied. Microstructure characterization was carried out with optical microscopy (OM) and scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS). Upon modification, the continuous eutectic carbide network structure was broken up and changed to a partly isolated and finer blocky structure in the as-cast alloy. Differential scanning calorimetry (DSC) revealed that the eutectoid temperature increased and the eutectic temperature decreased for the modified alloy. Modification also improved the impact toughness of the tempered steel, with a significant increase from 6.5 to 12.6 J cm −2, despite the hardness remaining around 66 HRC. Furthermore, in pure sliding under loads of 20, 60 and 100 N for 600 s against a zirconia ball, the modified alloy shows slightly higher friction coefficient at all loads than the non-modified one. In addition, the friction coefficient for the steel specimen decreased with load from 20 N to 100 N attributing to a reduction in metallic wear and the formation of a thicker oxide film on the surfaces.