Application of quenching–partitioning–tempering process and modification to a newly designed ultrahigh carbon steel

The applicability of quenching–partitioning–tempering (Q–P–T) process to an ultrahigh carbon steel (UHCS) has been investigated by means of optical microscopy (OM), scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and mechanical property tests. The molten steel was modified with a multi-component modifier-rare earth and a low melting point alloy (Al–Bi–Sb) before casting into iron molds. Observations showed that the carbide exists as partly isolated and fine blocky structure in as-cast microstructure, indicating good effect of modification. After the Q–P–T treatment, carbon was partitioned into austenite from martensite, creating a mixture of carbon-depleted martensite, carbon-enriched retained austenite and fine carbides. This kind of microstructure leads to a much higher impact toughness, 32 J/cm2, in comparison with the value, i.e., no more than 20 J/cm2, of the conventional quenching and tempering (Q–T) treatment at the same hardness level. Furthermore, wear-resisting property of the steel has also been investigated. It showed that the Q–P–T treated steel has better abrasive wear resistance, about 18% increased, compared with the Q–T treated alloy under high load conditions.