**Abstract**

Heat treatment route is an important route for the development of high-strength alloy steel. Many heat treatment processes are applied depending on alloy compositions and desired mechanical properties. There are various high-strength alloy steels, namely, austenitic stainless steel (16–26 wt%Cr, 0.07–0.15 wt%C, 8–10 wt%Ni, rest Fe), where the heat treatment adopted is the low-temperature plasma nitriding so as to achieve a strength in a range of 800–1000 MPa. In twinninginduced plasticity (TWIP) steel (>20 wt%Mn, <1 wt%C, <3 wt%Si, <3 wt%Al, rest Fe), high-temperature thermomechanical heat treatment provides a strength greater than 1000 MPa. High-speed steel (18 wt%W, 4 wt%Cr, 1 wt%V, 0.7 wt%C, 5–8 wt%Co, rest Fe) suits best for high-speed machining purpose, owing to secondary hardening. Besides, high-temperature annealing is performed with majorly ferritic structure to achieve a maximum bending strength of 4700 MPa. Furthermore, in Hadfield steel (11–14 wt%Mn, 1–1.4 wt%C), a fully austenitic phase is obtained with a strength level of 1000 MPa. High-alloy tool steel (5 wt%Mo, 6 wt%W, 4 wt%Cr, 0.3 wt%Si, 1 wt%V, rest Fe) is provided with austenitizing, quenching, and tempering treatment to achieve a maximum hardness of 1200–1400 HV.

**Keywords:** high alloy steel, heat treatment, strengthening mechanism, mechanical properties
