Much progress has been made in developing chromium-molybdenum steels with improved hardenability, elevated temperature strength, toughness, and resistance to environment. The new low alloy steels, intended for heavy-wall pressure vessels operating at temperatures in excess of 450°C and pressures up to 30 MPa, are fully bainitic for maximum strength and toughness, and are resistant to temper embrittlement, hydrogen embrittlement, and hydrogen attack. Microstructural studies have shown that alloys containing 3%Cr and 1.5%Mo exhibit improved heat treating characteristics as they rapidly temper to produce stable alloy carbides, which are resistant to hydrogen attack. Such steels could replace 2.25Cr-1Mo steel for service to 480°C. Higher creep strengths can be achieved in modifications of 2.25Cr-1Mo or 3Cr-Mo steels containing up to 0.25% vanadium.