Heat treatment of low-alloyed steel up to grade Q125

This work investigates the problem of how to reduce the prime cost of Grade Q125 Group 4 casing and tubing according to API 5CT and proposes process solutions to resolve it, consisting in the enhancement of series of mechanical properties without chemical alloying of steel with expensive elements such as chromium, molybdenum, vanadium, niobium and boron. During the investigation the process parameters of 9MnSi5 low-alloy steel heat treatment were developed, which confirmed the efficiency of this technology providing high values of strength and yield strengths which are minimum 931 MPa and 862 MPa respectively; at the same time maintaining the required cold resistance and resistance to hydrogen cracking of pipe. The suggested process parameters were based on heat cycle quenching with heating in induction furnace up to 1050–1100°С, 880–900°С and 740–780°С accordingly at each cycle and cooling rate in each case equal to 50°С/sec, during which the uniform martensite grain size ranging from 5 to 10 micron was obtained.