Predicting properties and minimizing residual stress in quenched steel parts

Abstract

Quenching refers to the process of rapidly cooling metal parts from the austenitizing or solution treating temperature, typically in the range of 845 to 870°C (1550 to 1600°F) for carbon and low alloy steels, for the purpose of forming martensite. Several factors determine whether a particular part can be successfully hardened, including the type, molecular weight, and thermal characteristics of the quenchant, the quenchant use conditions (such as velocity, temperature, and polymer concentration, etc.), section thickness of the part, and the transformation characteristics of the specific alloy being quenched. Successful hardening usually means achieving the required hardness, strength, or toughness while minimizing residual stress, distortion, and the possibility of cracking. Heat removal from parts during quenching can be described in terms of the effective interface heat transfer coefficient. A quenchant must impart a sufficiently high interface heat transfer coefficient to produce a cooling rate that will minimize transformation of austenite to ferrite or pearlite and yield the desired amount of martensite. A quench factorQ has been devised that interrelates quenching variables and transformation kinetics of steel to provide a single number indicating the extent to which a part can be hardened at various locations within the part. The quenchant and the quenchant operating conditions should be selected to provide the proper quench factor while minimizing thermal gradients that can cause distortion or cracking. Cooling curves, interface heat transfer coefficients, quench factors, and thermal gradients produced by a variety of quenchants are presented.