Failures due to improper hardening of steel components

Components made up of steel are often quenched, so that they become hard due to the formation of martensite. The quenching rates through out the component, however, changes depending on the section size. Wher¬ever the rate of cooling is greater than the critical cooling rate marten¬sitic microstructure is obtained. In a specified standard quenchant the depth of martensitic case being a characteristic of the steel is known as hardenability. Hardenability is characterized by critical diameter (D) or critical plate thickness (L) which corresponds to the formation of 50% martensite -50% pearlite at the centre. This, D1, is dependent on the car¬bon content, austenite grain size and various alloying elements. Pres¬ence of alloying elements lead to increase in hardenability by delaying the ferrite pearlite transformation. However, the differential cooling rates encountered by the component produces stresses leading to distor¬tion and cracking. To reduce these stresses, hardened materials are sub¬jected to tempering to make a balance between hardness and toughness required in service. This also determines the conditions of tempering for a given application. Case studies will be presented to show the effect of the quenching response and the subsequent tempering treatment for a component depending on its specific application requirements.