Overview of theMechanisms of Failure in Heat Treated Steel Components

Design deficiencies are a common source of component failure. Examples include the presence of a sharp notch in regions of high stress or a fillet radii that is too sharp. Using a component design for a new application can also lead to unanticipated failures. Higher stresses or unanticipated service conditions can cause unforeseen failure because of complex or increased stress fields. Stress concentrations may become more critical because of the increase in loading for the new application. Insufficient design criteria can also be the cause of unforeseen failures. Inadequate knowledge of the stress state in the component or inadequate stress calculations can contribute to failures. Much higher stress states than initially assumed or improper stress assumptions can result in premature service failures. Lack of consideration of severe environmental, fatigue, or impact conditions may contribute to failure. Material issues can usually be attributed to either selection of material or material imperfections rendering it unsuitable for service. Inadequate material data can also result in conditions that may contribute to failure. For example, adequate fatigue data, elevated-temperature tensile data, or creep or corrosion data may not be available, and the designer may have to extrapolate or estimate the effects or these properties. Other sources of failure can be attributed to material imperfections. For wrought products, this could be related to segregation, inclusions, porosity, laps, and seams. For castings, these imperfections could be cold shuts, inclusions, shrinkage, voids, and porosity. Forgings can have laps, seams, segregation, and anisotropy in properties from forging flow lines. In one example (Fig. 1), a large roll was heat treated, and several large cracks were observed after inspection. This was originally attributed to quench cracking. On further examination, it was determined that a lap was present in the forging, Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_43-86.pdf/Chap_02/ 18/8/2008 2:54PM Plate # 0 pg 43