M. Rawson, Rolls-Royce Corportation, Chantilly, VA; P. Novotny, Carpenter Technology Corporation, Reading, PA
For ultra-high strength steels mechanical properties such as fatigue and fracture toughness are very sensitive to cleanness, and every effort is made, therefore, to reduce the level of residual elements and associated inclusions during the manufacture of the material. This is typically achieved during melting by the use of vacuum induction melting (VIM) followed by vacuum arc re-melting (VAR) which remove most of the unwanted impurities and provide a fine segregation free microstructure. However, even with modern extraction and melting techniques, these cannot eliminate all the oxygen and sulphur from the liquid steel. The oxygen and sulphur must be gettered by adding small quantities of elements/alloys, with a large thermodynamic driving force to react with any remaining sulphur and oxygen in the melt to form sulphides, oxides or oxy-sulphides. The most common desulphuriser is manganese, which forms manganese sulphides (MnS). It has been shown that replacing manganese with mischmetal (an alloy of rare earth elements, mainly Cerium and Lanthanum) has significantly increased the fracture toughness in some high strength steels. The higher fracture toughness allows the steel to have a larger critical flaw size for a given stress. This is very useful when design life is based on damage tolerance. The use of clean high strength steels desulphurised with mischmetal has been proposed for future aeroengine shafts. This application requires high strength and fracture toughness but also good fatigue life. Testing on low cycle plain fatigue samples showed unexpected scatter in fatigue life. The fatigue cracks were shown to have initiated on the mischmetal oxy-sulphide inclusions. These were predominately found at the surface, although some tests showed subsurface crack initiation. Changing the desulphurising treatment to an alternative route has resulted in an improved fatigue life, with no evidence of sulphide inclusions at the fatigue initiation sites.
