On the strain hardening of austempered spheroidal graphite cast iron: Dissertation

The properties of austempered spheroidal graphite cast iron are surveyed.It has found many applications in engineering.The microstructure of its matrix consists essentially of metastable austenite and bainite, which in turn consists of ferrite laths and plates imbedded in the austenite.There is no cementite in the structure.Owing to the remarkable amount of the austenite in the structure the properties differ in many respects from the properties of ordinary spheroidal graphite cast irons and steels.Structural changes caused by deformation were studied by transmission electron microscopy.Ion bombardment technique was used'in preparing the thin foils for the TEM studies.The austenite amounts were measured by magnetic scales.The strain hardening mechanism due to monotonic compression at room temperature was studied in detail.Shear bands, identifed to be deformation twins in the austenite, are caused by plastic deformation.Intersections of twins are nucleation sites for #martensite.The structure of the strain-induced martensite is very fine, needle-like, due to the thinness of the twins.The increasing strain increases the number of the twin-twin intersections, and thus the number of martensite needles.The orientation relationship between the strain-induced martensite and parent austenite is the Kurdyumov-Sachs relationship.The strainhardening caused by cyclic loading due to rolling contact was studied also.It turned out to be caused essentially by high dislocation density.The strain-hardening characteristics were measured using plane strain compression and tensile tests.The characteristics were compared with those of austenitic steels referred to in the literature.The steels displayed higher strain-hardening exponents.Causes for the difference are proposed.