| Abstract |
High strength, unchanged ductility, good impact properties and superior fatigue
behavior were produced in Mn-Cr-B spring steel (SUP 11A) by modified ausforming in which
reductions in thickness up to 60% in the temperature range from 880°C to about 720°C were
followed by rapid quenching into martensite. Observations of fracture surfaces by electron
microscope have disclosed the existence of partial intergranular fracture along the prior
austenite grain boundary in conventionally heat-treated steel. The modified ausformed
steel, however, does not involve such the intergranular fracture at all but transgranular
fracture entirely even at the sub-zero temperature in the impact test as well as at room
temperature. Refinement of martensite laths, the improvements in the shape, the alignment
and the distribution of carbides in matrices, and the refined subgrains tempered at
higher temperature tempering, and so on, seemed to give beneficial effects on
strengthening and toughening of the steel. |