The crystalline structure of steel at fracture

Abstract

In a previous paper (Gough and Wood 1936) were described the results of a research into the characteristics of deformation and fracture of a mild steel (0·1% C) under static and fatigue stresses, in which precise methods of X-ray diffraction were used in a systematic study of the changes pro­duced in the crystalline structure by five stressing systems. It was established, for the first time, that failure by static and cyclic stressing was characterized by exactly the same kind of progressive deterioration of the crystalline structure, fracture in all cases being associated with a break­down—complete or partial—of the crystal grains to a mass of crystallites having a limiting size of between 10 -4 and 10 -5 cm. and a completely random orientation. Under static stressing the sequence of changes in structure were successfully studied, while, using cyclic stresses, the separate and combined influences of the range of stress and the superior stress of the cycle were investigated, also the essential differences on the structure between the effects of safe and unsafe ranges of stress were clearly established. The present paper describes the results of an investigation undertaken in the hope of obtaining a clearer insight into one of the features revealed by the previous research. In addition to a complete fragmentation of the structure into the limiting size of crystallite, it was considered that a state of marked lattice distortion in the fragmented material was also a necessary condition of the fracture stage, but, as stated in the previous paper, the recorded radial broadening of the reflexion spots could not be ascribed with certainty to the presence of distortion in the lattice of the crystallites. It is very difficult to accept the view that the fracture stage is reached as a consequence of fragmented structure alone, for the employment of severe deformations to obtain cold-worked materials possessing increased hardness and increased nominal strength is common practice. These considerations suggested that further light might be thrown on this very important problem by the investigation of a material similar to that used previously but commencing with a structure which had already been brought to a fragmented condition by cold-rolling.