Creep fracture and some low-alloy steels.

The processes involved in the high temperature-creep deformation and fracture of low alloy steels have been extensively reviewed. The fracture behaviour of two series of alloy steels, 2 ¼ Cr 1Mo and ½ Cr ½ Mo ¼ V, has been investigated over a range of temperature extending from the ambient to that experienced under typical power plant operating conditions. The effect of prior heat treatment, stress state and presence of tramp elements on the accumulation of creep damage has also been studied. In a heavily tempered condition, the 2 ¼ Cr 1Mo alloy steels exhibited a high ductility and similar rupture mode over a wide temperature range. The sliding of adjacent prior austenite and bainite plate boundaries contributed to the overall creep strain. The presence of wide precipitate free zones suppressed the nucleation of grain boundary cavities and the dominant mechanism of damage accumulation involved decohesion, after large plastic strains, at the interface of coarse M23C6 particles and the matrix. Growth of such damage occurred by -a viscous process that was inhibited by the introduction of a hydrostatic compressive component of stress. Additions of Sn had no effect on creep life or ductility over the range explored. The simulated heat affected zone structure of the ½ Cr ½ Mo ¼ V alloy steel possessed extremely limited ductility at typical power plant operating temperatures. Failure occurred by the nucleation, growth and interlinkage of cavities at the prior austenite grain boundaries. A nucleation mechanism has been invoked, involving the interaction of matrix dislocations and grain boundary precipitates. Growth of cavities probably occurred by vacancy transport. Detailed quantitative fractography has yielded empirical laws governing the nucleation and growth of such cavities which show some similarities to those suggested for the phenomenon in single phase materials. It was demonstrated that remaining creep life was a function of the previous stress history of the material.