Creep crack initiation and growth in 2.25% chromium - 1% molybdenum alloy steel

2.25%Cr-1 %Mo Steel has been utilised extensively for the manufacture of power plant components.

This study has highlighted the implication of the presence of pre-existing defects in such components and, in particular, the influence of the microstructure on the creep deformation response ahead of a defect during the crack initiation, or incubation, period.

The investigation was conducted by carrying out creep tests on compact tension specimens in three microstructural variations of the alloy and, subsequently, using optical, scanning and transmission electron microscopy techniques to evaluate the creep damage in the "defect" tip region up to a point where -1 mm of crack extension was detected.

Details of two image analysis routines devised specifically for this project have been presented and their suitability for purpose discussed. The first of these routines allowed the distribution and density of creep voids occurring ahead of the simulated defect to be quantified using back scattered electron imaging in the scanning electron microscope. The second allowed carbides, extracted from the alloy, to be classified in the STEM on the basis of their key element ratios.

The microstructural constraint imposed by the material ahead of the defect tip was shown to critically affect the crack initiation and growth process; the order of constraint observed being that of 100% bainite > mixed ferrite / bainite > mixed ferrite / pearlite.

A correlation between the carbide type, distribution and shape and the creep deformation and crack initiation process has been discussed and the implications of this to remanent life assessment suggested. Evolution of M₂₃C₆ grain boundary carbides to the equilibrium carbide, M₆C, and the affect of the grain boundary sliding on the shape of the M₆C, were proposed to be the most important microstructural phenomena contributing to the crack initiation process.

A detailed review of published literature relating to the topic studied and recommendations for further work have also been included in this thesis.