Dependence of creep crack propagation on columnar grain orientation in cast HK40 tube

Compact tension specimens having a columnar grain orientation inclined to the notch plane at an angle θ in the range 0-60° were prepared from a centrifugally cast HK40 tube. Constant load creep crack propagation tests were carried out on these specimens at 871⁰C. It was found that the creep cracks tended to advance along the columnar grain boundaries, and the crack propagation resistance increased with increasing θ. The dependence of creep crack propagation rate da/dt on the columnar grain orientation can be expressed as da/dt = A[K_r cos²(θ/2)]^m, where A and m are constants and K₁ is the stress intensity factor. A good correlation between the creep crack propagation rate and the path independent integral C* confirms that the integral C* is an appropriate controlling factor for creep crack propagation in cast HK40 alloy.