Low temperature creep crack growth in low alloy reactor pressure vessel steel

Uni-axial creep and creep crack growth (CCG) tests between 320 °C and 420 °C have been carried out in a low alloy reactor pressure vessel steel (ASTM A508 class 2). Both as-received material and simulated coarse grained heat affected zone (CGHAZ) microstructure have been tested. CCG does occur at tested temperatures. The lifetimes for the CCG tests are considerably shorter than those for the uni-axial creep tests at the same reference stress and temperature. This is more pronounced at longer lifetimes or lower stresses. Increasing temperature causes a significant reduction of lifetime, e.g. the CCG lifetime on the simulated CGHAZ is reduced by a factor of five as temperature raises from 320 °C to 340 °C at given reference stresses. Three distinct regions relating crack length and time are observed for the CCG tests. After incubation, the crack grows steadily until it accelerates to approach failure. For the longer time CCG tests, the cracks propagate intergranularly, independent of temperature and microstructure. An approximate linear extrapolation based on the stress-time results indicates that the reference stress causing failure in the simulated CGHAZ due to CCG at a given lifetime of 100 000 h at 320 °C is lower than both yield and tensile strengths, showing that the design stress should be based on creep crack growth property rather than static strength.