NEW OBSERVATIONS ON THE CRACK-GROWTH RATE OF LOW-ALLOY NUCLEAR GRADE FERRITIC STEELS UNDER CONSTANT ACTIVE LOAD IN OXYGENATED HIGH-TEMPERATURE WATER

Within the scope of reactor safety research attempts have been made ov er several decades to determine corrosion-assisted crack growth rates. National and international investigations have been performed on both an experimental and an analytical basis. A compilation of internation ally available experimental data for ferritic steels exhibits a scatte r of crack growth rates of up to 5 decades. This was one of the reason s for commencing further experimental investigations focused on the ev aluation of corrosion-assisted crack growth rates. These experimental studies were performed under constant, active, external load on 2T-CT specimens of the materials 20 MnMoNi 5 5 with 0.009 and 0.020% S (simi lar to A508 Cl.3), 22 NiMoCr 3 7 with 0.006% S (similar to A508 Cl.2) and 17 MnMoV 6 4 with 0.017% S. The tests were carried out in deionize d oxygenated high-temperature water (240 degrees C; 0.4 and 8.0 ppm O- 2). For K-1 values up to 60 MPa m(1/2), the experimental results showe d no significant dependence between corrosion-assisted crack growth ra tes and the stress intensity factor, the oxygen content of the medium or the sulphur content of the steel. Here it is important to note, tha t in this K-1 region the high crack growth rates after the onset of cr acking due to loading are decreasing and finally come to a standstill after a short period of time as compared with operational times of pla nts. Consequently, the determination of crack growth velocities as cor rosion-assisted crack advance divided by the test duration, so far pra ctised worldwide, results in wrong crack growth rate values in the abo ve-mentioned range of loading up to 60 MPa m(1/2). Based on a test dur ation of 1000 h, the average crack growth rates are below 10(-8) mm s( -1) for K-1 less than or equal to 60 MPa m(1/2). When applied to a sin gle start-up and a service period of one year, this would formally lea d to an average crack growth rate of 2.10(-9) mm s(-1) (equivalent to 0.06 mm per year). At K-1 values between 60 and 75 MPa m(1/2) the aver age corrosion-assisted crack growth rates increase significantly. It c an be observed experimentally that the crack propagates during the who le period of the test. Consequently the calculation of crack growth ve locities as corrosion-assisted crack advance divided by the test durat ion as mentioned earlier can be applied as a first estimate. Finally, for K-1 values greater than or equal to 75 MPa m(1/2) high crack growt h rates up to 10(-4) mm s(-1) can be observed. In this region the aver age crack growth rates are also in quite good agreement with a theoret ically based crack growth model. (C) 1997 Elsevier Science S.A.