K and da/dt relation Typical relationship between K and crack growth rate (da/dt) for 610MPa tensile strength steel is shown in Figure 5. Under RL condition, crack start to grow rapidly at low initiation point KIH, and continue to propagate at relatively constant speed of 10-4mm/sec order of magnitude for KI level ranging from 30 to 80MPa·m1/2 . After CMOD kept constant, KI and da/dt relationship can be drawn as steeper falling line, which finally reaches threshold slightly decreasing from holded KI. Since repetitional RL+CD result indicate that crack stops at any holded KI level (<KIC-H), threshold Kth may exist in between KIH and KIC-H affected by the holded CMOD value. Effect of temper embrittlement KIH were measured by RL condition for a temper embrittled old and newer high toughness heats and were plotted as a function of its material’s FATT in Figure 6. Regardless of FATT of its heat, majority of steels exhibits low initiation sensitivity of cracking; KIH=30MPa·m1/2. K-curves for Low J and Mid J steels are compared in Figure7. Initiation occur at low KI point in two heats, but cracking resistibility is higher for low J steel, whereas Mid J steel cracking propagates in low KI and led to KIC-H. Fracture appearances are compared in Figure 8. Intergranular cracking was dominant in Mid J steel whereas LowJ steel heat exhibits almost quasi-cleavage with some intergranular fracture surfaces. Although difference in fracture surfaces recognized, the role of hydrogen on the initiation kinetics should still to have to be studied. Effect steel strength level Crack growth measurement of RL+CD tests were conducted for enhanced and annealed heat and results are shown together in Figure 9. Three heats show entirely different cracking characteristics respectively where the enhanced heat exhibited a very aggressive propagation under long term CD condition, on the other hand, the only little cracking was observed in the annealed heat. These tendency indicates that the higher strength steel is susceptible to delayed type cracking under static CD condition falls on to lower Kth and lower strength steel as well as was already suggested by the previous studies.2) SUMMARY Crack growth characteristics for a variety of 2.25Cr-1Mo steels were clarified and summarized. Schematic illustration of K and da/dt relationship is shown in Figure 10. 1. Imposing of active rising load obviously enhances crack initiation and growth rate, whereas crack growth was deactivated by keeping CMOD constant. 2. Crack start to grow rapidly at low initiation point KIH, and continue to propagate at relatively constant speed and as KI level increases, fast fracture occurs at KIC-H. 3. Initiation of cracking KIH did not make much difference in temper embrittled and new high toughness steel. But cracking resistibility is higher for low J steel, whereas Mid J steel propagates cracking in low KI and reached KIC-H. 4. The higher strength steel is susceptible to delayed type cracking under static CD condition falling on to lower Kth and lower strength steel as well as was already suggested by the previous studies.
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