13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- The solution was acidified 0.1 M Na2SO4 pH 2 solution. Details about hydrogen charging, LIST and SEM sample preparation are as in [15]. After each LIST, the fracture surface was examined by SEM. Figure 1. The microstructure of the NiCrMo1 steel. Table 1. Chemical composition of NiCrMo1 (wt%) as determines by two independent measurements. Steel Composition, wt% C Ni Cr Mo V Mn Si S P Cu NiCrMo1 0.09 1.04 0.52 0.60 0.01 0.88 0.31 0.01 0.01 0.06 0.08 0.98 0.48 0.60 0.01 0.91 0.31 0.01 0.01 0.06 The stress rate was mostly the moderate stress rate: 0.02 MPa s-1. The yield stress in air, σy, or the threshold stress, σth, was determined by the PD measurement. The precision for σy or σth was ± 5 MPa. The precision of the fracture stress was ± 2 MPa. The reduction of area, RA, of each specimen was evaluated from the original specimen gauge diameter and the measured minimum diameter after each test. For the evaluation of σth or σf, in most cases the area used was the original specimen cross-section area. For tests in solution at Ecorr, corrosion reduced the cross section area of the specimen during the LIST. In those cases, the area used was the cross section area that had undergone uniform plastic deformation. The specimen designation can be explained by the following example: S1200.02a, where the letter ‘‘S’’ identified a specimen tested in solution (alternatively ‘‘A’’ identified a specimen tested in air), ‘‘1200’’ identified the applied potential (alternatively “ocp” identified a specimen tested in solution at the free corrosion potential), ‘‘.02’’ identified the applied stress rate, and the subsequent letter identified specimens tested under the same conditions. 3. Experimental procedure 3.1. Tensile properties Fig. 2 shows a typical PD record for NiCrMo1 tested at 0.02 MPa s-1 in acidified 0.1 M Na 2SO4 pH 2 solution at three negative potentials. Similar plots were obtained from the other experiments. Table 2 presents the measured data for σy, σth at which subcritical cracking appeared to initiate in solution, σf and RA. The Ecorr was about -500 mVAg/AgCl. The σy or σth in air or at negative applied potentials were in the range of 600 to 660 MPa, while the σf was varied from 690 to 760 MPa. The values of RA were about 70 ~76 %. LIST for specimens in solution at Ecorr produced values of the σth varying from 625 to 690 MPa, values of the σf varying from 710 to 800 MPa, and values of the RA varying from 69% to 79%.
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