Saber, Mohammed
(2011)
Experimental and finite element studies of creep and creep crack growth in P91 and P92 weldments.
PhD thesis, University of Nottingham.
Abstract
P91 and P92 steels are two ASTM grades of steel which have been used in high temperature applications, such as fossil-fuelled power stations, nuclear power stations and chemical plants. Operating under creep conditions, i.e. high temperature and/or high stress, the welds made from these steels are potential failure locations and, therefore, life limiting for the entire plant. In this thesis, the results of creep and creep crack growth (CCG) tests, which were carried out on P91 and P92 welds, are reported. These tests were carried out on welds constituents, i.e. parent materials (PM) and weld metal (WM), and across-welds, for the P91 material at 650°C and for the P92 material at 675°C. For the cross-weld tests, interest was focused on the Type IV region, a narrow zone at one end of the heat affected zone (HAZ) at the side of the PM. Also reported, in this thesis, are the results of the Finite Element analyses for predicting the creep and creep crack growth in the P91 and P92 materials.
For the P91 material, CCG tests were carried out on PM and cross-weld CT specimens, at 650°C. The results of the CCG tests for P91 show that the CCG rates for the cross-weld CT specimens are about ten times higher than those for the PM specimens. For P92, uniaxial and notched bar creep tests were carried out on the PM and WM, at 675°C. Cross-weld uniaxial, notched bar and waisted specimens were also tested in order to characterise the creep behaviour of the P92 HAZ material. Impression creep tests were also carried out on the P92 HAZ material. The results of the CCG tests for P92 show that the CCG rates for the cross-weld CT specimens are about two times higher than these for the PM CT specimens.
The fracture mechanics parameter, C*, was used to correlate the CCG rates in the P91 and P92 CT specimens. The load line displacement rates and the CCG data, for the CT specimens, were used to calculate C* values according to ASTM E 1457-00 (2001). The Reference Stress was also found to correlate the CCG rates in the P91 CT specimens. Further, FE analyses were carried out to obtain the values of C*, based on the steady state value of the contour integral C(t). Stationary crack and growing crack CT models were used to obtain the C* values.
Damage mechanics theory and equations were used to predict the creep and CCG for the P91 and P92 materials using the commercial FE package, ABAQUS. Both the Kachanov and the Liu and Murakami damage models were used. In order to use these models, material properties have to be determined. The results of creep and creep rupture tests were used to determine those properties. It was found that both damage models could be used to predict the creep behaviour of the tested materials. However, the Liu and Murakami model was favoured over the Kachanov model in predicting the CCG in the CT specimens. The effect of the material multiaxiality is also highlighted.
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