Weld Heat-affected Zone Material Research Articles

Effect of thermal aging on fracture toughness of RPV steel

The effect of thermal aging on mechanical properties and fracture toughness was investigated on pressure vessel steel of light water reactors. Submerged are welded plates of ASME SA508 C1.3 steel were isothermally aged at 350°C, 400°C and 450°C for up to 10,000 hrs. Tensile, Charpy impact and fracture toughness testings were conducted on the base metal and the weld heat affected zone (HAZ) material to evaluate whether thermal aging induced by the plant operation is critical for the integrity of the pressure vessel or not. Tensile properties of the base metal was not changed by thermal aging as far as the thermal aging conditions were concerned. Relatively distinct degradation was observed in fracture toughness J IC and J- resistance properties of both the base metal and the weld HAZ material, while only slight changes were observed in Charpy impact properties for both of them. However, it was concluded that the effect of thermal aging estimated by 40–80 years of plant operation on fracture toughness of both materials is small.

Crack growth in brittle materials exhibiting primary creep

Engineering materials such as ferritic steels for turbine rotors and in weld heat affected zones exhibit substantial primary creep, and low failure strains (less than 1%), at low operating stresses. In components containing initial defects, the possibility of the growth of the defect by creep must be considered. This paper initially examines time and strain hardening primary creep under conditions of stress relaxation in uniaxial specimens. Uniaxial failure by damage accumulation during stress relaxation is also considered for a brittle ferritic steel representative of weld heat affected zone material. An approximate analysis is presented to obtain near crack tip stress and strain rate amplitudes for time and strain hardening primary creep. The amplitudes are found for the transient state from the initial elastic to the fully primary creep stresses. Characteristic times for the transition from small scale to large scale primary creep are determined for time and strain hardening creep. It is found that the strain hardening transient amplitude and characteristic times are greater than for the time hardening case. The derived transient amplitudes and characteristic times are used to determine crack initiation times assuming that near crack tip damage accumulation takes place. Crack initiation times are shorter in strain hardening than in time hardening primary creep. Subsequent crack growth using experimental crack growth rate data is explored. The growth of a crack relative to the growth of the small scale creep zone in both time and strain hardening primary creep is determined. Results show that for strain hardening primary creep in a creep brittle weld heat affected zone steel, the crack grows rapidly out of the transient creep stress field into the surrounding elastic field.