Mechanisms of Underclad Intergranular Cracking in a Nuclear Pressure Vessel Steel

UoM administered thesis: Phd

  • Authors:
  • Alessandro Cattivelli

Abstract

Underclad cracking is a problem that can arise during the manufacturing of nuclear reactor pressure vessels. Many of the parts used to manufacture those components are forged form large low alloy steel cast ingots. Typically, the internal wall of the vessel is clad with stainless steel to protect the low alloy steel from the reactor coolant. The cladding process is usually performed by submerged-arc strip cladding. In the 1970s cracks were sometimes detected after the post weld heat treatment, under the cladding, in the heat affected zone of the low alloy steel base material. Since then, the problem has been solved from a practical point of view. However, the precise mechanism by which these cracks form has remained elusive. In recent years, some fabricators have considered a shift from submerged-arc strip cladding to electroslag strip cladding. To correctly assess the likelihood of underclad cracking reemerging, when shifting to a different deposition technique, it would be useful to have a better understanding of the cracking mechanism. Firstly, the thermal cycles to which the heat affected zone is subjected could create a microstructure that favours underclad cracking. Charpy sized coupons were extracted from a forged steam generator cylindrical outer structure. They were subjected to those thermal cycles and then fractured. The results, together with dilatometric measurements and microstructural observations, identified that a coarse grained heat affected zone, reheated to intercritical temperatures by a subsequent cladding thermal cycle, is likely to be brittle. However, fracture surface examination did not reveal extensive intergranular cracking in material that did not show obvious segregation regions. Secondly, the effects of segregation regions, specifically ghost lines, such as those that can be found in large steel cast ingots, were considered. Forged bars, of a composition that closely matched those typical of ghost lines, were manufactured intentionally. In these bars, microsegregation of some potentially embrittling elements was found in regions close to the former prior austenite grain boundaries. It is likely that this segregation occurred during solidification and that it persisted during subsequent thermal cycles. Heat affected zone conditions that produced a low density of high angle interfaces were susceptible to intergranular fracture after post weld heat treatment. The embrittlement most likely took place in the holding step. It is most likely to occur through a mechanism that is similar to temper embrittlement. Finally, a suitable stress state is necessary for underclad cracking to happen. Residual stresses were thought to build up during the heating ramp of the post weld heat treatment due to the mismatch between the thermal expansion coefficients of low alloy and stainless steel. Contour method measurements at room temperature and neutron diffraction measurements both at room temperature and after heating to 200 and 325 °C were performed on mockup plates from steam generator material. This was accompanied by tensile tests, at the same temperatures, with material in different heat affected zone conditions. High tensile longitudinal residual stresses, close to the yield strength, were present immediately after deposition. They persisted up to the mid-point of the post weld heat treatment heating ramp (i.e. up to 325 °C). However, the tensile tests showed that all of the investigated heat affected zone conditions retained sufficient ductility to make underclad cracking unlikely during the early stages of the heating ramp. Overall, the dominant influence of the cladding process itself on the likelihood for underclad intergranular cladding can be judged based on the prior austenite grain sizes that are achieved.

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Original languageEnglish
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Award date31 Dec 2020