• Paddea, Sanjooram

Abstract

The so-called creep strength enhanced ferritic (CSEF) 9-12% Cr steels have been identified as the most promising class of materials for some of the key components in ultra-supercritical fossil-fired power plants, including the main steam pipes, headers and superheater tubings. These steels are less costly, and they have a lower coefficient of thermal expansion and a higher thermal conductivity when compared with austenitic stainless steels, making them less susceptible to degradation through thermal fatigue. However, experience has shown that the weldments in these steels are particularly prone to premature creep failure, due to a localised form of cracking in the heat-affected zone {HAZl, which is referred to as Type IV cracking. The work presented in this thesis is concerned with the effects of residual stresses and constraint on Type IV cracking. In the first part of this work, the residual stresses in a 25.4 mm thick, 324 mm diameter pipe girth weld, made in a P91 steel pipe, have been measured in both the as-welded and post weld heat treated (PWHT) conditions using neutron diffraction, and compared with the corresponding metallurgical zones across each weld. It was found that the highest as-welded tensile stresses resided near the outer boundary of the HAZ, and towards t he weld root region and these were not fully relieved by the applied PWHT. In both conditions substantial tensile direct and hydrostatic stresses existed across the HAZ, including the fine-grained and intercritically-annealed regions, where premature Type IV creep failures manifest in 9-12% Cr steel welds. Compressive stresses were found in the weld metal coinciding with the last weld bead to be deposited. In the second part of the work, creep tests were conducted at 625°C on cross-weld and simulated fine-grained HAZ specimens. The contributions of specific influences on creep performance (such as residual stress, constraint and creep damage associated with relaxation of residual stresses during PWHT) were then systematically examined. It was found that the geometric constraint (introducing a triaxial stress state) was beneficial in improving creep rupture life and that residual stresses (of the order of 50 MPa) showed a. clear reduction in life. Moreover there was some evidence that residual stress relaxation associated with PWHT may introduce some creep damage. The digital image correlation (DIC) technique was applied to resolve tensile and time dependent creep deformation properties along the length of P91 cross-weld samples. The results demonstrated the capability of the Die technique for full field measurement of strain during both room-temperature-tensile and high temperature creep tests in the vicinity of welded joints, where the gradients in microstructure and mechanical properties can be steep.

Topics

• impedance spectroscopy
• microstructure
• stainless steel
• laser emission spectroscopy
• strength
• fatigue
• neutron diffraction
• thermal expansion
• thermal conductivity
• creep
• creep test