Stress and creep damage evolution in materials for ultra-supercritical power plants

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

Mitigating Cutting-Induced Plasticity Errors in the Determination of Residual Stress at Cold Expanded Holes Using the Contour Method

Background The split sleeve cold expansion process is widely used to improve the fatigue life of fastener holes in the civil and military aircraft industry. The process introduces beneficial compressive residual stresses around the processed hole, but uncertainties remain about the character of the stress field immediately adjacent to the bore of the hole.ObjectiveThe primary objective of this study was to implement the contour method with minimising error associated with cutting-induced plasticity to provide detailed and reliable characterisation of the residual stress introduced by the split sleeve cold expansion process.MethodsA systematic FE study of plasticity effects by simulating different contour cutting strategies (a single cut, two sequential cuts and a 6-cut sequence) for a cold expanded hole in an aluminium alloy coupon was conducted. The identified ‘optimum’ cutting strategy was then applied experimentally on coupons containing a hole that had been processed to 3.16% applied expansion. ResultsThe FE study of different cutting simulations show that the locations of the stress error is consistent with the location where cutting-induced plasticity accumulated and that the magnitude and locations of stress re-distribution plasticity can be controlled by an optimised cutting strategy. In order to validate this hypothesis a high quality contour measurement was performed, showing that accurate near bore stress results can be achieved by the proposed 6-cut approach that controls cutting induced plasticity.ConclusionsThe present work has demonstrated that detailed FE simulation analysis can be a very effective tool in supporting the development of an optimum cutting sequence and in making correct choices of boundary conditions. Through optimizing these key aspects of the cutting sequence one is much more likely to have a successful, low error contour residual stress result

Quantification of Residual Stress Relief by Heat Treatments in Austenitic Cladded Layers

The effect of the heat treatment on the residual stresses of welded cladded steel samples is analyzed in this study. The residual stresses across the plate’s square sections were determined using complementary methods; applying diffraction with neutron radiation and mechanically using the contour method. The analysis of the large coarse grain austenitic cladded layers, at the feasibility limits of diffraction methods, was only made possible by applying both methods. The samples are composed of steel plates, coated on one of the faces with stainless steel filler metals, this coating process, usually known as cladding, was carried out by submerged arc welding. After cladding, the samples were submitted to two different heat treatments with dissimilar parameters: one at a temperature of 620 °C maintained for 1 h and, the second at 540 °C, for ten hours. There was some difference in residual stresses measured by the two techniques along the surface of the coating in the as-welded state, although they are similar at the welding interface and in the heat-affected zone. The results also show that there is a residual stress relaxation for both heat-treated samples. The heat treatment carried out at a higher temperature showed sometimes more than 50% reduction in the initial residual stress values and has the advantage of being less time consuming, giving it an industrial advantage and making it more viable economically

Experimental Study of the Weld Residual Stress in Manually and Mechanically Fabricated Dissimilar Metal Weld

The dissimilar metal welds between the Inconel 600 flow element and the SA-106 Grade B carbon pipe with Alloy 82 or Alloy 182 filler material of some CANDU® designs have been identified as being susceptible to Primary Water Stress Corrosion Cracking (PWSCC). Initiation and growth of PWSCC in a Dissimilar Metal Weld (DMW) are driven primarily by Welding Residual Stresses (WRS). The present paper focuses on the experimental study of weld residual stress distribution in manually and mechanically fabricated DMWs with emphasis on the effect of repair. A series of DMW samples are firstly fabricated in accordance with the original welding procedures for those DMWs in the field, which were fabricated in 1970s and 1980s. Multiple thermocouples were used to record the temperature evolution during the entire welding process. These samples were then examined by ASME qualified personnel in accordance with the requirements for Class 1 weld in Article 9 of Section V of ASME BVPC using Visual Testing (VT) and Radiography Testing (RT). Repair was then performed in some samples, and further NDE examinations were performed. The qualified samples (with and without repair) were finally subject to destructive weld residual stress measurement using contour method. It is observed that weld repair dramatically changes the distribution of weld residuals tress. The use of a constant through-thickness WRS of 60,000 psi (415 MPa) is justified as the bounding case

A study of fracture behaviour of gamma lamella using the notched TiAl micro-cantilever

Microcantilevers were machined from a single $\gamma(TiAl)-\alpha_2(Ti_3Al)$ lamellae colony of a polycrystalline commercial Ti-45Al-2Mn-2Nb sample using the focussed ion beam. The long axis of the microcantilevers is perpendicular to the lamellar interface with the notch prepared in the same γ lamella. Bending test results demonstrated good repeatability and the fracture stress intensity factor of individual γ lamella was measured to be $3.1 \pm 0.2 MPa\ m^{1/2}$. $(\overline{1}\overline{1}1)[\overline{1}\overline{1}\overline{2}]$ twinning ahead of the notch (crack) tip plays a key role in the crack initiation. TEM analysis shows that the interaction of twinning with the lamellar interface leads to interfacial cracking and contributes to the work-hardening observed

Residual stresses in thick-section electron beam welds in RPV steels

Low pressure electron beam welding offers the prospect of large increases in productivity for thick section welds in RPV steels. However, it is important to understand how this welding process affects the structural performance of the completed weld. This paper reviews and presents key results from a programme of weldment manufacture, materials characterisation, residual stress measurements, and finite element modelling of EB welds made in plate of three thicknesses, 30mm, 130mm, and 200mm, and in three steels: SA508 Gr 2, SA508 Gr 3 C1 1, and SA516 Gr 70.</jats:p

Application of multiple residual stress determination methods to coarse-grained biomedical implant castings

ASTM F75 femoral knee implant components distort during manufacture due to residual stress re-distribution or inducement. X-ray diffraction, neutron diffraction, centre-hole drilling and the contour method residual stress determination techniques were applied to as-cast and/or shot-blasted components. The centre-hole drilling and contour methods can only be considered qualitative as a result of uncertainty associated with the elastic anisotropy of gauge volumes. Additionally, neutron diffraction experimentation returned unfeasible results. However, it was qualitatively identified that a shot-blasting shell-removal process has the ability to significantly alter the bulk residual stress state of the implants and induce a stress state which would cause distortion by re-distribution following material removal during manufacturing processes

Transforming e-waste into value: A circular economy approach to PCB recycling

This study delves into the pressing global issue of electronic waste (e-waste), with a specific emphasis on printed circuit boards (PCBs). PCBs, being a crucial component of electronic devices, contribute significantly to e-waste due to their intricate composition of hazardous substances and valuable metals such as gold, silver, and copper. The research explores the concept of the circular economy—an economic system aimed at eliminating waste through the continual use of resources—and its potential application in recycling PCBs. This involves a detailed investigation of the challenges and opportunities associated with various extraction methods and waste management strategies. The study also presents a case study on The Royal Mint’s innovative approach to gold extraction from PCBs. This practical example offers valuable insights into the application of circular economy principles in the context of PCB recycling, demonstrating how these principles can lead to improved resource efficiency, waste reduction, and economic benefits. The findings of this research underscore the need for further development and implementation of sustainable practices in e-waste management to mitigate environmental impact and capitalise on the economic potential of valuable materials in e-waste

Residual stress mapping in additively manufactured steel mould parts using asymmetric and multiple cuts contour method

The distribution of residual stress in additively manufactured (AM) complex structures can be intricate, exhibiting large gradients. Accurate measurements with high spatial resolution at significant depths are crucial for understanding the residual stress distribution in AM parts. In this study, the contour method, with asymmetric and multiple cuts, is employed to map residual stresses on various cross-sections of AM steel mould parts with inner channels. The resulting residual stress maps are compared for different states: as-built, shot-peened, and shot-peened plus heat-treated, in order to quantify the stress relaxation effect of each processing step. In the as-built state, elevated levels of tensile stress are observed around the perimeter, with a substantial stress gradient throughout the depth. Compressive stresses are distributed over a larger interior region, but with lower magnitudes compared to the tensile stresses. Upon shot-peening, the tensile stresses are reduced by approximately 300 MPa around the perimeter, with limited relaxation up to a depth of 1 mm compared to the as-built state. In the shot-peened and heat-treated state, a more significant stress relaxation is observed across the cross-sections. The maximum relaxation in tensile stress reaches approximately 1200 MPa, while the maximum compressive stress relaxation amounts to around 500 MPa, both compared to the as-built state. The stress maps effectively identify regions experiencing significant stress states, and the two-dimensional comparisons between different stress states provide crucial information for enhancing and validating AM models and processing techniques