Residual stresses are introduced into engineering components during welding and other manufacturing processes. Knowledge of these stresses can help to determine the structural integrity of the component and prevent stress corrosion cracking, therefore understanding the distribution and magnitude of residual stresses is very important. This paper presents the work carried out on a stainless steel pipe manufactured by cold rolling 17mm thick, 304 stainless steel plates to form three tubes attached with circumferential welds to create a full cylinder of roughly 3660mm axial length, 1710mm outer diameter and 17mm thickness.
In order to quantify the residual stresses in the cylinder the Contour, iDHD, ICHD and XRD measurements were carried out at different locations, including the circumferential weld, the seam weld and the repaired location. Additionally, ultrasonic residual stress measurements were carried out to detect weld repair locations and were particularly effective at identifying the hotspot locations.
The goal of the work described in this paper is to determine the stress states that exists at various locations within a stainless steel pipe by evaluating the properties of a full-diameter cylindrical mockup. This paper describes the design and procurement of the mockup and the characterization of the stress state associated with various portions of the container.
In order for SCC to be a viable degradation mode, three criteria must be met - there must be a sufficiently large tensile stress in the material to support crack growth, the material itself must be susceptible to SCC, and the environment must be sufficiently aggressive to support crack initiation and propagation. The work described in this paper is aimed at evaluating the first of these criteria for in-service pipe by characterizing the material properties of the base metal and weld zones on the canister mockup. Assessment of residual stresses associated with forming and welding was performed using a combination of five techniques. These include deep-hole drilling, the contour method, the centre-hole drilling x-ray diffraction, and ultrasonic testing. The deep-hole drilling technique allows measurement of residual stresses along a one-dimensional hole drilled through the wall of the cylinder; it allows the residual stresses within the container to be assessed while it is intact and hence, captures the effects of the cylindrical constraint on the stresses. The contour method provides a two-dimensional map of stresses along a cross section through a region of interest; however, the mockup must be cut into pieces to measure the face of the cross section, and stresses due to the constraint of the intact cylinder are lost. The centre-hole drilling method provides measurement at the surface while the X-ray diffraction allows assessment of very shallow near-surface stresses associated with shaping and grinding the mockup. It is also used to map stress components that are in-plane with the cross sectional surface, when using the contour method. Ultrasonic testing evaluates the change in sound velocity due to a change in local stress, and is able to evaluate stresses at a depth of 2.5mm within the stainless steel (as implemented here). The ultrasonic technique is non-destructive, and relatively new as a stress analysis method. It has been included here for comparison to the other techniques discussed above.