Residual Stress Analysis in Girth-welded Ferritic and Austenitic Steel Pipes Using Neutron and X-Ray Diffraction

Abstract

Abstract. This paper is dedicated to the thorough experimental analysis of the residual stresses in the vicinity of tubular welds and the mechanisms involved in their formation. Pipes made of a ferriticpearlitic structural steel and an austenitic stainless steel are each investigated in this study. The pipes feature a similar geometry and are welded with two passes and comparable parameters. Residual strain mappings are carried out using X-ray and neutron diffraction. The combined use of both techniques permits both near-surface and through-wall analyses of the residual stresses. The findings allow for a consistent interpretation of the mechanisms accounting for the formation of the residual stress fields due to the welding process. Since the results are similar for both materials, it can be concluded that residual stresses induced by phase transformations, which can occur in the structural steel, play a minor role in this regard. IntroductionCurrent fatigue design standards and recommendations, like the ones given by the International Institute of Welding (IIW) [1], are based on the assumption of yield strength magnitude tensile residual stresses if the actual residual stress state is unknown. This postulate reflects uncertainties about the initial residual stress state after welding, which may depend on numerous parameters, as well as about the possible relaxation of residual stresses, which occurs when the static or cyclic yield strength is exceeded locally. Therefore, research concerning both the development and the relaxation of residual stresses is needed to improve the generalized approach given in [1]. Experimental and numerical analyses of welding residual stresses hardly ever show that the conservative assumptions made in the IIW recommendations hold. In girth-welded pipes, the pipe geometry and the heat input have been identified as the governing factors for the residual stress development [2,3], apart from material parameters. Under suitable conditions, pipe wall bending can occur, leading to compressive axial residual stresses at the weld toe of girth welds, but also to tensile residual stresses at the weld root, which was shown by other authors, see e.g. [2,3], and in previous studies on ferritic-pearlitic and austenitic steel pipes using X-ray diffraction [4][5][6]. In this work, these will be supplemented by neutron diffraction measurements and the results obtained from the two different steels will be compared. The thorough experimental analysis of the residual stress state after welding will serve as a basis for the investigation of residual stress relaxation under loading and the validation of numerical simulations, both of which are subject of future work.

Study on the residual stress relaxation in girth-welded steel pipes under bending load using diffraction methods

“…Here, the XRD measurements on the inner surface of pipe sections yield different results than the ND measurements on intact pipes for both the original residual stresses after welding, see also [19], and the residual stress state after loading. In both cases, the ND measurements prove that the highest axial and hoop residual stresses occur in the weld metal or at the weld root notch, whereas the XRD measurements underestimate these stresses due to a relaxation effect occurring during the sectioning process.…”

“…It has been shown that the pipe geometry and the heat input are the governing factors for the residual stress development [15,16], apart from material parameters. The characterization of the welding residual stresses in the pipe welds analyzed in the current paper has already been carried out by the authors [17][18][19], using mainly X-ray and neutron diffraction. It was found that the circumferential contraction of the weld and the highly heated areas in its vicinity during cooling is constrained by the adjacent material, thus causing tensile hoop residual stress in the former and compressive hoop residual stress in the latter areas.…”

A Survey and Comparison of Methods for Quantitative Texture Analysis

“…The Second Generation Neutron Residual Stress Mapping Facility (NRSF2) within the High Flux Isotope Reactor (HFIR) is typically employed for bulk residual stress mapping of engineering materials (Hempel et al, 2016) and the Nanoscale Ordered Materials Diffractometer (NOMAD) at the Spallation Neutron Source (SNS) is typically employed for Pair Distribution Function (PDF) analysis of liquids and amorphous solids (Neuefeind et al, 2012). The goal of this study is to demonstrate that both of these instruments can be used to measure texture reliably.…”