Residual stress characterization of welds using x-ray diffraction techniques (1996)

Neglect of residual stresses created during welding processes can lead to stress corrosion cracking, distortion, fatigue cracking, premature failures in components, and instances of over design. Automated residual stress mapping and truly portable equipment have now made the characterization of residual stresses using x-ray difiaction (XRD) practical. The nondestructive nature of the x-ray diffraction technique has made the residual stress characterization of welds a useful tool for process optimization and failure analysis, particularly since components can be fieasfired hrire aqd afier I . I J ~ ! ~ ~ ~ ~ nnct yuV' tlrolrlinn .. w.Yn .b nrnrmxoc vw.3-w. This paper illustrates the importance of residual stress characterization in welds and presents examples where x-ray diffraction techniques were applied in the characterization of various kinds of welds including arc welds, TIG welds, resistance welds, laser welds and electron beam welds. Numerous techniques are available to help manage potentially harmful residual stresses created during the welding process thus, the effects of a few example post weld processes such as grinding, heat treating and shot peening are also addressed. INTRODUCTION The advantages of XRD and three areas of concern regarding residual stress and weld quality, stress corrosion cracking, fatigue, and stress concentrations, will be briefly discussed. F a t i ~ u e and Stress Concentrations Tensile residual stress fields created during the welding process often contribute to decreases in the fatigue life of welded components, especially when they exist in the HAZ. The residual stress state existing in certain weld toes and undercuts can also ba critical when stress concentration geometries exist which can magnify the effects of applied loads. fatigue cracking are considered, potentially When issues of harmfbl tensile residual stresses alone or in combination with stress concentrations can lead to fatigue crack initiation and propagation. This means that accurate residual stress characterization must be performed in key areas such as the toe and the I= of welds to understand fatigue failures, help in the experimental verification of stress concentration factors predicted by finite element models and to reduce instances of over-design and unneeded increases in weight. Environmentally Assisted Weld Cracking Also known as Stress Corrosion Cracking or SCC, environmentally assisted cracking is A rrajor source of potential failures in the process industries, in pulp mills, in storage vessels, and even in aircraft. Most often, SCC occurs in the heat affected zone (HAZ) immediately adjacent to a weld simply because the I N is left in a state of very high residual tensile stress as a result of the shrinkage and differential cooling occurring in most welds. Tensile stresses (residual or applied) are the main component of the stress corrosion cracking triangle: the other two are a susceptible metal and an environment that often needs to be only slightly corrosive to that metal. For instance, grade 316 stainless steel is essentially inert to the corrosive effect of common salt urtless tensile stresses are present, when it becomes very sensitive to chloride induced SCC. There are a number of possible solutions. The obvious one is to change the environment, but that is rarely possible. The next is to change the tnetal, but usually that is expensive and if the equipment is already built, impractical. Thermal stress relieving is a partial solution at best because, to relieve all the tensile stresses in the HAZ, it is necessary for the heating to reach the annealing temperature and the material properties will be lost. Corrosion engineers have long recognized that an effective solution for the retardation or even prevention of SCC is the introduction of compressive stresses in the I M . This can be done using controlled shot peening. Surface residual tensile stresses in the IIAZ may approach or even exceed the yield strength of the material but if the surface is shot peened, the dimpling action of the shot bombardtnent can reduce the tensile stresses and even replace them with residual stresses that are in compression if properly done. IIence, there is a clear need for accurate nondestructive residual stress characterization of welds to verify the residual stress state of weld HAZs in components that may be susceptible to SCC. This type of weld characterization can be performed t~ondestructively on components that are already in service and ideally, on welded components before they go into service. Advances in XRD Technology Accurate characterization of residual stresses in welds has been difficult and oflen impractical since most residual stress measurement techniques are destructive and lack the resolution to accurately characterize the steep stress gradients that exist in weld H M s . Recently, the measurement of residual stresses using x-ray diffraction techniques has becotne both practical and eflicient. In addition, the introduction of stress mapping techniques has allowed the quick and precise characterization of entire welds, including areas of interest such as steep stress gradients and their associated tensile residual stress maxima. 1%:: skess mzp dfsp!r;j! has given engineers i; cciiii;!zk and accurate visual analysis of the magnitude and distribution of residual stresses in their welded components. Since surface stress measurements using x-ray diffraction techniques are non-contact and nondestructive, meas~iren~ents can be performed at the same point before and after a process such as post weld heat treating or shot peening, so that residual stresses can be characterized at all steps of the manufacturing process. This can help in the optimization of these manufacturing processes and the management of their associated residual stresses. MEASUREMENT APPARATUS The residual stress measurements were performed at the Proto Mfg. Ltd. x-ray diffraction laboratory and in the field using a dual solid state detector diffractometer with automated stress mapping hardware and software. BASICS OF THE X-RAY DIFFRACTION STRESS MEASUREMENT TECHNIQUE The x-ray method does not measure stress directly but measures strain ffom which stress values are calculated. The x-ray method rather elegantly takes advantage of the crystalline structure of the material itself, by using the atomic lattice spacing as a strain gage. As a result, thousands of built in strain gages within the crystals which compose the material are available for strain measuretnent by the x-ray diffraction method. 'To phrase it more exactly, the surface strain present can be determined by the tneasurement of the elastic atomic lattice spacing or d-spacing as it is commonly called. Illis lattice spacing, the distance between the planes of atom;, is dependent upon the material and the stresses present in the material. The x-ray diffraction angle 0 for a given x-ray wavelength h can be used to determine the material spacing by means of Bragg's law: nh=2dsin 0 ( 1 ) For x-ray diffraction to occur, i.e, constructive wave interference, the path difherence traveled by the diffracted beam through the mateh l , as compared to a non-difl'racted beam, must be equal to nL (Noyan and Cohen, 1987). The presence of residual stresses in the material produces a shif? in the x-ray diffraction peak angular position (Cullity, 1978) which is directly measured by the detector. Once the lattice d-spacings are measured for the unstressed (do) and stressed (d,) material conditions, the atomic lattice strain can then be calculated by the following relationship (Hilley et. al., 1971): strain = (di-do)/do.. (2) For isotropic materials, strains can be converted to stress ... ....... ..&-, d l . , . :,I . , . . , l.,l,.., U I U L J U 3 l l l t j L11L LClu:IuII SLIUWII U b I U W . d -do E 1 stress ( c ~ ) = ( ~ ) ~ do s ~ n v (3) is the x-ray elastic col~stant, y is the angle where: (j+v) subtended by the bisector of the incident and diffracted beam and the surface normal, dv is the lattice spacing at a given yr tilt and do is the unstressed lattice spacing. Residual stresses are measured using either of two techniques. The first is the single exposure technique (SET), whereby a stress measurement is performed using only one tilt angle. This technique gives the user a very quick and eflicient method to perfom a stress measurement and is particularly suited to cases where many measurements are needed quickly. The second is the multiple exposure technique (MET), whereby multiple beta tilts are used in the analysis. The MET method is more revealing for material conditions for which the d vs. sin2\y relationship is not linear, as assumed in equation (3), but takes much longer than the SET (Klug and Alexander, 1974). Techniques are also available that deal with phase changes that may exist through the weld, the HAZ and the parent material (Noyan and Cohen, 1987).