IIW History 1948-1958

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All residual stress measurement methods depend on measuring marks which of course can be applied to the surface of an object, thns enabling residual stress measurements t o be made on the surface but not in the interior of a work-piece. There is, however, some hope that, by making certain a prio1'i assumptions about the stress distribution in the interior and by making more or less continuous measurement s during sub-division, some indication of the stress values in the interior may be obtained. These efforts are continuing. A) Methods of stress relieving. lviore progress has been made by the Commission in assessing the value of differe nt stress relieving methods than perhaps in any other field of its activities. The results of these investigations were incorporated in a number of resolutions, adopted at various times during the existence of the Commission, which give v aluable guidance t o engineers responsible for the choice of stress relieving methods to be adopt ed in practice. With regard to pre-heating at t emperatures normally used, i.e. up to values of the order of 250°-300° C, the Commission concluded that this does not produce any substantial reduction in the magnitude of the residual stresses in welded mild steel. For this reason pre-heating cannot generally be regarded as a substitute fo r the relief of stresse by post-heating to 600°-650° C. The Commission recognised, however, that pre-heating i of real value in some cases and that benefits not connect ed with stress relief are to be obtained in addition to the limited reduction in residual stresses . Never– theless, warning was given by the Commission that local pre-heating may in fact give rise to increased residual stresses under certain conditions. Similar conclusions were reached with regard to stress relieving heat treat– ment at lmv temperatures. The Commission found that heat treatment at 200°-300° C vvas not as effective as the more normal treatment at 600°-65 0° C and that, whilst there is an appreciable reduction in the highest stresses in a \velded structure, the average level of stress would be only slightly reduced. Nevertheless, the Commission agreed that heat treatment at t emperatures lower than 600°-650° C could with advantage be considered for those constructions that cannot be given the higher t emperature treatment. The Commission recently concluded that only heat treatment at 650° C in a furnace or mechanical stress relieving, by plastic straining, at a temperature above the fracture appear– ance t emperature of the notch t ens ile t est , are fully effective stress relieving methods to prevent brittle fr acture at applied stresses substantially below the yield point of the steel plate. The process of heating two bands of the material at each side of the weld to a temperature of 200° C followed by a water quench and pre-heating to 200° C before welding were fo und to have a limited beneficial effect : but heat treatment to 250° C after welding and local heating of the weld to 650° C had no beneficial effect in preventing brittle fracture. B) Stress relieving of alloy steels. The Commission instituted an international enquiry to find out the ex t ent to which l)ractices in the stress relieving of alloy steels varied in different coun– tries and what the primary objective of such stress relieving treatments was. This enqniry showed that there was considerable divergence of opinion, not only with regard to the correct procedure to be adopted for constructions made from different alloy steels, but also with regard to the objective to be attained by such treatment. The Commission concluded from this that it was necessarv first of all to obtain objective data on the degree of stress relief obtainable by treatment at different t emperatures fo r different alloy steels, and a large co-operative programme of research was put in h and. The experimental work was carried out in Belgium, Germany, Sweden and the United Kingdom, on a number of im-