IIW History 1948-1958

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stress. However, if high strain fatigue only were taken into account for the calculation of the allowable stress range, steels with a relatively low yield point, such as carbon steels, would be submitted, as a result of such high stresses, to severe strain hardening. There were indications that strain hardened materials had a tendaucy to much more severe corrosion than non-strain hardened material, the resulting surface cracks increasing the stresses locally to such a point that there was risk of failure after a few cycles. It was therefore decided that strain hardening \vas to be avoided in the interest s of safet y . It was also found that the possibility of strain hardening depended on the more or less local character of the stress . Sub-Commission B, under the Chairmanship of Dr. \V. B. CARLSON (United Kingdom), was able, having taken account of these phenomena, to calculate the allowable stress ranges without using a safety factor. These ranges could easily be plotted on a curve « conventional allowable stress t emperature » for carbon st eels and the usual low alloy boiler steels . The investigation has not, for the moment, been extended to the recently developed quenched and tempered steels . On comparing the curves of the allowable stresses obtained by calculation and the values drawn from the various codes, it was found that the maximum curve which could be traced by combining the practices of the different countries had never been exceeded. This supported the position of the Sub-Commission when, in 1955, it submitted the first t ext of its report to the members of the parent Commission . In 1956, at Madrid, the recommendations regarding allow– a ble stresses for carbon and low alloy steels were approved and fo rwarded t o ISO/TC II under the title Pressure Vessel Design . The Commission had a lready decided at a previous session in Oxford in l 95I that the use of a \veld efficiency factor did not rest on realistic premises and should be avoided. The peak stresses used for the determination of the allowable stress range had for the most part been measured on domed ends. The necessity fo r analysing these stresses made it possible to undertake the study of the calculation of domed ends. Although this study has no direct relationship with welding, Sub-Com– mission B decided to carry it out and domed ends we re added to the programme. During the Madrid assembly in 1956, Mr. GrLG (U.S.A.), who had been Chairman of the meetings of ISO/TC II held at Madrid in February of the same year, explained that it had been impossible for I SO to work out, for materials, international specifications which were entirely acceptable to the various national st andards institutes. He therefore asked Commission XI of the IIW to prepare a list of available materials for boilers and pressure vessels; this would form a substitute for an international specification, which would be too vague. A preliminary list of boiler quality st eels was prepared by the Chairman ; this list contained the principal standardised st eels from Belgium, France, Ger– many, Italy, the Netherlands, Sweden, the United Kingdom and the United States. In it we re given the mechanical properties of the materials, such as U.T.S., yield point, elongation, reduction of area, impact strength at room temperature, together with yield point, creep to rupture and creep strain values for the temperature ranges at which each steel is usable. The document also contained the chemical properties and references t o the reports from which were drawn the mechanical properties at higher temperatures. In the case of st eels whose properties at these high t emperatures are not known, curves indicat ed the minimum values guarantee ing safet y. The members of the Commission intend to ask their national st andards committees to examine