IIW History 1948-1958

110 -

E. Spot Welding of Mild Steel. \i\Tith mild steels, there are the same problems t o be solved as with low alloy high t ensile st eels; in the first place, it is necessary to lay down testing methods enabling the quality of the spot welds to be evaluat ed so as to make possible the definition of what is meant by a good quality weld, in relation to the testing methods adopted. H owever, whereas the use of high tensile steel is limited to high qualit y construction, the use of mild steel represents, perhaps, 95 % of the applications of spot welding in all types of products. It is quite obvious that the production of a railway carriage, or of a motor car, calls for the use of methods giving much greater reliability than that required in the manufacture of cigarette cases or of lampshades. Thus the problem arises whether, in the case of mild steels, it is necessary to provide for several qualities of spot weld. F inally, the designation (( mild steel »covers a very large quantity of materials of very variable quality, so that the weldability of mild st eel in spot welding must be taken into consideration in the case of products for \vhich a high degree of reliability is essential. Sub-Commission A was given the task of investigating these different questions; its title, when set up in 1954, ·was (( Spot Welding of Mild Steel ». Its Chairman is Mr. J o MAT (France) and it consists of representatives of Belgium, the United States, Switzerland and the United Kingdom. One of the first activities of this Sub-Commission was to consider to what extent the torsion shear test (1) could be used to evaluate a good weld on mild st eel. Many t ests were carried out by member countries of the Sub -Commission, as well as by Italy and Sweden. These t ests have shown that the convenient method of torsion shear testing, described in relation to high tensile steels, is suitable for parts whose thickness exceeds r.5 mm. For thinner pieces, fracture often took place by tearing and not by shearing through the plane of the joint, which makes the application of this method impossible. It was then possible to draft and publish a D efinit1;on of a F irst Quality S pot T!fl eld (z) which includes two parts : - one, general, contains specifications of dimensions (diamet er, indenta– tion , the magnitude and distribution of allowable faults) which are valid, indep– endent of the method of test, - the other describes the results which, in addition to the preceeding, should be obtained when the spot is tested by the torsion shear method. The comparative examination of the methods of e\ aluation of spot welds is being pursued and the British delegation has submitted a very well documented paper on this subj ect. This paper, the publication of which has been recom– mended, is entitled ( .-1 comparison of methods of assessing spot weld ductility ·in sheet steels » (3). In this comparison, the torsion shear test was carried out, not by hand, but on a specially designed machine which is described in the document « Torsion testing machine for spot welds» (4) which was also recommended for publication. Sub-Commission A was re-cast in 1957 and h as the title (( Weldability in Spot Welding», its main object being to find out, taking the hardness test as a basis of assessment, t he maximum carbon content for mild st eels above which heat treatment is necessary to obtain a good weld.

(r J Document III-35-54. (2) Document III-71-56. (3) Document III-77-57. (4) Document III-76-57.