Stress-Strain Characteristics of High Strength (60kg/mm²) Steel Welds Subjected to Plastic Bending

Abstract

Transverse-weld bending tests under alternating loads beyond the elastic range were carried out on the welds of a quenched and tempered high strength steel indicated in Table 1 for the purpose of evaluating the stress-strain characteristics of the individual zones of weld, such as base metal, heat-affected zone and deposit metal. The test specimens were cut out of different types of welds, viz. bead, semi-circle-groove and U-groove welds, as shown in Figs. 1 to 3. After hardness tests, microscopy and tension test, bending tests were performed, in which the strain at each zone and the deflection in parallel part were measured with a strain gage of 1 mm gage length and a dial gage respectively and then the elastic limits at 0.002% offset as well as the mode of hysteresis loops of the stress-strain diagrams, for example, represented in Fig. 8 were observed under repeated stress cycles. The results obtained are as follows:
Both the over-heated and the softened base metals in the heat-affected zones, -both of them having been most hardened and softened respectively in each weld, -exhibit generally lower elastic limits than the unaffected base metal and deposit metal, as illustrated in Figs. 6, 10 and 15. Figs. 10 and 15 show also that the reversal of direction of stress in cyclic bending noticeably decreases the elastic limits of all zones, especially those of the over-heated and the softened zones, to a fraction of the initial value of the base metal, owing to the Bauschinger effect and the internal stresses caused by the previous bending. And then, in Figs. 11 and 13, the widths of hysteresis loops of these heat-affected zones are considerably larger than those of the base metal at the same stress cycles. These properties of the over-heated zones seem to be accounted for by their microstructures consisting of coarse-grained proeutectoid ferrite and matrix of bainite plus martensite, as shown in Photo. 4 2 6, the former being responsible for the lower elastic limits as well as wider hysteresis loops and the latter for the high hardness.