An experimental study was made on the spot welding of 18Cr-10Mn stainless steels containing high concentration of nitrogen up to about 0.8%, prepared by a high-pressure melting process in nitrogen atmosphere. Main results obtained were as follows. (1) In the spot welding of high nitrogen 18Cr-10Mn stainless steels, if ordinary welding conditions recommended for welding commercial austenitic stainless steels are adopted, we shall usually be able to obtain excellent and sound weld nuggets. (2) The spot weldability of the steels scarcely depends upon nitrogen concentration, and is about the same as those of commerical austenitic stainless steels. (3) But, in the steel containing high concentration of nitrogen i.e., more than 0.5%, expulsion and blowhole in the weld nugget increase a little. Therefore, in case of welding such a steel, better properties of the nugget will be obtained by adjusting welding conditions to prevent them. (4) The nitrogen contained in base metals scarcely diffuses out of the nugget even in the steel containing about 0.8% nitrogen. Consequently, the spot weld shear strength rises in proportion to the increase in nitrogen content and any decrease of it does not occur on account of nitrogen diffusion.
In this report, the breakdown of oxide layer between the sheets and the subsequent temperature rise will be discussed in detail. Firstly, it is observed that there is a clear difference in the mode of temperature rise between mild steel and Al alloys under degreasing treatment. In the surface preparation for spot welding of Al alloys, degreasing and pickling treatment as well as degreasing was occasionally used. We compare the temperature pattern of degreased specimens with that of degreased and pickled specimens, and see how changes in surface preparation influence on the initial temperature rise. The temperature patterns of the degreased and pickled specimens have good reproducibility and symmetry. Scattering of weld strength is small, if degreasing and pickling treatment is applied to Al alloys. An interesting fact is that the temperature rising region increases its area remarkably within one or two cycles at 60 Hz, which is a general character of spot welding of Al alloys. Furthermore, it is found that a specimen of Al alloys (degreasing treatment) which is scratched with a scriber the checker on the contact surface between the specimens shows the tendency of temperature rise which closely resembles that of mild steel. Lastly, from the result of many observations, contribution of a contact surface between the specimens to temperature rise is examined, and it becomes evident that this contribution is mainly caused by the plastic deformation on the contact surface, i.e., sheet separation. Meaning of contact resistance in spot welding is also discussed.
Microscopic survey, measurement of grain size and electron probemicroanalysis of Fe-Al, Fe-Cr, Fe-W, Fe-V, Fe-Si, Fe-Ti Fe-Mo and Fe-Zr weld metals were performed. The results obtained are summarized as follows: (1) The solidification structure of Fe-Al, Fe-Cr, Fe-W, Fe-V or Fe-Si weld metals changes from equiaxed to columnar crystal with increasing contents of alloying elements. (2) The solidification structure of Fe-Ti weld metals changes from equiaxed to columnar crystal and again to equiaxed crystal attended with the segregation of titanium. (3) The solidification structure of Fe-Zr weld metals changes from equiaxed to cell structure and further to cellular dendrite with increasing zirconium content. (4) The segregation of alloying element is observed on sub-boundaries of the cell structure and cellular dendrite in Fe-Zr weld metals. (5) The solidification structure is disarranged during cooling through the γ-loop. (6) Concentration gradients of alloying elements of 10 to l00μ width are observed.near fusion lines. (7) The alloying elements in weld metals appear to diffuse into unmelted base metal from 5 to 10 μ. (8) The macroscopic segregations of alloying elements are not seen in the weld metals.
On the basis of results shown in reports 1-4, a method of predicting the reduction in residual stress due to stress relief heat treatment was studied. As the behaviour of welded part in stress relieving process is different from that of base metal, a proper method was applied individually for each part. Next, stress relieving process was divided into three stages, heating, holding and cooling. Estimation formulae were established for each stage besides cooling stage, in which stress drop was neglected. From these studies, the following findings are obtained. 1 Reduction of residual stress in the base metal part due to S.R. Treatment could be estimated from creep curves in transient region. 2 Reduction of residual stress in the welded part could be estimated from test results on HL specimen. 3 Estimated residual stress by the above method showed a little higher value than experimental value, especially for welded part. 4 Effect of heating rate became negligibly small after holding over 30 mimutes. Thus the stress drop in the heating stage is less-important when holding is made.
Processes of transverse angular distortion free from restraint and bending moment reacting to restraint of the distortion in bead-on-plate welding as well as in multi-layer groove welding of mild, steel plate have been observed (Figs. 1, 7 and 11). Thermal cycles at some points in welded region have been also recorded. The results are summarized as follows: Angular distortion and bending moment develop as the welded region cools from high temperatures, and then disappear when the distribution of temperatre becomes uniform across the thickness of weld, though the temperature of weld is still as high as 600°to 500°C (Figs. 8, 9, 12, 13 and 14). At cooling stage of first layer in groove welding, additional angular distortion and bending moment occur in the reverse direction to those of the normal ones. In progressive welding the shrinkage and angular distortion induced in welded region are unable to occur freely on account of the resistance of weld metal previously deposited, and finally residual bending moment, amount and direction of which vary along the seam, appears in theweldment (Figs. 5 and 6).