The effect of the residual plastic strain on the diffusivity of hydrogen was studied by means of the occlusion test and permeation test. Specimens were strained in four stages by plastic deformation. (residual plastic strain ε=4% 8%, 12% and 16%) Hydrogen was charged cathodically. From the results of these tests, it was found that the hydrogen diffusivity depends upon the residual plastic strain. Generally, hydrogen diffusion coefficient increases with an increasing strain. From these results and previous work, it is considered that diffusion coefficient of hydrogen in steel specimen under various applied stress or strain increases within the order of 10-7 and 10-6 cm2/sec.
Recently, steel structures are in a trend to be designed with consideration of their reserved strength in the plastic range. Accordingly, the strengths of welded connections are also to be discussed from this point of view. There are only few papers which dealt with elastic-plastic behavior of welded connections and consequently, present conventional design formulae of welded connections seem to be based on the simple elastic analysis or the experimental results on the fracture strength. In order-to clarify the elastic-plastic behavior of welded connections, a series of experiments was conducted on cruciform connections with fillet weld under uniaxial forces, pure bending, and pure shear, and their combination. From the results, it was found that the connection showed a sudden reduction of regidity at general yielding and reached its ultimate strength after a large amount of plastic deformation. Therefore, it is very reasonable to discuss the strength of welded connection in terms of the above two points, i.e., yield and ultimate strength. Theoretical analysis was also made on the above mentioned strength with the aid of plastic analysis method. The results of the analysis well coincide with those of the experiment and locate well the actual sections in which general yielding and ultimate strength are attained. Based on the results of the investigation, design fomulae are proposed for the yield and ultimate strength of welded connection.
An experimental study was made on the spot welding of 18Cr-7Ni stainless steels containing high concentration of nitrogen up to about 0.6%, 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-7Ni stainless steels, if ordinary welding condition recommended for welding commercial austenitic stainless steels are adopted, we shall obtain usually-excellent and sound weld nuggets. (2) The spot .weldability of, the steels scarcely depended upon nitrogen concentration. The weld defects such as weld cracking, welding distortion and large pit were not observed in any steels investigated. (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 high nitrogen steels, better properties of the nugget were obtained by taking electrode force comparatively high and weld time short with an increasing nitrogen content. (4) It was confirmed that the nitrofien contained in base metals scarcely diffuses out of the nugget even in the steel containing about 0.6% nitrogen. Consequently, the spot weld shear strength rises in pro-portion to the increase in nitrogen content and any decrease of the strength does not occur on account of nitrogen diffusion.
This report discusses mechanical properties, in particular ductility, at room temperature of steels subjected to prestrain during weld thermal cycles. Three kinds of steels, a plain carbon steel (SM-41A), 50 kg/mm2 high strength steel (SM 50) and a heat-treated high strength steel (HY 80) were used for experiments. (See Table 1.) Thermal cycle simulating weld heat-affected zone was applied to a round bar specimen shown in Fig. 1 and plastic strain was given at a temperature during cooling. Fig. 2 shows schematic explanation of the test procedure. High frequency induction heating was used to obtain a synthetic weld thermal cycle. Plastic strain was given for about 4 seconds at a temperature during cooling through the tensile load produced by compressed air. Tension test was done at room temperature on the specimensubjected to the thermal cycle and prestraining. Attention was paid to general elongation or strain at the maximum load in addition to usual mechanical properties such as reduction in area, yield stress, ultimate tensile strength and fracture stress. The mechanical properties at room temperature change with cooling conditions, prestraining temperature and magnitude of prestrain. Prestrainig at 400°C to room temperature, in particular in the temperature range of blue brittleness, increases flow stress and decreasees ductility at room temperature (See Figs. 10 and 11.) Serious effect was the decrease in general elongation at room temperature. (See Figs. 12 and 15.) General elongation of SM41A steel and HY80 steel decreased respectivity from 24% and 15% as received to 15% and 9.5% after thermal cycle and to only 8% and 1% after thermal cycle combined with 5% prestrain at 200-300°C during cooling. The general elongation decreased with the increase in ratio of yield stress to ultimate tensile strength. (See Fig. 24).