Refining mechanism of crystal grains in the upset butt weld is studied by some model experiments under various conditions. According to the experiment on the relation between the temperature of the weld interface and the behavior of crystal grains under constant upset pressure, it is clear that if the temperature of the weld, interface is above a certain value in the upset butt welding process, grains of the weld become fine, and so the boundary between fine grains and coarse grains makes nearly a line, and grains change abruptly from fine to coarse at this boundary. Fig. 3(c), 6. These phenomena show that there is a critical temperature for refining the crystal grains in the weld. Also this critical temperature is mainly determined by material, and is between 420-620°C for pure iron (0.003%) and 620-800°C for carbon steel (S15C). From these values, these critical temperatures seem to be recrystalization temperatures. Next, the stage of grain refining in welding process is investigated by stopping the platen movement at maximum temperature of weld interface. As the result, it is made clear that grain refining occurs when the weld is pressed continuously in the stage while the interface is being cooled, and that upset pressure influences the degree of grain refining. Furthermore, it is shown that if the weld is not deformed, grain refining does not occur. (Fig.13). From the above results, refining mechanism of crystal grains is as follows. In upset butt welding, the weld is deformed continuously by upset pressure until the temperature of the weld becomes low. At this cooling stage, crystal grains become fine by successive recrystalization without giving no chance of grain growth, because the deformation takes place up to the recrystalization temperature.
Some researchers have carried out, from a dynamics point of view, the investigations for various aspects of end cracking in one side automatic welding. Especially, the many studies with respect to the relation of rotational distortion and end cracking have been done heartily, since this, end cracking is found to be a kind of hot cracking induced above 1, 200°C. So it is comfirmed that the opening displacement of groove in the vicinity of plate edge is important factor for end cracking. In this paper, the authors give tensile load intentionally to the weld metal during solidification process on the assumption that the tack weld is melted and the groove is rapidly expanded, and discuss the effect of opening displacement and displacement rate on the end cracking behavior, from the viewpoint of the hypothesis of rotational distortion.
Absorptance of various metals in polished surface condition at wavelength 10.6μ was evaluated at both room and fusion temperature, and as the results it was emphasized that the absorptance was too low. The authors proposed to improve the absorptance by the surface treatment. The surface treatments were divided into four groups; (a) roughened, (b) coated with non-metallic thin layer, (c) coated with fine powder and (d) created by the focused laser beam heating. Conclusions obtained may be summarized as follows: (1) The absorptance of metal sheet in the surface condition supplied by a commercial vender, which was highly polished, was proportional to a square root of the dc resistivity and agreed well with the theoretical values calcurated from Hagen-Rubens's equation at both room and fusion temperatures. (2) Surface treatments such as superficial roughening or coating having a material of lower boiling point than base metal had little effect on the absorptance. (3) When the focused laser beam impinged upon the aluminium specimen in the surface condition (b) or (c) for a long time, a part of the surface-treated layer remained so as to keep the considerably high absorptance. However, in the case of SUS27 stainless steel specimen absorptance decreased with beaming time, at last down to the polished one due to complete removal of the surface-treated layer. (4) In the case of SUS27 there was large difference between the absorptance in different surface conditions, and coating with non-metallic fine powder had the largest effect on the absorptance. On the other hand, in the case of Al there was little difference between the absorptance in different surface condition except for coating with a material of very low boiling point. (5) Though, in polished surface condition, Al was very difficult for CO2 laser processing in comparison with SUS27, surface treatment made the processing easier for Al than SUS27. (6) In generel, the absorptance of metal had a tendency to increase with increasing the traveling velocity of the laser heat source and decreasing its power level in the surface conditions (b) and (c), but the tendency was completely contrary in the surface condition (d).
The conditions under which Stress Relief Cracking (SR cracking) may occur and characteristics of SR cracking have been investigated with welded high tensile strength steels of tensile strength 80 kg/mm2 and low alloy steels, lCr-0.5Mo-V, Nb, or Ti steels using the bend restraint SR cracking test specimens and y groove restraint SR cracking test specimens. The following results were obtained from. this investigation. (1) SR cracking is an intercrystalline cracking which occurs in the coarsened grains region in H.A.Z. during stressrelieving heat treatment and does not occur in deposited metal and unaffected parent metal. (2) The residual stress and local stress raisers are necessary to initiate this cracking. (3) The aging in H.A.Z. during stressrelieving heat treatment, especially precipitation hardening by the secondary hardening elements, Nb, Ti, V, etc., promotes the initiation of SR cracking. Therefore, the initiation of SR cracking is suppressed when micro structures of the upper bainite, ferrite and perlite are produced in coarnsened grain region by the increase of heat input and pre-heating during welding process, and secondary hardenings are limited during stressrelieving heat treatment.