Fracture toughness of explosive clad composed of SUS 304 stainless steel and HT80 high strength steel is investigated. To clarify the effect of metallurgical properties and composite structures on the fracture toughness, both V-notch Charpy test and bending COD test are carried out by using homogeneous and heterogenous specimens which are cut from various portions in thickness direction of the clad plate and have notch root or precrack leading edge parallel to the plate surface. The energy absorbed for fracturing the specimens and its component energy for crack initiation from notch root in V-notch Charpy test are affected by the material involving notch root and by the thickness ratio of two different steels in net section of specimen. In static bending test of the heterogeneous specimens with pre-crack tip in stainless steel, fracture occurs in the high strength steel adjacent to weld interface before the pre-crack extends although ductile fracture occurs at pre-crack tip over all temperature range. In relation to crack initiation, the critical COD, if the first fracture initiation in interface zone can be measured, and the stretched zone width, if fracture firstly occurs at pre-crack tip, can be a parameter of fracture toughness respectively. In the heterogeneous specimens with pre-crack tip in high strength steel, both the critical COD at low temperature where ductile fracture has not yet initiated and the stretched zone width over all tem-perature range can be fracture toughness parameters since the fracture toughness values depend largely on the material involving pre-crack tip, namely, high strength stecl.
For pressure hulls of Deep Submergence Vehicle (DSV) operating in seas of 2000 m depth, the most important point is the maximum load carrying capacity under the minimum weight. For this reason, spherical shell made of high strength steel is adopted for pressure hulls. Spherical shape is an optimum shape for external hydrostatic pressure loading and the most suitable steel for this shape is HT100 steel (5 Ni-Cr-Mo-V quenched and tempered steel with a yield strength of 90 kgf/mm2). However, in the application of HT100 steel to spherical pressure hulls, there are some difficult problems from the view point of fabrication; that is, (1) quenching and tempering procedure of hemisphere. (2) welding procedure of two hemispheres and penetrate joints. Then heat treatment conditions and TIG welding conditions were investigated. The results obtained are summarized as follows: (1) The quenching cooling rate of HT100 is needed over 3°C/sec for satisfying required property. (2) The mean quenching cooling rates of hemisphere (thickness: 60 mm, diameter 2320 mm) are 5.7-6.2°C/sec. Then it is possible to fabricate hemisphere by hot forming method. (3) It is necessary to preheat over 100°C for preventing cold crack. (4) The cooling rate (at 540°C) of weld metal is required over 20.5°C/sec for guarantee 90 kgf/mm2 proof stress.
Recently, architectural structure is occasionally designed to resist the large earthquake load in such a way that yielding of material would be permitted to occur locally in structural members. In such case, structural members and joints are required sufficient load carrying. capacity and ductility, especially to resist against cyclic loads. In this report, basic experiments have been carried out to investigate deformation and fracture behaviours of welded I-girder with notches in very low cycle regime. Specimens used are simple beam type that is a model of column-beam welded joints in framed steel steel structure. The butt welded joint of flange in the specimen contains notches and loacates near the center of beam span. The weld metal has high fracture toughness at room temperature (Critical COD δc=1.25 mm at 0°C). Tests were carried out under conditions that cyclic load were applied on the center of beam span, and reversed deflection at the loading point was controlled to be constant during fatigue. Under cyclic loading, ductile crack initiates from the tip of initial notches and propagates stably because of high-toughness weld metal. After ductile crack propagated, the load-deflection curve had negative slope in tension load range. This phenomenon was called failure. As deflection amplitude was small and initial notch length was short, number of cycles to failure increased. Crack propagation area per one cycle (ΔA) versus COD(δ) relation was given by ΔA=50δ3. δis crack opening displacement at the crack tip of previous cycle. This empirical equation gave the upper limit for experiment results. The application would be limited under experimental conditions. Total crack length (initial notch length+propagated crack length) at failure was decreasing linearly, as controlled deflection amplitude was increased.
The fatigue crack propagation rate and the stress intensity threshold level were measured on welded joints which were made by three kinds of welding processes, manual arc welding with covered electrode, gas metal arc welding, and submarged arc welding. The specimens were 200 mm wide center notched type. The material was a SM50B steel 20 mm in thickness. The fatigue crack propagation properties of the weld metal and the heat affected zone of these welded joints were found to be quite similar to each other and inferior to that of the base metal. The stress intensity threshold level of welded joints was about one fourth of that of the base metal. It is assumed that the inferiority of welded joints in crack propagation properties may be caused from the tensile residual stresses distributed in the middle part of the specimens. The removal of the tensile residual stresses by postweld heat treatment could improve welded joints in the fatigue crack propagation properties. The fatigue crack propagation properties and the stress intensity threshold level of welded joints after the postweld heat treatment are similar to those of the base metal.
With the increasing severity of operating conditions of welded structures, the improvement of their safety and reliability or their quality assurance becomes more and more important. As the types or kinds of welded structures become so multifarious recently and important structures, which is usually quite large-scaled, are produced only in small quantities, the establishment of reasonable methodology for estimating the reliability of such structures is strongly needed, because most of the past reliability analyses are oriented more towards mass produced products. This paper is an attempt to establish such a methodology by applying graph theory to the reliability analysis, and it was made clear that (1) the concept of directed graph is quite useful for clarifying the complicated failure logics in welding, (2) the causality of failure can be included into the data base system if connection matrices are utilized, and (3) connection matrices can be used to roughly evaluate the impor-tances of events.
The effects of Ti, B and N on the notch toughness of submerged arc weld metals were investigated and systematic studies on characterization of these elements were undertaken with various analytical techniques, including EPMA, EDAX and conventional ayalyses, boron autoradiography, hydrogen hot extraction analysis of N, infrared absorption analysis of BN and X-ray diffraction analysis of non-metallic inclusions. Based on the experimental results and some model calculation, it has been concluded that the coexistence of Ti and B has dual advantage in improving notch toughness of the weld metals because i) during terminal transient solidification, B has a unique ability to reduce "free N" while Ti protecting B from oxygen, and ii) during r to a transformation, active B being present at a weld metal's grain boundaries retards nucleation of proeutectoid ferrite, and titanium monoxide promotes nucleation of acicular ferrite intragranularly since it has quite small crystallographic disregistry with a iron, thus producing very fine and homogeneous microstructures.
The effect of testing temperature on the incubation time of hydrogen-induced weld cracking at low temperatures was investigated, based on such conception that the quality damage of materials due to hydrogen is subjected to rate process. The HT-80 steel was used in the previous work and a plain carbon steel in the present study. The results indicate that (1) the delayed fracture occurs at very low temperatures, such as -196°C, (2) the activation energy determined by the incubation time is 11, 000 cal/mol at the temperatures over -30°C, 2, 300 cal/mol in the temperature range of -30 to -130°C, and (3) at temperatures below -130°C the activation energy decreases below 2, 000 calf snot, therefore the potential barriers are probably overcome by an alternative mecanism such as tunneling.
Friction welding of different diameter bars is very important for practical applications. That is affected by the asymmetrical thermal conduction of friction heat and deformation, compared with the symmetrical behaviour of the friction welding of equi-diameter bars. In this study, friction welding phenomenon of different diameter bars of strainless steel SUS 304 was studied. The diameter of one bar was fixed as a standard, and the diameter of another bar was changed from the same diameter of the standard bar to large diameter enough to be considered as a plate, then the pair was friction welded. During friction welding, temperature of friction surface, friction torque and brun-off behaviour were examined. Effect of the ratio of diameter of each bar, d/d0 (d0=standard diameter, d=comparative diameter) on these friction welding behaviour was studied. Experimental results were as follows; (1) From the observation of the macrophotograph of joints, deformation of comparative diameter bar decreases with increase of d/d0=1.75 or above. This is the same state of stud to plate welding. (2) Steady state temperature of friction surface rises up with increase of d/d/, and it is close to the constant value with the d/d0 value of more than 2. (3) As for the friction torque, the initial torque is not so affected by d/d0, but steady torque and terminal torque increase with increase of d/d0 becoming to constant when d/d0 is more than 1.75. (4) Burn-off behaviours, for example, burn-off rate and total upset decrease with increase of d/d0, and these also become constant for d/d0 above 1.75. These constant value is larger than half of the values at d/d0=1.
The effects of various surface treatment methods such as emery paper polishing, buff polishing, argon ion sputtering, and silver ion plating on the diffusion weldability of copper have been investigated by scanning electron microscopy and Auger electron spectroscopy of the surfaces, in addition to mechanical and microscopic tests of the joints. Buff polishing was more effective than emery paper polishing because of giving the smaller surface roughness, but the both methods left alumina powder on the surfaces. Argon ion sputtering for five minutes removed the alumina and improved the weldability. Silver ion plating after argon ion sputtering showed remarkable improvement in the weldability, and the joint welded for ten minutes at 400°C after this treatment showed almost the same tensile strength as that of the base metal. This result demonstrates that silver ion plating is superior to silver foil interlayer method in improvement on the diffusion weldability of copper.
For MIG welding of thin Aluminium alloys sheets, the thyristor-controlled pulsed-arc welding machines have been conventionally used. By using a transistor-controlled welding device, droplet transfer phenomena caused by various pulse current wave were so examined that the relationships between pulsed current wave and bead appearances, mode and speed of droplet transfer, droplet detachment time and so on were clearly grasped. On constructing a proto-type welder applying this pulse current wave, arc stability against external disturbance had to be discussed. Examining the self-control characteristics of the power source, the enough arc stability is obtained at 2.5v/100A external characteristic. As a result of these studies, control of "one pulse-one droplet" transfer is successfully performed by a new proto-type power source, between 10 Hz (52 A) and 500 Hz (250 A). It was confirmed by results of butt welding of 0.8 mm thickness sheets and fillet welding in vertical up position of 1.2 mm thickness sheets.
Laser hardening process in various plain carbon steels containing 0.1-0.84%C has been analyzes based on hardness distributions, change in micro-structure observed and thermal conduction theory. Results obtained are summerized as follows: 1) In laser hardening with cooling rates faster than the critical values of Ar" transition, the hardness depends only on the maximum temperature attained during heating, not on heating and cooling rates and holding time above the transformation temperature. 2) Hardening occurs at heating temperatures above 750°C, independently of carbon content and carbide distribution of material. For hypo-eutectoid carbon steel having a ferrite and pearlite structure, two-step change in hardness has been observed whereas eutectoid steels having spherodized carbide gives onestep curve. 3) Carbon steel containing pearlite structure gives a narrower transient hardness region between maximum hardness layer at the surface and the mother matrix than that for steel having spherodized carbide. 4) The heating temperature θn corresponding to the maximum hardness in ferrite-pearlite steel decreases with increasing carbon content, approaching progressively about 880°C for eutectoid steel. θn in spherodized steel is higher than θn/ in eutectoid pearlite steel. 5) In laser hardening carbon steel with carhon content less than 0.84%, the effect of retained austenite on the hardness has been found to be negligible.