The sensitization behavior in austenitic stainless steel weld metal was simulated using the phase-field method. The simulations were performed for two cases that δ-ferrite does and does not exist in austenitic phase. The calculation results revealed that the Cr-depleted zones do not form both in the vicinity of γ/M23C6 and δ/M23C6 when δ-ferrite exists. On the other hand the Cr-depleted zones form in the interface of γ/M23C6 in the austenitic stainless steel when δ-ferrite dose not exist. The suppression of the formation of Cr-depleted zone can be explained by the model that carbon atoms in the austenitic phase are consumed due to rapid growth of M23C6 in δ-ferrite.
This study examined fatigue strength of cruciform joints fabricated with laser-arc hybrid welding. Load-carrying and non-load-carrying cruciform welded joints with a plate thickness from 8mm to 25mm were tested under tensile cyclic loads and out-of-plane bending cyclic loads. Measurements of weld toe profile, residual stress and hardness around a welded portion were also conducted. The test results revealed that the fatigue strength of the hybrid welded joints is equivalent to that of arc welded joints and satisfies the fatigue design curves for arc welded joints in JSSC recommendation. Besides, the thickness effect on fatigue strength was observed in the hybrid welded joints as well as in arc welded joints.
Zinc induced cracking (ZIC) occurs under excess tensile stress even in steels suitable for hot-dip galvanizing kettles. In this study, it was examined how the excess stress affects microstructure of the base metals and heat-affected zones (HAZs), that relates to molten zinc diffusion along the grain boundary leading to ZIC. Two kinds of steels, exhibiting two-phase (ferrite and perlite) structure (Steel A) and ferrite structure (Steel B), were selected. Tensile tests were conducted with a V-shaped notch, at which a piece of zinc wire adhered, in air at 450°C and 500°C. They failed with sudden drop in flow stress at their ultimate tensile strength (i.e. excess tensile stress). The local tensile strain at the failure point was estimated based on the aspect ratio change of grains beneath the V-shaped notch before and after the tests. The Steel A specimens with the base metal and HAZ composed of ultrafine bainitic structure failed without exhibiting ZIC at similar local tensile strain at 450°C, while the specimen with HAZ composed of bainitic structure exhibited ZIC. The two-phase structure seems to be strong enough, while the ultrafine bainitic structure prevented zinc diffusion along its grain boundary maybe with carbon and/or carbon precipitates. The Steel B specimens with the base metal and HAZs exhibited ZIC and the critical strain decreased with decreasing the grain size at 450°C. The intermetallic compound layer (IMCL) formed between steels and molten zinc at 500°C was thicker than at 450°C, and thus ZIC was hardly observed in similar tests at 450°C with holding time of 20 mins before starting the test. Thus, the tests were immediately started upon reaching 500°C. Consequently, the ZIC at 500°C was divided into three groups: no ZIC in microstructure with high strength and ZIC dependent and independent on existence of IMCL.
Effect of weld metals on the fatigue behavior of Nb-added ferritic stainless steel JFE429EX (15Cr-0.9Si-0.5Nb) welds in laboratory air and in 3% NaCl solution was investigated. Two filler metals with different chemical compositions, Filler I and II, had been used for the MIG butt welding of JFE429EX. Filler II contains Al and Ti to improve microstructure of fusion zone (FZ), resulting in the finer grains than Filler I. Fully reversed axial fatigue tests had been performed using smooth specimens of welds and base-metal specimens at ambient temperature in laboratory air and in 3% NaCl solution. In laboratory air, fatigue strengths of the welds with Filler II were higher than those of the base metal and welds with Filler I. Fatigue fracture occurred at HAZ and FZ in the welds with Filler I, while at base metal in the welds with Filler II. It could be attributed to the finer grains in the FZ of Filler II. In 3% NaCl solution, the fatigue strengths of both welds and base metal became lower than those in laboratory air due to the corrosive environment. However, the welds with Filler II had still the highest fatigue strengths compared with the base metal and welds with Filler I. It indicates that the addition of Al and Ti could suppress sensitization. In both welds, fatigue fracture took place at HAZ, and intergranular crack fractures were observed. It suggested that the heat input of welding led to the sensitization near HAZ.
The effect of titanium interlayer on dissimilar joining between an aluminum alloy and a magnesium alloy was evaluated. Mutual diffusion between aluminum and magnesium at the interface was suppressed by the interlayer. Ti3Al with the thickness of 200 nm was formed at the AZ92/Ti by the reaction of titanium and small amount of aluminum, whereas Al3Ti with the thickness of 5 μm was formed at Ti/A5052 interface by the reaction between titanium and local-melted aluminum, respectively. Tensile shear testing showed that a sound joint that fractured at the base metal was obtained and the maximum value of 191 N/mm achieved 880 W is about 81 % that of the A5052 base metal, which was significantly increased compared with the direct laser brazing between A5052 and AZ31 joint.
Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out by using 980MPa 0.13%C high strength steel sheets of 1.0mm, 1.6mm and 2.0mm thickness. At the 5mm of nugget diameter full plug fracture were observed in cross tension resistance spot welded joint of 1.0mm, on the other hand partial plug fracture were recognized in cross tension resistance spot welded joint of 1.6mm and interface fracture were indicated in cross tension resistance spot welded joint of 2.0mm. These test results imply that it is necessary to consider the thickness effect in order to prevent deterioration of strength in resistance spot welded joint. In case of interface fracture of cross tension resistance spot welded joint of 2.0mm, circumferential crack initiation due to separation of corona bond arose. The crack opening process without propagation (plateau crack length) was recognized until just before fracture and then the crack propagated to nugget immediately in brittle manner around CTS. Fracture toughness (Mode I stress intensity factor and crack tip opening displacement) at CTS of nugget diameter of 3√t, 3.5√t, 4√t and 5√t in cross tension resistance spot welded joint of 2.0mm thickness were almost the same to the previous study of nugget diameter of 3√t in cross tension resistance spot welded joint of 980MPa 0.13%C 1.6mm thickness. Average fracture facet size of interface fracture and average packet size of nugget (fully martensite) were almost the same with meaningful scatter. These experimental results imply that fracture toughness were almost same independent of nugget diameter, sheet thickness and strength of steel sheet because the nugget of same C contents showed same hardness and same packet size with meaningful scatter as long as brittle chemical elements (P and S) were extremely changed.
This study proposed the estimation method of the depth of wed penetration using the experimental measurement of the weld pool shape and the temperature on the front surface during the welding. This method is based on the simple heat conduction calculation, and does not require a flow-field calculation and a heat input information as seen in various welding simulations. In this paper, we first examined the applicability of this method to a bead on plate welding. As a result of application to TIG and A-TIG welding with SUS304 base metal, we obtained the estimated depth with an error of +14∼30% to the actual depth. We investigated this experimental result by the analytical heat conduction model, and found that utilizing the surface temperature with wider area has a possibility to estimate the depth with higher precision.
This study discusses the brittle fracture controlling parameter for the steel components under mixed mode loading with focusing on the Weibull stress concept proposed as an original Beremin model. The Weibull stress based on the original Beremin model, which is calculated by integrating a maximum principal stress over the fracture process zone around crack-tip, is widely demonstrated to be a fracture controlling parameter independent of crack-tip plastic constraint for 3PB (3-point bending) specimen under mode I loading. Critical Weibull stress obtained by 3PB test founds to be different from that obtained by 4PS (4-point shear) specimen under mixed mode loading (ratio of loading mode I and mode II is 0.51). The reason why the Weibull stress can not be a unified fracture controlling parameter is interpreted from the crack-tip stress fields that shows the different principal stress ratio between the specimens subjected to mode I and mixed mode loading conditions. A new fracture model considering stress components other than maximum principal stress is necessary to propose a fracture controlling parameter independent of plastic constraint and loading mode mixture.
Ductile crack initiation limit characteristic of metallic material, which is evaluated with stress triaxiality and equivalent plastic stain, shows material dependency. The main purpose of this study is to propose the evaluation approach of ductile crack initiation limit with the material dependency. Ductile crack initiation behaviors were experimentally and analytically comprehended using notched round bar specimens of aluminum alloy as non-ferrous metal in addition to two different steels. On the basis of analyses, the locus of stress triaxiality during loading was found to be dependent on strain hardening exponent which is one of the parameter characterizing stress-strain relationship of material, and the behavior was confirmed in the strain region after uniform elongation for smooth specimen. Focusing on Mohr-Coulomb fracture criteria, limit strain was derived using strain hardening exponent and stress triaxiality. In addition, considering ductility after uniform elongation, ductile crack initiation limit with material dependency could be evaluated and it was found that limit strain at ductile crack initiation could be predicted using basic material properties obtained from monotonic tensile test, which are strain hardening exponent and ductility after uniform elongation.
Fatigue strength of fusion-welded joints is lower than that of the base metal, due to stress concentration, tensile residual stress, and microstructural degradation at the weld toe. To improve these issues, friction stir processing (FSP) was applied to the weld toe of high-strength low-alloy steel joints using a newly developed tool with a conical shoulder. With use of FSP, the weld toe geometry and microstructure were successfully modified without defect formation in the stir zone (SZ). Hardness was increased due to significant grain refinement and compressive residual stress was produced on the weld surface. Fatigue strength and life of the FSP-modified welded specimens was improved, though not largely, since a new stress concentration region with reduced plate thickness and serrated surface was produced by the shoulder edge of the FSP tool, just beside the base metal. Fatigue cracks initiated there and propagated in the SZ, thermo-mechanically affected zone, and base metal for the FSP-modified welded specimens. The fatigue strength could be further increased by prevention of the stress concentration at the SZ edge.
In this study, a method was developed for the automatic generation of input-output relationship models that clearly indicate the role of input factors in determining the outputs. This proposed method was used to model the relationships between the welding conditions and penetration dimensions of TIG welds. The models developed using the proposed method clearly and explicitly indicate the effects of the welding current, arc length, and welding speed on the penetration dimensions. In addition, the errors of the predicted weld penetration dimensions obtained using the developed model were less than 3%.