Mikihito Hirohata

Effects of heat input ratio of laser–arc hybrid welding on welding distortion and residual stress

For predicting welding distortion and residual stress generated by laser–arc hybrid welding, a series of experiments and analyses were carried out. A bead-on-plate welding was performed on SM490 steel by using a fibre laser and CO2 arc welding by changing their heat input ratio. The experiment was simulated by thermal elastic–plastic analysis with the proposed simulation model considering the penetration shape by laser and arc separately. By using this model, the experimental results could be simulated with high accuracy. Therefore, the validity and generality of the numerical simulation model could be verified. The tendency and magnitude of angular distortion varied with the heat input ratio of laser and arc. The results indicated the possibility of the ideal heat input ratio of laser and arc for controlling angular distortion generated by hybrid welding. On the other hand, it was confirmed that the heat input ratio of laser and arc did not affect residual stress generated by hybrid welding.

Effects of Phase Transformation on Distortion and Residual Stress Generated by Laser Beam Welding on High-Strength Steel

In order to elucidate the effects of phase transformation on distortion and residual stress generated by laser beam welding (LBW) on high-strength steel (HT780), a one-pass bead-on-plate welding experiment was carried out. Then, the experiment was simulated by the thermal elastic-plastic analysis. Because the mechanical properties in the phase transformation range could not be specified, they were idealized while considering the transformation expansion and the transformation superplasticity in the simulation of the experiment. The validity of the treatment of the mechanical properties in the phase transformation range was verified. It was elucidated that welding distortion and residual stress were largely affected by the phase transformation in the cooling process, although the bead width of LBW was extremely narrow, Angular distortion and tensile residual stress in the weld metal were largely released by the phase transformation.

Anticorrosive performance of repair painting as remedy for deterioration in metallised steel

Abstract When the various metallic coating systems applied on steel bridges deteriorated, they may need to be repaired by painting because onsite metallic recoating is very difficult. In order to examine the performance of repaired painting on steel plates with the remains of four types of deteriorated metallic coating systems, cyclic corrosion tests were performed in combination with exposure to an acid rain spray. It was found that a residual coating of zinc hot dip galvanising acted as a helpful substrate for the repair painting. Residual coatings of a zinc–aluminium thermal spray and of a zinc–aluminium pseudoalloy did not have such an effect. The residue of an aluminium thermal spray had a positively harmful influence on the repair painting over it. The degree of this influence was correlated with the thickness of the residual coating.

Corrosion accelerated exposure experiment simulating under seawater environment for organic coated steel materials

Abstract In order to investigate and evaluate the corrosion characteristics of steel members more effectively under seawater conditions, the authors have developed an accelerated exposure experimental system to simulate under seawater environments. The acceleration coefficients of this experimental system were based on a study of the depths of corrosion actually found in structural steel members exposed to seawater environments for over 19 years. The accelerated coefficient for the splash zone was around 6·0. In the experiment reported, the system was used to investigate the corrosion behaviours of steel coated with epoxy resin, polyurethane resin and tar-epoxy resin and then subjected to damage from cross scribe lines or circular types of coating defect. Corrosion propagation was found to occur under the coating around the boundaries of these coating defects and macrocell corrosion was observed around the boundary areas. Methods of predicting corrosion propagation distances under the organic coating and the depths of corrosion under the coating defects were proposed based on the experimental results.

Effect of post weld heat treatment on steel plate deck with trough rib by portable heat source

A lot of cases have been reported that fatigue cracks occur from welded parts between steel deck plates and trough ribs of highway bridge structures. In order to improve fatigue life of this kind of welded parts, welding residual stress release by post weld heat treatment is examined. A general heat treatment method by a heating furnace is not available because steel plate deck structures are so large. Therefore, applicability of a sheet-type ceramic heater as a portable heat source is investigated. A heat treatment experiment was carried out on test specimens modeling a steel plate deck with a trough rib. The effect of tensile stress release was elucidated by measuring welding residual stress and stress distribution after the heat treatment. On the other hand, the experiment was simulated by a thermal elastic plastic analysis for proposing an analysis model for simulation of heat treatment process by the sheet-type ceramic heater. Based on the experimental and analytical results, a residual stress release method for the welded parts between the deck plate and the trough rib by the portable heater was examined. Furthermore, the effect of fatigue life improvement by the stress release due to heat treatment on the specimens was investigated.

Corrosion of aluminium alloy A6061-T6 members embedded in alkaline materials

ABSTRACT Owing to the corrosive conditions in coastal areas, aluminium alloy railings and similar structures are normally used on infrastructure works such as bridges and roads located in such areas because aluminium alloys are inherently corrosion resistant. However, to achieve rigid support, aluminium alloy members are often embedded in concrete. Consequently, the embedded parts undergo corrosion caused by the alkaline content of the cement used in the concrete. This research was carried out to study the corrosion behaviour on both the embedded and unembedded sides of aluminium alloy A6061-T6 members embedded in the concrete and also to investigate the ability of a combined coating of anodic oxide and organic film to inhibit corrosion. Additionally, the influence of scratch damage on the performance of this coating material was evaluated.

Elastic mechanical behavior of spliced joints assembled by fillet welding and bonding

For repair and reinforcement of steel structural members, patch plate by bolting is generally used. However, welding is not used for patch plate repair because fatigue cracks are anticipated to occur from the welded part between the existing members and the patch plates. In order to solve this problem, it was examined whether the stress around the fillet welded parts of spliced joints was reduced or not by assist with bonding. The tensile static strength and the elastic mechanical behavior of the spliced joints assembled by fillet welding and bonding were investigated through a series of experiments and analyses. The elastic analysis model for spliced joints by welding and bonding was proposed with considering the change of mechanical properties of the bond due to high temperature during welding. From the results of elastic analysis, the possibility was confirmed that the stress around the weld toe and the weld root of spliced joints became small compared with those of joints by only welding.

Corrosion deterioration characteristics of Sn-bearing steel and its applicability on steel bridges

Elegance in Structures IABSE Conference, Nara, Japan (May 13-15, 2015, Nara Prefectural New Public Hall in Nara, Japan) NM-11

Static tensile strength characteristics of fillet welding lap joints assisted with bonding

A series of tensile tests were carried out on fillet welded lap joints assisted with bonding for investigating the static tensile strength characteristics of the joints from the viewpoints of stress reduction effect around the welded part due to bonding. It was confirmed that the mechanical properties of epoxy resin bonding used in this study were not deteriorated by heating to less than 150°C. When the fillet welded lap joints with bonding were assembled, the bond layer 20 mm from the weld toe was subjected to heating to over 150°C. In other words, the mechanical properties in that region deteriorated. The strengths of the elastic limits of specimens with welding and bonding were higher than those of specimens with only welding by from 60 to 100 MPa. The ultimate tensile strengths of them were almost the same because they were broken at the base plate. The strains around the weld toe and the root of specimens with welding and bonding were smaller than those of specimens with welding by around 13% in the elastic region. The strengths of specimens with only bonding were 170 MPa, which could be explained by a theory of elastic stress distribution. Even if the bond layer 20 mm from weld toe of the specimens with welding and bonding was thermally damaged, the possibility was confirmed that the residual bond layer had around 100 MPa in strength. It could be concluded that the strength of the residual bonding assisted to decrease the stress around the welded part of the specimens with welding and bonding.