Yasushi Morikage

Development of new low transformation temperature welding consumable to prevent cold cracking in high strength steel welds

Abstract A low transformation temperature (LTT) welding consumable has been developed to prevent cold cracking in high strength steel welded joints without preheating. In the LTT welded joint, the residual tensile stress is reduced by martensitic expansion of weld metal formed by the LTT consumable. In the weld cracking tests, cold cracking in the LTT weld metal is successfully prevented under high restraint conditions, but cold cracking occurs at very low joint restraint strength in case the weld metal is fully martensitic. Chemical compositions of the consumable are designed to retain austenite in martensite in the newly developed weld metal to absorb the diffusible hydrogen into the austenite to prevent cold cracking. In the newly developed LTT weld metal, cold cracking is almost fully suppressed without preheating under every joint restraint condition.

Angular distortion of fillet welded T joint using low transformation temperature welding wire

Abstract A newly developed low transformation temperature welding wire, of which the transformation start temperature is lower than that of conventional welding wires, was applied to fabrication of fillet welded T joints. The welding angular distortion and the temperature profile of the weld metal were continuously measured during the welding process. The angular distortion of the fabricated T joint was reduced when the weld metal reached the martensitic transformation start temperature. The residual angular distortion was less with the low transformation temperature welding wire than that with the conventional welding wires. The welding distortion of T joints was calculated by a numerical simulation with consideration of the effect of phase transformation under weld thermal cycles. The welding distortion was reproduced with high accuracy in the numerical simulation. Results of the numerical simulation also determined that there was a direct correspondence between the transformation expansion of the weld metal and the angular distortion.

Study of residual stress reduction in welded joints using phase transformation behaviour of welding material. studies on numerical simulation of temperature, microstructure, and thermal stress histories during welding and their application to welded structures (2nd report)

Abstract The residual stress generated in welded structures is well-known to exert a strong influence on features such as brittle fracture,1 buckling,2 stress corrosion cracking,3 fatigue,4 etc. It is therefore important to develop appropriate measures for evaluation of the residual stress distribution at structural locations requiring prevention of deformation or fracture and for suitable reduction of residual stress in response to the weld configuration being used.

Effect of Material Properties on Welding Distortion History

Welding distortion of a T-joint is calculated numerically considering the effect of phase transformation. Welding distortion becomes smaller in the case of a T-joint with developed low-transformation temperature welding wire than with conventional wire, which has been produced for high accuracy fabrication. Results of weld distortion history by numerical simulation agree well with those by the measurement during the whole weld process. The effect of material properties such as martensitic transformation temperature on welding distortion is investigated by using numerical simulation.Copyright

Effect of Hammer Peening Processing on Fatigue Property of Welded Joints After Overload

One general method for improving the fatigue strength of welded joints is introduction of compressive residual stress by peening. However, there is concern that the fatigue strength of the welded joint may decrease if excessive preloading is applied after peening.It has been found that fatigue strength decreased after applying compressive preloading to a welded joint due to cancellation of the compressive stress at the weld toe.In the present research, the influence of excessive preloading on the fatigue strength of welded joints with compressive residual stress at the weld toe was clarified by experiments using hammer peening with an improved pin.When hammer peening was applied to welded joints, increasing the radius of the weld toe reduced the decrease of compressive residual stress due to excessive preloading. As a result, the decrease of the fatigue strength of the welded joint was also reduced.Copyright

Assessment of Fatigue Crack Growth in Imperfect Lap Joint: Part 2 — Crack Propagation Analysis Using “SCANP”

The deck plates of single-deck-type floating roofs for large oil storage tanks are joined by single-welded Full-fillet lap joints. In areas with frequent strong winds, fatigue cracks sometimes occur in the welds of the deck plate. The aim of the present study is to investigate the effect of the gap imperfection of the lap joints on the fatigue life. In the case that tensile load acted on Full-fillet lap joints, the stress at the crack face becomes larger by gap imperfection of the lap joint. The authors have developed a software system called “SCANP (Surface Crack Analysis Program)”, a software system to evaluate the stress intensity factor, K, and to simulate fatigue crack propagation for surface cracks for arbitrarily distributed surface stresses. The fatigue life of a lap joint was predicted by the “SCANP” using the calculated stresses at the root crack face and the toe crack face. The number of loading cycles to penetration for arbitrary bending load amplitudes and tensile load amplitudes are obtained.Copyright

Effect of Transformation Temperature of Weld Metal on Welding Distortion

Welding distortion during welding is an inevitable outcome of weld metal shrinkage due to non-uniform heating and cooling in welded joints. Several studies have investigated methods of estimating and controlling welding distortion in order to construct steel structures correctly, easily and efficiently. In the present study, the effect of transformation expansion of the weld metal on welding distortion was investigated in order to develop a welding wire that reduces welding distortion. Concretely, experiments and numerical simulations were performed on the effect of the transformation temperature of the weld metal on angular distortion of a horizontal fillet welded joint. The results showed that angular distortion of the welded joint decreases when the martensitic transformation start temperature of the weld metal decreases in the range from 350°C to 450°C. Based on this result, a welding wire that is effective in reducing welding angular distortion was developed considering usability, weldability and the mechanical properties of welded joints.Copyright

Residual Stress Redcution by Using Weld Metal With Property of Low-Temperature Phase Transformation

Residual stress in welded joints by using a new weld metal with the property of low-temperature phase transformation is numerically analyzed. 10 % Nickel and 10 % Chromium are involved in the developed welding material for producing the property of martensitic phase transformation at a low temperature and for generating compressive residual stress. A fillet-welded joint between plate and stiffener is used for the numerical simulation of the thermal elastic-plastic finite-element analysis with coupling phase transformation effect. Moving heat source is considered by using the gradual deposition of the finite-element during welding. Distribution of the computed residual stress mostly agrees with the measuring values by strain gauge. Compressive residual stress distributes in the weld metal for both longitudinal and transverse directions with weld line. The effects of the material of weld metal and welding pass sequence on residual stress are investigated. Residual stresses on the weld toe and root are improved lower by using the low-temperature transformation weld wire than the conventional one, regardless of the sequence of welding deposition. It is found that the weld metal with property of low-temperature phase transformation is effective to reduce residual stress near weld metal.Copyright