Naoya Hayakawa

Analysis of martensite transformation behaviour in welded joints of low transformation-temperature materials

In order to improve fatigue strength in welded joints, low transformation-temperature welding wire has been developed in which residual tensile stress can be reduced. In application of the low transformation-temperature welding wire, the prevention of cold cracking without preheating in high strength steel welded joints is expected and examined from the control of residual tensile stress. However, it is expected that residual stress distribution in a welded joint can be suggested by numerical analysis, because the residual stress cannot be measured simply and non-distractively. In this report, martensite transformation behaviour such as Ms point, transformation expansion, and so on is measured firstly by the Formaster test. And temperature dependence of several mechanical properties was measured in full-austenite and full-martensite microstructures, and temperature dependence of mechanical properties was estimated in dual phase microstructure of austenite and martensite. By these data, numerical analysis was carried out and martensite transformation behaviour was compared with measured and calculated results in a rigid model test. From the comparison, it was suggested that transformation superplasticity had to be considered in numerical analysis. Next, the increase of Ms point due to transformation induced plasticity was guessed from the comparison with measured data by laser speckle measurement and calculated data under transformation superplasticity consideration. From the all results, it was found that the measured transformation behavior and residual stress had good agreement with the calculated results under transformation superplasticity and transformation induced plasticity considerations.

Development of Heavy Wall High-Strength UOE Linepipe by Means of Microstructural Control in Base Metal and Seam Weld

The growing demand of the transport of gas and oil under severe conditions, such as in deepwater and onshore in cold climates, requires heavy wall thickness line pipes with both high strength and excellent toughness at low temperature in order to reduce the cost of gas transportation and constructions. In particular, when developing and efficiently manufacturing high strength line pipes with heavy wall thickness, achieving excellent heat affected zone (HAZ) toughness of seam weld is one of the key subjects. In general, steel plate for heavy wall thickness line pipes relatively contains large amount of alloy elements to secure required mechanical properties, whereas addition of alloy elements cause deterioration of HAZ toughness of seam weld. This paper deals with the method of improving HAZ toughness in UOE pipe seam weld. Relationship between microstructure and toughness of simulated HAZ was investigated, and volume fraction of martensite-austenite constituent was reduced by optimization of chemical composition of steel plates. Moreover, low heat input double submerged arc welding (DSAW) process with using welding wires with smaller diameter was newly developed to enhance HAZ toughness. With this new DSAW process, decrease of HAZ width and reduction of austenite grain size were also achieved. Based on the above knowledge, the manufacturing condition was optimized and heavy wall thickness X70 UOE pipe with excellent toughness was successfully developed.Copyright