Yasushi Hasegawa

Development of Laminar Plasma Shielded HF-ERW Process: Advanced Welding Process of HF-ERW 3

High frequency - electric resistance welded (HF-ERW) pipe has been successfully used for many years for a number of applications. The benefits of HF-ERW pipe are considerable, including a higher dimensional tolerance and lower prices than seamless pipe and UO pipe. The conventional weld seam produced by HF-ERW, however, often has a relatively low toughness. We have developed an automatic heat input control technique based on ERW phenomena that relies on optical and electrical monitoring methods and has been shown to result in a significant improvement in the toughness. Shielding of the weld area must also be considered as a key factor in the formation of a sound weld. It has been shown that an inert cold gas (e.g., at room temperature) shielding technique is effective for maintaining a stable low oxygen state in the weld area that inhibits the formation of penetrator, a pancake oxide inclusions. Compared to the cold gas shielding technique, high temperature gas shielding, due to its higher kinetic viscosity coefficient, should make it easier to sustain a higher laminar flow, thus leading to a rather low air entrainment in the shielding gas. In addition, plasma is a much higher temperature state (∼6000 K), and the dissociated gases can react with the entrained oxygen; plasma jets should, therefore, enhance the overall shielding effects. Moreover, oxides on the strip edges can be expected to melt and/or be reduced by the high temperature plasma jets. Nippon Steel has developed a plasma torch that can generate a long and wide laminar argon – nitrogen – (hydrogen) jet. This paper describes the results obtained from our investigation of the effects of a plasma jet shield on the weld area of high strength line pipe with a yield strength grade of X65. Preliminary attempts in applying this novel shielding technique has been found, as expected, to demonstrate extremely low numbers of weld defects and a good low temperature toughness of the HF-ERW seam.Copyright

Metallurgical Investigation of a Type IV Damage at the Heat Affected Zone of Weld for Tungsten Containing Martensitic Heat Resistant Steels

Metallographic analyses were carried out to clarify the micro-mechanism of Type IV failure at the outer edge of HAZ for W containing martensitic creep resistant 9 to 12% Cr steels. Microstructure observation and the creep rupture test determined the type IV failure location at fine grain HAZ through the welding thermal cycle simulation. Based on the comparison of three candidate mechanisms, grain size refinement, dislocation sub-structure evolution and precipitation morphology alteration, the microstructure evolution by thermal cycle of welding and PWHT was expected to be the main reason for the failure. TEM analysis of the microstructure elucidated the globular sub-grain microstructure formation by PWHT derived from the low dislocation density of the fine grain martensite with ambiguous lath structure. The precipitation morphology change is also one candidate for type IV failure. The quantification of the influence of V-Wing type MX disappearance at fine grain HAZ on creep rupture strength must be accurately estimated in further research.Copyright

Development of the New Welding Control Method for HF-ERW Pipes: Advanced Welding Process of HF-ERW 1

In recent years, the key application requirement of the ERW line pipe has been its toughness, including the weld seam.It is known that, among defects generated at the weld seam, the penetrator defect affects toughness and is difficult to control by welding condition[1–4].Generally speaking, ERW pipes are welded with exposure to air, and oxides are produced on the surface of the melted metal during the process. The discharge of this melted metal by electromagnetic force and squeezing produced at the current welding route is effective in eliminating the penetrator, and constantly optimizing the welding heat input means this defect can be constantly reduced.To optimize the welding heat input, therefore, it is important to determine the welding phenomena occurring at the welding spot and contrast them with the defect area ratio. We have studied (examined) the welding phenomena, optimum heat input power and the welding defect generation mechanism. Consequently, it was revealed that by varying the welding speed, Vee convergence angle and welding heat input, etc., a new categorization of welding phenomena as Types 1, 2, 3, and 2′ was possible.In the case of Type 2 and 2′ welding phenomena, the welding defect area ratio decreases, which resulted in a sound seam weld with high toughness. If these two welding phenomena are compared, the wider heat input power range of Type 2′ is preferable for the HF-ERW manufacturing process. The higher heat input of Type 2′ compared to Type 2 compensates for the abutting surface angle fluctuation, meaning it is also preferable for pipe manufacturing. Consequently, the control of the Type 2′ welding phenomenon is preferable for the HF-ERW manufacturing process.Copyright