Characteristics of high power LASER welded carbon steel pipe has been investigated. The heat-treated seam weld portion has the excellent low temperature toughness and the sour resistance. This excellent quality is due to the combined effects of LASER welding and its manufacturing process which means low welding heat input , same chemical composition of weld nugget as mother plate and in-line heat treatment on weldment
High power LASER welding process enables both high speed seam welding and smooth joining by combining high frequency induction preheating. This process has already been a plied to the ! mill o f manufacturing small diameter stainless pipes . The increase in maximum output power of LASER has been investigated for applying to a mill of manufacturing pipes with medium diameter and heavy wall thickness. 25kW C02 LASER of three orthogonal axes type has been introduced for this investigation. This LASER oscillator will be applied to the conventional electric resistance welding (ERW) mill as shown in Figure 1. Pipes with various chemical compositions, which ranged from carbon steel to stainless one have been partially manufactured by using the test mill with this 25kW LASER and the 300kW high frequency generator for preheating. And low temperature toughness, sour resistance and other corrosion resistance at the weld scam have been investigated. The characteristics and the quality of carbon steel LASER welded linepipe, are described in this paper.
CHARACTERISTICS OF HIGH POWER LASER WELDING PROCESS LASER
Welding is one of fusion welding as well as submerged arc welding (SAW) and is not a type of pressure welding such as ERW , which these two welding processes are widely used for manufacturing welded linepipes of carbon or low alloy steels. Figure 2 shows the general characteristics of these welding processes and the performance of weldment. Compared with the SAW of fusion welding, the heat input is lower and weld pool is shorter in LASER welding. No micro segregation occurs in the LASER weld metal due to instantaneous solidification. And the chemical composition of weld metal is the same as that of parent metal if filler metal is not used.
If this LASER welding process is applied to the conventional
ERW mill, in?line heat treatment on weld seam will be available. Therefore, microstructure in the welded portion can be improved in 1Ow temperature toughness and sour resistance, compared with SAW without post weld heat treatment (PWHT). Compared with the ERW of pressure welding, less upset is necessary in LASER welding. Therefore, no metal flow is formed at heat affected zone (HAZ) and no lamination crack due to the metal flow occurs. Although the welding defects called penetrator and the depletion zone of alloying elements such as Mn, which is called white band, are often focused in ERW seam, no such welding defects occur in LASER welding because of fusion welding. It is expected that better performance will be obtained at the weld seam by LASER welding than by other welding processes such as ERW or SAW.
