Abstract Scope |
Hybrid laser welding (HLAW) presents an important advancement in productivity due to high welding speeds. However, fast cooling rates are inherent to the process, which affects the resultant microstructures and joint performance. In this research, the performance of HLAW was evaluated on three API base materials with similar specifications. Material A: 19 mm thick API X65 grade carbon steel plate. Produced using Thermomechanical Controlled Processing followed by accelerated cooling (TMCP-AC); Material B: 19 mm wall thickness, API X65 grade seamless pipe 16 inches outer diameter produced by extrusion; Material C: 19 mm wall thickness, API X70 grade steel pipe with 38 inches diameter formed through the UOE process.
HLAW was only used on the root pass. Materials A and B used a 15mm depth HLAW root pass followed by two GMAW filling passes. Material C had a 12mm depth HLAW followed by 3 GMAW passes. The research focused on the HLAW weld metal and the effect of the filling passes over the HLAW root pass. Incomplete joints with only the hybrid root pass and completed joints (root and filling passes) were evaluated.
Due to the intrinsic fast cooling rate from two preheats, 100ºC and 300ºC, were used. High-resolution micro hardness maps were developed to properly identify critical regions for characterization within the weld HLAW metal. The hardness maps revealed that 100ºC preheat was not enough to provide a cooling rate capable to avoid martensite formation, resulting in hardness values as high as 370HV0.2. The 300ºC heat treatment has shown a peak of hardness field of only 275HV0.2 on the GMAW intercritical heat-affected zone (HAZ) over the HLAW weld metal. However, material B demonstrated a regular hardness field throughout the whole HLAW weld metal below 250HV0.2. SEM analysis denoted that using a 300ºC preheat produced a mainly bainitic microstructure; however, martensite and acicular ferrite can also be found in the microstructure. The chemical composition of the base material and filler metal combined with the thermal-cycle will be used to quantify other microstructures that will be formed.
Due to the small width of the weld metal region, a miniaturized charpy specimen from ISO 14556 was used to create ductile-to-brittle transition temperature (DBTT) curves. The curves showed a DBTT increase from the base metal to the welded condition. Interestingly, all the 300ºC preheated specimens resulted in a very similar DBTT regardless of its composition, microstructure, and microhardness. The fine sheaved bainitic microstructure is responsible for the similar DBTT results among the tested materials.
Fractography on the miniaturized CVN Specimen confirmed the abrupt transition from ductile-to-brittle behavior. The localized fracture toughness testing showed adequate joint performance for all tested conditions. Nevertheless, the hardness values meet the requirements, but only for higher preheating temperature conditions at 300ºC.
Acceptable results were found for initial research focused on an onshore pipeline application. These results demonstrated the technical feasibility of HLAW for the oil and gas industry, which can also be applied to other industries in the future. |