# Research Status and Progress of All-Position Narrow-Gap GMAW for Pipelines
## 2 Current Status of Research on Narrow-Gap All-Position GMAW
### Microstructure of Narrow-Gap GMAW  
During the welding process, the base material undergoes alternating thermal cycles of rapid heating and cooling. The microstructure of the weld undergoes significant changes during solid-state phase transformations under different cooling rates, leading to notable differences in properties between the heat-affected zone (HAZ) and the base material [39].  
The heat input involved in welding thermal cycles alters the microstructure of the base material, such as forming austenite grains, bainite, and martensite phases, which can result in reduced toughness in the HAZ at low temperatures [40]. [PERSON] et al. [9] analyzed the optical microstructures of weld metals under three typical droplet transfer modes, finding that more uniform grain distribution was achieved under the ODPP condition. Lower heat input, combined with stronger stirring effects, facilitate the growth of finer and more uniform grains. [PERSON] et al. [31] found that liquid convection reduces the temperature gradient in the welding pool, intensifies compositional undercooling, and significantly alters the growth direction of primary austenite grains. Stronger impacts increase the proportion of preoutectoid ferrite in the weld microstructure and enlarge the grain size of preoutectoid ferrite and acicular ferrite, highlighting the importance of considering arc and droplet impingement for optimizing weld microstructures during welding process design.  
Rapid cooling leads to the transformation of austenite into martensite in the weld and HAZ [41]. While martensite possesses high strength and hardness, it is brittle. Finite element analysis can be used to predict thermal history, deformation, residual stress and microstructure at any point in the material. Moreover, it has been identified that laser power has the greatest impact on thermal history during welding [39]. However, phenomena such as the movement of molten metal and latent heat released during solid-state phase transformations are often overlooked in numerical analyses. These factors affect temperature distribution and significantly influence the predicted microstructural composition [41; 42]. Therefore, controlling welding parameters is crucial during welding process design to avoid excessive formation of brittle phases, thereby enhancing the overall performance of pipeline welds. According to related studies [29], lower brittleness microstructures were observed in the upper region of hybrid laser-arc welds, indicating that the heat source generated by the arc acts as a cooling inhibitor in the fusion zone. Thus, incorporating hybrid laser-arc welding in process design can effectively reduce weld brittleness and improve joint performance. Additionally, carbides play a critical role in the formation of weld microstructures [29]; excessive carbide content can adversely affect the microstructure, potentially causing localized hardening or embrittlement and increasing the risk of crack formation.  
In narrow gap welding, a smaller groove angle can lead to uneven arc heating, making it easy to cause localized overheating or differences in cooling rates. These issues can further exacerbate problems such as inconsistent weld quality and increased risk of defects. It is essential to control these factors through appropriate welding procedures and heat treatment processes to avoid the occurrence of welding defects.