Microstructure and Mechanical Properties of G115 Steel Pipe Weld Joints Prepared by Submerged Arc Welding

Literature Overview and Research Context

The literature under review addresses the welding metallurgy and mechanical performance of G115-grade steel pipe joints fabricated through submerged arc welding (SAW). G115 steel, with a minimum yield strength of 1150 MPa and ultimate tensile strength exceeding 1300 MPa, belongs to the ultra-high-strength steel category used in demanding applications such as heavy-lift cranes, offshore structures, and high-pressure pipelines. The study systematically examines the microstructural evolution across the weld zone, heat-affected zone (HAZ), and base metal (BM), with particular attention to how welding parameters influence hardness distribution, impact toughness, and microhardness profiles. This work is significant because ultra-high-strength steels present unique challenges in weldability, including susceptibility to cold cracking, coarse grain formation in the HAZ, and difficulty in achieving adequate toughness without extensive preheat and post-weld heat treatment (PWHT).

Core Technical Findings

The investigation reveals that the HAZ of G115 steel pipe joints exhibits a pronounced transition in microstructure from fine acicular ferrite in the base metal to a mixture of lath martensite, retained austenite, and carbide precipitates in the severely affected zone. The weld metal, deposited using a matched flux-cored wire system, displays a tempered martensitic structure with dispersed carbide particles. The hardness distribution across the joint shows a peak hardness of approximately 450-520 HV in the HAZ, which exceeds the base metal hardness of 350-380 HV, indicating potential concerns for stress concentration and crack initiation under cyclic loading.

The study demonstrates that welding current and travel speed have a decisive influence on the HAZ width and hardness peak. Higher welding currents (280-320 A) with moderate travel speeds (250-350 mm/min) produce wider HAZ regions with more pronounced hardness peaks, while lower heat inputs (1.5-2.5 kJ/mm) help restrict the HAZ width to approximately 1.5-2.0 mm. The preheat temperature of 200-250°C was found to be critical for suppressing cold cracking sensitivity, as confirmed by the dilute nitric acid solution cracking test.

Welding Process Analysis and Defect Countermeasures

The SAW process parameters optimized in this study include a single-pass welding configuration with a flux-cored wire diameter of 1.6 mm, welding voltage of 28-32 V, and arc length maintained at 3-5 mm. The flux composition, rich in CaF₂ and SiO₂, provided adequate slag coverage and controlled the cooling rate of the weld pool. A key finding is that the cooling rate from 800°C to 500°C (t₈₀₀₋₅₀₀) in the HAZ ranges from 2-8 °C/s, which falls within the acceptable window for martensitic transformation but still produces a relatively hard microstructure.

Common defects identified include lack of fusion at the root pass, porosity in the cap pass, and microcracks in the HAZ under high heat input conditions. The countermeasures proposed include:

• Maintaining a minimum preheat temperature of 200°C for all passes to reduce hydrogen-induced cracking risk
• Using low-hydrogen flux with controlled moisture content below 1.0%
• Implementing interpass temperature control between 250-350°C for multi-pass welding
• Applying post-weld heat treatment at 600-650°C for 2 hours to temper the HAZ and reduce residual stress

Integration with Engineering Practice

In practical engineering applications, the weldability of G115-grade steel pipes requires careful attention to the carbon equivalent (CE) value, which typically ranges from 0.55-0.65% for this grade. The Pcm value (cold cracking susceptibility index) should be controlled below 0.25% through appropriate chemical composition design. The study's findings on HAZ hardness exceedances suggest that for pressure-containing applications governed by standards such as ASME B31.3 or GB/T 20801, the hardness limit of 350 HV (or 390 HV for certain materials) may be exceeded, necessitating PWHT as a mandatory requirement.

The dilution ratio between base metal and weld metal, estimated at 20-35% for single-pass SAW, plays a critical role in determining the final weld metal composition and properties. Engineers should consider using a multi-pass welding sequence with a low-carbon root pass to minimize dilution effects and improve the toughness of the first pass, which is most susceptible to cracking.

Key Reflections and Study Insights

The most compelling insight from this literature is the fundamental trade-off between strength retention and toughness in ultra-high-strength steel weldments. Achieving full strength matching (≥90% of base metal yield strength) inevitably leads to hard, brittle microstructures in the HAZ, particularly when welding without PWHT. The study underscores that for G115-grade applications, a combination of low heat input welding, controlled preheat, and mandatory PWHT represents the only viable path to producing joints that meet both strength and toughness requirements. Future research should explore the application of advanced flux formulations or multi-wire SAW configurations that can further refine the HAZ microstructure without compromising productivity.