Microstructure and Properties of Arc Welding Joints of 20G-316L Bimetallic Composite Pipe Welding Sequence Effects

Literature Overview

This study provides a complementary perspective to the previous investigation, focusing specifically on the effect of welding sequence and layering strategy on the microstructure and performance of arc welding joints in 20G-316L bimetallic composite pipe. While the previous study examined the overall joint characteristics, this paper delves deeper into how the order of welding layers—from the stainless steel side to the carbon steel side, or vice versa—affects the dilution profile, microstructural evolution, and final joint performance. This is a critical consideration in engineering practice, as the welding sequence directly influences the quality and reliability of the joint.

Core Technical Points

The study systematically compares three welding sequences: (1) starting from the 316L side and welding toward the 20G side, (2) starting from the 20G side and welding toward the 316L side, and (3) a balanced approach where both sides are welded simultaneously. The results demonstrate that the welding sequence has a profound impact on the dilution rate and, consequently, on the weld metal composition and corrosion resistance.

When welding starts from the 316L side, the first pass has minimal contact with the 20G base metal, resulting in very low carbon dilution (<10%). Subsequent passes gradually increase the dilution rate as the weld pool interacts more with the 20G base metal. This sequence produces a weld metal with the most favorable composition for corrosion resistance, with a carbon content below 0.03% and a chromium equivalent that maintains the austenitic structure.

Microstructural Analysis

The microstructural evolution across the weld cross-section is highly dependent on the welding sequence. In the 316L-first sequence, the weld metal near the 316L base metal exhibits a clean austenitic structure with fine grain boundaries and minimal carbide precipitation. The weld metal near the 20G base metal shows a slightly higher ferrite content and some chromium carbide formation along the grain boundaries, but the overall composition remains within acceptable limits.

In contrast, the 20G-first sequence produces a weld metal with significantly higher carbon content, leading to extensive chromium carbide precipitation throughout the weld metal and HAZ. The δ-ferrite content exceeds 20% in many regions, which, while beneficial for hot cracking resistance, promotes the formation of intermetallic compounds and reduces the corrosion resistance. The HAZ on the 20G side in this sequence exhibits a wider martensitic zone with higher hardness (up to 400 HV), increasing the risk of cold cracking.

The balanced (simultaneous) sequence produces intermediate results, with a dilution rate of approximately 25% and a weld metal composition that is acceptable but not optimal. This sequence is recommended when the welding equipment does not allow for sequential welding from one side.

Mechanical Properties and Engineering Considerations

The tensile strength of the 316L-first sequence weld is approximately 580–620 MPa, with an elongation of 38–42%, which meets the requirements of ASTM A263 and NB/T 47002. The hardness profile shows a gradual transition from the weld metal (200–220 HV) to the 20G HAZ (300–350 HV), with no abrupt hardness peaks that could cause stress concentration.

The 20G-first sequence weld exhibits a higher tensile strength (600–650 MPa) due to the higher carbon content, but with significantly reduced elongation (25–30%) and increased brittleness. The hardness profile shows a more abrupt transition, with a peak hardness of 380–420 HV in the HAZ, which is a concern for cold cracking in thick sections.

From a pressure vessel fabrication perspective, the 316L-first sequence is strongly recommended for 20G-316L composite pipe joints, as it provides the best combination of mechanical properties and corrosion resistance. The welding procedure specification (WPS) should explicitly define the welding sequence to ensure consistent quality across all joints.

Defect Analysis and Countermeasures

The primary defects in the 20G-first sequence are intergranular corrosion, excessive δ-ferrite, and cold cracking. The intergranular corrosion is caused by the high carbon dilution and can be mitigated by switching to the 316L-first sequence. Excessive δ-ferrite can be reduced by using a filler metal with higher nickel content (such as ER308L with 8–10% Ni) or by applying a post-weld solution heat treatment. Cold cracking in the 20G HAZ is mitigated by preheating to 150°C and maintaining interpass temperatures below 200°C.

Study Insights and Reflections

The study underscores the critical importance of welding sequence in bimetallic composite pipe fabrication. The 316L-first sequence is not merely a preference but a requirement for achieving acceptable corrosion resistance in the final joint. From a quality control perspective, the welding sequence should be documented in the WPS and verified during fabrication through dilution analysis (using optical emission spectrometry or XRF) at representative locations. The study also highlights the need for post-weld stress relief annealing at 1050°C for 30 minutes to dissolve any chromium carbides that may have formed during welding, which is a standard practice in pressure vessel fabrication per NB/T 47002. The key lesson for engineers is that the welding sequence is a simple but powerful lever for controlling joint quality, and it should be given due consideration in the welding procedure development phase.