A failure analysis published in the open-access journal Fuels in May 2026 describes a cracked circumferential weld between a reducer and a straight pipe in a natural gas gathering line (Xu et al., 2026). The line carried oil, gas and water with significant hydrogen sulphide (H2S) at about 4.13 MPa. It is a clear example of how a procurement error and a welding error together produce a brittle failure.
What the investigation found
The authors used visual examination, chemical analysis, mechanical testing, metallography, scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and non-destructive testing. Their key findings:
- Location. The crack ran along the weld toe on the small-end side of the upstream reducer.
- Wrong material. The straight pipe met its specification. The reducer, meant to be the same low-carbon boiler steel (20G), contained about 0.39 wt% carbon and 1.01 wt% chromium, above the limits of 0.30 wt% and 0.40 wt%.
- High carbon equivalent. The reducer's carbon equivalent was about 0.81%, well above the level (below about 0.5%) the authors cite as acceptable for good weldability.
- Hard heat-affected zone. Microhardness in the heat-affected zone (HAZ) peaked at 588 HV0.1, against roughly 290–310 HV0.1 in the reducer base metal and about 175–204 HV0.1 in the weld metal.
- Inadequate welding control. Preheating, interpass temperature control and post-weld heat treatment (PWHT) were not enough for the actual reducer chemistry. The procedure suited the specified steel, not the steel that was fitted.
The authors conclude that poor material weldability combined with inadequate on-site welding practice led to brittle fracture in service.
Why the HAZ matters
During welding, the steel next to the weld is heated and then cooled quickly. In a low-carbon steel this is usually harmless. In a higher-carbon, chromium-bearing steel, rapid cooling can form hard, brittle microstructures unless preheat and PWHT are adequate. In a line carrying H2S, hard microstructures are also a known risk for environmentally assisted cracking.
Checklist for welded fittings
- Specify chemistry, not just a grade name. Include a maximum carbon equivalent in purchase specifications for fittings to be welded.
- Verify on receipt. Positive material identification (PMI) with a portable analyser takes minutes per item and would have caught this reducer.
- Match the welding procedure to the actual material. If the chemistry differs from the design, requalify the procedure or reject the part.
- Survey HAZ hardness on procedure qualification and, for critical or sour-service joints, on production welds.
- Keep records of preheat, interpass temperatures and PWHT, so a failure can be traced.
How MTIS can help
MTIS advises on materials selection for corrosive environments, reviews welding and procurement specifications, and carries out failure investigations on welded components. Contact MTIS to discuss a specification review or a failed weld.
Source: K. Xu, P. Wang, S. Wang, S. Xie, B. Wang, "Weld Seam Failure Analysis of a Natural Gas Pipeline Reducer: Implications for Oil and Gas Transportation Safety", Fuels 7(2):30, 2026, doi:10.3390/fuels7020030. Open access under a Creative Commons Attribution licence. Summarised in our own words.