Remediation of Overlay Weld Cracks in Hydrogenation Reactor Manhole Nozzle

Literature Overview

This paper by Zhu Changhong and Zhang Taojun, published in Welding Technology (2013, Vol. 42, Issue 10, pp. 64-65), reports a real engineering case involving a hydrogenation reactor where multiple circumferential cracks were detected in the overlay weld layer of a manhole nozzle during the first comprehensive inspection. The authors conducted a systematic investigation using penetrant testing (PT) for initial detection and ultrasonic testing (UT) for depth assessment, ultimately identifying the root causes and proposing corrective measures. The case is particularly instructive for pressure vessel engineers and welding inspectors working in the hydrogenation and petrochemical sectors.

Core Findings and Defect Characterization

The overlay layer on the manhole nozzle consisted of 0Cr18Ni10Ti (equivalent to austenitic stainless steel 321) deposited on a 15CrMoR base material. During inspection, penetrant testing revealed multiple circumferential cracks in the overlay layer. Ultrasonic testing confirmed that the cracks did not penetrate through to the base material, meaning the defects were confined entirely within the overlay weld deposit.

The circumferential orientation of the cracks is significant. Circumferential cracks in overlay welds typically indicate longitudinal residual stress, which is a direct consequence of the welding sequence and the thermal contraction pattern during multi-pass welding. The fact that the cracks were confined to the overlay layer suggests that the interface between the base and overlay was intact, pointing to a metallurgical or thermal issue within the overlay deposit itself rather than a bonding failure.

Root Cause Analysis

The authors identified three primary contributing factors to the cracking:

1. Unstable interpass temperature control: During multi-pass overlay welding, failure to maintain consistent interpass temperatures leads to uneven thermal cycling. Excessively high interpass temperatures can cause softening of previously deposited layers, while too-low temperatures increase cooling rates and promote brittle microstructures. In this case, the inconsistency likely resulted in localized regions of rapid cooling where martensitic transformation occurred.
2. Alloy element depletion leading to hard and brittle martensite: The dilution from the 15CrMoR base material into the overlay weld deposit altered the effective composition of the overlay. The base material's carbon and chromium content diluted the austenitic overlay, lowering the effective austenite-stabilizing elements (nickel, titanium). This compositional shift pushed the weld metal into a regime where martensitic transformation upon cooling became thermodynamically favorable. The resulting martensite is hard, brittle, and highly susceptible to cracking, especially under residual stress.
3. Non-uniform post-weld heat treatment (PWHT) temperature distribution: The PWHT was intended to relieve residual stresses and temper any martensite that had formed. However, non-uniform heating of the large vessel geometry meant that some regions of the overlay layer experienced insufficient or excessive thermal exposure. Regions that were under-heated retained residual stress and possibly untempered martensite, while over-heated regions could suffer from grain coarsening or intergranular embrittlement.

Discussion on Hydrogen and Hydrogen Sulfide Corrosion Concerns

A key question in any hydrogenation service is whether the operating conditions pose a risk of hydrogen damage to the materials. The authors explicitly addressed this concern, concluding that at the given working pressure and temperature, neither molecular hydrogen nor trace amounts of hydrogen sulfide (H₂S) would cause hydrogen attack (HA) or sulfide stress cracking (SSC) in either the 0Cr18Ni10Ti overlay or the 15CrMoR base material.

This conclusion is consistent with the Nelson curves for hydrogen attack, which define the threshold pressure-temperature combinations above which high-carbon iron phases become susceptible to decomposition in the presence of atomic hydrogen. 15CrMoR, being a low-carbon Cr-Mo steel with controlled microalloying, sits well below the Nelson curve limits at typical hydrogenation reactor conditions. The austenitic overlay is inherently resistant to hydrogen embrittlement and sulfide stress cracking due to its FCC crystal structure, which does not facilitate hydrogen diffusion and trapping in the same manner as ferritic or martensitic structures.

Engineering Implications and Corrective Measures

This case study offers several lessons for engineers and inspectors:

• Interpass temperature monitoring is not optional: For overlay welding on dissimilar material combinations, especially involving Cr-Mo steels and austenitic stainless steels, interpass temperature must be tightly controlled. A typical recommended range for such combinations is 150-250°C, and each pass should be verified with calibrated thermocouples or infrared pyrometers.
• Pre-weld dilution analysis is essential: A Schaeffler diagram or equivalent dilution calculation should be performed before welding to predict the effective composition of the weld metal. If the predicted composition falls into a brittle phase region, additional passes with higher austenite-stabilizing content or a transition layer should be specified.
• PWHT uniformity must be verified: For large vessels, PWHT uniformity should be confirmed using multiple thermocouples at critical locations. The heating and cooling rates should comply with applicable codes (e.g., ASME Section VIII Div. 1 or GB/T 150), and the soak temperature must be sufficient to fully temper any martensite.
• Repair strategy: The recommended repair approach involves complete removal of the cracked overlay layer down to sound base material, followed by re-overlay using a revised welding procedure that includes controlled interpass temperatures, possibly a transition layer, and a properly executed PWHT.

Study Insights and Reflections

Having reviewed this case, I find it particularly instructive because it illustrates a common failure mode that is often underestimated in practice: the assumption that overlay welding is a straightforward surface treatment. In reality, overlay welding on dissimilar materials involves complex metallurgical interactions, including dilution, phase transformations, residual stress development, and thermal mismatch. The circumferential crack pattern is a textbook example of how residual stress orientation interacts with brittle microstructure to produce cracking. This case reinforces the importance of a systematic approach to overlay welding qualification, where the welding procedure specification (WPS) must account for all thermal, metallurgical, and mechanical factors rather than being treated as a simple welding exercise. The explicit discussion of hydrogen and H₂S corrosion also demonstrates good engineering practice—addressing not only the observed defect but also ruling out other potential failure mechanisms in the service environment.