CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis and Prevention of Cracks in Manhole Cover Weld Overlay Sealing Surfaces
Literature Overview
This technical paper, authored by Chen Kunshan and published in 2013 in Yunnan Chemical, addresses a critical field problem encountered during the fabrication of manhole cover weld overlay sealing surfaces at Yunnan Yuntianhua Co., Ltd. Manhole covers are integral components in pressure vessels and chemical process equipment, serving as access points for inspection, maintenance, and internal component replacement. The sealing surface of a manhole cover is typically produced by weld overlay (cladding) onto the base material to achieve the required corrosion resistance, hardness, and sealing integrity. Cracks in this overlay layer represent a severe quality defect that compromises the pressure boundary integrity and can lead to catastrophic leakage in aggressive chemical service environments.
Core Technical Content
The paper investigates the root causes of cracking in manhole cover weld overlay sealing surfaces and proposes systematic preventive measures. The fundamental metallurgical issue arises from the combination of high dilution between the overlay material and the base steel, residual stress accumulation during multi-pass welding, and the formation of brittle intermetallic phases at the weld interface. In chemical plant applications, manhole covers often require overlay with austenitic stainless steel (such as 304 or 316) or nickel-based alloys (such as Inconel 625 or Hastelloy C276) on carbon steel or low-alloy steel substrates. The coefficient of thermal expansion mismatch between these dissimilar materials generates significant thermal stresses during both welding and subsequent cooling, which can initiate microcracks that propagate under cyclic thermal loading.
Crack Formation Mechanisms
The primary mechanisms identified include hydrogen-induced cracking, solidification cracking in the overlay weld metal, and stress relief cracking in the heat-affected zone. Hydrogen-induced cracking is particularly problematic when the base material contains carbon equivalents exceeding 0.45 percent, as the diffusion of hydrogen atoms into the high-hardness martensitic microstructure at the fusion line creates localized stress concentrations. Solidification cracking occurs in the last-solidifying interdendritic regions when the overlay alloy composition falls within a composition range susceptible to hot cracking, particularly when sulfur and phosphorus impurities are present at elevated levels. Stress relief cracking manifests as intergranular fractures in the sensitized austenitic overlay layer, typically occurring in the 450 to 850 degrees Celsius temperature range where chromium carbide precipitation depletes the grain boundaries of chromium.
Process and Standards Analysis
The following table summarizes the key process parameters and control measures relevant to preventing overlay cracking on manhole cover sealing surfaces:
| Parameter | Acceptable Range | Risk if Exceeded |
|---|---|---|
| Preheating temperature (carbon steel base) | 150-250°C | Hydrogen cracking in HAZ |
| Interpass temperature | 100-200°C | Excessive grain growth, sensitization |
| Welding current (GTAW overlay) | 80-150 A | Excessive dilution, burn-through |
| Travel speed | 50-100 mm/min | Incomplete fusion, excessive dilution |
| Post-weld heat treatment | 1050-1100°C, 2h + PWHT 600°C | Residual stress, sensitization |
| Overlay layer thickness | 3-6 mm minimum | Insufficient corrosion allowance |
The applicable standards include GB/T 150 for pressure vessel fabrication requirements, NB/T 47014 for weld procedure qualification, and JB/T 4730 for non-destructive testing. According to these standards, the overlay weld procedure must be qualified with appropriate mechanical property tests, including impact testing at the fusion line, intergranular corrosion testing per ASTM A263, and hardness profiling across the overlay-to-base transition zone. The dilution rate between the overlay material and base material must be controlled to ensure the overlay layer maintains its required corrosion resistance properties; typically, dilution should not exceed 25 percent for stainless steel overlays on carbon steel substrates.
Engineering Practice and Defect Countermeasures
In practice, the preventive strategy follows a systematic approach combining material selection, process optimization, and quality verification. The selection of the appropriate overlay welding consumable is the first critical decision. For manhole covers in chemical service, a two-pass approach is often recommended: the first pass uses a transition alloy (such as E309L or ER309L) to buffer the dilution effect, followed by the second pass with the final overlay alloy (such as E316L or ER316L) to achieve the required corrosion resistance. This technique reduces the carbon equivalent at the fusion line and minimizes the risk of brittle phase formation.
The welding sequence is another critical control variable. A staggered welding pattern should be employed to minimize the accumulation of residual stresses in any single direction. The first weld pass should be placed at the center of the sealing surface, with subsequent passes progressing outward in a balanced manner. This approach distributes the thermal distortion evenly and prevents the buildup of unidirectional compressive stresses that could exceed the yield strength of the overlay material. Post-weld heat treatment is essential for relieving residual stresses and restoring the microstructural integrity of the overlay layer. For austenitic stainless steel overlays, a solution heat treatment at 1050 to 1100 degrees Celsius followed by rapid water quenching is recommended to dissolve precipitated carbides and restore full corrosion resistance.
Non-destructive testing must be performed after overlay completion and after post-weld heat treatment. Magnetic particle testing (MT) is the primary method for detecting surface and near-surface cracks in ferromagnetic base materials, while dye penetrant testing (PT) is used for non-ferromagnetic overlay surfaces. Ultrasonic testing (UT) with contact probes or phased array ultrasonic testing (PAUT) provides assessment of subsurface defects and bond integrity between the overlay and base material. Any detected cracks must be ground flush, re-inspected, and the overlay re-applied after root cause analysis confirms the corrective action is appropriate.
Key Reflections
This paper highlights a common but often underappreciated failure mode in pressure vessel fabrication. The sealing surface of a manhole cover is subjected to both mechanical loading from the bolting system and thermal cycling from process conditions, making it a fatigue-critical location. The overlay layer, while providing corrosion resistance, introduces a metallurgical discontinuity that must be carefully managed throughout the fabrication sequence. The lesson is clear: overlay welding is not merely a surface treatment but a structural modification that requires the same rigor as primary pressure boundary welds. Engineers must approach overlay qualification with the same discipline applied to butt welds, including full procedure qualification, documented operator certification, and comprehensive NDE coverage.