Crack Analysis and Prevention of Manhole Cover Cladding Sealing Surface

Literature Overview

This 2013 publication by Chen Kunshan from Yunnan Yuntianhua Co., Ltd. investigates cracking defects observed on the cladding sealing surfaces of manhole covers in chemical process equipment. Yunnan Yuntianhua, a major phosphate chemical producer, operates equipment exposed to aggressive media including phosphoric acid, sulfuric acid, and ammonia solutions. Manhole covers are critical pressure-retaining components that must maintain leak-tight integrity under cyclic thermal and pressure loading. The cladding sealing surface provides corrosion resistance, but cracking in this region constitutes a serious reliability concern. This case study offers valuable lessons for engineers dealing with overlay weld integrity in chemical environments.

Failure Mode Analysis

Cracking in cladding sealing surfaces of manhole covers can originate from multiple mechanisms. The following table summarizes the primary crack types identified or considered in such analyses:

Crack Type	Location	Primary Cause	Detection Method
Transverse cracks	Cladding weld bead	Thermal stress,拘束 stress	MT / PT
Longitudinal cracks	Cladding weld bead	HAZ microstructure, hydrogen	MT / PT
Interfacial cracks	Clad-base interface	Poor wetting, porosity	UT / MT
Reheat cracks	HAZ	Precipitation, slow cooling	MT / PT
Stress corrosion cracks	Clad surface	Corrosive medium + tensile stress	PT / ECT

The most common scenario in manhole cover applications involves transverse cracking in the overlay weld metal or at the weld root, driven by high thermal restraint combined with hydrogen pickup from the welding process. Manhole covers are typically thick-walled forgings or castings, and the high restraint factor amplifies the thermal stress during multi-pass cladding.

Root Cause Investigation Methodology

A systematic approach following the 5W2H framework was applied:

What: Transverse and longitudinal cracks in the stainless steel cladding layer on the sealing face of manhole covers.
Where: Primarily at the weld toe and in the root pass of multi-layer cladding.
When: Cracks detected during post-weld NDT (magnetic particle testing) or during hydrostatic testing.
Who: Affected covers from a specific heat of base material and a specific welding procedure.
Why: Combination of high carbon equivalent of base material, excessive welding heat input, insufficient preheating, and rapid cooling.
How: Cracks initiated at micro-voids or inclusions and propagated under residual tensile stress.
How much: Cracks up to 25 mm in length, with a 30–40% defect rate in the initial batch.

Metallographic analysis of extracted specimens typically reveals a coarse-grained HAZ with martensitic transformation in the dilution zone. In duplex stainless steel cladding, cracking may be associated with phase imbalance (excessive ferrite or austenite).

Preventive Measures

Measure	Implementation Detail
Preheating	150–250 °C for low-alloy steel base; controlled by thermocouple monitoring
Interpass temperature	Maintain 150–250 °C throughout multi-pass sequence
Heat input control	Limit to 0.8–1.5 kJ/mm for root pass; 1.5–2.5 kJ/mm for fill passes
Post-weld heat treatment	600–650 °C × 2 h for stress relief; or 1050 °C × 1 h + air cool for solution treatment of clad
Welding sequence	Symmetric, balanced sequence to minimize restraint; start from center outward
Consumable selection	Low-hydrogen flux or electrode; E309L-type consumable for carbon steel/stainless interface
Surface preparation	Thorough cleaning of sealing face; removal of oxide scale and contaminants
NDT coverage	100% MT or PT on cladding surface; 100% UT on weld root if accessible

Engineering Practice Integration

In chemical plant operations, manhole cover integrity directly affects safety and environmental compliance. A cracked cladding surface can lead to leakage of toxic or corrosive media, creating both safety hazards and regulatory violations. The preventive measures outlined above must be embedded into the welding procedure specification (WPS) and verified through welder performance qualification (WPQ).

A particularly important aspect is the interaction between the cladding process and the subsequent machining of the sealing surface. Post-weld machining removes the surface layer and can expose underlying microstructural features. If the clad layer is too thin or if cracking extends below the machined surface, the defect will not be detected during final inspection. Therefore, adequate cladding thickness (typically 3–5 mm for manhole cover sealing faces) and thorough NDT before machining are essential.

Key Questions and Reflections

An important question arises regarding the role of base material quality in cladding crack susceptibility. If the base forging has high sulfur content or contains MnS inclusions, these can act as crack initiation sites. The procurement specification for manhole cover forgings should therefore include strict limits on sulfur and phosphorus content, and the forging should be supplied with proper heat treatment documentation.

Another reflection concerns the applicability of these findings to other overlay applications. The same principles of thermal stress management, hydrogen control, and dilution management apply broadly to weld overlay in chemical equipment, regardless of the specific component geometry. Engineers should treat this case study as a template for systematic crack analysis in their own operations.

Summary

The cracking of cladding sealing surfaces on manhole covers is a multifactorial problem rooted in the interaction between material properties, welding process parameters, and thermal-mechanical constraints. The systematic analysis approach described in this literature provides a replicable framework for engineers facing similar defects. By implementing controlled preheating, optimized welding sequences, appropriate consumable selection, and thorough NDT, the defect rate can be reduced to negligible levels. The experience from Yunnan Yuntianhua underscores the importance of integrating metallurgical understanding with practical process control in chemical equipment fabrication.