CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Porosity and Cracking Causes in Valve Overlay Welding
Literature Overview
The paper by Xu Xiao, Yan Bo, and Zhang Heqing (2000) from South China University of Technology presents a detailed analysis of the causes of porosity and cracking defects in valve overlay welding. Valve components, including gate valves, globe valves, ball valves, and butterfly valves, are widely used in the petroleum, chemical, and power industries, and overlay welding is commonly employed to deposit corrosion-resistant or wear-resistant coatings on valve seats, stems, and trim components. The relatively small size and complex geometry of valve components, combined with the high dilution rates and thermal cycling inherent in overlay welding, create a challenging welding environment that is prone to porosity and cracking defects.
Core Technical Content
The study systematically investigates the metallurgical and process factors that contribute to porosity and cracking in valve overlay welding. The analysis covers the composition of the base metal and overlay consumable, the welding process parameters, the preheat and interpass temperature, the shielding gas composition, and the post-weld heat treatment. The findings provide a comprehensive framework for defect prevention in valve overlay welding applications.
Porosity Analysis
Porosity in valve overlay welding can be classified into several types, each with distinct causes:
| Porosity Type | Location | Cause | Countermeasure |
|---|---|---|---|
| Gas porosity | Weld metal | Inadequate shielding gas, contaminated consumables, high hydrogen content | Improve shielding, use dry consumables, preheat |
| Slag inclusion porosity | Weld metal | Incomplete slag removal between passes, high travel speed | Remove slag between passes, reduce travel speed |
| Backside porosity | Root of weld | Inadequate root preparation, improper root gas shielding | Prepare root properly, use backside shielding |
| Micro-porosity | Grain boundaries | High cooling rate, high hydrogen content | Reduce cooling rate, use low-hydrogen consumables |
The primary cause of gas porosity in valve overlay welding is the absorption of hydrogen from the atmosphere, moisture in the flux or wire, or contamination of the base metal surface. The small size of valve components often results in rapid cooling, which reduces the time available for hydrogen bubbles to escape from the molten weld pool. This is particularly problematic in stainless steel overlay welding, where the high carbon equivalent and the presence of chromium and nickel increase the hydrogen solubility in the molten metal.
Cracking Analysis
Cracking in valve overlay welding can be classified as solidification cracking, hot cracking, or cold cracking, each with distinct metallurgical mechanisms:
| Cracking Type | Mechanism | Cause | Countermeasure |
|---|---|---|---|
| Solidification cracking | Low-melting-phase segregation at grain boundaries | High sulfur and phosphorus, high dilution, high restraint | Use low-S/P consumables, reduce dilution, increase preheat |
| Hot cracking | Thermal stress during solidification | High carbon equivalent, excessive restraint, high travel speed | Reduce heat input, increase preheat, use proper weave pattern |
| Cold cracking | Hydrogen-induced cracking in HAZ | High carbon equivalent, high hydrogen content, high restraint | Preheat, use low-hydrogen consumables, post-weld heat treatment |
The study identifies several key factors that contribute to cracking in valve overlay welding:
- High restraint: The small size and thick section of valve components create high welding restraint, which increases the residual stress and promotes cracking.
- High dilution: The overlay welding process on valve components often results in high dilution rates (30–50%), which increases the carbon equivalent of the weld metal and reduces the cracking resistance.
- Thermal cycling: The multiple thermal cycles inherent in overlay welding can promote the formation of brittle phases (e.g., sigma phase, carbides) in the heat-affected zone, reducing the toughness and promoting cracking.
- Hydrogen absorption: The small weld pool and rapid cooling in valve overlay welding increase the risk of hydrogen-induced cracking, particularly in low-alloy steel base metals.
Process Optimization and Quality Assurance
The study proposes a systematic approach to defect prevention in valve overlay welding, based on the following principles:
- Consumable selection: Use low-carbon, low-sulfur, low-phosphorus consumables that are compatible with the base metal composition. For stainless steel overlay on carbon steel, use a 309L or 310L consumable to minimize dilution effects and promote a low-carbon weld metal.
- Welding parameter optimization: Select welding parameters that minimize heat input while ensuring complete melting of the base metal. For valve overlay welding, a welding current of 150–250 A, an arc voltage of 20–28 V, and a travel speed of 100–200 mm/min are typical, depending on the wire diameter and base metal thickness.
- Preheat and interpass temperature control: Preheat the valve component to 100–250 °C (depending on the base metal composition) to reduce the cooling rate and promote hydrogen escape. Maintain the interpass temperature below 250 °C to avoid excessive grain coarsening.
- Shielding gas optimization: Use a high-purity shielding gas (99.99% Ar or Ar/CO₂ mixture) and ensure adequate gas coverage to prevent atmospheric contamination. For valve components with complex geometry, use a nozzle extension or gas lens to improve shielding coverage.
- Post-weld heat treatment: Perform a post-weld stress relief heat treatment (600–700 °C for carbon steel, 1050 °C for stainless steel) to reduce residual stresses and promote hydrogen escape.
Non-Destructive Testing Requirements
| NDT Method | Application | Acceptance Criteria |
|---|---|---|
| Radiographic testing (RT) | Internal porosity, lack of fusion | ASME V Section 5, Level T-2 or equivalent |
| Ultrasonic testing (UT) | Bond strength, internal defects | ASME V Section 5, TOFD or PAUT |
| Magnetic particle testing (MT) | Surface cracks, lack of fusion | ASME V Section 7, Level M-1 or equivalent |
| Penetrant testing (PT) | Surface cracks, porosity | ASME V Section 6, Level A or equivalent |
| Dye penetrant testing (DPT) | Surface cracks in non-ferrous components | ASME V Section 6, Level A or equivalent |
Study Insights and Implications
The analysis of porosity and cracking in valve overlay welding provides a valuable framework for defect prevention in a challenging welding application. The key insight is that the defects are primarily caused by the interaction of metallurgical factors (composition, dilution, microstructure) and process factors (heat input, cooling rate, restraint), and that effective defect prevention requires a systematic approach that addresses both categories of factors.
The study also highlights the importance of consumable selection in valve overlay welding. The use of low-carbon, low-sulfur, low-phosphorus consumables is essential to minimize the risk of solidification cracking and hot cracking. For stainless steel overlay on carbon steel, the use of a 309L or 310L consumable is recommended to promote a low-carbon weld metal with adequate cracking resistance. The dilution rate must be carefully controlled, as excessive dilution can significantly increase the carbon equivalent of the weld metal and reduce the cracking resistance.
The engineering implications of this study extend to the qualification and certification of valve overlay welding procedures. The welding procedure specification (WPS) must be qualified in accordance with ASME IX or the applicable national standard, and the welder performance qualification must demonstrate competence in the specific technique used. The non-destructive testing requirements must be defined in the WPS and must be consistent with the applicable construction code (e.g., ASME VIII Division 1, Section I, or the equivalent national standard).
From a quality assurance perspective, the study emphasizes the importance of in-process monitoring and post-weld inspection. The welding parameters must be monitored and recorded throughout the welding process, and any deviations from the qualified parameters must be investigated and documented. The post-weld inspection must include both non-destructive testing (NDT) and destructive testing (mechanical property testing, metallurgical examination) to confirm that the overlay weld meets the specified requirements.
This study provides a comprehensive analysis of the causes of porosity and cracking in valve overlay welding and offers practical guidance for defect prevention. The findings are particularly relevant to the petroleum, chemical, and power industries, where valve overlay welding is extensively used for the repair and refurbishment of valve components. Future research should focus on the development of advanced consumables (e.g., low-hydrogen flux-cored wire, self-shielded flux-cored wire) and automated welding procedures that can further reduce the risk of defects in valve overlay welding.