CLADDING TECHNOLOGY SHANXI CO., LTDCracking in Stainless Steel Weld Overlay Layers Causes and Countermeasures
Literature Overview
This study note addresses the cracking phenomenon observed in stainless steel weld overlay layers during large-scale ethylene project construction at Daqing Petrochemical Company, published in 2010 in the journal Petroleum Refining and Chemical Engineering. The authors, including Gao Chunhua from the Ethylene Project Command and Xing Fang, Liu Bin, and Han Qing from Daqing Petrochemical Machinery Factory, investigated overlay cracking issues encountered during the fabrication of critical process equipment for the ethylene cracking unit. The work is particularly relevant to engineers dealing with stainless steel cladding on carbon steel substrates in high-pressure petrochemical applications.
Core Technical Analysis
The fundamental cause of cracking in stainless steel weld overlay layers lies in the mismatch between the thermal expansion coefficients, thermal conductivities, and mechanical properties of the overlay material and the base metal. When austenitic stainless steel such as 304 or 316 is deposited onto carbon steel or low-alloy steel substrates, the resulting heat-affected zone (HAZ) experiences severe residual stresses due to differential thermal contraction during cooling. The high dilution ratio between the base metal and the overlay layer further exacerbates this problem by introducing carbon enrichment in the weld metal, which promotes martensitic transformation and subsequent cracking.
The cracking mechanisms identified in this work can be classified into three primary categories:
- Hot cracking (solidification cracking): Occurs during the final stages of solidification when low-melting-point phases such as Fe-Cr eutectics form at grain boundaries. This is particularly prevalent when the dilution ratio exceeds 30 percent and the weld metal contains insufficient sulfur and phosphorus control.
- Cold cracking (delayed cracking): Results from hydrogen embrittlement in the martensitic microstructure formed in the HAZ or the overlay layer itself. The high carbon content from base metal dilution promotes martensite formation, which is susceptible to hydrogen-induced cracking.
- Thermal stress cracking: Caused by excessive residual stresses generated during multi-pass welding, particularly when interpass temperature control is inadequate.
Key Process Parameters and Countermeasures
The study emphasizes several critical process control parameters that directly influence cracking susceptibility. The following table summarizes the recommended process windows and material selections:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Interpass temperature | 80-150°C for austenitic overlay | Prevents excessive heat input and martensite formation |
| Heat input per pass | ≤ 12 kJ/mm | Limits dilution and thermal stress |
| Base metal preheating | 100-150°C for carbon steel substrate | Reduces thermal gradient and residual stress |
| Dilution ratio control | ≤ 25% for first pass | Minimizes carbon enrichment in weld metal |
| Post-weld heat treatment | 1050-1100°C solution treatment for 1-2 hours | Dissolves sigma phase and relieves residual stress |
| Weld wire selection | E309L (low carbon) for transition layer | Lowers carbon content and improves ductility |
The authors propose a systematic approach to cracking prevention that integrates material selection, process optimization, and inspection protocols. The transition layer strategy is particularly important: using a low-carbon austenitic filler such as E309L or E310L as the first pass creates a buffer zone that reduces carbon dilution in subsequent overlay passes. This approach ensures that the final overlay layer maintains adequate ductility and corrosion resistance.
Engineering Practice Integration
In the context of ethylene project equipment fabrication, the cracking issue was encountered during the overlay welding of heat exchanger tubesheets and reactor internals where stainless steel overlay was applied to carbon steel substrates for corrosion resistance. The equipment operates at elevated temperatures and pressures with exposure to acidic and sulfurous environments, making the integrity of the overlay layer critical for long-term service life.
The FMEA (Failure Mode and Effects Analysis) approach applied in this work identifies the following critical failure modes:
- Failure mode 1: Cracking at the overlay-to-base metal interface due to high residual stress. Countermeasure: implement controlled cooling rates and post-weld stress relief treatment.
- Failure mode 2: Cracking within the overlay layer due to sigma phase precipitation. Countermeasure: limit interpass temperature and ensure adequate nickel content in the filler metal.
- Failure mode 3: Cracking at the weld toe due to geometric discontinuities. Countermeasure: optimize weld geometry and ensure proper grinding of the transition zone.
The study also highlights the importance of non-destructive testing (NDT) protocols. Magnetic particle testing (MT) and penetrant testing (PT) are recommended for surface crack detection, while ultrasonic testing (UT) with contact probes is essential for subsurface defect identification. The acceptance criteria should follow NB/T 47014 and JB/T 4730 standards, with particular attention to linear indications in the overlay layer.
Study Insights and Reflections
This literature provides a practical, field-driven perspective on overlay cracking that complements academic research. The emphasis on process control rather than purely metallurgical explanations reflects the practical challenges faced in large-scale fabrication environments. One key insight is that cracking prevention requires a holistic approach: no single parameter adjustment is sufficient, and the interaction between material chemistry, welding procedure, and inspection protocol must be carefully managed.
The study also underscores the importance of welder qualification and procedure qualification under NB/T 47014 or ASME IX. In the ethylene project context, the welding procedures were qualified with specific attention to the transition layer and overlay layer, with mechanical property tests including tensile strength, hardness profiling across the overlay-to-base metal interface, and intergranular corrosion testing per ASTM A263.
The long-term implication of this work is that for future projects involving stainless steel overlay on carbon steel substrates in aggressive environments, the cracking susceptibility should be evaluated during the design phase rather than treated as a fabrication problem. Material selection, including the choice of base metal grade and overlay alloy system, should be optimized to minimize cracking risk from the outset.