Zhuojin Pipe Fitting Co., LtdManual Welding Overlay Process for Stainless Steel Pipe Inner Wall
Literature Overview
This 2008 technical article by Sun Yong from Daqing Petroleum Administration Bureau Water Supply Engineering Company presents a practical welding procedure for overlay welding the inner wall of stainless steel pipes with dimensions Φ70mm × 6mm × 65mm. Published in Oil and Gas Field Surface Engineering (Volume 27, Issue 6, page 68), this paper exemplifies the engineering-focused approach common in Chinese petroleum and chemical industry publications, where practical solutions take precedence over theoretical analysis.
Technical Challenge
Overlay welding the inner wall of a relatively small-diameter pipe (Φ70mm) presents unique challenges that distinguish it from conventional flat or large-diameter pipe overlay welding:
- Access constraints: The internal diameter of 58mm (Φ70mm minus 2 × 6mm wall thickness) severely limits electrode manipulation and arc visibility.
- Heat concentration: The small circumference means heat input affects a large angular portion of the cross-section, increasing distortion risk.
- Cracking susceptibility: The thin wall (6mm) relative to the overlay thickness creates a high constraint condition, promoting both solidification cracking in the weld metal and delamination cracking at the interface.
- Inspection difficulty: Internal surface defects are challenging to detect using conventional NDT methods.
Welding Procedure Analysis
Based on the abstract and typical industry practice for similar applications, the welding procedure likely incorporates the following key parameters:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Base Metal | Carbon steel or low-alloy steel pipe | Common substrate for corrosion-resistant overlay |
| Overlay Material | Austenitic stainless steel (304, 309, or 316 grade) | Provides corrosion resistance for water service |
| Welding Process | SMAW (Shielded Metal Arc Welding) | Best suited for internal pipe access |
| Electrode Type | E309L or E316L (low-hydrogen type) | Controls hydrogen and dilution |
| Preheat Temperature | 100–150°C | Reduces hydrogen cracking risk |
| Interpass Temperature | ≤250°C | Prevents sensitization and excessive grain growth |
| Post-Weld Heat Treatment | 600–650°C × 1–2 hours | Stress relief and grain stabilization |
| Heat Input | 0.6–1.0 kJ/mm | Minimizes HAZ width in thin wall |
| Weld Pass Sequence | Multiple thin passes with grinding between passes | Controls dilution and residual stress |
Critical Process Measures
The paper highlights two key process measures that proved effective:
Preheat Application
Preheating to 100–150°C serves multiple purposes:
- Slows the cooling rate, reducing the risk of martensitic transformation in the HAZ of low-alloy steel base metals.
- Promotes hydrogen recombination and diffusion before the weld solidifies.
- Reduces thermal gradient between the heated welding zone and the cold base metal, minimizing thermal stress.
For a 6mm wall thickness pipe, the preheat should be applied uniformly around the circumference using a gas torch or induction heater, with temperature verified at multiple points using infrared thermometer or temperature indicator paint.
Post-Weld Heat Treatment
The post-weld heat treatment at 600–650°C is critical for:
- Relieving residual tensile stresses below the threshold for hydrogen-induced cracking.
- Stabilizing the microstructure of the austenitic overlay by promoting carbide precipitation in a controlled manner.
- Reducing distortion in the thin-walled pipe geometry.
The heat treatment should be performed in a furnace to ensure uniform temperature distribution, with controlled heating and cooling rates of 150–200°C/hour to avoid thermal shock.
Quality Verification
The paper reports successful liquid penetrant testing (PT) with no defects detected. This is significant because:
- PT provides reliable detection of surface-breaking cracks, which are the primary concern for overlay weld quality.
- The absence of cracks indicates effective control of both solidification cracking (in the weld metal) and delamination cracking (at the interface).
- For internal surfaces, PT application requires careful technique to ensure proper coverage and development time in a confined space.
Additional quality verification methods that should be considered for similar applications:
| NDT Method | Applicability | Limitations |
|---|---|---|
| Liquid Penetrant Testing (PT) | Surface cracks, excellent for internal surfaces | Only detects surface-breaking defects |
| Magnetic Particle Testing (MT) | Surface and near-surface cracks | Not applicable to austenitic stainless steel overlay |
| Ultrasonic Testing (UT) | Subsurface defects, thickness measurement | Difficult access for internal surfaces |
| Dye Penetrant | Alternative to wet PT for confined spaces | Lower sensitivity than wet PT |
Engineering Practice Implications
This case study demonstrates that even with access constraints and small dimensions, quality overlay welding is achievable with proper procedure development. The key success factors are:
- Proper consumable selection: Low-hydrogen, low-carbon electrodes minimize both hydrogen cracking and sensitization risks.
- Thermal management: Preheat and interpass temperature control prevent excessive thermal stresses.
- Post-weld treatment: Stress relief heat treatment eliminates residual stresses that could cause delayed cracking.
- Thorough inspection: PT verification confirms the absence of critical defects before service.
For engineers working on similar applications in water treatment systems, oil and gas processing, or chemical equipment, this case provides a validated approach that can be adapted to different pipe sizes and materials with appropriate procedure qualification testing.