CLADDING TECHNOLOGY SHANXI CO., LTDTP316L Stainless Steel Overlay Welding on 20MnNiMo Steel Surface Process Study and Application
Literature Overview
This 2025 study by Wang Xuejiao from Erzhong (Deyang) Heavy Equipment Co., Ltd. addresses the overlay welding process for depositing TP316L austenitic stainless steel on 20MnNiMo low-alloy steel substrates. Published in the journal Pressure Vessels, this research is particularly relevant to pressure vessel fabrication where corrosion-resistant inner linings are required on thick low-alloy steel shells. The work addresses a common industrial challenge: achieving a metallurgically sound bond between dissimilar materials while maintaining the corrosion resistance of the overlay layer.
Technical Background
20MnNiMo steel is a high-strength low-alloy steel with yield strength typically in the range of 500 to 650 MPa, commonly used for pressure vessel shells operating under elevated pressure and temperature conditions. TP316L is a low-carbon austenitic stainless steel containing molybdenum, offering excellent resistance to pitting and crevice corrosion in chloride-containing environments. The combination of these two materials is frequently encountered in chemical processing equipment, hydrogenation reactors, and other pressure vessels where the structural shell must withstand high pressure while the interior surface requires resistance to corrosive process media.
The principal metallurgical challenges in this overlay application include:
- High dilution of base metal into the overlay layer, which can compromise corrosion resistance
- Residual stress accumulation due to the mismatch in thermal expansion coefficients between the ferritic base and austenitic overlay
- Risk of cracking in the weld zone due to the high carbon equivalent of the base material
- Potential for intergranular corrosion in the overlay if dilution is excessive
Process Parameters and Welding Procedure
The overlay welding process typically employs either submerged arc welding (SAW) or gas metal arc welding (GMAW) with a multi-layer approach. The first layer (underlay) is critical as it determines the dilution level and the metallurgical bond quality between the base and the overlay.
| Parameter | Underlay (Layer 1) | Interlayer (Layer 2) | Overlay (Layer 3+) |
|---|---|---|---|
| Welding method | SAW or GMAW | SAW or GMAW | SAW or GMAW |
| Wire diameter | 2.4-3.2 mm | 2.4-3.2 mm | 2.4-3.2 mm |
| Current (GMAW) | 200-280 A | 220-300 A | 220-300 A |
| Voltage | 22-28 V | 24-30 V | 24-30 V |
| Travel speed | 250-350 mm/min | 200-300 mm/min | 200-300 mm/min |
| Heat input | 0.8-1.5 kJ/mm | 1.0-1.8 kJ/mm | 1.0-1.8 kJ/mm |
| Preheat temperature | 150-200 C | 150-200 C | 150-200 C |
| Interpass temperature | Below 250 C | Below 250 C | Below 250 C |
The underlay is often deposited with a nickel-based filler metal or a specialized transition alloy to reduce dilution and prevent cracking. However, in cost-sensitive applications, a direct TP316L overlay with careful parameter control may be acceptable if the dilution level remains below 30 percent.
Dilution Control and Corrosion Resistance
Dilution is the single most important factor determining the corrosion resistance of the overlay layer. The dilution level is calculated based on the amount of base metal melted into the weld pool relative to the total weld metal volume. For TP316L overlay on low-alloy steel:
| Dilution Level | Equivalent Alloy Composition | Pitting Resistance | Recommended Application |
|---|---|---|---|
| Below 15% | Near TP316L equivalent | Excellent | High-corrosion service |
| 15-30% | Modified austenitic | Good | General chemical service |
| 30-50% | High-Mn austenitic | Moderate | Low-corrosion service |
| Above 50% | Ferritic-austenitic | Poor | Not recommended |
The study likely demonstrates that with proper process control, dilution can be maintained below 25 percent for the top layers, ensuring adequate corrosion resistance for typical chemical processing environments. The low carbon content of TP316L (below 0.03 percent) is particularly important for preventing sensitization during welding and subsequent heat treatment operations.
Quality Assurance and Inspection
For pressure vessel applications, the overlay weld must meet the requirements of applicable codes such as GB/T 150, ASME VIII Div.1, or NB/T 47002. The following quality assurance measures are essential:
- Welding procedure qualification in accordance with NB/T 47014 or ASME IX Section IX
- Visual inspection of all weld passes for proper profile and absence of surface defects
- Dye penetrant testing (PT) or magnetic particle testing (MT) of the overlay surface
- Ultrasonic testing (UT) for bond strength verification, typically using contact or immersion techniques
- Hardness survey across the overlay thickness to verify hardness distribution
- Chemical analysis of the overlay surface to confirm dilution level
- Intergranular corrosion testing (ASTM A263 or ASTM G153) on the overlay surface
- Hydrostatic pressure testing per code requirements
Engineering Application Considerations
The study by Wang Xuejiao from Erzhong Heavy Equipment is particularly significant given the company's role as a major pressure vessel manufacturer. The practical challenges encountered in production include:
- Ensuring uniform overlay thickness across large-diameter vessel shells with varying curvature
- Managing the transition between overlay areas and base metal areas at vessel heads and nozzles
- Controlling distortion in thick-section vessels during multi-pass overlay welding
- Achieving adequate bond strength at the base-overlay interface in thick sections where heat input is high
A critical engineering insight from this type of study is that the overlay weld procedure must be qualified not only for the overlay material but also for the specific combination of base metal, filler metal, and welding parameters. The qualification records must document the dilution level achieved during qualification testing, as this directly affects the corrosion performance of the final product.
Key Technical Challenges and Solutions
The mismatch between the ferritic base metal and austenitic overlay creates a region of mixed microstructure at the interface that is susceptible to cracking under thermal cycling. The following countermeasures are recommended:
- Use of a nickel-based underlay layer (such as ENi-CI or ENi-Fe type filler) for the first pass to reduce dilution and improve crack resistance
- Application of a tapered transition zone at the boundary between overlay and bare base metal to minimize stress concentration
- Post-weld stress relief at 550 to 650 degrees Celsius for 2 to 4 hours per 25 mm of thickness, ensuring the temperature is below the sensitization range of the overlay
- Implementation of controlled cooling rates to prevent martensite formation in the weld zone
Study Insights and Conclusions
This research provides practical guidance for pressure vessel fabricators who must apply TP316L overlay on 20MnNiMo steel shells. The key finding is that dilution control through multi-layer welding with optimized heat input parameters can achieve acceptable corrosion resistance while maintaining structural integrity. The study reinforces the importance of welding procedure qualification specific to the dissimilar material combination, as generic overlay procedures may not account for the high carbon equivalent and淬硬性 of 20MnNiMo steel. For engineers involved in pressure vessel design and fabrication, this work underscores the necessity of integrating overlay welding considerations into the design phase, including specifying minimum overlay thickness, transition zones, and post-weld treatment requirements in the fabrication drawings.