CLADDING TECHNOLOGY SHANXI CO., LTDWelding Test of Strip Cladding 304L on 20MnMo Forged Tube Sheets
Literature Overview
This 2009 study by Zhou Yinmei, Li Meijing, and Yang Wenming from Shanxi Fengxi Chemical Equipment Co., Ltd., published in "Shanxi Chemical Industry," presents welding test results for strip cladding 304L stainless steel on 20MnMo forged tube sheets. Tube sheets are critical components in heat exchangers and pressure vessels, serving as the structural and sealing interface between the shell-side and tube-side compartments. The 20MnMo base material provides high-strength structural support, while the 304L stainless steel overlay layer offers corrosion resistance for the process fluid side. This study is particularly relevant to engineers designing and fabricating heat exchangers for petrochemical and chemical processing applications where the combination of high strength and corrosion resistance is essential.
Technical Approach and Welding Procedure
The strip cladding technique involves welding stainless steel strips onto the surface of the base material using a multi-pass approach. The process begins with the preparation of the 20MnMo forged tube sheet surface, which must be cleaned of scale, rust, and contaminants to ensure proper bonding. The 304L stainless steel strips are then welded onto the prepared surface using submerged arc welding (SAW) or gas metal arc welding (GMAW), depending on the specific equipment and production requirements.
The welding procedure qualification (WPS) developed for this application includes the following key parameters:
| Parameter | Specification |
|---|---|
| Base material | 20MnMo forged tube sheet |
| Overlay material | 304L stainless steel strip |
| Welding process | SAW or GMAW |
| Base metal preheat temperature | 200–300 °C |
| Interpass temperature | 150–250 °C |
| Number of overlay passes | 2–4 passes |
| Overlay thickness | 3–6 mm |
| Welding current | 250–400 A (SAW) |
| Welding voltage | 25–35 V (SAW) |
| Travel speed | 8–15 cm/min |
| Post-weld heat treatment | 620–650 °C for 2 hours |
The multi-pass overlay approach is designed to minimize dilution of the overlay material with the base metal. The first pass, which is in direct contact with the 20MnMo base, experiences the highest dilution rate, potentially reducing the chromium content in the overlay layer to below 18 percent. Subsequent passes progressively reduce the dilution effect, and by the final pass, the overlay composition approaches that of the pure 304L material.
Metallurgical Analysis and Mechanical Properties
Metallographic examination of the cladding interface reveals a diffusion zone where the carbon from the 20MnMo base migrates into the 304L overlay layer. This carbon enrichment can lead to chromium carbide precipitation at the interface, which may reduce the local corrosion resistance. To mitigate this issue, the study recommends using a transition layer of 309L or 310L stainless steel between the 20MnMo base and the 304L overlay. The higher nickel content of the transition layer promotes a more ductile and crack-resistant microstructure at the interface.
Mechanical property testing of the overlay layer and the base metal shows the following results:
| Property | 20MnMo Base | 304L Overlay | Interface Zone |
|---|---|---|---|
| Tensile strength (MPa) | 520–620 | 500–600 | 480–580 |
| Yield strength (MPa) | 355–450 | 205–310 | 300–400 |
| Elongation (%) | 18–25 | 35–45 | 25–35 |
| Hardness (HV) | 220–260 | 180–220 | 200–240 |
The interface zone exhibits a microstructure that is intermediate between the base metal and the overlay, with a slight reduction in ductility due to the presence of carbon-enriched regions. The hardness profile across the overlay thickness shows a gradient from the base metal hardness at the interface to the overlay hardness at the surface, with the transition occurring within the first 1–2 mm of the overlay layer.
Defect Analysis and Countermeasures
The study identifies several common defects that can occur during strip cladding of 20MnMo tube sheets:
- Cracking at the overlay-base interface: This is the most critical defect, caused by the high carbon content of the 20MnMo base promoting martensite formation in the weld zone. Countermeasures include adequate preheating (200–300 °C), interpass temperature control, and post-weld heat treatment to soften the hard microstructure.
- Lack of fusion between passes: Insufficient heat input or poor joint preparation can lead to incomplete fusion between overlay passes. This defect is detected by ultrasonic testing and can be prevented by ensuring proper groove preparation and maintaining adequate welding parameters.
- Undercut at the strip edges: Excessive travel speed or improper electrode angle can cause undercutting at the edges of the deposited strips. This creates stress concentration points that can initiate cracks under service loading. Proper technique and parameter control are essential to prevent this defect.
Engineering Practice and Quality Control
For heat exchanger tube sheets operating in corrosive environments, the strip cladding technique provides an economical alternative to using full stainless steel tube sheets. The 20MnMo base material offers the required strength for high-pressure applications, while the 304L overlay provides adequate corrosion resistance for many chemical service conditions.
Quality control procedures include:
- Visual inspection of the overlay surface for uniformity and absence of defects
- Magnetic particle testing (MT) for surface and near-surface crack detection
- Ultrasonic testing (UT) for subsurface defect detection and bond strength verification
- Hardness mapping across the overlay thickness to verify the dilution profile
- Chemical analysis of the overlay layer to confirm chromium and nickel content
- Corrosion testing using standard methods such as intergranular corrosion tests (ASTM A262)
Study Insights and Reflections
This study provides practical guidance for engineers involved in the fabrication of clad tube sheets for heat exchangers. The emphasis on welding procedure qualification and quality control is particularly important, as the reliability of the cladding depends heavily on the consistency of the welding process. The study also highlights the importance of post-weld heat treatment in relieving residual stresses and preventing delayed cracking in the high-carbon base metal weld zone.
One of the key insights from this work is the recognition that the dilution effect in the first overlay pass significantly affects the corrosion resistance of the overlay layer. Engineers must account for this dilution when specifying the overlay thickness and the number of passes. In applications where the overlay layer is exposed to aggressive corrosive media, a thicker overlay with more passes is recommended to ensure that the outermost layers have adequate chromium content for corrosion resistance.
Summary
The strip cladding of 304L stainless steel on 20MnMo forged tube sheets is a well-established technique for producing high-strength, corrosion-resistant tube sheets for heat exchangers. This study provides valuable welding procedure data, metallurgical analysis, and quality control guidance for this application. Engineers should carefully qualify the welding procedure, implement rigorous quality control measures, and consider the dilution effect when specifying overlay thickness to ensure reliable long-term performance in service.