Welding Test of Strip Cladding 304L on 20MnMo Forged Tube Sheets

Literature Overview and Background

Tube sheets in heat exchangers and pressure vessels are critical components that must withstand high pressure, thermal cycling, and often corrosive process media. The combination of a strong, creep-resistant base material such as 20MnMo with a corrosion-resistant stainless steel overlay such as 304L is a common design strategy for heat exchanger tube sheets exposed to aggressive service environments. The literature under review presents welding qualification and production trials for strip cladding 304L stainless steel onto 20MnMo forged tube sheets, addressing the challenges of weldability, dilution control, and mechanical integrity.

Core Technical Findings

The strip cladding process was performed using submerged arc welding (SAW) with a multi-layer multi-pass technique. The 20MnMo base material is a low-alloy steel with good strength and creep resistance, while 304L provides excellent resistance to intergranular corrosion due to its low carbon content. The key challenge in this cladding application is controlling the dilution of the base metal into the overlay layer to ensure that the final overlay composition remains within the acceptable range for the intended corrosion service.

Metallographic examination of the cladding layer revealed a predominantly austenitic-ferritic microstructure with a ferrite content in the range of 8-15%, which is within the acceptable range for 304L to prevent hot cracking. The dilution ratio was measured to be approximately 10-15% in the first layer, decreasing to less than 5% in the final layer. The chemical composition of the final overlay layer met the requirements of ASTM A263/A263M for 304L stainless steel.

Process Analysis and Standards Considerations

The welding procedure was qualified in accordance with NB/T 47014 and ASME Section IX. The qualification coupon consisted of a 20MnMo base plate with the same thickness as the production tube sheet, with the 304L strip cladding deposited in the same number of layers as the production procedure. Mechanical property tests included tensile tests on the overlay layer, bend tests on the overlay layer, and impact tests on the base metal. Intergranular corrosion testing per ASTM A263/A263M-24 was performed on the final overlay layer to confirm resistance to intergranular corrosion.

The preheat temperature of 150-200°C was selected to reduce the cooling rate in the heat-affected zone (HAZ) of the 20MnMo base metal, preventing the formation of brittle martensite and reducing the risk of hydrogen-induced cracking. The interpass temperature was maintained below 250°C to avoid excessive grain growth in the overlay layer and to minimize the risk of sensitization in the 304L stainless steel.

Engineering Practice and Defect Analysis

During the welding trials, the following defects were observed and analyzed:

• Undercut at the strip edge: Occurred when the travel speed was too high or the welding current was insufficient. The countermeasure was to optimize the welding parameters and use a slight weaving technique to ensure full penetration at the strip edge.
• Cracking in the HAZ of the 20MnMo base: This defect was associated with insufficient preheat and rapid cooling. Increasing the preheat temperature to 200°C and reducing the welding current slightly resolved the issue.
• Porosity in the overlay layer: Caused by moisture in the flux or inadequate flux coverage. Proper flux drying at 300°C for 2 hours before use and maintaining adequate flux coverage prevented porosity formation.

The literature emphasized that the forged nature of the 20MnMo tube sheet introduces directional variations in microstructure and mechanical properties. The welding procedure must account for the anisotropy of the forged material, particularly in terms of residual stress distribution and hydrogen-induced cracking susceptibility.

Study Insights and Reflections

This literature provides practical guidance for the strip cladding of 304L on 20MnMo tube sheets, a common but technically challenging application in heat exchanger fabrication. The key takeaway is that dilution control is the primary concern in this cladding application, and a multi-layer approach with progressive dilution reduction is essential for achieving the required overlay composition. The PWHT temperature of 620°C is carefully selected to be below the sensitization range of 304L while still providing adequate stress relief for the 20MnMo base metal. Engineers should note that the forged tube sheet geometry, with its large diameter and varying thickness, requires careful thermal management during welding to prevent distortion and residual stress buildup. The qualification procedure must include testing representative of the actual production geometry and material condition. This study reinforces the importance of rigorous welding procedure qualification and the need for process-specific optimization rather than generic procedure application in bimetal cladding fabrication.