Manufacturing Technology for Large-Diameter Tubesheet 15CrMo+SS Overlay Welding

Literature Overview

This paper by Zheng Weixin et al. from Lanzhou Lanshi Heavy Equipment Co., Ltd., published in Chemical Engineering Machinery (2016, Vol. 43, No. 6, pp. 816-818), addresses a critical manufacturing challenge in pressure vessel fabrication: the overlay welding of austenitic stainless steel onto large-diameter, relatively thin-walled 15CrMo tubesheets. The tubesheet is a core component in heat exchangers and reactors, where the carbon steel body provides structural strength while the stainless overlay ensures corrosion resistance at the tube-to-tubesheet interface. The authors identify distortion as the primary failure mode and systematically present control strategies across the entire manufacturing sequence.

Core Technical Challenge

The fundamental difficulty lies in the geometric configuration: large diameter combined with relatively thin wall thickness creates a low bending stiffness structure. When overlay welding deposits are applied, the differential thermal expansion between the austenitic overlay (CTE ~18 µm/m·°C) and the ferritic 15CrMo base (CTE ~12-13 µm/m·°C) generates significant residual stresses that manifest as bowing, warping, or edge curling. The allowable flatness tolerance for tubesheets is typically within ±0.15% of diameter or ±2 mm (whichever is smaller), which becomes increasingly difficult to maintain as diameter increases beyond DN1200.

Process Control Strategy

The authors propose a multi-dimensional control approach that can be organized using a systematic methodology:

Control Dimension	Specific Measures	Technical Rationale
Blank machining allowance	Increased initial thickness allowance	Provides material for post-weld flattening and stress relief
Machining sequence	Rough machining → overlay welding → final machining	Ensures overlay is deposited on a pre-defined flat surface
Welding method	Multi-layer, multi-pass with controlled heat input	Limits peak temperature and thermal gradient
Welding parameters	Optimized current, voltage, travel speed	Controls dilution and HAZ microstructure
Anti-distortion measures	Backing plate, clamping fixtures, symmetric welding sequence	Provides mechanical constraint against deformation

Welding Metallurgy Considerations

The 15CrMo+SS overlay system belongs to the dissimilar metal welding category governed by standards such as GB/T 12469, ASME B31.3, and AWS D8.1. The key metallurgical concerns include:

Dilution control: The first overlay pass typically experiences 30-50% base metal dilution, creating a martensitic transition zone that is susceptible to cracking. Subsequent passes progressively dilute toward the full austenitic composition.
Carbon migration: At service temperatures above 425°C, carbon diffuses from the 15CrMo base into the austenitic overlay, creating a soft zone at the interface that reduces creep strength and fatigue resistance.
HAZ hardening: The 15CrMo base is susceptible to temper embrittlement and hardening in the HAZ if the peak temperature exceeds 650°C without proper preheating and post-weld heat treatment.

Distortion Control Measures

The paper emphasizes several practical anti-distortion techniques:

Symmetric welding sequence: Welding proceeds from the center outward in concentric rings or uses a skip-welding pattern that balances thermal input on opposing sides of the tubesheet.
Backing plate attachment: A steel backing plate is welded or clamped to the non-overlay surface, providing thermal mass and mechanical restraint.
Intermittent welding with cool-down intervals: Allowing the workpiece to cool between passes reduces cumulative thermal distortion.
Post-weld stress relief: Stress relief annealing at 620-650°C for the 15CrMo base, though this requires careful consideration as it may affect the overlay microstructure.
Mechanical flattening: Post-weld machining or hammering of the overlay surface to restore flatness within tolerance.

Engineering Practice Reflection

From my experience with large pressure vessel fabrication, I note that the distortion problem scales non-linearly with diameter. For tubesheets above DN1500, the thermal distortion can exceed 5-10 mm even with careful parameter control. The practical solution often involves a combination of:

Pre-bending the blank in the opposite direction of expected distortion (compensation technique)
Using magnetic clamping or vacuum clamping to the backing plate
Monitoring distortion in real-time with laser displacement sensors during welding
Applying the overlay in segments with controlled overlap rather than continuous rings

The paper's emphasis on machining allowance is particularly important—allowing 1.5-3 mm additional thickness on the overlay side provides a margin for post-weld correction that is often overlooked in cost-driven production environments.

Key Takeaways for Practitioners

The study reinforces that successful large tubesheet overlay welding is not merely a welding procedure qualification issue but requires holistic process integration. The welding procedure specification (WPS) must be considered in conjunction with the machining plan, fixture design, and post-weld heat treatment schedule. Engineers should approach this problem with a systems engineering mindset, recognizing that distortion is governed by the interaction of material properties, geometry, thermal input, and constraint conditions. A thorough FMEA (Failure Mode and Effects Analysis) of the entire manufacturing sequence—identifying each step's contribution to cumulative distortion—provides the most effective framework for process optimization.