Large Diameter Tube Sheet Surfacing Manufacturing Technology

Literature Overview

This 2016 paper published in "Chemical Engineering Machinery" by Zheng Weixin, Xu Wenkui, Ning Xingsheng, Duo Yuanchai, and Wang Zhigang from Lanzhou Lanchen Heavy Equipment Co., Ltd. addresses the manufacturing challenges of large-diameter, large-thickness 15CrMo+SS surfaced tube sheets. Tube sheets are critical components in heat exchangers and reactors, and the combination of a 15CrMo structural steel base with a stainless steel overlay creates significant technical challenges related to deformation control, weld quality, and dimensional accuracy.

Core Technical Challenges

The primary difficulty in manufacturing large-diameter tube sheets with stainless steel surfacing lies in the combination of large diameter, relatively thin relative thickness, and the high thermal input required for the 15CrMo+SS surfacing weld. These factors together create severe distortion risks that can compromise both the geometric integrity of the tube sheet and the quality of the surfacing layer.

Deformation Mechanisms

The 15CrMo base material is a low-alloy steel with moderate carbon content, while the stainless steel overlay introduces a dissimilar metal interface with significantly different thermal expansion coefficients and thermal conductivity. During surfacing, the localized heating creates asymmetric thermal stresses that cause the plate to warp, bow, or develop angular distortion. The large diameter amplifies these effects because the peripheral regions are farther from the heat input zone and experience greater differential thermal contraction.

Manufacturing Strategy

The authors describe a comprehensive manufacturing approach that addresses deformation control through multiple interconnected measures:

Detailed Process Analysis

Blank Preparation and Machining Allowance

The machining allowance on the blank must be calculated to account for the expected total distortion from surfacing and subsequent heat treatment. This is not a simple additive calculation because distortion is not uniform across the plate; it varies with position relative to the weld sequence. Engineers must use experience-based estimates or finite element analysis to predict the distortion pattern and allocate machining allowance accordingly.

Welding Method and Parameter Selection

For 15CrMo+SS surfacing, the choice of welding process is critical. Processes with high energy density and narrow heat-affected zones—such as plasma arc welding or TIG welding—are preferred over submerged arc welding or manual arc welding because they minimize the thermal cycle severity and reduce distortion. However, high energy density processes may have lower deposition rates, which must be balanced against productivity requirements.

The welding parameters must be carefully controlled to maintain the integrity of the 15CrMo+SS interface. Excessive heat input can cause excessive dilution of the base metal into the surfacing layer, reducing the corrosion resistance of the overlay. Conversely, insufficient heat input can lead to incomplete fusion and lack of bond. The optimal parameter window is narrow and requires careful process development.

Anti-Deformation Measures

The paper emphasizes multiple anti-deformation strategies. Mechanical restraint using clamping fixtures and backing plates is the most direct approach, but excessive restraint can create residual stresses that lead to cracking. Backing plates provide both thermal mass to reduce cooling rate and mechanical support to limit bowing. Pre-bending the blank in the anticipated distortion direction is a more advanced technique that requires accurate prediction of the distortion pattern.

Engineering Practice Integration

This paper is directly applicable to engineers designing and manufacturing large heat exchanger tube sheets, reactor head covers, and similar components requiring 15CrMo+SS dissimilar metal surfacing. The systematic approach to deformation control—combining allowance planning, process optimization, and mechanical restraint—reflects the PDCA (Plan-Do-Check-Act) methodology applied to manufacturing process development.

The key lesson is that deformation control cannot be achieved through any single measure alone. It requires a holistic approach where machining allowance, welding sequence, welding parameters, and mechanical restraint are all optimized together. Changing one parameter without adjusting the others can lead to unexpected distortion patterns.

Key Questions and Reflections

A critical question not fully addressed in the paper is the long-term performance of the 15CrMo+SS interface under thermal cycling. The coefficient of thermal expansion mismatch between 15CrMo (approximately 12 x 10^-6 /K) and austenitic stainless steel (approximately 17 x 10^-6 /K) creates cyclic stresses during thermal cycling in service. While the paper focuses on manufacturing deformation, the residual stresses from surfacing combined with thermal cycling can lead to interfacial fatigue cracking over time.

Another consideration is the post-weld heat treatment (PWHT) requirement. 15CrMo typically requires PWHT to relieve welding residual stresses and improve toughness, but the PWHT temperature range must be compatible with the stainless steel overlay to avoid sensitization and intergranular corrosion susceptibility. The interaction between PWHT and the dissimilar metal interface is a complex metallurgical issue that requires careful control.

Summary

This paper provides a practical and comprehensive framework for manufacturing large-diameter 15CrMo+SS surfaced tube sheets, with particular emphasis on deformation control through integrated process measures. The systematic approach of combining machining allowance optimization, welding method selection, parameter control, and mechanical restraint offers a replicable methodology for similar large dissimilar metal surfacing applications. Engineers working on heat exchanger and pressure vessel components should study this approach carefully, as deformation control in large surfacing operations is one of the most persistent challenges in heavy equipment manufacturing.