Manufacturing Technology for Large Diameter 15CrMo+SS Surfaced Tube Sheets

Literature Overview

This paper by Zheng Weixin, Xu Wenkui, Ning Xingsheng, Duo Yucai, and Wang Zhigang from Lanzhou Lanchuan Heavy Equipment Co., Ltd., published in Chemical Machinery in 2016, details the manufacturing technology for large-diameter, thick 15CrMo+stainless steel (SS) surfaced tube sheets. The work is particularly relevant to engineers involved in heat exchanger and reactor construction, where tube sheets serve as critical pressure-containing components that must withstand both mechanical loading and corrosion from process fluids.

Technical Challenge Analysis

The fundamental challenge described in this paper is the susceptibility of large-diameter tube sheets to deformation during the surfacing operation. Because the tube sheet diameter is large relative to its thickness, the component exhibits a high aspect ratio that makes it particularly prone to warping and distortion under the thermal cycling of welding. The 15CrMo base material, while providing good high-temperature strength and moderate creep resistance, is also susceptible to distortion due to its relatively high thermal expansion coefficient and the residual stresses developed during welding.

The 15CrMo+SS dual-material configuration is typical for tube sheets exposed to corrosive process media on one side while requiring high-temperature mechanical strength. The stainless steel overlay provides corrosion resistance, while the 15CrMo base provides the necessary mechanical properties for high-pressure service. However, the dissimilar material interface introduces additional complexity in terms of thermal expansion mismatch, potential galvanic coupling in service, and the challenge of achieving a metallurgically sound bond between the two materials.

Manufacturing Process Control Strategy

The authors identify five key control areas that must be addressed to prevent deformation:

The emphasis on blank machining allowance is particularly important. In my experience with large fabricated components, providing generous machining stock is often the most practical and cost-effective strategy for managing distortion. Rather than trying to prevent all distortion during welding, which is rarely achievable, it is more effective to design the process so that the final machining operation removes any residual warping. The key is ensuring that the allowance is sufficient without being so excessive as to create economic waste.

Welding Process Considerations

The selection of welding method for 15CrMo+SS tube sheet surfacing requires careful consideration of several factors. Low-heat-input processes such as TIG (GTAW) or pulsed GTAW are generally preferred for the first layers to minimize thermal distortion and to ensure good fusion at the dissimilar interface. However, for large-diameter components, the productivity of TIG welding may be insufficient, and hybrid approaches using TIG for the root layers followed by higher-deposition-rate processes for the fill layers are common.

The welding sequence is critical. For large-diameter tube sheets, a symmetric welding pattern that progresses from the center outward, or from the inner diameter outward, helps distribute thermal stresses more uniformly. Skip welding or back-step welding techniques may be employed to further reduce local heat concentration. The interpass temperature must be carefully controlled; for 15CrMo, a preheat of 150-250°C is typically required to prevent cold cracking, but excessive interpass temperatures can promote grain growth and reduce the hardenability of the heat-affected zone.

Engineering Practice and Quality Assurance

From a quality assurance perspective, the welding of 15CrMo+SS tube sheets requires comprehensive non-destructive testing. Surface-mounted cracks at the weld interface are a particular concern due to the thermal expansion mismatch between the carbon alloy base and the austenitic stainless overlay. Ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) is essential for detecting subsurface defects at the interface. Radiographic testing (RT) may also be employed, though the geometry of tube sheets can make RT interpretation challenging.

The paper's focus on deformation control aligns with practical experience. In large tube sheet fabrication, distortion is often the primary quality issue rather than weld defects per se. A tube sheet that has warped beyond acceptable flatness tolerances cannot be machined to specification and must be rejected, regardless of the quality of the weld metal itself. The economic impact of tube sheet rejection is substantial, given the material cost of large 15CrMo forgings and the long lead times for replacement.

Key Insights and Reflections

The systematic approach to deformation control presented in this paper reflects a mature manufacturing philosophy where process design is guided by an understanding of the root causes of quality problems. Rather than relying on post-weld correction, which is always a compromise, the authors focus on preventing distortion through careful process planning. This philosophy is consistent with the PDCA (Plan-Do-Check-Act) methodology applied to welding process development.

One area that deserves further discussion is the metallurgical compatibility at the 15CrMo/SS interface. The formation of martensitic phases in the dilution zone can compromise both corrosion resistance and mechanical properties. The use of transition layers, such as low-carbon austenitic or nickel-based alloys, may be necessary in some applications. The paper does not extensively address this aspect, which may be a limitation for engineers seeking to understand the full metallurgical picture.

Summary

This paper provides a practical and comprehensive guide to the manufacturing of large-diameter 15CrMo+SS surfaced tube sheets, with particular emphasis on deformation control. The five-area control strategy covering machining allowance, operation sequencing, welding method, parameters, and anti-deformation measures represents a systematic approach that can be applied to similar large fabrication challenges. For engineers involved in heat exchanger and reactor manufacturing, this work reinforces the principle that successful fabrication of large dissimilar-material components requires integrated process planning that addresses thermal, mechanical, and metallurgical considerations simultaneously.