Weld Overlay Process Development for Waste Heat Boiler Tube Sheets

Literature Overview

This technical article, published in China Chemical Equipment in 2009 by Liu Baoxiang from Qingdao Soft Control Heavy Industry Co., Ltd., addresses the weld overlay process development for waste heat boiler tube sheets. Waste heat boilers are critical components in cement manufacturing, steel production, and other high-temperature industrial processes, recovering thermal energy from exhaust gases for power generation or process heating. The tube sheet serves as the structural and sealing interface between the boiler shell and heat transfer tubes, operating under severe conditions of high temperature, corrosive flue gas, and thermal cycling. This research focuses on developing reliable weld overlay processes to protect tube sheet surfaces from corrosion and erosion.

Core Technical Content and Research Objectives

Waste heat boiler tube sheets face a unique combination of degradation mechanisms:

High-temperature corrosion from SOₓ and NOₓ in flue gas
Ash erosion from particulate matter in exhaust streams
Thermal fatigue from cyclic temperature variations (typically 200–400 °C operating range)
Oxidation at elevated temperatures
Creep deformation under sustained high-temperature stress

The weld overlay process must deposit a corrosion-resistant and erosion-resistant layer on the tube sheet surface while maintaining structural integrity and ensuring proper sealing at tube joints. The challenge is compounded by the large size of boiler tube sheets (typically 1000–3000 mm diameter) and the need for uniform coverage across the entire tube sheet face.

Weld Overlay Process Parameters

The following table presents the process parameters developed for waste heat boiler tube sheet overlay:

Parameter	Specification	Rationale
Base material	16Mn or 20G boiler steel	Structural strength requirement
Overlay material	310SS or 309SS	High-temperature oxidation resistance
Welding process	Multi-pass submerged arc welding (SAW)	High deposition rate for large surfaces
Electrode type	Flux-cored wire with 310SS core	Controlled dilution and microstructure
Welding current	400–550 A	High deposition rate
Arc voltage	28–35 V	Adequate penetration and fusion
Travel speed	200–400 mm/min	Balance between deposition and quality
Number of passes	3–5 passes	Achieve required overlay thickness
Overlay thickness	3–5 mm total	Adequate protection for service life
Preheat temperature	200–300 °C	Reduce residual stress and cracking
Post-weld heat treatment	620–650 °C × 2h	Stress relief and microstructure homogenization

Overlay Material Selection and Performance

The selection of overlay material is critical for waste heat boiler service. The following table compares typical overlay materials for this application:

Overlay Material	Cr (%)	Ni (%)	Service Temperature (°C)	Corrosion Resistance	Erosion Resistance	Cost Factor
309SS	22–25	12–15	≤1100	Good	Good	Baseline
310SS	24–26	19–22	≤1200	Excellent	Excellent	1.5–2.0×
347SS	18–20	9–13	≤1150	Good	Moderate	0.8–1.0×
Inconel 625	20–23	58–65	≤1000	Excellent	Excellent	4.0–6.0×
Hastelloy C276	14–16	56–62	≤900	Excellent	Excellent	5.0–8.0×

For most waste heat boiler applications, 310SS provides the optimal balance of performance and cost. Inconel 625 is reserved for extremely aggressive service conditions involving halide contamination or very high sulfur content in flue gas.

Quality Control and Inspection Requirements

The following table outlines the quality control requirements for tube sheet overlay:

Inspection Method	Acceptance Criteria	Standard Reference
Visual inspection (VT)	No cracks, undercuts, or excessive porosity	JB/T 4730
Magnetic particle testing (MT)	No linear indications >2 mm	JB/T 4730.4
Ultrasonic testing (UT)	No lack of fusion or cracks at overlay/base interface	JB/T 4730.3
Hardness testing	Overlay: 150–250 HV; HAZ: ≤350 HV	ASTM E18
Chemical composition	Overlay Cr: ≥24%, Ni: ≥19% (for 310SS)	ASTM E415
Intergranular corrosion	No intergranular attack after 72h in 65% HNO₃	ASTM A263

Engineering Practice Implications

For boiler manufacturers and maintenance engineers, this research provides critical process development guidance:

Multi-pass SAW is the most economical process for large tube sheet overlay, achieving deposition rates of 20–30 kg/h compared to 3–5 kg/h for manual processes
The 200–300 °C preheat is essential to prevent cold cracking in the HAZ, particularly for higher-strength base materials like 16Mn
Post-weld stress relief at 620–650 °C is mandatory to reduce residual stresses that could cause creep deformation during high-temperature service
Overlay thickness of 3–5 mm provides adequate protection for typical waste heat boiler service lives of 10–15 years

The practical challenges include:

Maintaining uniform overlay thickness across large tube sheet diameters
Ensuring adequate gas protection at tube sheet edges and near tube holes
Managing thermal distortion during multi-pass welding on thick tube sheets
Achieving proper fusion at tube sheet edges without excessive penetration that could weaken tube joints

Key Questions and Reflections

A critical consideration is the long-term performance of the overlay layer under cyclic thermal loading. While laboratory tests demonstrate excellent corrosion resistance, the fatigue behavior of the overlay/base material interface under thermal cycling deserves further investigation. Thermal fatigue cracking at the interface could lead to premature overlay detachment and loss of corrosion protection.

Another important question is the repairability of overlaid tube sheets. If a tube leaks during service, the replacement tube must be welded through the overlay layer, which requires specialized welding procedures and consumables. The compatibility of repair welding with the existing overlay is a significant engineering consideration that must be addressed in the design phase.

Study Insights and Implications

This research provides practical, field-proven process development for waste heat boiler tube sheet protection. The systematic approach to material selection, process parameter optimization, and quality control establishes a reliable methodology that can be adapted to different boiler configurations and service conditions. For engineers in the cement and steel industries, the key insight is that weld overlay is a cost-effective and reliable solution for extending boiler tube sheet service life, with 310SS providing optimal performance for most applications. The emphasis on multi-pass SAW for large surface coverage represents a practical solution to the economic challenges of protecting large boiler components, making this technology accessible for both new construction and retrofit applications.