Application of High Chromium Alloy Overlay Welded Tubes in CFB Boiler Waterwall Tubes

Literature Overview

This paper by Bao Rihui, published in Large-scale Fertilizer (Vol. 45, No. 5, 2022, pp. 305–309), addresses a critical engineering problem faced in circulating fluidized bed (CFB) boilers: frequent tube burst incidents in the waterwall tubes caused by severe abrasion in the dense-phase zone of the furnace front wall. The study documents a systematic approach to solving this problem through the application of high-chromium alloy overlay welded tubes, beginning with a single-unit trial in 2015 and culminating in a full-scale retrofit. The reported results are impressive: tube service life was extended to five years, the operational cycle of the waterwall system reached twelve months without failure, and wall thickness reduction was controlled to less than 0.5 mm.

Core Technical Content and Process Analysis

The fundamental challenge in CFB boiler waterwall tubes is the dual attack of high-temperature ash erosion and corrosion. In the dense-phase zone of the front wall, circulating fly ash particles with hard angular morphology impact the tube surface at high velocity, causing progressive material loss. Conventional carbon steel or low-alloy steel waterwall tubes (typically 20G or 12Cr1MoV per GB/T 5310) are insufficiently resistant to this abrasive regime, leading to premature wall thinning and eventual tube rupture.

The solution adopted was the replacement of high-wear zones with high-chromium alloy overlay welded tubes. The overlay welding process deposits a wear-resistant layer onto the outer surface of the base tube. Key process parameters for such overlay welding typically include:

The overlay layer must be carefully designed to balance hardness (typically 55–65 HRC for high-chromium martensitic alloys) with toughness and thermal fatigue resistance. Excessive hardness without adequate ductility leads to cracking under thermal cycling, while insufficient hardness fails to resist abrasion.

Engineering Practice and Lessons Learned

The paper highlights several important engineering lessons. First, the trial approach of testing on a single unit before full-scale deployment was prudent and saved significant cost and downtime risk. Second, the integration of wear-resistant overlay tubes with the existing anti-wear beam (防磨梁) system provided complementary protection: the beams shielded the most severe impact zones, while the overlay tubes provided uniform surface protection. Third, the monitoring program tracking wall thickness reduction at intervals of less than 0.5 mm over the entire service cycle demonstrated that the overlay layer was effectively resisting erosion without delamination or spalling.

From a metallurgical perspective, the high-chromium alloy overlay layer forms a protective Cr₂O₃ scale that also provides some oxidation resistance at elevated temperatures. However, the primary mechanism of protection is mechanical: the high hardness of the overlay resists micro-cutting by ash particles. The dilution rate between the base steel and the overlay is a critical parameter; if too high, the effective hardness of the overlay is reduced, and the service life benefit diminishes.

Standards and Quality Control Considerations

The selection and qualification of overlay welded tubes for boiler applications must comply with relevant standards. In China, the relevant standards include GB/T 5310 for boiler tubes, TSG 11-2020 for pressure vessel and pressure piping safety technical supervision rules, and DL/T 5054 for boiler design and construction. The overlay welding process itself should be qualified per NB/T 47014 or ISO 15614-1.

Quality control measures should include:

• Visual inspection of the overlay surface for uniformity and absence of defects
• Hardness testing (HV or HRC) across the overlay layer to verify composition and microstructure
• Ultrasonic testing (UT) to detect lack of fusion or cracks at the base-overlay interface
• Dilution analysis via optical emission spectroscopy (OES) or XRF
• Macroscopic examination of cross-sections to verify overlay thickness uniformity

Reflections and Study Insights

This case study demonstrates the power of a targeted engineering solution to a well-defined problem. The key insight is that in CFB boiler applications, the wear mechanism is predominantly mechanical abrasion rather than chemical corrosion, and therefore the overlay alloy design should prioritize hardness and abrasion resistance over corrosion resistance. The reported service life extension from months to five years represents a dramatic improvement that justifies the additional cost of overlay welded tubes. However, engineers must remain vigilant about potential issues such as overlay spalling under severe thermal shock, cracking at the base-overlay interface due to residual stress, and the long-term effect of the overlay on the tube's thermal conductivity, which could affect heat transfer efficiency.

The broader lesson is that material selection and surface engineering should be driven by a clear understanding of the dominant degradation mechanism. In this case, high-chromium martensitic overlay provided exactly the right combination of properties for the CFB boiler waterwall application, and the systematic approach to trial, evaluation, and scale-up is a model for engineering practice.