Application of Ultrasonic Testing in Quality Inspection of Stellite Alloy Weld Overlay for Ultra-Supercritical Power Units

Literature Overview

This 2014 publication by Yang Jing, Chen Jie, Lu Yun, and Yu Gang represents a significant contribution to power industry quality assurance. The authors from State Grid Shanghai Electric Power Company, Shanghai Electric Wai Gao Qiao Power Plant, State Grid Electric Power Research Institute, and Shanghai Pudong New Area Special Equipment Supervision and Inspection Institute address the critical challenge of non-destructive evaluation (NDE) for Stellite alloy weld overlay coatings applied to components in ultra-supercritical (USC) power units operating at temperatures exceeding 600 °C and pressures above 25 MPa.

Core Technical Content

Ultra-supercritical boilers and heat exchangers require corrosion-resistant overlay layers on critical components such as superheater tubes, reheater tubes, and economizer headers exposed to high-temperature flue gas corrosion. Stellite alloy (typically Stellite 6, composition: 60Co-28Cr-6W-3Fe-2Mo-1Si-1C) is widely specified for these applications due to its exceptional resistance to high-temperature oxidation, sulfidation, and abrasive corrosion. However, the NDE of these overlays presents unique challenges owing to the coarse grain structure of the cobalt-based alloy, which causes significant ultrasonic attenuation and scattering.

Ultrasonic Testing Methodology

Defect Detection Capabilities

The study systematically evaluated the ability of ultrasonic testing to detect the following defect types in Stellite overlay welds:

1. Interface lack of fusion: Detected using angle beam probes at 45° orientation; minimum detectable defect size of 1.5 mm flat bottom equivalent.
2. Internal porosity: Volumetric defects detected using pulse-echo technique; sensitivity to individual pores ≥1.0 mm diameter.
3. Cracks (hot cracks and cold cracks): The most critical defect category; detected using both straight beam and angle beam techniques with sensitivity to crack lengths ≥2.0 mm.
4. Undercut and geometry irregularities: Detected using edge probes or surface waves; critical for assessing stress concentration sites.

Standards Framework and Acceptance Criteria

The inspection was conducted in accordance with multiple standards simultaneously:

The acceptance criteria followed a modified approach where the coarse grain structure of Stellite alloy necessitated higher gain settings and reduced acceptance thresholds compared to conventional austenitic stainless steel welds. The authors proposed that for critical power plant components, the acceptance level should be one level more stringent than the standard allows, reflecting the safety-critical nature of USC applications.

Engineering Practice and Challenges

Key Technical Challenges Identified

• Grain boundary scattering: The large grain size (typically 0.5–1.5 mm) of cast Stellite deposits causes significant signal attenuation, reducing effective inspection depth to approximately 3–4 times the grain size. For overlay thicknesses exceeding 15 mm, dual-probe through-transmission techniques are recommended.
• Interface reflection complexity: The impedance mismatch between the Stellite overlay and the carbon or low-alloy steel base creates a strong interface echo that can mask near-interface defects. The authors recommend using a combination of contact and immersion techniques for comprehensive coverage.
• Temperature effects: Post-weld heat treatment and in-service elevated temperatures alter acoustic properties; the study emphasizes the importance of testing at representative service temperatures or applying temperature correction factors.

Practical Recommendations

The authors established a multi-technique NDE protocol:

1. Primary screening: Magnetic particle testing (MT) or penetrant testing (PT) for surface-breaking defects.
2. Interface inspection: Angle beam ultrasonic testing (ABUT) at 45° and 60° for lack of fusion and cracks.
3. Volumetric inspection: Straight beam pulse-echo for internal porosity and inclusions.
4. Supplementary: Eddy current testing for surface and near-surface defects in conductive overlays.

Study Insights and Reflections

This publication makes a valuable contribution to the power industry NDE community by addressing a long-standing practical gap. The Stellite overlay inspection problem has historically been considered one of the most challenging applications in weld NDE, and the systematic approach presented here—combining proper reference block selection, multi-probe angle coverage, and modified acceptance criteria—provides a practical solution. The involvement of both operating companies and regulatory inspection bodies in this collaborative study lends additional credibility to the proposed methodology. The findings are directly applicable to similar cobalt-based overlay applications in petrochemical, pulp and paper, and marine industries where high-temperature corrosion resistance is required.

Reference Value and Outlook

The methodology can serve as a template for developing qualification procedures for other difficult-to-inspect overlay materials, including nickel-based superalloys (Inconel 625, Hastelloy C-276) and titanium overlays. Future developments should incorporate phased array ultrasonic testing (PAUT) and total focusing evaluation (TFE) imaging techniques, which offer superior defect characterization capabilities and digital record-keeping. The integration of data analysis-based signal processing algorithms for automated defect classification would further improve inspection efficiency and consistency, though such developments must be validated against established manual techniques before regulatory acceptance.