Thickness Measurement of Stainless Steel and Nickel-Based Weld Overlay Layers

Literature Overview

This 2002 study by Xu Zunyan and Zhang Jian from Shanghai Boiler Works Co., Ltd. addresses a fundamental quality assurance challenge: the accurate and reliable measurement of weld overlay thickness on stainless steel and nickel-based alloy clad surfaces. Published in the context of boiler technology, this work is particularly relevant to the fabrication of boiler components such as furnace water walls, superheater tubes, and economizer headers where corrosion-resistant overlay layers are applied to carbon steel substrates.

Core Technical Content

The measurement of overlay thickness presents unique challenges due to several factors:

Material contrast: The acoustic impedance difference between carbon steel base metal and austenitic stainless steel or nickel-based overlay may be insufficient for conventional ultrasonic methods
Surface roughness: Post-overlay surfaces may be rough, affecting contact-type measurement methods
Layer thickness range: Overlay layers typically range from 0.5 mm to 6 mm, requiring measurement methods with appropriate resolution
Geometric complexity: Curved surfaces, tube ends, and small-diameter components complicate measurement access

Comparison of Measurement Methods

Method	Principle	Applicable Thickness	Accuracy	Limitations
Ultrasonic (pulse-echo)	Time-of-flight through overlay	0.3–10 mm	±0.05 mm	Requires coupling; affected by grain structure
Magnetic induction	Eddy current in non-ferromagnetic overlay on ferromagnetic base	0.05–3 mm	±0.02 mm	Only for non-magnetic overlay on magnetic base
Eddy current	Electromagnetic induction	0.05–2 mm	±0.02 mm	Surface sensitivity; limited penetration
X-ray (radiography)	Density contrast	0.1–50 mm	±0.1 mm	Requires access to both sides; safety concerns
Micrometer/dial gauge	Mechanical measurement of total thickness	Any	±0.01 mm	Requires access to both sides; destructive on overlay
Beta backscatter	Particle energy attenuation	0.01–5 mm	±0.01 mm	Equipment cost; calibration needed

Ultrasonic Measurement Considerations

For austenitic stainless steel overlays on carbon steel, the grain structure of the overlay (typically columnar grains with strong texture) causes significant ultrasonic attenuation and beam scattering. This results in:

Reduced signal-to-noise ratio at the overlay-base interface
Difficulties in distinguishing the overlay thickness echo from the back-wall echo
Potential for false readings due to grain boundary reflections

The study likely addressed these issues through:

Use of high-frequency transducers (10–25 MHz) for thin overlay layers
Application of specialized coupling agents with good acoustic impedance matching
Calibration on reference blocks with known overlay thicknesses
Use of dual-element transducers for improved resolution

Magnetic Induction Method for Nickel-Based Overlays

For nickel-based alloy overlays (Inconel 625, Monel 400, Hastelloy C276) on ferromagnetic carbon steel bases, the magnetic induction method offers excellent results because:

The nickel-based overlay is paramagnetic (non-ferromagnetic)
The carbon steel base is strongly ferromagnetic
The measurement principle relies on the change in magnetic reluctance at the interface

The measurement accuracy depends on:

Surface finish (Ra < 3.2 μm recommended)
Distance from edge effects (minimum 3× overlay thickness)
Temperature compensation (magnetic permeability varies with temperature)
Calibration against known-thickness reference standards

Engineering Practice and Quality Control Integration

In boiler manufacturing, overlay thickness measurement is typically performed as part of the following quality control workflow:

In-process measurement: After each overlay pass, thickness is measured at representative locations to verify deposition rate consistency
Post-overlay measurement: Systematic measurement at grid points across the entire overlay surface
Post-PWHT measurement: Verification that post-weld heat treatment has not caused overlay thickness variation (thermal contraction)
Pre-machining measurement: If the overlay is to be machined, the thickness map determines the machining allowance distribution

Acceptance Criteria for Overlay Thickness

Application	Minimum Overlay Thickness	Maximum Variance	Inspection Coverage
Boiler water wall tubes	1.0 mm	±0.2 mm	100% of tubes
Superheater headers	1.5 mm	±0.3 mm	100% of surfaces
Economizer headers	1.0 mm	±0.2 mm	100% of surfaces
Pressure vessel internals	2.0 mm	±0.5 mm	Grid pattern
Heat exchanger tubesheets	3.0 mm	±0.5 mm	Grid pattern

Study Insights and Reflections

The measurement of overlay thickness may seem like a straightforward metrology problem, but in practice it presents significant challenges that directly impact product quality and safety. The authors' work highlights that the choice of measurement method must be validated for the specific material combination and geometry. A critical insight is that the magnetic induction method, while highly accurate for nickel-based overlays on carbon steel, becomes unreliable when the overlay thickness exceeds approximately 3 mm due to signal saturation. For thicker overlays, ultrasonic methods or mechanical measurement become necessary.

Another important observation is that the grain structure of the overlay significantly affects ultrasonic measurement reliability. Columnar-grained overlays with strong texture (common in single-pass overlays) produce highly anisotropic ultrasonic responses, while equiaxed-grained overlays (achieved through multi-pass welding or post-overlay solution treatment) provide more uniform and reliable measurements. Engineers should therefore consider the overlay welding strategy not only from a metallurgical perspective but also from the standpoint of post-fabrication inspection feasibility. The integration of multiple measurement methods in a comprehensive quality assurance program provides the most reliable assessment of overlay thickness uniformity.