Effect of Steel Pipe Thickness on Leeb Hardness Test Results

Literature Overview

This paper by Shi Zhigang et al., published in Foundry Technology (2016, Vol. 37, No. 9, pp. 2043–2045), addresses a critical metrological issue in the quality assurance of steel piping systems. The study investigates how pipe wall thickness, specifically for thicknesses at or below 25 mm, influences the accuracy of Leeb hardness measurements. The authors selected three commonly used pipeline steel grades—P91 (a 9Cr-1Mo-V-Nb high-temperature creep-resistant alloy), A106B (a carbon steel for boiler, heat exchanger, and superheater service), and A672B70CL32 (a low-temperature carbon steel for cryogenic service)—to conduct comparative hardness testing.

The motivation for this research is rooted in the revision of the Chinese national standard GB/T 17394.1-2014, which raised the minimum specimen thickness requirement for Leeb hardness testing from 5 mm to 25 mm when no backing plate is used. This change was significant because it effectively excluded a large number of thin-walled pipe sections from field-portable hardness verification, raising concerns among quality engineers and inspectors about the practicality of in-service inspections.

Core Technical Points

Impact of Thickness on Hardness Reading Accuracy

The fundamental physics behind portable hardness testing involves the rebound of a hardened impact body against the test surface. When the tested material is thin, the energy absorption characteristics of the substrate differ from those of a thick, homogeneous block, leading to systematic deviations in the measured hardness value. The authors systematically compared three measurement configurations:

1. External wall surface testing — Leeb hardness measured directly on the pipe's outer surface.
2. End-face testing — Leeb hardness measured on the cut end of the pipe, where the full wall thickness is engaged.
3. Laboratory Brinell hardness testing — Bench-type Brinell hardness measured on a prepared end-face specimen, serving as the reference method.

The results demonstrated that for pipes with wall thicknesses at or below 25 mm, the external wall Leeb hardness readings exhibited measurable deviations from the laboratory Brinell reference values. The magnitude of this deviation varied with thickness, with thinner walls showing larger discrepancies. For P91 pipes, the authors established a quantitative correspondence between Leeb hardness values obtained from thin-walled sections and the laboratory Brinell hardness reference values.

Standard Revision Justification

The study provides empirical support for the GB/T 17394.1-2014 revision. The authors concluded that raising the minimum thickness requirement from 5 mm to 25 mm is technically justified because hardness readings on specimens thinner than 25 mm cannot be reliably correlated to bulk material hardness without correction factors. This finding carries important implications for field inspection protocols in power plants, refineries, and petrochemical facilities where P91 piping is extensively used for high-temperature service.

Process and Standards Analysis

Comparison of Hardness Testing Methods

Grade-Specific Considerations

• P91 steel — This 9Cr-1Mo-V-Nb alloy is critical for ultra-supercritical power plant applications, operating at temperatures up to 620°C. Its hardness is sensitive to tempering conditions and microstructural evolution, making accurate hardness verification essential for remaining life assessment. The study found that P91 thin-walled pipes showed the most pronounced thickness effect among the tested grades, likely due to its higher base hardness and more complex microstructure.
• A106B carbon steel — Widely used in heat exchanger and boiler tubing, this grade exhibited moderate thickness sensitivity. Its lower base hardness and simpler microstructure (ferrite-pearlite) resulted in smaller deviations compared to P91.
• A672B70CL32 low-temperature steel — Designed for cryogenic service down to −46°C, this grade showed thickness effects consistent with its moderate hardness level. The fine-grained microstructure achieved through controlled rolling contributed to relatively stable hardness readings even at reduced thicknesses.

Engineering Practice Integration

Implications for In-Service Inspection Programs

For engineers responsible for in-service inspection of power plant piping systems, this study highlights a practical challenge: many P91 pipe sections in service have wall thicknesses that have been reduced by corrosion or erosion to below 25 mm. Under the revised GB/T 17394.1-2014, these sections cannot be reliably tested with portable Leeb hardness equipment without a backing plate. This necessitates alternative approaches:

1. Use of backing plates — Installing a rigid backing plate behind the test point to simulate an infinite-thickness condition. This requires surface accessibility and proper coupling between the backing plate and the pipe wall.
2. End-face sampling — Cutting a sample from a pipe end and testing the end face, which provides the full remaining wall thickness. This is destructive and typically limited to spare piping or sections scheduled for replacement.
3. Correction factor application — Using the correspondence relationships established in this study to convert Leeb readings from thin-walled sections to equivalent bulk hardness values. This approach requires grade-specific calibration data and introduces additional uncertainty.

FMEA Perspective on Hardness Verification

Applying Failure Mode and Effects Analysis (FMEA) to the hardness verification process for thin-walled piping:

The high Risk Priority Number (RPN) for overestimation of hardness underscores the critical importance of adhering to the minimum thickness requirements in GB/T 17394.1-2014. Engineers should not rely on uncorrected Leeb hardness readings from pipes with wall thicknesses below 25 mm for fitness-for-service assessments.

Key Questions and Reflections

The study raises several important questions for the engineering community. First, while the GB/T 17394.1-2014 revision is technically justified, it creates a practical gap in field inspection capabilities for thin-walled piping. Power plant operators frequently encounter P91 pipe sections with remaining wall thicknesses well below 25 mm due to internal corrosion or erosion. How should these sections be hardness-verified in a manner that is both technically defensible and practically feasible?

Second, the study focuses on new or relatively well-characterized pipe grades. In service, P91 piping undergoes microstructural evolution including temper embrittlement, creep damage, and precipitate coarsening. The thickness-hardness relationship established for as-received material may not directly apply to aged material. Further research is needed to extend the correspondence relationships to service-aged P91 piping.

Third, the study does not address the effect of pipe curvature on Leeb hardness readings. The curvature of thin-walled pipes introduces additional geometric effects that may compound the thickness effect. Future work should consider the combined influence of wall thickness and diameter-to-thickness ratio on portable hardness measurement accuracy.

Study Insights and Implications

This study makes a valuable contribution to the quality assurance framework for steel piping systems. By providing empirical evidence for the GB/T 17394.1-2014 revision, it strengthens the metrological foundation of hardness-based fitness-for-service assessments. The grade-specific correspondence data for P91 thin-walled pipes is particularly useful, as it provides engineers with a basis for applying correction factors when direct laboratory testing is not feasible.

The work also highlights the importance of standard revisions in maintaining technical integrity. The original 5 mm minimum thickness requirement was likely based on early portable hardness tester capabilities and insufficient understanding of thickness effects. The revision to 25 mm reflects improved scientific understanding and the need for more conservative, reliable measurement practices. However, standard revisions must be accompanied by practical guidance for engineers who must work within the constraints of the revised requirements.

In summary, this paper reinforces the principle that measurement uncertainty must be rigorously managed in engineering quality assurance. For thin-walled steel piping, particularly P91 alloy pipes in high-temperature service, hardness verification requires careful attention to specimen thickness, measurement configuration, and the application of appropriate correction factors. Engineers should integrate these findings into their inspection procedures to ensure that hardness-based assessments remain technically valid and defensible.