Experimental Study on Linear Expansion Coefficient of Q235B Spiral Steel Pipes

Literature Overview

This paper by Hu Yueyue, Li Guanglu, and Liang Yahong, published in Hot Working Technology (2017, Vol. 46, Issue 7, pp. 47-50), presents an experimental investigation into the linear expansion coefficient of Q235B spiral steel pipes. The study was conducted in collaboration between Xi'an University of Architecture and Technology and a power generation company, reflecting a practical engineering need to accurately predict pipe deformation under thermal loading in district heating systems. The authors used a similar triangle amplification method to measure the small elongation of steel pipes subjected to thermal expansion, testing DN300 and DN400 spiral pipes. This work is directly relevant to engineers designing thermal expansion loops, supports, and anchorages for heating pipelines.

Core Technical Findings

The experimental results reveal several important deviations from theoretical expectations:

1. DN300 spiral pipes exhibited a linear expansion coefficient that was 47.38% lower than the theoretical value for Q235B steel.
2. DN400 spiral pipes exhibited a linear expansion coefficient that was 51.19% lower than the theoretical value.
3. DN400 pipes had a linear expansion coefficient 7.24% lower than DN300 pipes, indicating a diameter-dependent effect.
4. Microstructural analysis revealed that the base metal and heat-affected zone (HAZ) consisted of ferrite and pearlite with different proportions, while the weld zone contained Widmanstätten structure.
5. The presence of Widmanstätten structure in the weld zone reduced plasticity and toughness, contributing to the deviation in linear expansion behavior.

Comparison of Expansion Coefficients

Interpretation of Technical Points

The significant deviation between the experimental and theoretical linear expansion coefficients is a critical finding that has direct implications for the design of thermal piping systems. The theoretical linear expansion coefficient for Q235B steel is approximately 12 × 10⁻⁶/°C, but the experimental values are roughly half of this. This deviation is attributed to the following factors:

1. Microstructural heterogeneity — the spiral weld introduces a zone of Widmanstätten structure, which has different thermal expansion characteristics compared to the base metal's ferrite-pearlite structure. The Widmanstätten structure, formed during rapid cooling of the weld, consists of acicular ferrite plates within a pearlite matrix, which restricts grain boundary movement and reduces thermal expansion.
2. Spiral weld geometry — the helical weld path creates a continuous reinforcement zone around the pipe circumference, which constrains the thermal expansion of the pipe wall. The helical angle of the weld affects the distribution of residual stresses and the effective constraint on thermal deformation.
3. Residual stress field — the welding process introduces a complex residual stress field that interacts with thermal stresses during heating. The compressive residual stresses in the HAZ and weld zone can partially offset thermal expansion stresses, reducing the net elongation.
4. Diameter effect — the larger diameter pipe (DN400) exhibits a lower expansion coefficient, likely because the thicker wall and larger circumference provide more constraint against thermal deformation. The spiral pitch and weld angle may also differ between diameters, affecting the constraint mechanism.

Microstructural Analysis

The metallographic examination revealed important microstructural differences:

The Widmanstätten structure in the weld zone is a direct consequence of the welding thermal cycle. During submerged arc welding (SAW), which is commonly used for spiral pipes, the heat input is relatively high, and the cooling rate in the weld zone can be moderate to slow, promoting the formation of coarse Widmanstätten ferrite. This microstructure has lower plasticity and toughness compared to the equiaxed ferrite-pearlite structure of the base metal, which is consistent with the reduced linear expansion coefficient observed in the experimental results.

Integration with Engineering Practice

For engineers designing district heating pipelines and other thermal piping systems, the following practical implications arise from this study:

1. Thermal expansion loop design — using the theoretical linear expansion coefficient would overestimate the required loop length, leading to oversized loops and unnecessary space consumption. The experimental values suggest that the actual thermal expansion is significantly less than predicted by the theoretical coefficient.
2. Support and anchorage design — the reduced expansion coefficient means that the thermal forces transmitted to supports and anchors are lower than expected. However, engineers should not simply reduce the support design without considering the safety margin and the variability of the expansion coefficient with temperature, pipe diameter, and welding parameters.
3. Material selection — the study highlights the importance of considering the microstructural effects of welding on thermal properties. For critical applications where thermal expansion is a design constraint, alternative materials or welding processes that minimize Widmanstätten formation should be considered.
4. Quality control — the presence of Widmanstätten structure in the weld zone should be monitored during production. Heat input control, interpass temperature control, and post-weld heat treatment can be used to minimize Widmanstätten formation and improve the thermal expansion behavior of the pipe.

Key Questions and Reflections

A critical question is whether the experimental setup accurately captured the true thermal expansion behavior of the pipe. The similar triangle amplification method, while clever for measuring small elongations, may introduce systematic errors due to friction, misalignment, or thermal gradients in the measurement apparatus. The authors conducted three tests for each diameter, which is a reasonable number for experimental validation, but the statistical variability of the results is not reported. Engineers should seek additional experimental data from independent sources before adopting the experimental expansion coefficients for design.

Another important consideration is the temperature range of the experiments. The linear expansion coefficient is not constant over all temperatures; it varies with temperature, particularly near phase transformation temperatures. The study does not specify the temperature range of the experiments, which limits the applicability of the results to specific operating conditions. For district heating systems operating at temperatures between 60°C and 150°C, the linear expansion coefficient may vary within this range, and the experimental values should be interpreted with this in mind.

Furthermore, the study focuses on Q235B steel, which is a low-carbon structural steel commonly used in China. The results may not be directly applicable to other steel grades (such as Q345B, Q355, or API 5L grades) used in other countries or applications. The microstructural response to welding is grade-dependent, and the Widmanstätten formation tendency varies with carbon equivalent and cooling rate.

Study Insights and Implications

This paper provides valuable experimental data that challenges the conventional assumption that the linear expansion coefficient of a welded pipe is the same as that of the base material. The finding that the actual expansion coefficient is roughly half the theoretical value is a significant result that, if confirmed by additional studies, could lead to more efficient and economical designs of thermal piping systems. The microstructural analysis provides a mechanistic explanation for the deviation, linking the Widmanstätten structure in the weld zone to the reduced thermal expansion. For engineers, the key takeaway is that the thermal expansion behavior of welded pipes should be experimentally verified rather than assumed from theoretical values, particularly for critical applications where thermal stresses and deformations are design-critical. Future research should extend this work to other steel grades, welding processes, and temperature ranges to build a comprehensive database of experimental expansion coefficients for welded steel pipes.