Experimental Study on Linear Expansion Coefficient of Q235B Spiral Welded Steel Pipes

Literature Overview

This paper published in Hot Working Technology (2017, Vol. 46, No. 7, pp. 47-50) by Hu Yueyue, Li Guanglu, and Liang Yahong from Xi'an University of Architecture and Technology investigates the linear expansion coefficient of Q235B spiral welded steel pipes. The study reveals a significant deviation between the measured expansion coefficients of spiral pipes and the theoretical values for homogeneous steel, attributed to microstructural variations introduced by the welding process.

Core Technical Content

Experimental Methodology

The study employs a magnification method based on similar triangles to measure the small elongation of spiral steel pipes under thermal loading. Test rigs were constructed using DN300 and DN400 spiral welded pipes, with three repeated tests conducted for each configuration to ensure statistical reliability.

Microstructural Analysis

Root Cause Analysis

The substantial reduction in linear expansion coefficient (approximately 50% below theoretical) is attributed to several factors:

1. Widmanstatten structure in weld metal: The acicular ferrite morphology has a different thermal expansion behavior compared to equiaxed ferrite, particularly at elevated temperatures where the coarse grain structure restricts thermal strain accommodation.
2. Residual stress field: Welding-induced residual stresses (typically tensile in the weld zone, compressive in adjacent HAZ) partially counteract thermal expansion, effectively reducing the apparent CTE.
3. Compositional segregation: Microsegregation in the weld metal and HAZ alters local phase fractions, affecting the overall expansion behavior.
4. Spiral geometry effects: The helical weld configuration distributes thermal strain differently compared to longitudinal welds, with the spiral geometry providing additional strain accommodation through geometric flexibility.

Engineering Practice Integration

Implications for Thermal Stress Analysis

The measured CTE values have direct implications for engineering calculations involving:

• Thermal stress in buried pipelines: When designing for thermal cycling (seasonal temperature variations), using the theoretical CTE of 12.5 × 10^-6/°C would overestimate thermal stresses by approximately 50%, leading to unnecessarily conservative design.
• Expansion joint sizing: In heating systems using spiral pipes, expansion joints and compensators sized based on theoretical CTE would be oversized, increasing cost without benefit.
• Welding procedure development: Understanding the actual CTE helps in predicting weld distortion and designing appropriate clamping and backing arrangements.

Quality Control Considerations

Thermal Cycling Performance

For applications involving repeated thermal cycling (district heating, power plant steam lines), the reduced CTE of spiral welded pipes is beneficial as it decreases thermal fatigue stresses. However, the Widmanstatten structure in the weld metal raises concerns about low-temperature toughness and hydrogen-induced cracking susceptibility. The following mitigation measures are recommended:

• Preheating temperature ≥ 100°C for Q235B spiral pipes in cold environments
• Interpass temperature control between 150-250°C
• Post-weld heat treatment (PWHT) at 550-620°C for critical applications
• Hydrogen control: limit electrode moisture content and use low-hydrogen consumables

Key Questions and Reflections

The observation that DN400 pipes exhibit a 7.24% lower CTE than DN300 pipes raises an interesting geometric question. The larger diameter pipe has a longer helical weld path per unit length of pipe, meaning a greater proportion of the cross-section contains weld metal and HAZ material. This suggests that the CTE reduction is not merely a material property effect but is also governed by the geometric ratio of weld zone to total cross-section area.

A critical unanswered question is the temperature dependence of this CTE reduction. The study appears to measure CTE at specific temperature ranges, but in engineering practice, pipes may experience temperatures from -20°C to 300°C or higher. The Widmanstatten structure and residual stress field may exhibit different thermal expansion behaviors at various temperature levels, potentially leading to non-linear CTE-temperature relationships.

Additionally, the study does not address the effect of pipe manufacturing parameters (spiral angle, welding speed, current, voltage) on the CTE variation. These parameters directly influence weld metal microstructure and residual stress levels, and therefore represent important process control variables for achieving consistent CTE values in production.

Study Insights and Implications

This research provides critical experimental data that challenges the conventional assumption that spiral welded pipes can be treated as homogeneous steel for thermal analysis purposes. The approximately 50% reduction in CTE is substantial enough to affect engineering calculations and design decisions. For district heating pipeline design, where thermal expansion and contraction are primary design considerations, using the measured CTE values rather than theoretical steel values would lead to more accurate and economical designs.

From a manufacturing perspective, the findings highlight the importance of controlling welding parameters to minimize Widmanstatten formation in the weld metal, as this microstructure not only reduces CTE but also compromises toughness and crack resistance. Engineers should consider incorporating CTE measurement into their quality assurance programs for spiral welded pipes used in thermally demanding applications, and should develop specific thermal analysis models that account for the heterogeneous material properties of spiral pipes rather than treating them as uniform steel sections.