Microstructure and Cracking Analysis of Q235 Hot-Rolled Plate Longitudinal Seam Welded Pipe Elbows

Literature Overview

This paper by Zhang Li, Ji Lianghao, Liu Ying, Wang Hanting, Wang Ling, and Liu Sihan (2025), published in Ansteel Technology (Issue 3, pp. 32-38), presents a comprehensive failure analysis of cracking in Q235 hot-rolled plate longitudinal seam welded pipes during elbow forming operations. The study employs macroscopic fracture analysis, microscopic examination, and chemical composition analysis to identify the root causes of cracking at the weld region during hot expansion forming. Funded by Yingkou Institute of Technology (QNL202210) and the Liaoning Provincial College Student Innovation Training Program (X202414435029), this work provides critical insights into material selection and process control for pipe elbow manufacturing.

Failure Analysis Methodology

The investigation follows a systematic failure analysis approach:

Root Cause Analysis

The failure analysis identifies a multi-factorial cause for the elbow cracking:

Crack Initiation Mechanism

Cracks primarily initiate in the coarse grain zone (CGZ) of the heat-affected zone on the outer wall, particularly in regions containing:

• Pearlite: High carbon microconstituent with limited ductility
• Ferrite: Soft phase providing matrix but limited strength
• Widmanstätten structure: Needle-like ferrite formed during rapid cooling, creating stress concentrations

Crack Propagation Path

The crack propagates along inclusion paths from the outer wall to the inner wall, creating microcracks in both the outer and inner wall HAZ regions. This transverse crack pattern indicates that the inclusion-rich paths provide preferential crack propagation channels with lower fracture resistance.

Contributing Factors

1. Welding overheating: Excessive heat input during welding causes weld metal embrittlement and coarse grain formation
2. Widmanstätten structure: Coarse Widmanstätten ferrite further promotes crack propagation by creating stress concentrations at needle tips
3. Hot expansion process: The forming process affects inclusion distribution and promotes void formation
4. Inclusion-induced voids: Non-metallic inclusions act as nucleation sites for voids during hot deformation

Microstructural Characteristics

The Q235 material exhibits the following microstructural features relevant to the failure:

Process Control Recommendations

Based on the failure analysis, the following process improvements are recommended:

Welding Process Optimization

• Heat input control: Limit heat input to prevent excessive grain growth in the HAZ
• Preheating: Apply appropriate preheating to control cooling rates and prevent Widmanstätten formation
• Interpass temperature: Maintain interpass temperature to prevent excessive cooling between passes
• Post-weld treatment: Consider post-weld heat treatment to refine grain structure

Material Selection and Quality

• Inclusion control: Specify low-sulfur grades or add calcium treatment to modify inclusion morphology
• Grain size control: Ensure proper grain size in the base material through appropriate rolling practices
• Surface quality: Maintain clean surface conditions to prevent surface-initiated cracking

Forming Process Control

• Hot expansion parameters: Optimize temperature and strain rate to minimize void formation
• Forming temperature: Maintain adequate forming temperature to ensure sufficient ductility
• Strain distribution: Design forming sequences to distribute strain evenly and avoid localized deformation

Engineering Practice Implications

This failure analysis has significant implications for pipe elbow manufacturing:

1. Quality control: Enhanced inspection of weld HAZ microstructure before forming operations
2. Material specifications: Consider upgrading from Q235 to higher-grade materials for critical applications requiring hot expansion forming
3. Process validation: Implement systematic process validation including microstructural evaluation at critical process steps
4. Supplier qualification: Qualify steel suppliers based on inclusion content and grain size control capabilities

FMEA Analysis Integration

Applying Failure Mode and Effects Analysis (FMEA) to this case:

The high Risk Priority Numbers (RPN) for HAZ cracking and Widmanstätten formation indicate that these are the primary failure modes requiring immediate attention in process improvement efforts.

Reflections

This failure analysis exemplifies the importance of integrating metallurgical understanding with process engineering knowledge in pipe manufacturing. The multi-factorial nature of the failure—combining material microstructure, welding parameters, and forming conditions—demonstrates that failures in manufacturing processes rarely have single root causes.

The finding that cracks propagate along inclusion paths highlights the critical role of material cleanliness in hot forming operations. Even in relatively low-carbon steels like Q235, inclusion morphology and distribution can be decisive in determining forming success or failure. This has implications for material specifications that may need to be more stringent than currently specified for pipe elbow manufacturing applications.

The study also underscores the importance of process integration—welding, forming, and inspection must be considered as an integrated system rather than isolated process steps. A failure in one process step can propagate through subsequent operations, ultimately leading to product failure. This systems-level perspective is essential for effective quality management in pipe manufacturing.

This research provides valuable lessons for engineers working in pipe fitting manufacturing, emphasizing the need for comprehensive material characterization, careful process parameter control, and systematic failure analysis to prevent costly production failures.