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

Literature Overview

This paper by Zhang Li, Ji Lianghao, Liu Ying, Wang Hanting, Wang Ling, and Liu Sihan, published in Angang Technology (2025, Issue 3, pp. 32-38), investigates the cracking mechanism in Q235 hot-rolled plate longitudinal seam welded pipe elbows. The study employs macroscopic fracture analysis, microscopic examination, and chemical composition analysis to identify the root causes of cracking at the weld zone during the elbow forming process. The findings reveal that cracks initiate in the coarse grain zone of the outer wall heat-affected zone (HAZ), particularly in regions containing pearlite, ferrite, and Widmanstätten structures, and propagate along inclusion paths toward the inner wall, causing micro-cracks in both inner and outer wall HAZ regions.

Core Technical Content and Methodology

Q235 is a widely used carbon structural steel with good weldability and formability, commonly employed in water supply and drainage piping systems. The manufacturing process involves hot-rolled plate forming into longitudinal seam welded pipe, followed by hot expansion to achieve the elbow geometry. The cracking issue arises during the hot expansion forming stage, where the weld zone experiences significant plastic deformation under elevated temperatures.

Fracture and Microstructural Analysis

The investigation revealed several critical findings:

• Cracks primarily initiate in the coarse grain zone of the outer wall HAZ, where the microstructure consists of pearlite, ferrite, and Widmanstätten structures. The Widmanstätten morphology, characterized by plate-like ferrite growing into the prior austenite grains, creates a brittle microstructure susceptible to cracking under plastic deformation.
• Cracks propagate along inclusion paths from the outer wall toward the inner wall, indicating that non-metallic inclusions serve as crack initiation and propagation sites. The inclusion distribution is influenced by the hot expansion process, which redistributes inclusions and creates voids.
• Welding overheating causes weld metal embrittlement, and the coarse Widmanstätten structure further accelerates crack propagation.
• The hot expansion process affects inclusion distribution and void formation, ultimately contributing to elbow weld cracking.

Engineering Practice Implications

This study provides critical insights for preventing cracking in Q235 welded pipe elbows during hot expansion forming. Several practical recommendations can be derived:

• Welding parameters should be optimized to minimize HAZ overheating, thereby reducing the formation of coarse Widmanstätten structures. This may involve reducing heat input, increasing travel speed, or using preheating and interpass temperature control.
• The base material should be inspected for inclusion content and distribution, as inclusions are the primary crack initiation sites. Steel grades with lower inclusion content or inclusion-modified compositions should be specified for elbow applications.
• Hot expansion process parameters should be controlled to minimize void formation and inclusion redistribution. This may involve optimizing heating temperature, expansion speed, and cooling rate.
• Non-destructive testing (NDT) should be performed after forming to detect any micro-cracks that may have formed during the process.

Study Insights and Reflections

This paper provides a comprehensive metallurgical analysis of a common manufacturing defect in welded pipe elbow production. The systematic approach of combining macroscopic fracture analysis, microstructural examination, and chemical composition analysis is exemplary for failure investigation work. The identification of Widmanstätten structures as a key factor in crack initiation is particularly important, as this microstructure is commonly formed in HAZ regions of carbon steels when welding heat input is excessive. The finding that cracks propagate along inclusion paths reinforces the importance of controlling inclusion content and distribution in the base material.

I find the study particularly relevant to the water supply and drainage industry, where Q235 welded pipe elbows are widely used and cracking during manufacturing can lead to costly rework and potential service failures. The recommendations for optimizing welding parameters and controlling hot expansion conditions are practical and actionable. One area for further investigation would be the development of specific welding procedure specifications (WPS) for Q235 elbow manufacturing that explicitly limit HAZ Widmanstätten formation, and the establishment of acceptance criteria for inclusion content in base plate material. This work serves as a valuable reference for metallurgists and manufacturing engineers involved in welded pipe elbow production.