Analysis of Surface Peeling Defects in Cold-Rolled 10 Steel Pipes

Literature Overview

This paper by Sun Bo, Sun Weilian, and Li Ying, published in the Journal of Thermal Processing of Materials (Volume 34, Issue 5, 2013, pages 121-124), investigates the root cause of surface peeling and roughness defects observed after cold forming of 10 steel pipes. The study was supported by the Hebei Provincial Natural Science Foundation (Grant No. E2009000646) and conducted at the College of Mechanical and Electrical Engineering, Hebei Agricultural University. The classification code TG335.12 places this work squarely within the domain of cold plastic deformation of metals, making it highly relevant to engineers dealing with cold-drawn, cold-rolled, or cold-formed steel tubing products.

Core Technical Findings

The research team employed a systematic investigative approach combining direct reading spectrometry for chemical composition verification, scanning electron microscopy (SEM) and optical microscopy for microstructural characterization, and energy-dispersive spectroscopy (EDS) for phase identification at grain boundaries. The key finding is that the raw 10 steel material contained tertiary cementite (Fe₃C) phases distributed along ferrite grain boundaries. These intergranular cementite films, being inherently brittle, served as preferential crack initiation sites during cold deformation, leading to surface peeling.

Microstructural Root Cause

The presence of tertiary cementite at ferrite grain boundaries is a well-known metallurgical phenomenon that occurs when steel is cooled slowly from the austenite region or when the carbon content approaches the upper limit of the grade specification. In 10 steel (a carbon steel with nominal 0.10% C), the equilibrium microstructure should consist of fine ferrite with minimal pearlite. However, if the rolling mill cooling rate is inadequate or if the steel slab experienced prolonged holding at intermediate temperatures, cementite can precipitate preferentially along grain boundaries as a continuous or semi-continuous film.

Remediation Strategy

The authors demonstrated that normalizing treatment effectively eliminated the intergranular cementite network by dissolving the cementite phases during austenitization and allowing their redistribution during controlled cooling. This is consistent with established heat treatment practice: normalizing at temperatures between 830-870°C for 10 steel, followed by air cooling, produces a uniform fine-grained ferrite-pearlite structure without intergranular brittle phases.

Engineering Practice Implications

In my decades of experience with cold-formed steel pipe production, surface peeling defects represent one of the most economically damaging quality issues, particularly for small-diameter tubing used in automotive and appliance applications. The paper's findings reinforce several critical practices that I advocate in production environments:

1. Incoming material inspection: Every batch of cold-forming raw material should undergo metallographic examination at 400x-1000x magnification with etching at 2-4% nital to reveal intergranular cementite films. A simple quantitative criterion—no more than 10% of grain boundary length showing continuous cementite—should be established as an acceptance limit.
2. Heat treatment verification: For materials that fail the initial inspection, normalizing should be specified as a corrective action. The normalizing temperature window for 10 steel is typically 830-870°C, with a minimum holding time of 2 minutes per millimeter of thickness, followed by furnace cooling or air cooling depending on the section size.
3. Process parameter control: Cold forming reduction ratios should be limited to 25-30% per pass for materials with borderline microstructural quality. Multi-pass forming with intermediate annealing is preferable to single-pass high-reduction operations.

Study Insights and Reflections

This paper, while methodologically straightforward, provides a valuable case study in the application of the 5W2H analytical framework to production quality problems. The authors clearly established What (surface peeling), Where (along grain boundaries), Why (brittle cementite films), How (cold deformation cracking mechanism), and What to do (normalize). The use of EDS for phase identification at the grain boundary is particularly instructive—it demonstrates that even in a simple low-carbon steel, microstructural characterization at the nanometer scale is essential for root cause analysis.

From a quality control perspective, this case underscores the importance of implementing a PDCA cycle: Plan the incoming material microstructural verification, Do the normalizing treatment on affected heats, Check the cold-formed product surface quality, and Act by updating the supplier qualification requirements. The economic argument for normalizing treatment before cold forming is compelling when one considers that surface defects in finished tubing often require complete rejection or costly rework, whereas pre-treatment normalizing adds minimal cost per ton of material.