Crack Analysis of Hardfacing Layer on Continuous Casting Rolls

Literature Overview

The paper by Hong Yongchang (2006), published in Hot Working Technology, presents a detailed failure analysis of cracks in the hardfacing layer of continuous casting rolls. Using chemical composition analysis, metallographic examination, scanning electron microscopy of fracture surfaces, and microhardness measurements, the author identifies the root causes of cracking and proposes corrective measures. This case study is particularly relevant to engineers in the steel industry who deal with the maintenance and repair of continuous casting equipment, where hardfacing of roll surfaces is a common practice to extend service life.

Failure Analysis Methodology

The analytical approach employed in this study follows a systematic failure analysis methodology:

Analysis Method	Purpose
Chemical composition analysis	Determine weld metal and HAZ composition
Metallographic examination	Identify microstructural features
SEM fracture surface analysis	Characterize crack initiation and propagation mode
Microhardness measurement	Map hardness distribution and identify hard phases

Microstructural Findings

The metallographic examination revealed the presence of coarse high-carbon martensite in the overheated zone of the heat-affected zone. This microstructure forms when the local temperature exceeds the austenitization temperature significantly, resulting in excessive grain growth and rapid transformation upon cooling. The high-carbon martensite is extremely hard (typically HV 800-1000) but very brittle, making it susceptible to cracking under thermal cycling and mechanical loading.

The SEM fracture surface analysis indicated a mixed-mode fracture with both intergranular and transgranular features. The intergranular cracking suggests the presence of brittle phases along grain boundaries, while the transgranular cracking indicates stress concentration at hard carbide particles. The crack initiation sites were found to be at the interface between the hardfacing layer and the HAZ, where the microstructural gradient is most severe.

Root Cause Analysis

The primary causes of cracking identified in this study are:

Inappropriate welding procedure specification resulting in excessive heat input
Formation of coarse high-carbon martensite in the overheated HAZ due to insufficient preheating and improper interpass temperature control
Residual welding stresses that were not adequately relieved by post-weld heat treatment

The combination of these factors creates a highly stressed, brittle region at the weld-heat affected zone interface that cannot withstand the thermal cycling experienced during continuous casting operations.

Corrective Measures and Recommendations

Based on the failure analysis findings, the following corrective measures are recommended:

Process Parameter Optimization

Parameter	Original (Defective)	Recommended (Corrected)
Preheating temperature	100-150°C	250-350°C
Interpass temperature	Uncontrolled	≤200°C
Heat input	2.5-3.5 kJ/mm	1.5-2.5 kJ/mm
Post-weld heat treatment	None or inadequate	600-650°C for 2-4 hours
Welding sequence	Single direction	Multi-direction with staggered overlap

Electrode and Consumable Selection

The selection of welding consumables should consider the carbon equivalent of the base material and the required hardness of the hardfacing layer. For continuous casting rolls, low-carbon transition layers should be used first to reduce carbon pickup in the HAZ. The hardfacing alloy should be selected to provide adequate wear resistance without creating excessive hardness gradients at the interface.

Engineering Practice Integration

In my experience with continuous casting roll repair, the lessons from this failure analysis are directly applicable. The key insight is that hardfacing is not merely a matter of depositing a hard overlay; it requires careful control of the entire thermal cycle to prevent the formation of brittle microstructures. The residual stress component is often underestimated in practice, as operators tend to focus on the weld deposit quality while neglecting the stress state of the entire roll.

A practical approach that has proven effective in my engineering practice is the use of a multi-step welding procedure: first, a low-carbon transition layer with controlled heat input; second, the hardfacing layer with reduced current and faster travel speed; and finally, immediate stress relief annealing. This approach addresses both the microstructural and residual stress aspects of the problem simultaneously.

The economic implications of hardfacing failure on continuous casting rolls are significant. A single roll failure can result in production shutdown costs exceeding the cost of proper repair by orders of magnitude. Therefore, investing in proper procedure development and quality control is not optional but essential for cost-effective operation.

Study Insights and Implications

This failure analysis paper exemplifies the importance of systematic failure investigation in welding engineering. The methodology followed—starting from macroscopic observation and progressing to microstructural and fracture surface analysis—is a model for other failure analysis investigations. The findings highlight the critical role of the heat-affected zone in hardfacing applications, a topic that receives less attention than the weld metal itself in many engineering practices.

The corrective measures proposed are straightforward but require discipline in implementation. In many production environments, the temptation to skip preheating or post-weld heat treatment due to production pressure is strong, but this paper demonstrates clearly that such shortcuts lead to premature failure and higher overall costs. Engineers must advocate for proper procedure adherence and provide data-driven arguments when challenging production-driven compromises.