Cracking Analysis of Surfacing Layers on Continuous Casting Rolls

Literature Overview

The paper by Hong Yongchang (2006), published in Hot Working Technology (Volume 35, Issue 3, pp. 70-72), presents a detailed failure analysis of cracking in the surfacing layer of continuous casting rolls. The author employs a comprehensive analytical approach combining chemical composition analysis, metallographic examination, scanning electron microscopy (SEM) fracture analysis, and microhardness measurement to identify the root causes of surfacing layer failure. The findings reveal that improper surfacing process design, specifically the formation of high-carbon coarse martensite in the overheated zone of the heat-affected zone (HAZ), combined with retained welding residual stresses, are the primary causes of surfacing layer cracking.

This paper is highly relevant to engineers working in metallurgical equipment maintenance, surface engineering, and failure analysis, as continuous casting rolls are critical components in steel production and their failure leads to significant production losses.

Core Technical Points

Analytical Methodology

The author employed a multi-technique analytical approach, which is a model for systematic failure analysis:

The combination of these techniques provides a comprehensive picture of the failure mechanism, demonstrating that macroscopic cracking is the result of microstructural degradation in the HAZ combined with excessive residual stress.

Root Cause Analysis

The failure mechanism can be described through the following chain of events:

1. Excessive heat input during surfacing: The surfacing process parameters were not optimized for the roll material, resulting in excessive heat input to the base metal.
2. Overheated zone formation: The excessive heat input caused the HAZ to exceed the upper critical temperature (A3) by a large margin, leading to significant austenite grain growth.
3. Coarse martensite formation: Upon cooling, the coarse austenite grains transformed into coarse martensite (type III martensite), which is characterized by high hardness but extremely low toughness.
4. Residual stress accumulation: The high cooling rate and thermal mismatch between the surfacing layer and base metal generated high tensile residual stresses.
5. Crack initiation and propagation: The combination of brittle microstructure and high residual stress exceeded the fracture toughness of the HAZ, leading to crack initiation at the surfacing layer-HAZ interface and subsequent propagation through the brittle martensitic structure.

Microstructural Characteristics of the Failure Zone

The metallographic examination revealed distinct microstructural zones:

The sharp hardness gradient between the base metal (HV 200-250) and the overheated HAZ (HV 750-850) creates a severe stress concentration zone, which is the primary crack initiation site.

Heat Input and Cooling Rate Control

The paper implicitly addresses the critical relationship between heat input, cooling rate, and HAZ microstructure. For surfacing on high-carbon steel or cast iron substrates, the following parameters are critical:

• Heat input per pass: Should be kept below 15 kJ/mm for high-carbon substrates
• Interpass temperature: Maintain at 300-400°C to control cooling rate
• Preheat temperature: 300-500°C depending on carbon equivalent of substrate
• Post-weld heat treatment: Stress-relief annealing at 600-650°C for 2 hours minimum

The failure in this case was attributed to insufficient preheating and lack of interpass temperature control, which allowed cooling rates to exceed the critical cooling rate for martensite formation in the high-carbon HAZ.

Improvement Measures and Recommendations

Based on the failure analysis, the author proposes the following improvement measures:

1. Process parameter optimization: Reduce heat input per pass by using smaller diameter electrodes or wires, and increase the number of passes with thinner individual layers.
2. Preheat and interpass temperature control: Implement strict temperature monitoring and control throughout the surfacing operation.
3. Electrode selection: Use low-carbon, high-alloy electrodes that promote austenitic or ferritic weld metal with lower hardness and better toughness.
4. Post-weld heat treatment: Apply stress-relief annealing immediately after surfacing to reduce residual stresses and temper any martensite formed in the HAZ.
5. Multi-layer surfacing strategy: Use a transition layer of low-carbon austenitic alloy between the base metal and the final surfacing layer to reduce dilution and stress concentration.

Engineering Practice Implications

The failure analysis presented in this paper has direct implications for the maintenance of continuous casting rolls and similar metallurgical equipment:

• Preventive maintenance: Regular hardness mapping and ultrasonic testing of surfacing layers can detect early-stage cracking before catastrophic failure.
• Process documentation: Detailed process records including preheat temperatures, interpass temperatures, and heat input calculations should be maintained for each surfacing operation.
• Operator training: Welders performing surfacing operations on metallurgical equipment must be trained in the metallurgical consequences of process parameter deviations.
• Material selection: For high-carbon substrates, the surfacing material should be selected to minimize carbon pickup in the HAZ. Nickel-based austenitic electrodes are preferred over high-carbon iron-based electrodes.

Key Reflections and Study Insights

This paper exemplifies the power of systematic failure analysis in identifying root causes and developing effective corrective measures. The author's approach of combining multiple analytical techniques provides a comprehensive understanding of the failure mechanism, which is essential for developing reliable improvement measures.

The finding that coarse martensite in the overheated HAZ is the primary cause of cracking is a critical insight. In many industrial surfacing operations, attention is focused on the surfacing layer itself, while the HAZ is often overlooked. This paper demonstrates that the HAZ, not the surfacing layer, is often the weakest link in the surfacing joint.

The practical recommendations for process improvement are directly applicable to industrial settings. The emphasis on preheat and interpass temperature control, combined with post-weld stress-relief annealing, represents a comprehensive approach to preventing HAZ cracking. These measures should be incorporated into standard operating procedures for all surfacing operations on high-carbon substrates.

The paper also highlights the importance of understanding the metallurgical consequences of welding process parameters. Engineers must recognize that heat input is not merely a process variable but a metallurgical control parameter that directly determines the microstructure and properties of the HAZ. This understanding is essential for developing reliable surfacing processes for critical equipment.