Zhuojin Pipe Fitting Co., LtdCrack Analysis of Overlay Weld Layer on Continuous Casting Rolls
Literature Overview
This failure analysis paper by Hong Yongchang (2006), published in Hot Working Technology (Vol. 35, No. 3, pp. 70-72), investigates the cracking mechanism in the overlay weld layer of continuous casting rolls. The study employs a comprehensive analytical approach combining chemical composition analysis, metallographic examination, scanning electron microscopy (SEM) of fracture surfaces, and microhardness measurement.
Failure Analysis Methodology
Analytical Techniques Employed
| Technique | Purpose | Key Findings |
|---|---|---|
| Chemical composition analysis | Determine elemental distribution | Carbon enrichment in HAZ |
| Metallographic examination | Identify microstructure | Coarse martensite in overheated zone |
| SEM fractography | Analyze crack propagation mode | Mixed mode with cleavage features |
| Microhardness measurement | Map hardness distribution | Sharp hardness gradient at interface |
Root Cause Identification
The analysis reveals two primary failure mechanisms:
- Overheated zone formation: The excessive heat input during overlay welding produced a coarse-grained, high-carbon martensite structure in the heat-affected zone adjacent to the base metal. This structure exhibits extremely low fracture toughness and is highly susceptible to cracking under residual stress.
- Residual stress retention: Inadequate post-weld stress relief left significant tensile residual stresses in the weld overlay layer. Combined with the brittle microstructure in the HAZ, this created conditions favorable for crack initiation and propagation.
The SEM fractography showed mixed-mode fracture features including cleavage facets and intergranular cracking, confirming that the failure was stress-driven rather than purely fatigue-related.
Technical Analysis of the Microstructure
Microstructural Evolution in the HAZ
The continuous casting roll substrate typically consists of a ductile iron or low-carbon steel base. During overlay welding, the thermal cycle produces distinct microstructural zones:
| Zone | Temperature Range | Microstructure | Hardness (HV) |
|---|---|---|---|
| Weld metal | >1500°C | Fine martensite + retained austenite | 500-650 |
| Overheated zone | 1100-1350°C | Coarse martensite, large grains | 600-750 |
| Partially transformed zone | 800-1100°C | Mixed ferrite and martensite | 350-500 |
| Unaffected zone | <800°C | Original microstructure | 200-300 |
The critical issue lies in the overheated zone, where grain growth combined with carbon diffusion from the base metal produces a high-carbon martensite structure with hardness exceeding 700 HV. This zone has essentially no ductility and serves as the crack initiation site.
Stress State Analysis
The residual stress distribution in the overlay weld layer is governed by:
- Thermal contraction during cooling from welding temperature
- Phase transformation stresses from austenite to martensite
- Differential thermal expansion between weld metal and base metal
Without adequate stress relief, the peak tensile residual stress can exceed 400 MPa, which is sufficient to initiate cracks in the brittle overheated zone.
Improvement Measures and Recommendations
Process Optimization
Based on the failure analysis, the following improvement measures are proposed:
- Heat input control: Reduce welding current and increase travel speed to limit the thermal cycle severity. For multi-pass overlay welding, ensure that each subsequent pass partially reheats the previous pass, promoting transformation of coarse martensite into tempered products.
- Preheating optimization: Implement a higher preheat temperature (300-400°C) to slow the cooling rate and promote bainitic rather than martensitic transformation in the HAZ.
- Interpass temperature management: Maintain interpass temperature above 200°C to prevent formation of untempered martensite.
- Post-weld stress relief: Perform complete stress relief annealing at 550-650°C for sufficient duration to reduce residual stresses below 100 MPa.
- Electrode/wire selection: Choose overlay materials with lower carbon equivalent to reduce the carbon content in the dilution zone.
Engineering Practice Implications
This failure analysis provides valuable lessons for engineers working with overlay welding on critical components. The continuous casting roll is a high-value component where failure results in significant production downtime. The systematic approach of combining multiple analytical techniques to identify the root cause is a model for failure investigation in other applications.
The key insight is that overlay welding is not merely a deposition process but a thermomechanical process that fundamentally alters the microstructure of the substrate near the weld interface. Engineers must consider the entire thermal history, not just the weld metal properties, when designing overlay welding processes.
Key Reflections
The paper demonstrates that the most dangerous zone in an overlay weld is often not the weld metal itself but the heat-affected zone in the base metal. This is a frequently overlooked aspect in process design. The coarse martensite in the overheated zone, while providing high hardness, completely eliminates the toughness needed to resist crack propagation.
For engineers designing overlay welding processes for other critical components—such as pipe mill rolls, press tools, or pump components—the lesson is clear: process design must include explicit consideration of HAZ microstructure control, not merely weld metal composition and hardness. The thermal cycle parameters should be optimized to produce a bainitic or tempered microstructure in the HAZ rather than allowing untempered martensite to form.