Failure Analysis of Support Roll Surface Overlay Layer Cracking

Literature Overview

This paper by Wang Hui, Meng Xiaoxia, Yu Zhiwei, Xu Xiaolei, and Guo Xiaoyan, published in the journal "Welding" (2011, Issue 5, pp. 50-55), presents a detailed failure analysis of cracking in the overlay weld layer of a large support roll. The research was conducted jointly by Dalian Huarui Heavy Industry Special Spare Parts Manufacturing Co., Ltd. and Dalian Maritime University. The study identifies both material and mechanical factors contributing to two distinct crack types: crystallization hot cracks and reheat cracks.

Equipment and Service Context

Support Roll Function

In rolling mill operations, support rolls (also called backup rolls) provide the necessary backing support for work rolls during the hot rolling process. They operate under:

Extreme bending loads: The roll's own weight creates significant bending moments, particularly at bearing seats
High contact stresses: Force transmission from work rolls to support rolls
Thermal cycling: Temperature variations from hot rolled material and cooling systems
Fatigue loading: Cyclic stress from rolling operations

Overlay Welding Purpose

The overlay layer on support rolls serves to:

Provide corrosion resistance against cooling water and mill scale
Enhance wear resistance at bearing seat areas
Extend service life of expensive large-diameter rolls
Allow surface repair of worn or damaged areas

Failure Analysis Methodology

Macroscopic Examination

The initial examination revealed:

Crack locations: Both on the roll surface and within the overlay layer interior
Crack morphology: Distinct patterns characteristic of different crack types
Crack propagation direction: Aligned with principal stress directions
Crack length and depth: Variable, with some cracks extending through the full overlay thickness

Microscopic Analysis

Metallographic examination of crack surfaces and cross-sections revealed:

Feature	Observation	Interpretation
Crack surface morphology	Intergranular, with dendrite boundaries visible	Crystallization (hot) cracking
Secondary crack patterns	Transgranular with grain boundary branching	Reheat cracking
Grain boundary composition	Enriched with Cr, Mn, V carbides	Segregation and carbide precipitation
P and S content	Elevated at grain boundaries	Sulfide and phosphide formation
Microstructure	Coarse grains near crack origins	Slow cooling, insufficient refinement

Crack Type Identification and Mechanism

Crystallization Hot Cracking

Crystallization hot cracks (solidification cracking) form during the final stages of solidification when:

Low-melting-point phases: Form at grain boundaries due to segregation of P, S, and other impurities.
Stress conditions: Residual stress from differential solidification contraction exceeds the tensile strength of the interdendritic liquid films.
Constraint: The thick overlay layer on a massive roll substrate creates high constraint, promoting crack formation.

The mechanism involves:

Liquid film formation at dendrite boundaries during solidification
Stress concentration at liquid film locations
Crack initiation when stress exceeds the cohesive strength of the liquid-solid interface
Crack propagation along the weakest interdendritic paths

Reheat Cracking

Reheat cracks form during post-weld heat treatment or during subsequent thermal cycling:

Grain boundary embrittlement: Precipitation of Cr, Mn, V carbides at grain boundaries during PWHT or service exposure.
P and S segregation: Phosphorus and sulfur enrich at grain boundaries, reducing boundary cohesion.
Residual stress: Residual stresses from welding provide the driving force for crack initiation.
Thermal activation: Elevated temperatures during PWHT or service accelerate carbide precipitation and embrittlement.

The reheat cracking mechanism is analogous to temper embrittlement in alloy steels, where grain boundary precipitation reduces intergranular fracture resistance.

Contributing Factors Analysis

Material Factors

Factor	Effect	Severity
High P content	Promotes hot cracking, reduces hot strength	High
High S content	Forms low-melting sulfides, promotes hot cracking	High
Cr, Mn, V content	Promotes grain boundary carbide precipitation	Medium-High
Grain size	Coarse grains increase crack susceptibility	Medium
Inclusion content	Serves as crack initiation sites	Medium

Mechanical Factors

The support roll's own weight creates a significant bending moment, particularly at the bearing seats where the roll is supported. This bending load:

Creates tensile stresses on the convex side of the roll
Compresses the concave side
Generates shear stresses at neutral axis
Interacts with welding residual stresses

The combination of service bending stress and welding residual stress can exceed the fracture resistance of the overlay layer, particularly at grain boundaries weakened by segregation and precipitation.

Remediation and Prevention Strategies

Material Selection Optimization

Reduce P and S content: Specify low-P, low-S overlay materials to minimize hot cracking susceptibility.
Control Cr, Mn, V levels: Balance hardenability and wear resistance against reheat cracking susceptibility.
Consider alternative compositions: Overlay materials with lower reheat cracking sensitivity may be available.

Process Parameter Optimization

Parameter	Optimization Direction	Rationale
Heat input	Reduce	Finer grains, lower residual stress
Preheat temperature	Increase moderately	Reduce thermal gradient, slow cooling
Interpass temperature	Control within limits	Prevent excessive grain growth
Layer thickness	Reduce per pass	Better cooling control
Travel speed	Increase	Lower heat input per unit length

Heat Treatment Optimization

PWHT temperature: Avoid temperatures that promote maximum carbide precipitation (typically 450-600°C for Cr-Mn-V steels).
PWHT time: Minimize soak time to reduce precipitation extent.
Cooling rate: Controlled cooling to avoid additional thermal stress.

Stress Management

Stress relief welding: Apply low-heat-input stress relief welds to relieve residual stresses.
Peening: Apply shot peening or hammer peening to introduce compressive surface stresses.
Mechanical stress relief: Controlled mechanical deformation to relieve residual stresses.

Engineering Lessons and Reflections

This failure analysis provides critical lessons for engineers involved in overlay welding of heavy equipment:

Crack type identification is essential: Different crack types require different prevention strategies. Confusing crystallization hot cracks with reheat cracks leads to ineffective corrective actions.
Material chemistry matters: The interaction between P, S, and carbide-forming elements creates a complex cracking susceptibility that must be understood and managed.
Service loading must be considered: Overlay welding procedures developed in the laboratory may not account for the actual service stress states experienced by the component.
Systematic analysis is required: Effective failure analysis requires integration of macroscopic, microscopic, chemical, and mechanical evidence to establish the complete failure mechanism.
Prevention is preferable to repair: Implementing appropriate material selection, process parameters, and heat treatment from the outset is more effective than attempting to repair cracked overlays.

This case study exemplifies the importance of understanding the interaction between material properties, welding processes, and service conditions in overlay welding applications. For engineers responsible for specifying or approving overlay welding procedures for heavy equipment, this analysis provides a comprehensive framework for evaluating cracking risks and implementing effective prevention measures.