CLADDING TECHNOLOGY SHANXI CO., LTDCrack Mechanisms in Weld Overlay of Transfer Rolls
Literature Overview
The study by Wang Qingbao, Li Zhuoxin, and Shi Yaowu (Beijing University of Technology and CSM Welding Technology Co., published in the Welding Journal, 2010) investigates the cracking phenomena observed during weld overlay cladding of transfer rolls in hot rolling mills. Transfer rolls are critical components in continuous casting and hot rolling operations, subjecting them to extreme thermal cycling, mechanical abrasion, and chemical attack from scale and oxides. The overlay process aims to extend roll life by depositing a hard, wear-resistant surface layer, but cracking during and after overlay remains a persistent challenge that limits the reliability of the repair process.
Core Crack Types and Their Mechanisms
The authors classify the overlay cracks into three principal categories based on their origin and propagation timing: hot cracks, reheat cracks, and cold cracks. Each type arises from distinct metallurgical mechanisms tied to the specific composition of the overlay material, the thermal cycle parameters, and the residual stress state in the deposited layer.
Hot Cracking
Hot cracks form at temperatures above 600 °C, typically within the solidification range of the deposited metal. In transfer roll overlay, the base material is usually a high-chromium cast iron or a low-alloy steel, while the overlay material may contain chromium, molybdenum, and carbon at elevated levels to achieve hardness above HRC 50. The hot crack susceptibility is governed by the mushy zone width during solidification. A wide solidification range promotes interdendritic segregation of low-melting-point eutectics (such as Cr-rich eutectics containing Cr₇C₃ and Fe₃C), which creates a weakened grain boundary network susceptible to tensile stress. The high sulfur and phosphorus content in the base cast iron further exacerbates this effect through hot shortness mechanisms.
| Crack Type | Formation Temperature | Primary Cause | Typical Location |
|---|---|---|---|
| Hot crack | >600 °C | Low-melting eutectic segregation, high restraint | Between deposited layers, near fusion boundary |
| Reheat crack | 500–700 °C | Precipitation-induced embrittlement, residual stress relaxation | Along prior austenite grain boundaries |
| Cold crack | <200 °C | Hydrogen embrittlement, martensitic transformation | Near HAZ or in overlay layer |
Reheat Cracking
Reheat cracks in transfer roll overlay are particularly insidious because they appear during or after post-weld heat treatment (PWHT). The overlay layer, often containing high carbon and chromium, undergoes carbide precipitation along grain boundaries during the tempering or stress-relief cycle. This precipitation-induced embrittlement (PIE) mechanism is analogous to temper embrittlement in low-alloy steels. The chromium carbides (M₇C₃ and M₂₃C₆) preferentially precipitate at grain boundaries, depleting the adjacent matrix of carbon and chromium, creating a brittle intergranular film. When combined with the residual tensile stresses from differential thermal contraction between the overlay and base material, these embrittled boundaries become crack initiation sites.
Cold Cracking
Cold cracks in the overlay layer of transfer rolls are associated with hydrogen-induced cracking (HIC) and delayed cracking due to martensitic transformation. When the overlay material has a high hardenability (high carbon equivalent, CE), rapid cooling after welding produces a high volume fraction of martensite, which is inherently brittle and susceptible to hydrogen-assisted cracking. The base cast iron may contain absorbed hydrogen from the welding process or from prior surface contamination, which diffuses into the overlay layer during cooling. The combination of martensitic microstructure, residual tensile stress, and diffused hydrogen creates the classic "hydrogen cracking triangle."
Process Countermeasures
Based on the crack mechanisms identified, the following engineering countermeasures are recommended:
- Preheating and interpass temperature control: A preheat temperature of 250–350 °C is typically required to reduce the cooling rate below the martensite start temperature (Ms) for most overlay compositions, thereby minimizing martensite formation and reducing residual stress. The interpass temperature should be maintained above 200 °C to avoid cold cracking in subsequent layers.
- Post-weld heat treatment: A PWHT at 550–650 °C for 2–4 hours per 25 mm of weld thickness is essential to relieve residual stresses and transform any retained austenite or high-carbon martensite into tempered structures. However, the PWHT temperature must be carefully controlled to avoid reheat cracking, which can be mitigated by using overlay materials with lower carbon content or by adding niobium and vanadium to stabilize carbide precipitation.
- Welding process selection: Submerged arc welding (SAW) with rutile-type flux is preferred for its low hydrogen input and good arc stability. The flux should have low moisture content (<0.5% free water) to minimize hydrogen absorption. Multi-pass welding with appropriate layer thickness (3–5 mm per pass) helps distribute heat input and reduce thermal gradients.
- Overlay material design: Reducing the carbon equivalent of the overlay material below 0.45% significantly improves cold crack resistance. The use of low-carbon, high-chromium martensitic stainless steel wires (such as those based on AISI 410 with reduced carbon) provides an acceptable compromise between hardness and crack resistance.
Engineering Practice Insights
In my experience with transfer roll repair programs at hot strip mills, the most critical factor in preventing overlay cracks is the thorough preparation of the base surface. Scale and oxide contamination on the roll surface not only promote hot shortness but also act as hydrogen sources. Plasma cutting or mechanical grinding to bare metal, followed by solvent cleaning within 4 hours of welding, is essential. Additionally, the geometric design of the overlay area should incorporate a gradual transition (such as a chamfer or fillet) to avoid stress concentration at the edge of the overlay deposit, which is a common crack initiation site.
The study by Wang et al. provides a valuable systematic classification of crack mechanisms that directly informs the selection of welding procedures and quality control protocols. For engineers involved in transfer roll maintenance, understanding the interplay between composition, thermal cycle, and residual stress is fundamental to developing reliable overlay repair procedures that extend roll life without introducing new failure modes.