CLADDING TECHNOLOGY SHANXI CO., LTDMeasures to Prevent Hot Cracking in Continuous Casting Roller Cladding
Literature Overview
The 1995 study by Miao Hailiang from the Tianjin No. 2 Metallurgical Machinery Repair Center addresses the critical challenge of hot cracking during the weld overlay cladding of continuous casting rollers. Continuous casting rollers are key components in steel continuous casting machines, where they support and guide the solidifying steel shell as it travels through the mold and secondary cooling zones. These rollers are subjected to extreme thermal cycling, mechanical loading, and corrosion from the molten steel and cooling water. Cladding is applied to extend the service life of the rollers, but the formation of hot cracks during the cladding process has been a persistent quality issue that limits the reliability and service life of the cladded rollers.
Root Cause Analysis of Hot Cracking
Hot cracking in weld overlay occurs during the solidification stage when the weld metal is in the mushy zone (between the liquidus and solidus temperatures). The formation of hot cracks is governed by three primary factors: the chemical composition of the weld metal, the welding process parameters, and the restraint conditions imposed by the base metal.
The following table summarizes the key factors contributing to hot cracking and their respective countermeasures:
| Factor | Description | Countermeasure |
|---|---|---|
| Sulfur and phosphorus content | Lowers solidus temperature, promotes liquid film formation | Use low-S, low-P weld metal (S < 0.01%, P < 0.02%) |
| High carbon equivalent | Increases solidification range, promotes cracking | Select low-carbon or low-carbon-equivalent consumables |
| High restraint | Base metal restricts shrinkage during solidification | Preheat base metal; use low-heat-input process |
| Excessive cooling rate | Rapid cooling increases thermal stress | Preheat to 200–300 °C; use back-heat technique |
| Poor welding sequence | Asymmetric thermal input causes uneven stress | Use symmetric welding pattern; weld in small sections |
Detailed Process Strategies
The study proposes several specific measures to prevent hot cracking, which can be categorized into consumable selection, process parameter optimization, and thermal management strategies.
Consumable selection is the most fundamental measure. The weld metal composition must be carefully controlled to minimize the solidification range and to suppress the formation of low-melting-point phases. The sulfur content should be limited to below 0.01%, and the phosphorus content below 0.02%. The carbon equivalent (CE) should be kept low, ideally below 0.4%, to ensure good ductility during solidification. For continuous casting rollers, a typical overlay material is a Cr-Ni austenitic stainless steel such as 309 or 310, which has a wide solidification range but excellent hot cracking resistance due to its austenitic structure.
Process parameter optimization involves controlling the heat input, travel speed, and current density. A lower heat input reduces the solidification range and minimizes the time the weld metal spends in the critical mushy zone. The travel speed should be increased moderately to reduce the dwell time of the arc on any given section. The welding current should be kept at the minimum level that ensures proper penetration and wetting. For GTAW cladding, a typical current range is 100–150 A with a travel speed of 50–80 mm/min, depending on the wire diameter and overlay thickness.
Thermal management includes preheating the base metal to 200–300 °C, maintaining the interpass temperature at 200–350 °C, and applying back-heat treatment during multi-pass welding. Back-heat involves heating the previously deposited weld passes to 300–400 °C before depositing the next pass, which allows the residual stresses to relax and reduces the restraint on the newly solidifying metal.
Quality Verification and Service Performance
After cladding, the rollers undergo rigorous quality verification including visual inspection for surface cracks, dye penetrant testing (PT) for surface and near-surface cracks, and magnetic particle testing (MT) for ferromagnetic substrates. The overlay layer is also subjected to hardness testing and, if necessary, intergranular corrosion testing to verify the metallurgical quality. In service, the cladded continuous casting rollers should exhibit uniform wear without cracking or spalling of the overlay layer.
Study Insights and Implications
This study is particularly valuable because it addresses a defect that was, at the time, a significant barrier to the widespread adoption of cladding technology for continuous casting rollers. The systematic approach to hot cracking prevention—combining consumable selection, process optimization, and thermal management—provides a template that can be applied to other challenging cladding applications. The emphasis on low sulfur and phosphorus content in the weld metal is a reminder that even trace impurities can have a disproportionate effect on weldability. Engineers should also recognize that the prevention of hot cracking is not a one-time solution but requires continuous monitoring and adjustment as welding conditions, consumable batches, and environmental factors change. This literature remains relevant today as a foundational reference for weld overlay quality control in metallurgical equipment manufacturing.