Zhuojin Pipe Fitting Co., LtdMeasures to Prevent Hot Cracks in Surfacing Welds on Continuous Casting Rolls
Literature Overview
The paper by Miao Hailiang from the Tianjin Second Metallurgical Machinery Repair Center, published in the journal "Welding" in 1995 (Issue 7, p. 24), addresses a critical practical problem in steel plant operations: the prevention of hot cracks during the surfacing repair of continuous casting rolls. Continuous casting is recognized as an effective route to increase steel output, reduce energy consumption, and improve yield ratios, making continuous casting rolls a key spare part whose high-temperature strength and service life directly influence casting productivity. In 1993, the author's factory introduced continuous casting roll repair technology from P.E. Company in the United States, along with surfacing equipment from Lincoln Electric, including flux-cored wires and fluxes.
Core Technical Content and Process Parameters
The continuous casting rolls in question are made of 15CrMo and 30CrMo steel, which are low-alloy heat-resistant steels widely used in high-temperature applications. The original surfacing process employed a two-layer approach:
| Parameter | Underlay Layer | Surface Layer |
|---|---|---|
| Consumable | Lincore 8620 (flux-cored wire, 3.2 mm) | Lincore 423Cr (flux-cored wire, 3.2 mm) |
| Flux | Lincoln 802 | Lincoln 802 |
| Welding Current | 400–450 A | 400–450 A |
| Welding Voltage | 30–32 V | 30–32 V |
| Wire Feed Speed | 120–130 mm/min | 120–130 mm/min |
The underlay layer serves as a transition zone between the base material (15CrMo/30CrMo) and the final surfacing layer, while the surface layer provides the functional properties required for continuous casting service, including resistance to thermal fatigue and thermal shock.
Analysis of Hot Crack Mechanisms
Hot cracking in surfacing welds on continuous casting rolls is primarily a solidification cracking phenomenon driven by the following mechanisms:
- Solute segregation at grain boundaries: During solidification, low-melting-point phases (such as sulfides and silicides) segregate to the intergranular regions of the weld metal, creating weak boundaries susceptible to cracking under tensile stress.
- Thermal strain: The rapid cooling of the surfacing deposit on a relatively massive roll body generates significant thermal contraction stresses that exceed the limited ductility of the interdendritic regions during solidification.
- High dilution from base metal: The high carbon and alloy content of 30CrMo base material can dilute into the weld metal, increasing susceptibility to cracking.
- Multiple thermal cycles: Sequential pass welding introduces repeated heating and cooling, which can cause re-melting of previously deposited metal and exacerbate cracking tendencies.
Countermeasures and Engineering Practice
Based on the literature and engineering experience, the following measures are recommended for hot crack prevention:
- Preheating: Applying preheat of 150–250°C to the roll surface reduces the cooling rate and thermal gradient, thereby decreasing solidification cracking susceptibility.
- Reduced heat input per pass: Lowering the welding current and increasing the travel speed reduces the total thermal input and minimizes the liquid fraction time during solidification.
- Interpass temperature control: Maintaining interpass temperatures between 150–250°C prevents excessive cooling between passes while avoiding re-heating of the previous pass into the brittle temperature range.
- Flux selection: Using fluxes with appropriate alkalinity and controlled sulfur/phosphorus content helps prevent low-melting-point phase formation at grain boundaries.
- Wire composition optimization: Selecting flux-cored wires with controlled S, P, and C content, and adequate Mn and Si to tie up residual sulfur as MnS rather than FeS.
- Post-weld heat treatment: A low-temperature stress relief anneal (300–400°C) after surfacing can relieve residual stresses and improve crack resistance.
Study Insights and Implications
This paper, though published in 1995, remains relevant because the fundamental metallurgical mechanisms of hot cracking have not changed. The two-layer surfacing strategy—using a transition alloy (Lincore 8620) followed by a functional alloy (Lincore 423Cr)—is a classic approach to managing dilution effects and ensuring compatibility between base and deposit. The practical value of this work lies in its demonstration that systematic control of consumable selection, welding parameters, and thermal management can effectively prevent hot cracking in demanding repair applications. For modern continuous casting operations, where roll life directly impacts plant availability and productivity, the lessons from this literature translate directly into current repair protocols. Engineers should note that the specific consumable designations (Lincore 8620, Lincore 423Cr, Lincoln 802) may have been superseded by newer formulations, but the underlying principles of alloy design and thermal management remain universally applicable.