CO2 Gas-Shielded Automatic Cladding of Cast Iron Drying Cylinders

Literature Overview

This paper, published in Welding Technology in 1991 by Wang Jichang, Xue Jiren, and Liu Shuqing from Dalian Institute of Railway Technology and Dandong Paper Mill, documents the application of automatic CO2 gas-shielded arc welding (GMAW) for cladding repair of cast iron drying cylinders in the paper manufacturing industry. Drying cylinders are large-diameter, thin-walled cast iron components that play a critical role in the paper drying process by conducting hot steam to evaporate moisture from the paper web. Damage to the cylinder surface, including wear grooves, steam leakage cracks, and corrosion pits, can lead to paper quality defects, energy waste, and potential safety hazards.

The paper represents an early and pioneering application of automated welding technology for the repair of large industrial cast iron components. The use of CO2 as a shielding gas, combined with automatic wire feeding, represents a cost-effective and practical approach to cladding repair that was particularly suitable for the industrial conditions of the time. The research collaboration between an academic institution and an industrial user demonstrates the practical orientation of the work and its direct relevance to industrial maintenance needs.

Technical Challenges of Cast Iron Drying Cylinder Cladding

Cast iron drying cylinders present several unique challenges for cladding repair:

The high carbon equivalent of cast iron is the primary metallurgical challenge. During welding, the rapid heating and cooling of the heat-affected zone (HAZ) can produce a hard, brittle martensitic microstructure that is highly susceptible to cracking. The graphite flakes in the cast iron matrix further exacerbate this problem by acting as crack initiation sites and crack propagation paths.

Process Design and Parameter Optimization

The automatic CO2 GMAW cladding process was designed with careful attention to controlling heat input and minimizing the risk of cracking. The process parameters were optimized through systematic trial welding and microstructural analysis:

The use of a low-carbon filler wire (H08Mn2SiA) with a carbon content below 0.1% is critical for minimizing the carbon equivalent of the weld metal and reducing the risk of cold cracking. The manganese and silicon in the filler wire provide adequate strength and hardness while maintaining good ductility.

Multi-Pass Cladding Strategy

Given the typical depth of wear grooves or corrosion damage on drying cylinders (5-15 mm), a multi-pass cladding strategy is required. The process typically involves:

1. Surface preparation: Grinding the damaged area to remove all defective material, creating a smooth, uniform surface with a slight concave profile to improve fusion.
2. First pass (root pass): A shallow pass with low current and high travel speed to establish fusion with the base metal while minimizing heat input.
3. Subsequent passes: Progressive buildup with slightly increased current and reduced travel speed to achieve the required overlay thickness.
4. Final pass (cap pass): A finishing pass with optimized parameters to achieve a smooth, uniform surface profile.

The total number of passes depends on the depth of the damage and the desired overlay thickness. For typical drying cylinder repairs, 3-5 passes are usually sufficient to achieve an overlay thickness of 5-10 mm.

Microstructural Analysis and Defect Prevention

Metallographic examination of the clad repair area reveals a multi-layer microstructure. The weld metal consists of a ferritic-pearlitic matrix with fine grain structure, achieving a hardness of 180-220 HV. The HAZ exhibits a transition from the original cast iron microstructure to a partially transformed structure, with hardness typically in the range of 250-350 HV. The critical zone is the weld metal-base metal fusion line, where the microstructure transitions from the ductile weld metal to the brittle cast iron HAZ.

The primary defects to be controlled include:

Engineering Practice and Industrial Implementation

The industrial implementation of this cladding technology at Dandong Paper Mill demonstrated several practical advantages. The automatic welding setup, consisting of a wire feed mechanism, torch travel system, and gas supply, was mounted on a simple rotating fixture that allowed the torch to travel circumferentially around the drying cylinder. This automation ensured consistent weld quality and reduced operator fatigue compared to manual welding.

The repair turnaround time was significantly reduced compared to the previous practice of replacing damaged cylinders. A typical cylinder repair that previously required complete replacement (with a lead time of 3-6 months) could now be completed in 2-3 days, including preparation, welding, inspection, and reinstallation. The cost savings were substantial, with repair costs estimated at less than 10% of the cost of a new cylinder.

Post-repair inspection included visual examination, magnetic particle testing (MT) for surface cracks, and hydrostatic pressure testing at 1.5 times the design steam pressure to verify structural integrity. The repaired cylinders demonstrated reliable service performance with no reported failures during the subsequent 12-month follow-up period.

Key Technical Insights and Reflections

This paper is historically significant as one of the early documented applications of automated CO2 GMAW for cast iron repair. The use of CO2 as a shielding gas, while not the most metallurgically favorable choice (pure argon or argon-helium mixtures provide better arc stability and reduced spatter), was a pragmatic decision driven by cost considerations and equipment availability. The results demonstrate that with proper parameter optimization, CO2-shielded welding can produce acceptable weld quality on cast iron substrates.

The emphasis on low heat input and controlled thermal cycling is consistent with the fundamental principle of cast iron welding: minimize the extent and severity of microstructural changes in the HAZ. The combination of preheating, low-carbon filler metal, and controlled interpass temperature represents a well-established strategy for managing the high carbon equivalent of cast iron.

The automation aspect of the process is particularly noteworthy. Automated welding ensures parameter consistency, reduces operator variability, and enables the repair of large-diameter components that would be difficult to weld manually. The rotating fixture concept is simple but effective, and could be adapted for other cylindrical components such as shafts, rollers, and pipes.

Reference Value and Outlook

This paper provides a valuable historical reference for the application of automated welding technology in industrial repair applications. The process parameters and quality control measures documented here remain relevant for similar repair applications, although modern practice would likely incorporate improved filler metals, advanced monitoring systems, and more sophisticated process control. The fundamental principles of low heat input, controlled thermal cycling, and thorough surface preparation remain valid and continue to guide engineering practice in the welding of high-carbon materials.