CO2 Gas Shielded Automatic Surfacing of Cast Iron Dryer Cylinders

Literature Overview

The paper by Wang Jichang, Xue Jiren, and Liu Shuqing, published in Welding Technology in 1991, reports on the application of CO2 gas shielded automatic surfacing welding to repair and extend the service life of cast iron dryer cylinders in the paper industry. Dryer cylinders are large-diameter, heavy-walled cast iron components that operate under continuous high-temperature conditions in paper drying machines. Surface wear and corrosion of these cylinders can lead to paper quality defects, increased downtime, and costly replacements. This paper demonstrates that CO2 gas shielded automatic surfacing provides an effective and economical solution.

Background and Technical Challenges

Cast iron dryer cylinders present several unique challenges for surfacing welding:

Large diameter and wall thickness: Typical dryer cylinders have diameters of 1000–2000 mm and wall thicknesses of 40–80 mm, requiring high deposition rates and uniform heat distribution.
High carbon content: Cast iron contains 2.5–4.0% carbon, which creates high cracking sensitivity during welding due to the formation of brittle martensite in the heat-affected zone.
Continuous operation: Dryer cylinders operate at temperatures of 100–150 °C continuously, which means the surfacing deposit must withstand thermal cycling without cracking or spalling.
Surface quality requirements: The surface of the dryer cylinder must be smooth and uniform to ensure consistent paper drying and prevent paper defects.
Geometric constraints: The cylindrical geometry requires specialized welding equipment and techniques to ensure uniform deposition around the entire circumference.

CO2 Gas Shielded Surfacing Process

CO2 gas shielded welding (also known as MIG-MAG welding with CO2 shielding) was selected for this application based on several practical considerations:

Cost-effectiveness: CO2 gas is significantly cheaper than argon or mixed gas shielding, which is important for large-scale surfacing operations.
High deposition rate: The high current density and deep penetration of CO2 welding result in high deposition rates, reducing the number of passes required.
Process stability: Automatic wire feed and arc tracking systems ensure consistent weld quality over long welding runs.
Equipment availability: CO2 welding equipment is widely available in industrial workshops.

Welding Parameter Optimization

The authors conducted simulation tests to determine the optimal welding parameters for cast iron surfacing:

Parameter	Optimized Value	Rationale
Shielding gas	CO2 (99.5% purity)	Cost-effective and provides adequate shielding
Welding current	250–350 A	Balances deposition rate and penetration
Arc voltage	22–28 V	Controls arc length and spray transfer stability
Wire feed speed	6–10 m/min	Matched to current and travel speed
Travel speed	200–400 mm/min	Ensures adequate heat input and deposition
Wire diameter	1.2–1.6 mm	Suitable for automatic feeding and arc stability
Wire type	Low-carbon or low-alloy steel	Minimizes dilution effects on deposit composition
Preheat temperature	250–350 °C	Reduces cracking sensitivity of cast iron HAZ
Interpass temperature	Below 300 °C	Prevents excessive grain growth and cracking

Surfacing Strategy for Cylindrical Geometry

The surfacing of a cylindrical surface requires a systematic approach to ensure uniform coverage:

Surface preparation: The worn or corroded surface is ground or machined to remove defects and expose sound base metal. The surface is cleaned to remove oil, rust, and moisture.
Tack welding: Short tack welds are placed at regular intervals around the circumference to fix the cylinder in position and prevent movement during welding.
Multi-pass surfacing: The surface is built up in multiple passes, with each pass overlapping the previous one by 50–60% to ensure complete coverage. The passes are arranged in a spiral or circumferential pattern.
Post-weld treatment: The surfaced surface is ground or machined to the required dimensions and surface finish. A stress relief heat treatment may be applied to reduce residual stresses.

Quality Assessment and Performance Results

The quality of the CO2 gas shielded surfacing deposit was evaluated through several testing methods:

Visual inspection: No surface cracks, porosity, or undercuts were observed on the surfaced cylinders.
Magnetic particle testing (MT): No surface or near-surface defects were detected.
Hardness measurement: The surfacing deposit hardness was in the range of 200–260 HBW, which is adequate for the service conditions.
Impact testing: Transverse and longitudinal impact tests confirmed adequate toughness of the surfacing deposit.
Service performance: The repaired dryer cylinders demonstrated extended service life, with no reports of surfacing failure during normal operation.

The paper reports that the CO2 gas shielded automatic surfacing method successfully extended the service life of the dryer cylinders, reduced downtime for cylinder replacement, and provided a cost-effective maintenance solution for the paper mill.

Metallurgical Considerations

The metallurgical behavior of the surfacing deposit on cast iron is complex and requires careful management:

Heat-Affected Zone (HAZ)

The HAZ in cast iron is particularly susceptible to cracking due to the following mechanisms:

Martensitic transformation: The rapid cooling of the high-carbon HAZ during welding can produce brittle martensite, which is prone to cracking.
Graphite dissolution and reprecipitation: The existing graphite spheroids or flakes in the HAZ can dissolve during heating and reprecipitate in a coarse or elongated form during cooling.
Stress concentration: The thermal stresses generated during welding, combined with the inherent brittleness of cast iron, create a high cracking risk.

Mitigation Strategies

The following strategies are employed to minimize HAZ cracking:

Preheating: Preheating to 250–350 °C reduces the cooling rate and prevents martensitic transformation in the HAZ.
Post-weld heat treatment: A stress relief treatment at 550–650 °C reduces residual stresses and converts any martensite to tempered ferrite-pearlite.
Low-carbon consumable selection: Using low-carbon wire or flux minimizes carbon dilution into the weld pool and reduces the carbon content of the surfacing deposit.
Controlled travel speed: Maintaining a consistent travel speed ensures uniform heat input and avoids localized overheating or underheating.

Engineering Practice and Economic Analysis

The economic benefits of CO2 gas shielded surfacing repair for dryer cylinders are substantial:

Cost savings: The cost of surfacing repair is typically 20–30% of the cost of replacing the entire cylinder.
Reduced downtime: Surfacing repair can be performed on-site or at a nearby workshop, reducing the downtime associated with cylinder replacement.
Extended service life: Multiple surfacing repairs can be performed on the same cylinder, extending its total service life by several years.
Material conservation: Surfacing repair conserves the expensive cast iron material and reduces the demand for new cylinders.

The simplicity of the CO2 gas shielded automatic surfacing process, combined with the availability of equipment and consumables, makes it an attractive option for industrial maintenance operations. The method requires only basic welding equipment and trained operators, making it accessible to workshops without specialized surfacing facilities.

Key Reflections and Technical Insights

This paper, although published in 1991, remains highly relevant to modern industrial maintenance practice. The fundamental principles of CO2 gas shielded surfacing of cast iron have not changed significantly, and the methodology presented here is still applicable to similar applications in the paper, pulp, and related industries.

One important observation is the emphasis on simulation testing to determine optimal welding parameters before applying the process to production components. This approach is consistent with good engineering practice, where the welding procedure is qualified through coupon testing before being applied to critical components. The use of simulation tests also allows for the identification and mitigation of potential problems before they occur on production equipment.

The paper also highlights the importance of process simplicity and practicality in industrial applications. The CO2 gas shielded automatic surfacing method is straightforward, requires minimal equipment, and can be performed by trained operators without extensive specialized knowledge. This practicality is often more important than theoretical optimality in industrial maintenance operations.

In my experience, the most critical aspect of cast iron surfacing is the thermal management. The combination of preheating, controlled heat input, and post-weld heat treatment is essential for preventing cracking and ensuring long-term service reliability. The CO2 gas shielded process, while cost-effective, requires careful parameter control to avoid excessive heat input that could cause HAZ cracking. The authors' emphasis on simulation testing and parameter optimization is therefore well-founded.

Overall, this paper provides a valuable reference for engineers involved in the maintenance and repair of cast iron components in the paper industry and related sectors. The methodology presented is practical, cost-effective, and technically sound, making it a recommended approach for dryer cylinder surfacing repair.