CO2 Gas-Shielded Automatic Surfacing of Cast Iron Dryer Cylinder

Literature Overview

The paper published in Welding Technology (1991, Vol. 20, No. 5, pp. 2–4) by Wang Jichang, Xue Jiren, and Liu Shuqing presents an early and pioneering application of CO2 gas-shielded automatic surfacing welding to repair and extend the service life of cast iron dryer cylinders used in the paper industry. The authors conducted systematic simulation experiments to determine optimal process parameters and demonstrated that the resulting surfaced cylinders exhibited reliable quality and extended operational life.

Application Background

Cast iron dryer cylinders are large-diameter rotating components used in paper-making machines to dry paper webs through contact heating. These cylinders are typically made of gray cast iron (HT200 or equivalent) and are subjected to:

Continuous thermal cycling (operating temperatures of 100–150°C)
Mechanical loading from paper web tension
Corrosive attack from moisture and acidic paper solutions
Abrasive wear from paper web contact

The typical failure modes include surface cracking, corrosion pitting, and material loss at high-temperature zones. Replacement of entire cylinders is extremely costly due to their large size (typically 1.5–2.5 m diameter, 4–8 m length) and the specialized manufacturing requirements.

CO2 Gas-Shielded Surfacing Process

The CO2 gas-shielded arc welding process (GMAW with CO2 shielding) was selected for several practical reasons:

Equipment availability: CO2 welding equipment was already widely deployed in industrial workshops
Cost-effectiveness: CO2 shielding gas is inexpensive and readily available
Process flexibility: Suitable for large-area surfacing of curved surfaces
Deposition efficiency: High deposition rates suitable for thick surfacing layers

Optimized Process Parameters

Parameter	Value	Rationale
Shielding gas	Pure CO2	Available, cost-effective
Wire type	H08Mn2SiA or equivalent	Good weldability with cast iron
Wire diameter	φ1.2 mm	Balance of deposition and control
Welding current	250–320 A	Adequate penetration without excessive dilution
Arc voltage	24–28 V	Stable arc, minimal spatter
Travel speed	300–500 mm/min	Controlled heat input
Nozzle diameter	φ16–φ20 mm	Adequate gas coverage
Gas flow rate	15–20 L/min	Prevents atmospheric contamination
Preheat temperature	300–400°C	Reduces cracking tendency
Interpass temperature	≤400°C	Controls HAZ hardness

Cast Iron Weldability Challenges

Welding cast iron presents unique metallurgical challenges that must be addressed:

Common Defects and Countermeasures

Defect Type	Cause	Countermeasure
Cracking	High carbon content, brittle matrix	Preheating, low carbon filler, post-weld heat treatment
White cast	Rapid cooling, carbon precipitation as cementite	Preheating, controlled cooling, iron powder flux
Porosity	Gas evolution from carbon-gas reaction	Clean surface, adequate gas coverage
Excessive dilution	High carbon transfer to weld	Low carbon filler selection
Distortion	Thermal expansion mismatch	Sequential welding pattern, back-step welding

Metallurgical Control Strategy

The key to successful CO2 surfacing of cast iron lies in controlling the carbon activity at the weld pool:

Preheating to 300–400°C: Reduces the thermal gradient and slows the cooling rate, allowing carbon to precipitate as graphite rather than cementite.
Low-carbon filler metal: Using low-carbon wire (H08Mn2SiA) limits the carbon content of the weld metal, reducing the tendency for white cast formation.
Controlled cooling: The thick surfacing layers provide thermal mass that moderates cooling rates.

Performance Results

The surfaced dryer cylinders demonstrated the following improvements:

Surface hardness: 180–220 HV (compared to 150–180 HV for worn original surface)
Crack-free welds after thermal cycling tests
Extended service life of 2–3 times compared to un-repaired cylinders
Acceptable surface finish for paper-making applications after light machining

The process was validated through both laboratory simulation tests and field trials on actual dryer cylinders at the Dandong Paper Mill.

Engineering Practice and Process Development

The systematic approach adopted in this study follows a classic process development methodology:

Simulation testing: Parameter optimization conducted on test coupons representing the cylinder material and geometry
Full-scale validation: Application to actual production cylinders with monitoring of weld quality
Service evaluation: Long-term tracking of repaired cylinders in operational conditions

The process is particularly well-suited to the industrial environment because:

It requires no specialized equipment beyond standard CO2 welding machines
The process can be performed on-site without dismantling the cylinder from the machine
The parameters are robust and tolerant of minor variations
The results are immediately verifiable through visual and dimensional inspection

Study Insights

This 1991 publication represents an important milestone in the application of gas-shielded welding to cast iron repair applications. The selection of CO2 shielding—rather than the more expensive argon or helium mixtures—demonstrates a pragmatic engineering approach that prioritizes cost-effectiveness without sacrificing quality. The success of this application owes much to the careful attention paid to preheating and cooling control, which are the critical factors in preventing cracking and white cast formation in cast iron welds.