Zhuojin Pipe Fitting Co., LtdOverlay Welding Process Study for Gray Cast Iron
Literature Overview
This paper by Liu Cheng, Li Jiangong, and Liu Huipeng, published in Hot Working Technology in 2013, investigates the overlay welding repair process for a worn winch drum made of HT200 gray cast iron. The research was supported by the Hebei Provincial Department of Education Research Project (Z2011296). The study developed a three-layer overlay welding approach using self-developed flux-cored welding wires with CO₂ gas shielded welding, and systematically evaluated six wire combinations through microstructural analysis, hardness testing, and wear testing to determine the optimal combination.
Technical Background
Gray cast iron, represented by the HT200 grade in this study, is widely used in machinery components due to its excellent castability, vibration damping, and wear resistance. However, gray cast iron has inherently poor weldability due to its high carbon content (2.5–4.0%), the presence of graphite flakes that disrupt the microstructure, and the tendency to form brittle white cast iron in the heat-affected zone.
Winch drums are subjected to significant wear from rope contact, and when worn beyond acceptable limits, they require repair or replacement. Overlay welding provides a cost-effective repair method that can restore the surface dimensions and improve wear resistance simultaneously.
Three-Layer Overlay Welding Strategy
The study employs a three-layer approach, which is a standard strategy for welding gray cast iron:
| Layer | Function | Wire Composition | Purpose |
|---|---|---|---|
| Base layer (打底層) | Metallurgical transition | High Mn, high Si alloy steel flux-cored wire | Improve fusion, plasticity, and toughness at the interface |
| Transition layer (過渡層) | Property gradient | Low carbon, low Cr flux-cored wire | Bridge the property difference between base and wear layer |
| Wear layer (耐磨層) | Surface protection | High carbon, high Cr cast iron type flux-cored wire | Form hard carbide microstructure for wear resistance |
Welding Process Parameters
The CO₂ gas shielded welding process (GMAW with CO₂ shielding) was selected for its:
- High deposition rate compared to SMAW.
- Good arc stability and penetration control.
- Relatively low cost of consumables.
- Suitability for automated or semi-automated operation.
Metallurgical Analysis
Base Metal Characteristics
HT200 gray cast iron has the following characteristics:
- Carbon equivalent: CE = C + Si/3 ≈ 3.0–3.5%, indicating high crack susceptibility.
- Microstructure: Ferrite-matrix with flake graphite.
- Tensile strength: ≥ 200 MPa.
- Poor ductility and toughness.
Base Layer Metallurgy
The high Mn, high Si alloy steel flux-cored wire for the base layer serves multiple functions:
- Manganese acts as a deoxidizer and helps dissolve carbon in the dilution zone, reducing white cast iron formation.
- Silicon also acts as a deoxidizer and promotes the formation of pearlite rather than cementite.
- The alloy composition promotes a ductile microstructure at the interface, accommodating the thermal stresses generated during welding.
Transition Layer Metallurgy
The low carbon, low Cr flux-cored wire for the transition layer provides:
- A gradual transition in carbon content from the base metal to the wear layer.
- Adequate toughness to resist cracking under thermal cycling.
- Sufficient hardness to provide some wear resistance while maintaining ductility.
Wear Layer Metallurgy
The high carbon, high Cr cast iron type flux-cored wire for the wear layer forms:
- Hard carbide phases such as M₇C₃, M₃C, and M₂₃C₆.
- A martensitic or austenitic matrix depending on the cooling rate and composition.
- Hardness in the range of 50–70 HRC, providing excellent abrasion resistance.
Six Wire Combinations Evaluation
The study evaluated six combinations of the three wire types through:
| Evaluation Method | Parameter Measured | Acceptance Criteria |
|---|---|---|
| Microstructural examination | Phase composition, carbide morphology | No brittle phases, uniform carbide distribution |
| Hardness testing | Vickers or Rockwell hardness profile | Wear layer ≥ 50 HRC, base layer ≤ 30 HRC |
| Wear testing | Weight loss under standardized abrasion | Minimum wear rate for the wear layer |
Key Findings and Optimal Combination
The optimal wire combination was determined by balancing the following criteria:
- Hardness gradient: A smooth transition from the base metal hardness to the wear layer hardness, avoiding abrupt changes that could lead to cracking.
- Microstructural uniformity: No segregation of brittle phases, no excessive grain growth, and uniform carbide distribution.
- Wear resistance: The wear layer must achieve the target hardness and maintain it under service conditions.
- Crack resistance: The entire weld cross-section must be free of cracks, particularly at the base metal/weld interface.
Engineering Practice and Quality Control
Pre-Weld Preparation
- Surface preparation: Grind the worn surface to remove all damaged material and expose fresh metal. Clean thoroughly to remove dust, oil, and moisture.
- Preheating: Preheat the HT200 base metal to 200–300°C to reduce thermal stress and slow the cooling rate. For thicker sections, higher preheat temperatures (up to 500°C) may be required.
- Interpass temperature: Maintain interpass temperature at or below 300°C to prevent excessive dilution and cracking.
Post-Weld Treatment
- Controlled cooling: Allow the weld to cool slowly, preferably by wrapping in insulating material or using a post-weld heating blanket. This reduces residual stresses and prevents cracking.
- Stress relief: For critical applications, perform a stress relief annealing at 500–550°C to relieve residual stresses without causing graphitization.
Quality Verification
- Visual inspection: Check for uniform bead profile, no undercut, no porosity, and good fusion.
- Hardness profile: Measure hardness across the weld cross-section to verify the gradient.
- Wear test coupon: Deposit a coupon using the same parameters and test under simulated service conditions.
- Service monitoring: Monitor the repaired winch drum during operation to verify the improvement in service life.
Study Insights and Conclusions
This study demonstrates the practical application of multi-layer overlay welding for gray cast iron repair. The three-layer approach, with carefully selected wire compositions for each layer, provides a systematic method to overcome the inherent weldability challenges of gray cast iron. The use of self-developed flux-cored wires offers the advantage of tailored compositions optimized for the specific application.
The systematic evaluation of six wire combinations through microstructural analysis, hardness testing, and wear testing exemplifies a rigorous engineering approach to process development. The results provide a validated process for winch drum repair that can be extended to other gray cast iron components requiring surface hardening or dimensional restoration.
The key engineering lesson from this study is that successful welding of gray cast iron requires a holistic approach: proper preheating, appropriate consumable selection, controlled cooling, and thorough quality verification. The three-layer strategy, with its gradual transition from ductile base metal to hard wear layer, is a proven methodology that balances the competing requirements of crack resistance and wear resistance. This research provides a valuable reference for maintenance engineers and welders working on gray cast iron repair applications in heavy industry.