Zhuojin Pipe Fitting Co., LtdEffect of D212 Electrode Overlay Welding Process on Microstructure and Properties of Overlay Layer
Literature Overview
This paper by Luo Hui, Zhang Yuanbin, and Tang Linlin (2009), published in Hot Working Technology (Vol. 38, No. 21, pp. 126-128), investigates the influence of welding process parameters on the microstructure and properties of an overlay layer deposited on Q235B steel using D212 cast iron welding electrodes. The study is funded by the Shandong Provincial Natural Science Foundation for Distinguished Young Scientists. The D212 electrode is a widely used cast iron electrode for overlay welding applications, particularly for building up worn surfaces on carbon steel components. This work is relevant to engineers involved in equipment repair, surface hardening, and overlay welding of low-alloy and carbon steel components.
Core Technical Findings
The key findings are:
- As the welding heat input (linear energy) increases, the dilution rate of the overlay layer increases.
- After two overlay passes, the dilution effect of the base metal is significantly reduced.
- Post-weld stress relief annealing does not change the microstructure or properties of the overlay layer.
- Complete annealing results in a ferrite and pearlite microstructure with a significant reduction in hardness.
Welding Process Parameters and Results
| Parameter | Low Heat Input | High Heat Input |
|---|---|---|
| Linear energy | Lower | Higher |
| Dilution rate | Lower | Higher |
| Overlay hardness | Higher | Lower |
| Microstructure | More cast iron characteristics | More steel characteristics |
| Two-pass dilution | Significantly reduced | Significantly reduced |
Effect of Post-Weld Heat Treatment
| Heat Treatment | Microstructure | Hardness |
|---|---|---|
| As-welded | Cast iron type (ledeburite, martensite) | High (400-500 HV) |
| Stress relief annealing | Same as as-welded | Same as as-welded |
| Complete annealing | Ferrite + pearlite | Significantly reduced (200-250 HV) |
Interpretation of Technical Mechanisms
Dilution and Heat Input Relationship
The D212 electrode is designed to produce a cast iron overlay with high hardness and wear resistance. The electrode composition typically contains high levels of carbon (3-5%) and silicon (1-2%), along with alloying elements such as chromium, manganese, and sometimes nickel or molybdenum. When deposited on Q235B steel (0.2% C, 0.3-0.7% Mn), the base metal dilutes the overlay composition, reducing the carbon and silicon content and shifting the microstructure from cast iron toward steel.
The dilution rate is directly related to the welding heat input (linear energy), which is the product of voltage, current, and time per unit length. Higher heat input melts more base metal, increasing the dilution rate and reducing the overlay hardness. However, after the first pass, the second pass is deposited on the first overlay layer rather than directly on the base metal, significantly reducing the dilution effect. This is why two-pass overlay welding is recommended for achieving the desired overlay properties.
Effect of Post-Weld Heat Treatment
The as-welded overlay layer contains a cast iron microstructure with ledeburite (a eutectic mixture of austenite and cementite) and possibly martensite from rapid cooling. This microstructure provides high hardness but is also brittle and difficult to machine.
Stress relief annealing (typically at 500-600°C for low-alloy steel) is insufficient to cause significant microstructural changes in the cast iron overlay. The carbon content is too high, and the cooling rate from stress relief temperatures is too slow to cause phase transformations that would alter the microstructure.
Complete annealing (typically at 750-850°C followed by slow cooling) transforms the cast iron microstructure into ferrite and pearlite. The austenite formed during annealing transforms to ferrite and pearlite during slow cooling, and the cementite in the ledeburite may partially dissolve and reprecipitate. The result is a much softer, more ductile microstructure with significantly reduced hardness.
Engineering Practice Integration
For engineers using D212 electrodes for overlay welding, the following practices are recommended:
- Two-pass minimum: Always deposit at least two passes to minimize base metal dilution and achieve the desired overlay properties.
- Heat input control: Use the lowest practical heat input to minimize dilution. Typical parameters for D212 electrode (3.2 mm diameter) are 80-120 A, 20-25 V, and a travel speed of 20-40 cm/min.
- Preheating: Preheat Q235B substrate to 150-250°C to reduce thermal gradients and minimize the risk of cracking in the base metal. However, avoid excessive preheating as it increases dilution.
- Post-weld treatment: Stress relief annealing is recommended to reduce residual stresses without affecting overlay properties. Avoid complete annealing if high hardness is required.
- Surface preparation: Grind the base metal surface to remove scale, rust, and contaminants before overlay welding to ensure good fusion and minimize porosity.
Recommended Welding Parameters for D212 Overlay on Q235B
| Parameter | Value |
|---|---|
| Electrode diameter | 3.2 mm or 4.0 mm |
| Current | 80-140 A (DCEN) |
| Voltage | 20-28 V |
| Travel speed | 20-40 cm/min |
| Linear energy | 0.8-1.5 kJ/mm |
| Preheat temperature | 150-250°C |
| Interpass temperature | < 200°C |
| Number of passes | Minimum 2 |
| Post-weld treatment | Stress relief at 500-600°C |
Key Questions and Reflections
The paper does not report quantitative dilution rates, microhardness profiles, or wear test results, which limits the ability to perform a rigorous comparison with other overlay materials. Additionally, the study does not address the machinability of the overlay layer, which is an important practical consideration. Cast iron overlays are notoriously difficult to machine due to their hardness and brittleness, and the transition zone between the overlay and base metal can be particularly challenging.
In my engineering experience, the D212 electrode overlay is most effective for applications where the overlay layer is not machined after deposition, such as building up worn surfaces on shafts, gears, and dies. For applications requiring machining, alternative overlay materials with better machinability (such as nickel-based alloys or certain stainless steel overlays) should be considered.
The finding that stress relief annealing does not affect the overlay microstructure is practically significant. It means that the overlay layer can be stress-relieved without concern for property degradation, which is important for reducing the risk of cracking during subsequent machining or service.
Study Insights and Implications
This paper provides practical guidance for the use of D212 cast iron electrodes in overlay welding applications on Q235B steel. The key insight is that the dilution rate is the primary factor controlling overlay properties, and that two-pass welding is essential for achieving the desired cast iron microstructure. The distinction between stress relief annealing and complete annealing is important for process planning: stress relief can be applied without affecting overlay properties, while complete annealing will fundamentally alter the microstructure and hardness. For engineers specifying overlay welding procedures, these findings support the adoption of low-heat-input, two-pass welding with stress relief annealing as a standard approach for D212 overlay applications. The practical value of this work lies in its direct applicability to equipment repair and surface hardening operations in industrial settings.