CLADDING TECHNOLOGY SHANXI CO., LTDEffect of D212 Electrode Overlay Process Parameters on Overlay Layer Microstructure and Performance
Literature Overview
The paper by Luo Hui, Zhang Yuanbin, and Tang Linlin from the School of Materials Science and Engineering, Shandong Jianzhu University, published in 2009 in the journal Hot Working Technology, investigates the influence of D212 electrode overlay welding process parameters on the microstructure and mechanical properties of the overlay layer. This research was supported by the Shandong Provincial Science and Technology Outstanding Youth Fund (2007BS04010). D212 is a widely used hardfacing electrode in China, designed for depositing a chromium-carbide hard overlay on carbon steel or low-alloy steel substrates, commonly applied in mining, cement, and material handling industries where severe abrasive wear is encountered.
Core Technical Content
D212 Electrode Composition and Overlay Characteristics
D212 is a cast-iron type hardfacing electrode with a deposited layer composition typically containing 5-7% Cr, 1.5-2.5% C, and balanced Fe. The hard carbides formed are predominantly Cr7C3 and Cr23C6, providing high hardness (typically 58-65 HRC) and excellent abrasion resistance. The overlay layer is designed to be deposited on low-carbon steel substrates such as Q235, 20#, or 16Mn.
Process Parameters Investigated
The study examines how variations in welding current, arc voltage, welding speed, and inter-pass temperature affect the overlay microstructure and performance. Key process parameters typically considered in D212 overlay welding include:
| Process Parameter | Typical Range | Effect on Overlay |
|---|---|---|
| Welding current | 100-200 A (for 4.0 mm diameter electrode) | Higher current increases dilution and reduces carbide formation |
| Arc voltage | 22-30 V | Affects bead width and penetration |
| Welding speed | 80-200 mm/min | Higher speed reduces heat input and dilution |
| Inter-pass temperature | Below 150°C | Prevents excessive grain growth in previous layers |
| Number of passes | 2-4 layers | Multi-pass reduces overall dilution |
Microstructural Analysis
The overlay layer microstructure consists of hard carbides embedded in a pearlitic-ferritic matrix. The primary carbides are Cr7C3, which appear as polygonal or dendritic particles in the microstructure. The morphology and distribution of these carbides are strongly influenced by the cooling rate, which in turn is governed by the welding process parameters.
At higher welding currents, increased heat input leads to slower cooling rates, which promotes the formation of larger, coarser carbides and increases the dilution ratio of the base metal into the overlay. This results in reduced hardness and a decrease in the volume fraction of hard carbides. Conversely, lower currents and higher welding speeds produce finer carbide distributions and higher hardness values but may introduce more residual stresses and increase the risk of cracking.
Standards and Quality Considerations
The overlay welding process must comply with relevant standards including NB/T 47014 (qualification of welding procedures and welders for pressure vessels) and ASME Section IX. Key quality criteria for D212 overlay include:
- Hardness: The overlay layer should achieve a minimum of 58 HRC for satisfactory abrasion resistance.
- Dilution: The base metal dilution should be controlled below 30% to maintain adequate carbide content.
- Bond strength: The interface between the overlay and the substrate must withstand tensile and shear loading without delamination.
- Crack resistance: The overlay should be free from transverse cracks, which are common in high-carbon overlay welds due to the high carbon equivalent of the deposited material.
Engineering Practice Insights
In practical application, D212 overlay welding is frequently used for repairing or protecting components such as ball mill liners, conveyor rollers, excavator bucket teeth, and pump impellers. A critical engineering challenge is managing the residual stress in the overlay layer, which can lead to spalling under cyclic loading. Post-weld stress relief at 600-650°C for 1-2 hours per 25 mm thickness is often recommended, though care must be taken not to exceed temperatures that would cause carbide coarsening or temper softening.
A common defect encountered in D212 overlay is the formation of transverse cracks in the final pass. This occurs because the high-carbon deposited material has limited ductility, and the thermal contraction during cooling generates tensile stresses that exceed the crack initiation threshold. Countermeasures include using a backing layer of a more ductile material (such as D206 or D207), reducing the welding current in the final pass, and applying a slightly convex bead profile to reduce stress concentration at the edges.
Key Reflections and Study Insights
The fundamental trade-off in D212 overlay welding is between hardness (which requires high carbide content and low dilution) and crack resistance (which requires lower carbon activity and more ductility). This study highlights that optimizing process parameters—particularly maintaining moderate heat input and controlling inter-pass temperature—is essential for achieving both high hardness and acceptable integrity. Engineers should also consider the deposition sequence: a two-layer approach with a transition layer followed by the D212 hardfacing layer provides better overall performance than a single thick D212 deposit. The research contributes valuable data for welding procedure specification (WPS) development and welder qualification testing in industrial hardfacing applications.