Microstructure and Properties of D172 Electrode Overlay Layer on 45 Steel Substrate

Literature Overview and Research Background

The D172 electrode is a widely used hardfacing electrode in China, designed to produce a high-hardness overlay layer with excellent wear resistance on carbon and low-alloy steel substrates. This literature investigates the microstructure, mechanical properties, and wear performance of D172 electrode overlay layers deposited on 45 steel (a medium-carbon structural steel with 0.42–0.50 wt% C) substrates. The study is particularly relevant for field repair applications where arc welding with consumable electrodes remains the most practical and economical method for restoring worn components.

Core Technical Findings

The D172 electrode produces an overlay layer with a complex microstructure consisting of martensite, retained austenite, and a high density of hard carbides (primarily Cr₇C₃ and Fe₃C). The study examines the microstructural evolution and mechanical properties of the overlay layer as a function of welding parameters and substrate condition.

The microstructure of the D172 overlay layer on 45 steel features a dendritic martensite matrix with retained austenite (typically 5–15%) and dispersed Cr₇C₃ carbides. The retained austenite content is influenced by the cooling rate, which is affected by the substrate thermal conductivity and preheating temperature. Higher retained austenite content improves toughness but reduces hardness, while lower retained austenite increases hardness but may compromise ductility.

The hardness of the D172 overlay layer is primarily determined by the martensite structure and the volume fraction of hard carbides. The typical hardness range of 58–62 HRC is achieved through the combination of high carbon content in the overlay (0.8–1.2 wt%) and the formation of hard Cr₇C₃ carbides. The hardness distribution across the overlay thickness shows a gradient, with the top layer being harder than the lower layers due to the higher cooling rate at the surface.

Process Analysis and Parameter Optimization

The D172 electrode welding process parameters require careful optimization to achieve the desired microstructure and properties:

• Arc current: Lower currents (120–150 A) produce finer grain structures with higher hardness but may result in incomplete fusion. Higher currents (150–180 A) improve fusion but may increase dilution and reduce hardness.
• Travel speed: Slower travel speeds (30–50 mm/min) produce thicker deposits per pass but increase heat input and dilution. Faster speeds (50–80 mm/min) reduce dilution but may produce thinner deposits with more defects.
• Preheating: Preheating the 45 steel substrate to 150–250°C reduces the risk of cracking in the heat-affected zone and improves the toughness of the overlay layer. However, excessive preheating may reduce the hardness of the overlay by slowing the cooling rate.
• Interpass temperature: Maintaining interpass temperature below 200°C is critical for preserving the martensitic structure and achieving the target hardness. Higher interpass temperatures promote tempering and reduce hardness.

The dilution rate from the 45 steel substrate is a critical factor in determining the final overlay properties. For a single-pass deposit, the dilution rate is typically 20–30%, while for multi-pass deposits with a total thickness of 3–5 mm, the dilution rate decreases to 10–20% as the upper layers have less contact with the base metal. The dilution introduces additional carbon and alloying elements from the substrate, which can affect the carbide formation and martensite stability in the overlay.

Standards and Quality Control

For D172 electrode overlay applications on pressure vessel and heavy equipment components, the following standards and quality requirements apply:

• NB/T 47014: Qualification of welding procedures and welders for pressure equipment
• JB/T 4730: Non-destructive testing methods including MT, UT, and PT
• ASTM A263: Performance requirements for overlay welding
• GB/T 150: Pressure vessel design and fabrication standards
• GB/T 3375: Welding terminology and definitions

The quality control plan for D172 overlay welding should include:

1. Visual inspection: Check for surface porosity, slag inclusion, and incomplete fusion
2. Magnetic particle inspection (MT): Detect surface and near-surface cracks per JB/T 4730
3. Hardness testing: Verify hardness uniformity across the overlay surface (target 58–62 HRC)
4. Sectional examination: Verify dilution rate and microstructure at the overlay-substrate interface

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