Heat-Affected Zone Microstructure Evolution in Arc Overlay Welding of Ductile Iron

Literature Overview

This 2022 study by Zeng Daxin, He Zhongpu, and Shi Qiuyue from Hubei University of Automotive Technology investigates the microstructure evolution in the heat-affected zone (HAZ) of ductile iron subjected to arc overlay welding with iron-based alloys. Published in the journal Foundry (Volume 71, Issue 10, 2022, pages 1209–1215), this research provides a detailed metallographic and analytical characterization using optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The work addresses a critical and often poorly understood aspect of overlay welding on cast iron substrates: the complex microstructural transformations that occur in the HAZ during the welding thermal cycle.

Ductile Iron Weldability Challenges

Ductile iron (nodular cast iron) presents unique challenges for welding and overlay welding due to its microstructure, which consists of a ferritic, pearlitic, or ferrite-pearlite matrix with graphite nodules dispersed throughout. The graphite nodules are the primary source of welding difficulty because:

• Graphite burns off during melting: At welding temperatures (1500–1700°C), graphite oxidizes and escapes as CO/CO₂ gas, creating porosity and reducing carbon content in the melt.
• Carbon concentration gradients: The vicinity of graphite nodules is carbon-depleted, creating microstructural heterogeneity in the HAZ.
• Thermal cycling effects: The rapid heating and cooling of the welding thermal cycle causes multiple phase transformations that are strongly influenced by local carbon concentration.

The carbon equivalent of ductile iron typically ranges from 3.5% to 4.0%, far exceeding that of steel. This extremely high carbon content, combined with the presence of graphite, creates a weldability challenge that is fundamentally different from steel welding.

HAZ Zone Classification and Microstructural Evolution

The authors identified two distinct HAZ zones in the ductile iron overlay weld: the partially melted zone (PMZ) and the austenitization zone (AZ). This classification is critical for understanding the welding defects and mechanical properties of the repaired component.

Partially Melted Zone (PMZ)

The PMZ forms due to non-equilibrium melting that occurs when graphite dissolves into the iron matrix under rapid heating conditions. Unlike equilibrium melting, which requires the temperature to reach the solidus line, the PMZ forms because the carbon diffusion from graphite nodules to the surrounding matrix is kinetically limited under the rapid heating rates of arc welding.

The key metallurgical events in the PMZ are:

1. Rapid heating: The welding thermal cycle heats the HAZ at rates of 100–1000°C/s, depending on the welding process and parameters.
2. Graphite dissolution: Carbon from graphite nodules diffuses into the surrounding iron matrix, creating a locally carbon-enriched zone.
3. Non-equilibrium melting: When the local carbon concentration reaches the eutectic composition, melting occurs at temperatures below the equilibrium solidus temperature. This melting is localized around the graphite nodules and does not require the entire region to reach the liquidus temperature.
4. Rapid solidification: Upon cooling, the melt solidifies rapidly, forming a distinctive double-shell morphology: an inner austenite layer adjacent to the graphite nodule and an outer ledeburite layer.

The double-shell structure — austenite (γ) inner shell and ledeburite (γ + Fe₃C) outer shell — is a signature microstructure of the PMZ in ductile iron welding. The ledeburite layer is extremely hard and brittle, with hardness values typically exceeding 1000 HV, making it a crack initiation site under tensile or cyclic loading.

Austenitization Zone (AZ)

The AZ forms at locations farther from the weld where the peak temperature exceeds the A₁ transformation temperature (approximately 727°C) but does not reach the melting range. The microstructural evolution in the AZ depends on the original matrix microstructure of the ductile iron:

For pearlitic or ferrite-pearlite ductile iron:

• During heating, pearlite and the ferrite surrounding graphite nodules transform to austenite.
• The austenite near graphite nodules is carbon-enriched (due to carbon diffusion from graphite), while austenite farther away is carbon-depleted.
• Upon cooling, the carbon-enriched austenite near graphite nodules transforms to martensite, while the carbon-depleted austenite farther away transforms to ferrite or pearlite.
• The resulting microstructure is a gradient: martensite near graphite nodules, transitioning to ferrite or pearlite farther from the weld.

For ferritic ductile iron:

• During heating, the ferrite surrounding graphite nodules transforms to carbon-enriched austenite, while ferrite far from graphite nodules either transforms to carbon-poor austenite or remains unchanged.
• Upon cooling, carbon-enriched austenite transforms to martensite, while carbon-poor austenite transforms back to ferrite.
• The resulting microstructure is: martensite near graphite nodules, with the remainder being ferrite.

HAZ Zone Comparison

Element Segregation and EDS Analysis

The EDS analysis revealed significant carbon and alloy element segregation in the HAZ. Carbon concentration increases dramatically near graphite nodules in the PMZ, reaching values of 4.0–4.5 wt% in the ledeburite regions. Chromium and other alloying elements from the overlay weld metal also diffuse into the HAZ, creating compositional gradients that further influence phase formation.

The segregation of alloying elements has important implications for the mechanical properties of the HAZ. Regions with high carbon and chromium concentrations form hard, brittle carbides and martensite, while regions with low carbon concentrations remain relatively soft and ductile. This microstructural and mechanical heterogeneity is a primary source of weldability problems in ductile iron welding.

Engineering Practice and Repair Strategies

For engineers involved in the repair of ductile iron components, the following strategies are recommended based on the findings of this study:

1. Preheating: Preheat to 300–400°C to reduce cooling rates and minimize martensite formation in the HAZ. This is particularly important for thick-section components where cooling rates are inherently lower.
2. Post-weld heat treatment: A stress-relief anneal at 550–650°C can temper the martensite in the HAZ, reducing hardness and cracking susceptibility.
3. Welding parameter optimization: Use lower heat input per pass to minimize the extent of the PMZ and AZ. Multi-pass welding with low interpass temperatures is preferred.
4. Filler metal selection: Iron-based filler metals with appropriate carbon and alloy content should be selected to match the base metal composition and minimize HAZ microstructural extremes.
5. Mechanical preparation: Machining or grinding the weld surface can remove the brittle ledeburite-rich PMZ layer, significantly improving fatigue resistance.

Study Insights and Reflections

This research makes a significant contribution to our understanding of HAZ metallurgy in ductile iron welding. The identification of the non-equilibrium melting mechanism in the PMZ — which does not require the temperature to reach the equilibrium solidus line — is a particularly important finding. This mechanism explains why cracking in ductile iron welds often originates from the PMZ rather than from the weld metal itself, and why conventional weldability assessments based on carbon equivalent calculations are insufficient for cast iron substrates.

The double-shell microstructure (austenite + ledeburite) identified in the PMZ is a critical defect that must be addressed in any repair strategy. For critical applications, the PMZ should be mechanically removed by machining or grinding after welding, followed by post-weld heat treatment to relieve residual stresses. This approach, while labor-intensive, is the only reliable method to eliminate the brittle ledeburite layer.

The broader implication for the engineering community is that welding of ductile iron requires a fundamentally different approach from welding of steel. The presence of graphite nodules creates a microstructural complexity that cannot be addressed by simply adjusting welding parameters or filler metal selection. A comprehensive strategy that includes preheating, controlled cooling, post-weld heat treatment, and mechanical preparation is essential for producing reliable, crack-free repairs.