Electroslag Surfacing of High Chromium Cast Iron Interface Temperature Field and Microstructure Properties

Literature Overview

This study by Wang Hao and colleagues from the Naval Engineering University investigates the electroslag surfacing process applied to deposit high chromium cast iron hardfacing layers onto D32 low-alloy steel substrates. The work was funded by the Naval Engineering University's self-initiated research project (2022501110) and published in the Transactions of the China Welding Institution in 2023 (Vol. 44, No. 3, pp. 98–105). The research addresses a critical engineering challenge: achieving a metallurgically sound bond between dissimilar materials while maintaining the wear-resistant characteristics of the high chromium overlay.

Core Technical Findings

Thermal Field Characteristics

The electroslag surfacing process exhibits distinctly different thermal behavior compared to conventional arc welding methods. The relatively slow heating and cooling rates are inherent advantages of the slag pool heat transfer mechanism. During the steady-state deposition phase, the substrate temperature distribution remains remarkably uniform across the heated zone. The maximum temperature gradient measured in the surfacing direction was only −21.25 °C/mm, which is significantly lower than typical values observed in GTAW or GMAW processes (typically exceeding 50–100 °C/mm). This low thermal gradient directly correlates to the maximum thermal stress within the D32 substrate, which was calculated to be 53.4 MPa — well below the tensile strength of the low-alloy steel. This is a critical finding because it demonstrates that electroslag surfacing can effectively avoid crack initiation in the heat-affected zone, a persistent concern in hardfacing applications.

Microstructural Evolution

The interface morphology is particularly noteworthy. The flat and clear composite interface with a well-defined austenite band region of approximately 50 μm width indicates a controlled melting and resolidification process. The M₇C₃ type carbides, which are characteristic of high chromium cast irons, are fine and uniformly distributed along austenite grain boundaries. This microstructural configuration provides an excellent combination of toughness and wear resistance, as the austenite matrix offers ductility while the carbide network provides hardness.

Mechanical Properties

The composite interface bond strength was measured at 96 MPa, which represents a satisfactory metallurgical bond between the dissimilar materials. The impact absorption energy at the fusion zone (53 J) is substantially higher than that of the hardfacing layer (10.7 J), indicating a favorable toughness gradient that can absorb impact loads without catastrophic failure propagation into the overlay.

Strain-Induced Martensitic Transformation

A particularly significant finding is the observation that the hypoeutectic high chromium cast iron hardfacing layer undergoes martensitic phase transformation under high wear loads. This in-situ transformation leads to a localized increase in hardness, thereby enhancing the resistance to abrasive wear under severe loading conditions. This mechanism is analogous to the TRIP (Transformation-Induced Plasticity) effect utilized in advanced high-strength steels, and it represents a self-reinforcing wear resistance mechanism that is highly desirable in industrial applications.

Engineering Practice Integration

In the context of pipeline and equipment maintenance, this technology has direct applications for:

• Mining and mineral processing equipment: Chutes, hoppers, and screens subjected to severe abrasive wear from ore particles
• Power generation components: Boiler tubes, air preheater elements, and fan blades exposed to fly ash erosion
• Petrochemical equipment: Pump casings, valve seats, and impellers in slurry service

The low thermal stress characteristic of electroslag surfacing (53.4 MPa maximum) is particularly advantageous for thick-walled components where conventional arc welding hardfacing often produces cracking due to high cooling rates and residual stresses. For engineers working with large-diameter pipes and heavy fittings, this technique offers a viable alternative to overlay welding with consumable electrodes or strips.

Key Questions and Reflections

One question that arises from this research is regarding the long-term stability of the strain-induced martensite. While the initial hardness increase under load is beneficial, repeated cyclic loading may lead to fatigue cracking in the transformed regions. Further investigation into the fatigue behavior of the hardfacing layer under cyclic abrasive conditions would strengthen the engineering case for this technology.

Another consideration is the dilution ratio between the D32 substrate and the high chromium cast iron overlay. The study does not explicitly quantify the dilution, yet the presence of the austenite band region at the interface suggests some degree of substrate melting and mixing. In practical applications, controlling dilution is essential to maintain the desired carbon and chromium content in the effective hardfacing zone.

Study Insights and Implications

The fundamental insight from this work is that the electroslag process, with its inherently low thermal gradients and slow cooling rates, provides an ideal thermal cycle for depositing hardfacing alloys on thick-section substrates without inducing cracking. The combination of low thermal stress, a well-defined interface microstructure, and strain-induced hardening under service conditions makes this approach particularly attractive for heavy-duty industrial applications. For engineers involved in pipeline repair and equipment refurbishment, this research provides both the thermodynamic justification and the metallurgical evidence needed to confidently specify electroslag surfacing for critical wear-resistant overlays.