Microstructure Analysis of Austenitic Stainless Steel Strip Electrode Electroslag Surfacing Layer

Literature Overview

This study, published in the journal Electric Welder (2014, Vol. 44, No. 1, pp. 77-80), investigates the microstructure of an austenitic stainless steel surfacing layer produced by strip electrode electroslag surfacing on Q235 low carbon steel substrate. The authors, Yang Qin, Yang Ke, and Bao Yefeng from Hohai University, employed a self-developed flux in conjunction with an austenitic stainless steel strip electrode. The primary objective was to characterize the microstructural evolution within the surfacing layer, with particular emphasis on the content and morphological distribution of delta-ferrite (δ-ferrite), and to assess the implications for intergranular corrosion resistance.

Core Technical Findings

The research demonstrates that the strip electrode electroslag surfacing process achieves excellent bead formation quality with a low dilution rate, which is a critical advantage when surfacing dissimilar substrates such as Q235 carbon steel. The surfacing layer microstructure consists predominantly of austenite with a small fraction of δ-ferrite. Three distinct morphological forms of δ-ferrite were identified within the surfacing layer:

The coexistence of all three morphologies indicates that the surfacing process involves both primary solidification and subsequent solid-state phase transformations. The presence of a controlled amount of δ-ferrite is essential for ensuring good intergranular corrosion resistance in the surfacing layer, as it serves to tie up carbon in the form of ferrite carbides and thereby reduces the tendency for chromium carbide precipitation at grain boundaries.

Technical Interpretation and Standards Context

The importance of δ-ferrite control in austenitic stainless steel welds and surfacing layers is well established in welding standards and codes. According to ASME Section IX and AWS D1.6, the recommended δ-ferrite content for austenitic stainless steel weld metal typically ranges from 5% to 30% (measured in Ferrite Number, FN, per ASTM E1072). The ferrite content can be estimated using the DeLong diagram or the more refined WRC-1992 (ISO 8136) ferrite number calculator.

In the context of surfacing applications, the dilution rate between the base metal and the surfacing material plays a decisive role in determining the final ferrite content. For Q235 substrate with typical composition of approximately 0.14-0.22% C, 1.2-1.6% Mn, and negligible Cr and Ni, the dilution into the austenitic surfacing layer will tend to increase the ferrite-stabilizing effect of manganese and carbon, while the absence of nickel in the base metal will further promote ferrite formation. The self-developed flux used in this study likely provided additional alloying elements to compensate for dilution effects.

Engineering Practice Implications

For engineers working on corrosion-resistant surfacing applications on carbon steel substrates, several practical considerations emerge from this research:

1. Dilution control: The low dilution rate achieved by the strip electrode electroslag process is advantageous for maintaining the intended alloy composition of the surfacing layer. This is particularly important when surfacing carbon steel with austenitic stainless steel to create corrosion-resistant barriers.
2. Multi-pass strategy: The coexistence of multiple δ-ferrite morphologies suggests that multi-pass surfacing is involved, with each pass experiencing different thermal histories. Engineers should consider pass sequencing and interpass temperature control to manage ferrite evolution.
3. Heat treatment considerations: Post-weld solution annealing (typically at 1050-1150°C followed by rapid quenching) can dissolve δ-ferrite and homogenize the microstructure, but this must be balanced against distortion concerns for large components.
4. Corrosion testing: While the presence of δ-ferrite improves intergranular corrosion resistance, excessive ferrite can reduce overall corrosion resistance in certain aggressive environments, particularly those containing chloride ions.

Key Questions and Reflections

This study raises several important questions for further investigation. First, the specific δ-ferrite content is not quantified numerically in the abstract, which limits the ability to directly correlate microstructure with performance. Second, the corrosion testing methodology and results are not detailed, leaving the practical corrosion resistance claims somewhat unvalidated. Third, the comparison with alternative surfacing processes such as GTAW, GMAW, and submerged arc surfacing would provide valuable context for process selection.

From a standards perspective, this research aligns with the requirements of ASTM A403 for stainless steel pipe fittings and the corrosion resistance expectations outlined in ASME B31.3 for process piping. For engineers designing corrosion-resistant overlays on carbon steel equipment, this work reinforces the principle that controlled δ-ferrite content is not a defect but a beneficial microstructural feature that should be actively managed rather than eliminated.

Study Insights and Implications

The fundamental insight from this research is that the strip electrode electroslag surfacing process offers a viable and efficient route for producing corrosion-resistant austenitic stainless steel overlays on carbon steel substrates, with the inherent advantage of low dilution and good bead geometry. The coexistence of multiple δ-ferrite morphologies reflects the complex thermal history of multi-pass surfacing and underscores the importance of understanding solidification and transformation mechanisms in overlay metallurgy. For practitioners in the piping and equipment industry, this work provides a metallurgical foundation for specifying and qualifying surfacing procedures where corrosion resistance is a primary design requirement, particularly in applications governed by standards such as ASME B31.3, API 5L, and EN 10216. The emphasis on δ-ferrite as a beneficial phase rather than an undesirable constituent is a message that should be communicated clearly to quality assurance personnel who may otherwise interpret ferrite presence as a quality deficiency.