TIG Arc Surfacing of Iron-Based Superalloy Overlay Microstructure and High-Temperature Performance

Literature Overview

This 2016 paper by Li Ainong et al., published in Hot Working Technology (Vol. 45, No. 8, pp. 1–6), investigates the microstructure and high-temperature performance of iron-based superalloy overlays deposited by TIG arc surfacing onto Q235 carbon steel substrates. Based on the GH2135 superalloy composition, the study evaluates overlay candidates (designated 1A, 10J, 11K, and H13 as reference) for their temper resistance and high-temperature oxidation resistance. This work is directly relevant to engineers designing thermal barrier coatings and hot-section components in gas turbines, heat exchangers, and high-temperature processing equipment.

Core Technical Findings

Microstructural Characterization

The overlay microstructure consists primarily of dendritic γ-Fe(Cr, Ni) solid solution with inter-dendritic α-Fe, dispersed intermetallic compounds, and minor special carbides at grain boundaries. The grain size is relatively fine, which is beneficial for both mechanical properties and corrosion resistance. The TIG surfacing process, with its low heat input and precise heat control, produces a microstructure that is more homogeneous than alternatives such as plasma spraying or flame spraying.

Feature	Description	Engineering Significance
Primary phase	Dendritic γ-Fe(Cr,Ni)	Provides matrix strength and oxidation resistance
Secondary phase	α-Fe inter-dendritic	May affect ductility and thermal expansion
Intermetallics	Dispersed throughout	Contribute to hardness and creep resistance
Boundary carbides	Minor amount at grain boundaries	Can affect creep and intergranular corrosion
Grain size	Fine	Improves strength and oxidation kinetics

Temper Resistance Performance

A notable and somewhat counterintuitive finding is that, unlike conventional H13 hot work steel, the overlay hardness increases with tempering temperature. After tempering at 700°C, the 1A overlay achieves 346 HV₀.₁, which is approximately 50 HV higher than the base formula (10J) and 82 HV higher than H13 steel. The 11K overlay variant achieves the highest hardness of 349 HV₀.₁ after tempering, demonstrating superior temper resistance. This behavior is attributed to precipitation hardening of intermetallic compounds during tempering, which compensates for and exceeds the softening effect of dislocation recovery.

High-Temperature Oxidation Resistance

The 11K overlay demonstrates exceptional oxidation resistance at elevated temperatures:

Temperature	11K Weight Gain (mg/cm²)	H13 Weight Gain (mg/cm²)	Ratio (11K/H13)
600°C	Reference value	~8× higher	1/8
700°C	Reference value	~3× higher	1/3

This dramatic improvement in oxidation resistance is attributed to the formation of a protective chromium oxide (Cr₂O₃) scale on the overlay surface, facilitated by the high chromium and aluminum content in the GH2135-based composition. The fine grain structure also contributes by providing more grain boundary sites for protective oxide nucleation.

Process Analysis

TIG Surfacing Process Parameters

The TIG (GTAW) process is particularly suitable for superalloy surfacing due to its low heat input, minimal dilution, and precise control of the weld pool. Typical parameters for iron-based superalloy surfacing include:

Current: 80–150 A (DCEN)
Travel speed: 100–200 mm/min
Shielding gas: Pure argon (15–25 L/min)
Wire feed: Manual or semi-automatic, 1.2–1.6 mm diameter
Interpass temperature: Controlled below 150°C
Preheating: 100–200°C for thick substrates

The low heat input of TIG surfacing minimizes substrate dilution (typically 5–15%), preserving the alloying composition of the overlay. This is critical for maintaining the high-temperature properties that depend on specific Cr, Ni, and Al concentrations.

Multi-Pass Considerations

For thick overlay layers (>3 mm), multi-pass TIG surfacing is required. Each subsequent pass reheats the previous pass, potentially causing grain coarsening and precipitate dissolution. The study's finding of fine grain structure suggests that the authors employed careful interpass temperature control and possibly a technique of weaving or oscillating the arc to distribute heat evenly.

Comparison with Alternative Overlay Methods

Method	Dilution	Heat Input	Microstructure	Cost	Suitability for Superalloys
TIG Surfacing	Low (5-15%)	Low	Fine grain, homogeneous	Moderate	Excellent
Plasma Spraying	None	Low	Porous, columnar	High	Good (for thermal barrier)
HVOF	None	Low	Dense, columnar	High	Good
Powder Metallurgy	None	Moderate	Fine grain, homogeneous	Very High	Excellent
Flame Spraying	None	Moderate	Porous, coarse	Low	Poor (porosity)

Study Insights and Engineering Implications

The finding that overlay hardness increases with tempering temperature is particularly significant for components that undergo post-weld heat treatment or operate at elevated temperatures where tempering occurs in service. For gas turbine hot-section components, this means the overlay will actually harden during operation rather than soften, providing a self-improving wear resistance characteristic. The 11K variant's superior oxidation resistance at both 600°C and 700°C makes it a strong candidate for applications such as heat exchanger tubes, boiler components, and furnace parts operating in oxidizing environments. From a practical standpoint, the TIG surfacing approach offers a cost-effective alternative to investment casting of superalloy components, particularly for repair and retrofit applications where replacing entire components is impractical. The methodology of systematically comparing overlay compositions (1A, 10J, 11K) against a conventional reference (H13) provides a clear framework for overlay material selection that can be adapted to other high-temperature applications. Future work should investigate the long-term creep resistance and thermal cycling fatigue behavior of these overlays, as these properties are often more critical than static oxidation resistance in real service conditions.