TIG Arc Overlay of Iron-Based Superalloy Coating on Q235 Steel

Literature Overview

This paper published in Hot Working Technology (2016, Vol. 45, No. 8, pp. 1-6) by Li Ainong, Hu Jianhua, Wang Huajun, Fu Lili, and Tan Guowei investigates the microstructure and high-temperature performance of iron-based superalloy coatings produced by TIG arc overlay welding on Q235 steel substrates. The research was supported by the National Natural Science Foundation of China (51475346) and the Foshan Shunde Economic and Technology Promotion Bureau (2012CX040). The overlay compositions were based on the GH2135 iron-based superalloy system, with variations designated as 1A, 10J, 11K, and H13 steel for comparison.

Technical Approach and Materials

Overlay Compositions

The study examined multiple overlay compositions:

• 1A: Modified iron-based superalloy composition
• 10J: Base formulation coating
• 11K: Optimized formulation with superior temper resistance
• H13: Hot work steel (reference material)
• GH2135: Base iron-based superalloy (reference)

Substrate and Process

• Substrate: Q235 carbon structural steel
• Process: TIG (GTAW) arc overlay welding
• Characterization: Metallography, microhardness testing, tempering experiments, high-temperature oxidation testing

Key Technical Findings

Microstructure of Overlay Coatings

The overlay microstructure consisted of:

• Primary phase: Dendritic γ-Fe(Cr, Ni) solid solution
• Interdendritic phase: α-Fe (ferrite)
• Dispersed phases: Intermetallic compounds
• Grain boundary phases: Small amounts of special carbides
• Grain size: Relatively fine

The presence of both γ (austenite) and α (ferrite) phases indicates a dual-phase microstructure that provides a balance of strength and toughness. The fine grain size is beneficial for both room temperature and elevated temperature mechanical properties.

Tempering Behavior

A remarkable finding was that the 1A and 11K overlay coatings exhibited increasing hardness with increasing tempering temperature, which is contrary to the normal behavior of most steels and alloys. Specifically:

The 1A coating hardness was approximately 50 HV0.1 higher than the 10J base formulation after 700°C tempering, and approximately 82 HV0.1 higher than H13 steel. The 11K coating achieved the highest hardness of 349 HV0.1 after 700°C tempering, indicating superior temper resistance.

High-Temperature Oxidation Resistance

The 11K overlay coating demonstrated excellent high-temperature oxidation resistance:

This represents a dramatic improvement in oxidation resistance compared to the H13 hot work steel reference material.

Interpretation of Technical Points

Anomalous Hardness Increase with Tempering

The observation that overlay hardness increases with tempering temperature (up to 700°C) is unusual and indicates secondary hardening. This phenomenon occurs when:

1. Fine precipitates (carbides, intermetallics, or silicides) form during tempering, providing precipitation strengthening
2. The matrix retains sufficient carbon and alloy content to support precipitation hardening
3. The tempering temperature is below the temperature at which precipitate coarsening or dissolution occurs

The iron-based superalloy compositions likely contain sufficient chromium, molybdenum, and/or tungsten to form stable, fine precipitates during tempering. The precipitation of these phases during tempering compensates for and exceeds the softening normally associated with martensite tempering.

High-Temperature Oxidation Resistance

The superior oxidation resistance of the 11K coating is attributed to:

1. Chromium content: Chromium forms a stable Cr₂O₃ oxide layer that protects the underlying metal
2. Aluminum content: If present, aluminum forms an even more protective Al₂O₃ layer
3. Fine grain structure: Reduces diffusion pathways for oxygen ingress
4. Intermetallic compound distribution: May form protective surface films

The 1/8 reduction in oxidation weight gain at 600°C compared to H13 steel represents an order-of-magnitude improvement, which is highly significant for applications involving prolonged exposure to oxidizing environments at elevated temperatures.

TIG Arc Overlay Process Advantages

The TIG (GTAW) arc overlay process offers several advantages for superalloy coating applications:

• Precise heat input control: Allows careful management of dilution and microstructure
• Clean weld pool: No flux contamination, preserving alloy composition
• Good dilution control: With appropriate technique, dilution can be kept below 10%
• Applicability to thin coatings: Enables precise control of coating thickness

Engineering Practice Integration

The 11K overlay coating is particularly suitable for:

• Hot work tool components exposed to elevated temperatures
• Mold surfaces subject to thermal cycling
• Components requiring both wear resistance and oxidation resistance at 600-700°C
• Replacement or repair of H13 steel components with superior performance

Key Questions and Reflections

1. Dilution effects: The paper does not specify the dilution ratio achieved during TIG arc overlay on Q235 steel. Since Q235 is a low-alloy carbon steel, excessive dilution would significantly reduce the effective alloy content of the overlay and compromise its high-temperature performance. The practical dilution must be kept below 10-15% for optimal performance.
2. Multi-pass overlay: For thicker coatings, multiple passes are required. The thermal history of subsequent passes affects the microstructure of previously deposited layers. The tempering behavior and oxidation resistance may vary with coating thickness and pass sequence.
3. Thermal cycling fatigue: The overlay must withstand repeated thermal cycling in service. The coefficient of thermal expansion mismatch between the overlay and Q235 substrate creates thermal stresses during cycling. The paper does not address thermal fatigue behavior.
4. Creep resistance: At 600-700°C, creep deformation becomes relevant for structural applications. The paper focuses on oxidation resistance but does not address creep strength or creep rupture life.
5. Cost considerations: Iron-based superalloys are significantly more expensive than H13 steel. The economic justification for using the 11K overlay must account for the extended service life and reduced maintenance frequency.

Study Insights and Implications

This research demonstrates that TIG arc overlay welding of iron-based superalloy compositions can produce coatings with exceptional high-temperature performance, including anomalous temper hardening and dramatically improved oxidation resistance. The 11K formulation, with its 349 HV0.1 hardness after 700°C tempering and oxidation resistance 8 times better than H13 steel at 600°C, represents a significant advancement in surface engineering for hot work applications. For engineers involved in hot work tool manufacturing and repair, this technology offers a viable alternative to complete component replacement, with the potential to extend service life while maintaining or improving performance. The TIG arc overlay process provides the precision and control necessary to achieve the required microstructure and composition, making it a practical choice for industrial implementation.