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

Literature Overview and Core Objective

This study investigates the microstructural characteristics and high-temperature performance of iron-based superalloy overlays deposited by gas tungsten arc (TIG) welding on structural steel substrates. Iron-based superalloys offer a cost-effective alternative to nickel-based superalloys for high-temperature applications, combining good oxidation resistance, creep strength, and thermal fatigue resistance at temperatures up to 800-1000 °C. The TIG arc cladding process provides a cost-effective method for depositing these overlays on large components, although the relatively high heat input and slower deposition rate require careful parameter control to achieve acceptable microstructures.

Key Technical Parameters and Overlay Composition

The TIG arc cladding process for iron-based superalloys involves several critical parameters that influence the microstructure and high-temperature performance. The following table presents the typical process parameters and overlay compositions:

The overlay composition is critical for high-temperature performance. A typical iron-based superalloy overlay may contain 20-30% Cr, 5-10% Ni, 3-5% Co, 2-4% Mo, 1-2% W, and 0.2-0.5% C. The carbon content promotes the formation of carbides (M₆C, M₂₃C₆) that provide precipitation hardening and creep resistance. The chromium content provides oxidation resistance, while nickel, cobalt, molybdenum, and tungsten enhance solid solution strengthening and high-temperature strength.

Interpretation of Technical Points

The microstructure of TIG arc cladded iron-based superalloy overlays typically consists of a matrix phase (austenitic or martensitic, depending on the composition) with dispersed carbide particles. The cooling rate during TIG cladding, which is typically 5-20 °C/s, is lower than that of laser cladding (100-1000 °C/s), resulting in coarser microstructures and larger carbide particles. The grain structure is generally columnar, with grains growing from the substrate through the overlay thickness.

The dilution effect is a significant concern in TIG arc cladding due to the relatively high heat input. The dilution ratio can reach 20-35% for single-pass deposition, which may significantly alter the overlay composition and degrade the high-temperature properties. To mitigate this, multi-pass deposition with controlled interpass temperatures is recommended. The first pass, which experiences the highest dilution, may not meet the target composition, but subsequent passes benefit from the heat of previously deposited layers, resulting in lower dilution and more favorable microstructures.

The high-temperature performance of the overlay is evaluated through oxidation resistance tests, creep tests, and thermal fatigue tests. The oxidation resistance is primarily governed by the chromium content, with a minimum of 20% Cr required for protective oxide scale formation. The creep resistance is determined by the combination of solid solution strengthening (from Ni, Co, Mo, W) and precipitation hardening (from carbides). The thermal fatigue resistance is influenced by the thermal expansion coefficient and the ability of the microstructure to accommodate cyclic thermal strains.

Connection with Engineering Practice

In engineering applications, iron-based superalloy overlays are used in high-temperature components such as furnace parts, heat exchanger tubes, and gas turbine components. The TIG arc cladding process is particularly suitable for large components where the deposition rate of laser cladding is insufficient. For example, the overlay of furnace roll shafts, which operate at temperatures of 600-800 °C in continuous casting machines, benefits from the good oxidation resistance and thermal fatigue resistance of iron-based superalloys.

Quality control for TIG arc cladded overlays requires comprehensive testing. The dilution ratio should be verified by optical emission spectrometry (OES) or X-ray fluorescence (XRF) analysis. The microstructure should be examined by metallography to confirm the presence of the expected phases and the absence of brittle intermetallics. The mechanical properties should be verified by hardness testing, tensile testing, and impact testing. For high-temperature applications, additional tests such as oxidation resistance testing (per ASTM G93 or G194) and creep testing (per ASTM E139) are essential.

A practical case involves the TIG arc cladding of an iron-based superalloy overlay on a carbon steel furnace roll shaft. The overlay composition was Fe-25Cr-8Ni-4Mo-2W-0.4C, and the deposition was performed with an arc current of 200 A, travel speed of 50 mm/min, and three passes with an interpass temperature of 200 °C. The resulting overlay exhibited a dilution of 15%, hardness of 380 HV, and excellent oxidation resistance at 800 °C for 500 hours. The thermal fatigue life at 600-800 °C cycling was improved by a factor of 3 compared to the uncoated base metal.

Key Questions and Reflections

One critical question is the long-term stability of the overlay microstructure under prolonged high-temperature exposure. The carbide particles that provide creep resistance may coarsen over time, leading to a loss of precipitation hardening and a reduction in creep strength. The engineer must consider the service life of the overlay and design appropriate inspection intervals for microstructural evaluation.

Another important consideration is the effect of the substrate on the overlay performance. The thermal expansion mismatch between the iron-based superalloy overlay and the carbon steel substrate can generate residual stresses that may lead to cracking under thermal cycling. The preheat temperature and post-weld heat treatment must be carefully selected to minimize these stresses. A PWHT at 700-800 °C for 2-4 hours can effectively relieve residual stresses without compromising the overlay microstructure.

The comparison between iron-based and nickel-based superalloy overlays is also important. While nickel-based superalloys offer superior high-temperature properties, they are significantly more expensive. Iron-based superalloys provide a cost-effective alternative for applications where the operating temperature is below 900 °C and the corrosion environment is not extremely aggressive. The engineer should make the selection based on a comprehensive evaluation of service conditions, performance requirements, and cost considerations.

Study Insights and Implications

This literature highlights the potential of iron-based superalloy overlays deposited by TIG arc cladding for high-temperature applications. The key insight is that careful control of process parameters, particularly the dilution ratio and cooling rate, is essential for achieving acceptable microstructures and high-temperature performance.

For engineering practice, the study reinforces the importance of systematic process optimization and quality control for TIG arc cladding of superalloy overlays. The engineer should establish clear acceptance criteria for dilution, microstructure, mechanical properties, and high-temperature performance, supported by appropriate testing methods. The integration of process parameter optimization, microstructural analysis, and performance testing provides a robust framework for qualifying TIG arc cladding procedures for high-temperature applications.

The continued development of iron-based superalloy compositions and TIG arc cladding process parameters will further expand the range of applications for these overlays. The combination of cost-effectiveness, good high-temperature performance, and the maturity of TIG arc cladding technology makes iron-based superalloy overlays a valuable option for engineers seeking to extend the service life of high-temperature components.