Failure Analysis of Co-Based Superalloy and TIG Overlay Welding Process Optimization

Literature Overview

This 2022 publication in Hot Working Technology by Guo Ying, Yan Chao, Gu Ming, and Liu Jinyong from Beijing Hanghua Energy Saving and Environmental Protection Technology Co., Ltd. and Datang Hulunbuir Fertilizer Co., Ltd. addresses two interconnected issues: the failure analysis of cobalt-based superalloy components and the subsequent optimization of gas tungsten arc welding (GTAW/TIG) overlay processes to prevent recurrence. Cobalt-based superalloys such as Stellite 6, Stellite 21, and Hastelloy X are extensively employed in high-temperature, high-wear, and corrosive environments including chemical reactors, fertilizer plant components, and power generation equipment.

Failure Mechanism Analysis

The investigation begins with a systematic failure analysis of Co-based superalloy components in service, employing metallographic examination, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and hardness mapping. The identified failure modes include:

Failure Mode	Microstructural Evidence	Root Cause	Service Condition
Thermal fatigue cracking	Intergranular crack initiation at MC carbide clusters	Thermal cycling with insufficient thermal conductivity	Repeated heating/cooling cycles
Intergranular corrosion	Grain boundary attack along M6C carbide networks	Chromia depletion zones at grain boundaries	High-temperature oxidizing environment
Wear-induced spalling	Delamination along coating-substrate interface	Poor bond strength due to excessive dilution	Slurry erosion and impact loading
Hydrogen-assisted cracking	Microvoids at prior austenite boundaries	Hydrogen embrittlement from welding residual hydrogen	Welding without post-weld hydrogen removal

The root cause analysis reveals that many failures are attributable to suboptimal overlay welding parameters during original fabrication or repair, rather than material selection errors. Excessive heat input during TIG overlay leads to coarse microstructures, carbide network formation, and reduced toughness at the coating-substrate interface.

TIG Overlay Process Optimization

Process Parameter Optimization

The authors optimized the TIG overlay parameters through systematic trial welding and microstructural evaluation. The following parameter ranges were identified as critical:

Parameter	Original (Problematic)	Optimized Range	Rationale
Welding Current	180–220 A	120–160 A	Reduce heat input, minimize dilution
Travel Speed	100–150 mm/min	200–300 mm/min	Lower heat input per unit length
Arc Length	3–5 mm	1.5–2.5 mm	More stable arc, reduced dilution
Shielding Gas Flow	8–10 L/min	12–15 L/min	Improved shielding for reactive Co alloys
Wire Diameter	1.6 mm	1.0–1.2 mm	Finer wire enables lower current
Preheat Temperature	200–300°C	100–150°C	Reduce thermal stress, minimize cracking
Interpass Temperature	>250°C	<150°C	Control grain growth, prevent carbide coarsening

Microstructural Control

The optimized parameters yield a cladding layer with significantly finer grain structure (average grain size reduced from 120–180 μm to 40–80 μm) and reduced MC carbide volume fraction. The carbide distribution shifts from continuous intergranular networks to dispersed particles, improving the toughness and corrosion resistance of the coating. The dilution ratio is maintained below 20%, ensuring that the Co-based alloy retains its intended composition and properties.

Engineering Practice Implications

For Co-based superalloy overlay applications in fertilizer plants and chemical reactors, the following engineering practices should be adopted based on the findings of this study:

Pre-weld preparation: Ensure substrate surface cleanliness and preheat to 100–150°C to minimize thermal shock.
Parameter control: Maintain welding current in the 120–160 A range with travel speeds of 200–300 mm/min for single-layer deposits.
Interpass temperature management: Monitor interpass temperature with infrared thermometers and keep below 150°C to prevent grain coarsening.
Post-weld treatment: Apply post-weld heat treatment (PWHT) at 800–900°C for 1–2 hours to relieve residual stresses and refine carbide distribution.
Quality verification: Perform hardness mapping (target HV 350–450 for Stellite-type alloys), metallographic examination of the interface, and intergranular corrosion testing per ASTM G108.

Study Insights

This study demonstrates that failure prevention in Co-based superalloy overlays is fundamentally a process control challenge. The optimized TIG parameters provide a practical framework for engineers involved in the fabrication and repair of Co-alloy clad components. The emphasis on low heat input and fine-grained microstructure aligns with modern cladding philosophy where thermal management takes precedence over deposition rate. Future work should extend this optimization to multi-layer, multi-pass configurations and evaluate long-term creep and fatigue performance under actual service conditions.