Cracking Mechanism of Inconel 625 Overlay Metal

Literature Overview

This study, published in the Transactions of the China Welding Institution in 2023 by Guo Xiao, Gu Yu, Han Ying, Xu Kai, Wang Yan, and Jiang Yinglong, investigates the cracking mechanisms in Inconel 625 (UNS N06625) weld overlay deposits. The research is conducted jointly by the Harbin Welding Research Institute and the Taiyuan Iron and Steel Group's State Key Laboratory of Advanced Stainless Steel Materials, and is supported by the Shanxi Province Key Core Technology R&D Program. Inconel 625 is one of the most widely used nickel-based alloys for weld overlay applications in the chemical, petrochemical, and nuclear industries, and understanding its cracking susceptibility is essential for reliable fabrication.

Core Technical Content

Inconel 625 Composition and Properties

Inconel 625 is a precipitation-hardenable nickel-chromium-molybdenum alloy with the following nominal composition:

Key properties of Inconel 625 overlay deposits:

• Hardness: 200–250 HV (as-welded), up to 400 HV (after aging at 720°C for 8h)
• Yield strength: 690 MPa (as-welded), 1035 MPa (aged)
• Elongation: 30–40% (as-welded)
• Thermal conductivity: ~12 W/(m·K)
• CTE: 13.1 × 10⁻⁶ /K

Types of Cracking in Inconel 625 Overlay

The study systematically identifies and characterizes several crack types that can occur in Inconel 625 overlay deposits:

1. Hot Cracking (Solidification Cracking)

Hot cracking occurs during solidification when the weld pool is in a mushy state (between liquidus and solidus temperatures). The driving force is the combination of:

• Thermal shrinkage: As the weld pool solidifies, volumetric contraction occurs.
• Restraint: The surrounding solidified material and base metal restrain this contraction.
• Low ductility in the mushy zone: The liquid films between dendrites provide paths for crack propagation.

Conditions favoring hot cracking in Inconel 625:

• High heat input (creating wide, shallow welds with slow cooling rates)
• High restraint conditions (thick base plates, rigid fixtures)
• Impurities (S, P, C) that form low-melting eutectics
• Rapid solidification rates that increase dendritic segregation

2. Reheat Cracking (Weld Decay)

Reheat cracking occurs during subsequent thermal cycles (PWHT or service exposure) in the HAZ of the overlay or at the overlay/base metal interface. This is particularly relevant for Inconel 625 because:

• The HAZ experiences peak temperatures of 1100–1300°C, causing grain growth.
• During subsequent cooling, the coarse grains in the HAZ are susceptible to intergranular cracking.
• The Nb-rich γ'' precipitates (Ni₃Nb) can form preferentially along grain boundaries, embrittling the microstructure.

3. Hydrogen-Induced Cracking (Cold Cracking)

Although less common in nickel-based alloys than in high-strength steels, hydrogen-induced cracking can occur in Inconel 625 overlay deposits under certain conditions:

• Hydrogen pickup from the welding environment (moisture in flux, wet base metal)
• High residual stresses from the welding process
• Susceptible microstructure (martensitic transformation in Fe-rich areas)

4. Stress Corrosion Cracking (SCC)

In service, Inconel 625 overlay deposits can be susceptible to SCC in certain environments:

• Chloride-containing environments at elevated temperatures
• Sulfur-containing environments
• Ammonia environments

The susceptibility is influenced by the welding procedure, as different welding methods produce different microstructures with varying SCC resistance.

Cracking Mechanism Analysis

The study provides detailed metallographic and fractographic analysis of cracks observed in Inconel 625 overlay deposits:

Fractographic evidence:

• Interdendritic cracking: Characteristic of solidification cracking, with cracks propagating along the interdendritic regions of the weld microstructure. The fracture surface shows a "cracked" appearance with liquid-metal-embrittled features.
• Intergranular cracking: Observed in the HAZ and near-weld zone, with cracks propagating along prior austenite grain boundaries. The fracture surface shows a "sugar cube" appearance typical of intergranular fracture.
• Transgranular cracking: Less common, but observed in cases of high restraint with low ductility. The fracture surface shows cleavage-like features.

Chemical analysis of crack surfaces:

• Enrichment of S, P, and C along crack paths, confirming the role of low-melting eutectics in hot cracking.
• Presence of Nb-rich precipitates along grain boundaries in reheat cracks, confirming the role of γ'' precipitation in weld decay.

Engineering Practice Applications

Prevention Strategies for Inconel 625 Overlay Cracking

Based on the cracking mechanisms identified in this study, the following prevention strategies are recommended:

Welding Procedure Optimization

Material Selection and Preparation

• Filler metal quality: Use low-S, low-P filler metals (S < 0.01%, P < 0.02%) to minimize hot cracking susceptibility.
• Base metal cleanliness: Remove all coatings, rust, and contamination from the base metal surface before overlay welding.
• Preheat control: For thick sections (>50 mm), controlled preheat (100–150°C) reduces thermal gradients and residual stresses.

Post-Weld Treatment

• Stress relief: Low-temperature stress relief (425–480°C for 2–4 hours) can reduce residual stresses without promoting harmful precipitation.
• Avoid high-temperature PWHT: Temperatures above 600°C can promote γ'' precipitation at grain boundaries, increasing susceptibility to reheat cracking.

Inspection and Acceptance

Key Questions and Reflections

The most challenging aspect of Inconel 625 overlay fabrication is the inherent tension between achieving adequate weld penetration (which requires higher heat input) and minimizing cracking susceptibility (which requires lower heat input). This tension is particularly acute in thick-section applications where deep penetration is required.

The study highlights several areas where further research is needed:

1. Multi-pass overlay procedures: The cracking behavior of multi-pass Inconel 625 overlay deposits is complex, as each subsequent pass reheats the previous passes. The cumulative thermal cycling can promote grain growth and precipitation, increasing cracking susceptibility.
2. Interaction with base metal: The cracking behavior of Inconel 625 overlay is strongly influenced by the base metal properties. When overlaying Inconel 625 onto carbon steel or stainless steel, the dilution and intermetallic formation at the interface can create additional cracking susceptibility.
3. Service environment effects: The cracking mechanisms identified in this study are primarily welding-related. However, in service, additional cracking mechanisms (SCC, fatigue cracking, creep cracking) can operate, and the welding-induced microstructure can influence susceptibility to these mechanisms.

Study Insights and Implications for Practice

This research provides a comprehensive understanding of the cracking mechanisms in Inconel 625 overlay deposits, which is essential for the reliable fabrication of clad components in critical applications. The identification of specific cracking types and their mechanisms enables engineers to develop targeted prevention strategies.

For engineers in the bimetal pressure vessel industry, this work underscores the importance of:

1. Rigorous procedure qualification: Inconel 625 overlay procedures must be qualified through extensive testing, including cracking sensitivity evaluations under the specific restraint conditions of the intended application.
2. Integrated design-fabrication approach: The cracking susceptibility of Inconel 625 overlay must be considered in the design phase, with appropriate design margins and fabrication tolerances specified.
3. Quality control focus: Enhanced inspection protocols are required for Inconel 625 overlay joints, with particular attention to the HAZ and overlay/base metal interface regions.

The practical implication is that Inconel 625, while an excellent overlay material for corrosion resistance, requires careful fabrication to avoid cracking. The benefits of Inconel 625 overlay (excellent corrosion resistance, high temperature strength, good weldability) must be balanced against the fabrication challenges (cracking susceptibility, high cost, machining difficulty). For critical applications where Inconel 625 is the only suitable overlay material, the fabrication process must be meticulously controlled to ensure crack-free deposits.