Microcrack Analysis of Nickel-Based 690 Alloy GTAW Overlay Layer

Literature Overview

Nickel-based Alloy 690 (UNS N06690) is a critical material for nuclear power plant steam generator tubes, providing excellent resistance to stress corrosion cracking in primary water environments. When used as an overlay material for repair or surface protection, the weld overlay layer is susceptible to microcracking during solidification and cooling, which can compromise the integrity of the overlay. This study note provides a detailed analysis of microcrack formation mechanisms, characterization, and prevention strategies for Alloy 690 GTAW overlay layers.

Material Characteristics of Alloy 690

Alloy 690 is a nickel-chromium-iron alloy with the following key characteristics:

The high chromium content (25–30%) provides excellent corrosion resistance but also promotes microsegregation during solidification, which is a primary driver of microcrack formation.

Microcrack Classification and Formation Mechanisms

Type 1: Solidification Cracking (Hot Cracking)

Solidification cracking occurs during the final stages of solidification when the weld metal is in a mushy state with insufficient liquid film to accommodate thermal shrinkage strains.

Formation mechanism:

• High chromium content causes microsegregation of chromium, molybdenum, and iron to the last solidifying dendrite arms.
• The segregated regions have lower melting points, forming liquid films between solid dendrites.
• Thermal shrinkage during cooling creates tensile stresses that exceed the strength of the solid dendrite arms.
• Cracks form along the grain boundaries where liquid films are present.

Characteristics:

• Intergranular crack morphology
• Located in the weld centerline or near the weld surface
• Often associated with inclusions (oxides, sulfides) at grain boundaries
• Can be detected by MT or PT inspection

Type 2: Reheat Cracking (Weld Decay)

Reheat cracking occurs during post-weld heat treatment or during subsequent welding passes when the weld metal is reheated to temperatures in the sensitization range (800–1100°C).

Formation mechanism:

• Prior sensitization of the weld metal during solidification leads to chromium depletion at grain boundaries.
• Reheating to the sensitization range causes further precipitation of chromium-rich carbides.
• The chromium-depleted grain boundaries have reduced cohesion and are susceptible to cracking under stress.

Characteristics:

• Intergranular crack morphology
• Located along prior austenite grain boundaries
• Often associated with carbide precipitation at grain boundaries
• May not be visible on the surface but can be detected by MT or UT

Type 3: Thermal Cracking (Cold Cracking)

Thermal cracking occurs during cooling from the welding temperature to room temperature, driven by the combination of thermal stress and hydrogen embrittlement.

Formation mechanism:

• High thermal gradients during welding create tensile stresses in the weld metal and heat-affected zone.
• Hydrogen absorbed during welding diffuses to regions of high stress and low toughness.
• The combination of stress, hydrogen, and susceptible microstructure causes crack initiation and propagation.

Characteristics:

• Transgranular or intergranular crack morphology
• Located in the weld metal or HAZ
• Often associated with martensitic transformation in susceptible microstructures
• May be delayed (hours to days after welding)

Welding Process Parameters and Their Effect on Microcracking