Microstructure and Mechanical Properties of Inconel 600 Nickel-Based Alloy PAW+TIG Joint

Literature Overview

This recent study by Qiao Xiaoli and colleagues, published in the Transactions of the China Welding Institution in 2024, investigates the microstructure and mechanical properties of Inconel 600 nickel-based alloy weld joints produced using a combined plasma arc welding (PAW) and TIG welding process. The research was conducted at Lanzhou Lanshi Heavy Equipment Co., Ltd., in collaboration with Lanzhou University of Technology and the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. Funded by the Gansu Provincial Science and Technology Major Project (22ZD6GA008) and the Gansu Provincial Key R&D Program (20YF3GA041), the study addresses the practical challenge of welding Inconel 600 for petrochemical equipment applications.

Core Technical Findings

The study examines 6 mm thick Inconel 600 plates welded using a PAW+TIG process combination, where PAW provides the primary penetration and TIG provides the finishing pass. The key findings are summarized below:

Property	Value	Assessment
Tensile strength	589 MPa	Comparable to base material
Fracture type	Ductile fracture	Favorable for service
Bend performance	Good	Acceptable for fabrication
Weld bead morphology	Smooth, even fish-scale pattern	Excellent surface quality
HAZ grain structure	Fine equiaxed austenite	Coarser than base material
Weld metal grain structure	Dendritic	Normal for arc welding
Local corrosion sensitivity	Increased in HAZ	Requires consideration
Post-annealing second phase	Needle-shaped precipitation	Provides strengthening

The study demonstrates that the PAW+TIG process produces joints with good mechanical properties and thermal stability, suitable for high-temperature service conditions in petrochemical equipment. The post-annealing treatment further enhances the joint's performance by promoting the formation of strengthening second-phase precipitates at grain boundaries and twin boundaries.

Microstructural Analysis

Base Metal and Heat-Affected Zone

The base metal of Inconel 600 exhibits a fine equiaxed austenitic grain structure, which is characteristic of the alloy's thermomechanical processing history. The heat-affected zone (HAZ) also exhibits an equiaxed austenitic structure, but with grain sizes larger than those in the base metal. This grain coarsening is a direct consequence of the welding thermal cycle, which exposes the HAZ to temperatures above the austenite recrystallization temperature, allowing grain growth. The extent of grain coarsening depends on the peak temperature and the time above the recrystallization temperature, both of which are determined by the welding parameters.

The PAW process, with its high energy density and deep, narrow penetration, produces a relatively narrow HAZ compared to conventional arc welding processes. This is advantageous because it limits the volume of material exposed to grain coarsening, preserving more of the base metal's original fine-grained structure. The TIG finishing pass further modifies the surface layer, creating a thin region with different thermal history and potentially different microstructural characteristics.

Weld Metal

The weld metal exhibits a dendritic microstructure, which is typical of arc-welded austenitic alloys. The dendritic structure forms because the weld pool solidifies from the cooler edges toward the hotter center, creating a temperature gradient that favors directional solidification. The dendrite arm spacing is influenced by the cooling rate, which in turn is determined by the welding parameters and the thermal properties of the base material.

The fish-scale pattern observed on the weld bead surface is a surface manifestation of the dendritic growth pattern. The uniform distribution of the fish-scale pattern indicates consistent welding parameters and stable arc conditions throughout the welding process. This surface morphology is aesthetically pleasing and also provides a visual indication of process stability, which is valuable for quality assessment.

Mechanical Properties and Fracture Behavior

The tensile strength of 589 MPa for the PAW+TIG joint is comparable to the base material strength of Inconel 600, which typically exhibits a tensile strength in the range of 550-650 MPa depending on the heat treatment condition. This indicates that the welding process has not significantly weakened the joint, which is a critical requirement for structural applications.

The ductile fracture type observed in tensile testing is favorable, indicating that the joint has adequate ductility to accommodate plastic deformation before failure. This is particularly important for petrochemical equipment applications, where the joints may be subjected to thermal cycling, pressure fluctuations, and mechanical loading that could otherwise cause brittle fracture.

The good bend performance further confirms the joint's ductility and toughness. Bend testing is a sensitive indicator of weld joint quality, as it subjects the weld and HAZ to significant plastic deformation. A joint that passes bend testing without cracking demonstrates adequate ductility and freedom from brittle phases or significant porosity.

Corrosion Behavior and Post-Annealing Treatment

Local Corrosion Sensitivity

The electrochemical corrosion testing reveals increased local corrosion sensitivity in the HAZ region. This is a common issue in nickel-based alloy weldments and is attributed to several factors: the grain coarsening in the HAZ, which reduces the grain boundary area and may affect the distribution of precipitation hardening phases; the possible segregation of alloying elements at grain boundaries during the welding thermal cycle; and the residual stresses introduced by welding, which can promote stress corrosion cracking in aggressive environments.

The increased corrosion sensitivity in the HAZ is a critical concern for petrochemical applications, where the equipment is exposed to corrosive media such as acids, chlorides, and sulfides. The HAZ, being the region of lowest corrosion resistance in the joint, becomes the preferential site for corrosion initiation, which can lead to premature failure if not properly addressed.

Post-Annealing Treatment

The post-annealing treatment applied to the weld joint promotes the precipitation of needle-shaped second-phase particles at grain boundaries and twin boundaries. These precipitates serve multiple beneficial functions: they provide a strengthening effect through precipitation hardening; they act as barriers to grain growth, stabilizing the microstructure against further coarsening during subsequent thermal exposure; and they modify the local chemistry at grain boundaries, potentially reducing the susceptibility to intergranular corrosion.

The formation of these precipitates at twin boundaries is particularly significant, as twin boundaries are generally considered to be less susceptible to intergranular corrosion than random grain boundaries. By promoting precipitation at twin boundaries, the annealing treatment may effectively redirect the corrosion attack away from the more vulnerable random grain boundaries.

Treatment Condition	Tensile Strength	Corrosion Sensitivity	Microstructural Stability
As-welded	589 MPa	Increased in HAZ	Lower (grain coarsening)
Post-annealed	Maintained or slightly improved	Reduced in HAZ	Higher (precipitate stabilization)

Engineering Practice and Process Optimization

The PAW+TIG process combination offers several advantages for welding Inconel 600 in petrochemical equipment fabrication:

Process flexibility: PAW provides the deep penetration needed for thick plates, while TIG provides the surface quality and fine control needed for the finishing pass. This combination allows optimization of both penetration and surface quality independently.