Microstructure and Mechanical Properties of 308L Stainless Steel Fabricated by TIG Arc Additive Manufacturing

Literature Overview

The paper by Huang Jialei et al. from Jiangsu University of Technology, published in Hot Working Technology in 2023, presents a systematic investigation into the microstructure and mechanical properties of 308L austenitic stainless steel fabricated through TIG arc additive manufacturing (AM). This work is particularly significant in the context of the growing industrial interest in additive manufacturing for repair, remanufacturing, and complex component fabrication in the energy, oil and gas, and petrochemical industries. The authors fabricated straight-wall specimens and conducted comprehensive metallographic analysis using SEM and EDS, along with mechanical testing including hardness profiling and tensile testing.

Process Characteristics of TIG Arc Additive Manufacturing

TIG arc AM, also known as wire arc additive manufacturing (WAAM) in the broader literature, differs from conventional welding in several fundamental aspects. In additive manufacturing, the weld bead serves as a deposited layer that builds up a component layer by layer, rather than joining two pre-existing pieces. This distinction has profound implications for the thermal history and resulting microstructure:

The repeated thermal cycling in AM leads to significant grain coarsening in the upper layers, as each subsequent layer reheats the previously solidified layers. This is a well-documented phenomenon in WAAM and represents one of the primary challenges in achieving uniform microstructure throughout an additively manufactured component.

Microstructural Analysis and Solidification Mode Prediction

The authors employed the solidification mode prediction method for austenitic stainless steels, which is based on the solidification cracking susceptibility models developed by Brockett, Heisterkamp, and others. The predicted microstructure consisted of austenite with a small amount of δ-ferrite, which was confirmed by SEM and EDS analysis of the actual specimens.

The presence of δ-ferrite in 308L stainless steel is metallurgically expected and beneficial. During solidification, the Cr and Mo enrichment in the remaining liquid promotes ferrite formation, while the subsequent transformation of ferrite to austenite upon cooling through the critical temperature range is governed by the alloy's chemical composition. In 308L (a low-carbon variant of 308), the carbon content is limited to ≤0.03%, which reduces the risk of chromium carbide precipitation at grain boundaries and sensitization during cooling.

The microstructural evolution in AM-deposited 308L follows a characteristic pattern:

• Columnar grains: Formed in the initial layers due to the high thermal gradient directed toward the substrate.
• Equiaxed grains: May develop in the upper layers where the thermal gradient decreases as the deposited height increases.
• Grain coarsening: Progressive grain growth in lower layers due to repeated thermal exposure from subsequent layers.

The agreement between predicted and actual microstructures validates the applicability of conventional solidification models to AM processes, which is an important finding for process development and qualification.

Mechanical Properties and Hardness Distribution

The mechanical characterization revealed several important findings:

The hardness distribution pattern — high at bottom, high at top, low in the middle — is characteristic of AM-deposited components and can be explained as follows:

1. Bottom layers: These layers experience the highest number of thermal cycles and the highest peak temperatures from subsequent layers. However, they also have the highest cooling rates due to heat extraction into the substrate, which can lead to finer microstructures and higher hardness in some cases. The high hardness at the bottom may also be attributed to the presence of columnar grain structures and the constraint from the substrate.
2. Top layers: The upper layers have fewer thermal cycles but may exhibit higher hardness due to the absence of grain coarsening from subsequent layers and the relatively rapid cooling after the final deposition.
3. Middle layers: These layers experience moderate thermal cycling and may have undergone significant grain coarsening without the rapid cooling benefit of the top layers, resulting in the lowest hardness.

The tensile properties are particularly noteworthy. A minimum tensile strength of 520.19 MPa with an elongation of 41.7% represents excellent ductility for a weld-deposited material. For comparison, the typical tensile properties of wrought 308L stainless steel are approximately 515-620 MPa tensile strength with 35-45% elongation (per ASTM A269 or equivalent). The AM-deposited material achieves properties comparable to or exceeding the wrought base metal, which is remarkable given the non-equilibrium solidification conditions and the presence of residual stresses.

The ductile fracture mode confirms that the microstructure is free from significant brittleness-inducing features such as excessive δ-ferrite, intermetallic compounds, or porosity. This is a critical quality indicator for engineering applications where load-bearing capacity and fatigue resistance are paramount.

Engineering Practice Integration

For pipe and pipe fitting fabrication, the findings from this study have direct relevance to several industrial applications:

• Pipe repair and overlay: TIG AM can be used to build up worn pipe surfaces or repair localized damage in service, with the deposited material exhibiting properties comparable to the base metal.
• Custom fitting fabrication: Complex geometries that are difficult to produce by conventional forming or machining can be fabricated using AM, reducing material waste and manufacturing time.
• Heat exchanger tube fabrication: The ability to deposit thin-walled tubular components with controlled microstructure opens possibilities for custom heat exchanger designs.

However, several challenges must be addressed for industrial implementation:

1. Residual stress management: The cumulative residual stresses in AM components can lead to distortion and cracking, particularly in thick sections. Post-weld stress relief or in-situ heat treatment during deposition may be necessary.
2. Porosity control: Gas porosity and lack-of-fusion porosity are common defects in AM. Process parameter optimization, including travel speed, wire feed rate, and layer thickness, is critical.
3. Scale-up challenges: The properties demonstrated in small straight-wall specimens may not be directly transferable to large, complex components due to differences in thermal history and constraint conditions.

Study Insights and Reflections

This research contributes valuable data to the growing body of knowledge on TIG arc AM of austenitic stainless steels. The agreement between predicted and actual microstructures is particularly encouraging, as it suggests that conventional metallurgical models can be applied to AM processes with appropriate modifications. The excellent mechanical properties achieved — particularly the high ductility — indicate that the AM process can produce materials with competitive performance for structural applications.

From a quality assurance perspective, the consistent achievement of ductile fracture across all tested specimens suggests that the process is robust and not sensitive to minor parameter variations. This is an important characteristic for industrial deployment, where process stability and repeatability are essential. The study also implicitly highlights the importance of microstructural prediction in process development — by understanding the expected solidification behavior, engineers can optimize process parameters to achieve desired microstructural and mechanical outcomes.