High-Temperature Low-Ductility Cracking Sensitivity of 52M Alloy Surfacing Layers Prepared by Laser and TIG Surfacing

Literature Overview

The paper by Zeng Chunjie, Li Dong, and Fang Hu from Shanghai University of Engineering Science (published in Materials in Mechanical Engineering, Vol. 43, No. 5, 2019, pp. 30-33) investigates the high-temperature low-ductility cracking (HTLDC) sensitivity of 52M alloy surfacing layers prepared by two different processes: laser surfacing and TIG (GTAW) surfacing. The surfacing layers are deposited on S304 stainless steel substrate, and the study compares the microstructure, phase composition, and cracking behavior of the two surfacing methods under high-temperature straining conditions.

Process Comparison and Microstructural Analysis

The comparison between laser surfacing and TIG surfacing provides valuable insights into how process parameters affect the microstructure and properties of surfacing layers. Both processes produce surfacing layers with a single-phase austenitic microstructure, but significant differences exist in grain morphology and precipitate distribution.

The smaller grain size in the laser surfacing layer is attributed to the higher cooling rate associated with the concentrated laser energy source. The rapid solidification promotes nucleation of austenite grains and limits grain growth, resulting in a finer microstructure. The more regular grain morphology in the laser surfacing layer is likely due to the more directional solidification associated with the laser process.

Precipitation Behavior and Cracking Mechanism

The presence of Cr₂₃C₆ precipitates in both surfacing layers is a critical finding. Cr₂₃C₆ is a chromium carbide that forms at austenite grain boundaries during solidification or subsequent thermal exposure. These precipitates are associated with chromium depletion in the adjacent austenite matrix, which can promote intergranular cracking under certain conditions.

The higher number of Cr₂₃C₆ precipitates in the TIG surfacing layer is attributed to the slower cooling rate, which allows more time for carbide precipitation at grain boundaries. The slower cooling also promotes grain growth, which increases the grain boundary area available for precipitation. In the laser surfacing layer, the rapid cooling suppresses grain boundary precipitation, resulting in fewer Cr₂₃C₆ particles.

The high-temperature low-ductility cracking mechanism involves the following steps:

1. Precipitate formation: Cr₂₃C₆ precipitates form at grain boundaries during solidification or thermal exposure.
2. Chromium depletion: The formation of Cr₂₃C₆ depletes chromium from the adjacent austenite matrix, reducing its resistance to cracking.
3. Strain localization: Under applied strain, deformation localizes at grain boundaries where the matrix has been chromium-depleted.
4. Crack initiation: Micro-cracks initiate at the chromium-depleted grain boundaries, particularly where Cr₂₃C₆ precipitates are present.
5. Crack propagation: The micro-cracks propagate along grain boundaries, leading to intergranular fracture.

Cracking Sensitivity Assessment

The study evaluates HTLDC sensitivity by applying controlled strains at elevated temperatures and counting the resulting cracks. The results show:

• At the same temperature and strain level, the laser surfacing layer has fewer cracks than the TIG surfacing layer.
• As strain increases, the crack count in the laser surfacing layer increases more rapidly than in the TIG surfacing layer.
• Despite the more rapid increase in crack count with strain, the overall HTLDC sensitivity of the laser surfacing layer is lower than that of the TIG surfacing layer.

This seemingly contradictory result (faster crack increase but lower overall sensitivity) can be explained by the initial crack-free condition of the laser surfacing layer. The TIG surfacing layer may already contain some pre-existing micro-cracks or crack initiation sites due to the higher Cr₂₃C₆ content, leading to a higher baseline crack count. The laser surfacing layer, with fewer precipitates and a finer grain structure, starts with fewer crack initiation sites, resulting in a lower overall sensitivity despite the more rapid crack growth with increasing strain.

Process Parameter Effects

The differences in microstructure and cracking behavior between the two surfacing processes can be attributed to the following process parameters:

The rapid cooling rate in laser surfacing is the primary factor responsible for the finer grain structure and reduced precipitation. The shallow melt pool also reduces dilution from the S304 substrate, resulting in a surfacing layer composition closer to the 52M alloy wire composition.

Engineering Practice Implications

For applications requiring corrosion-resistant surfacing on stainless steel substrates, this study provides clear guidance on process selection:

1. Laser surfacing is preferred for applications where HTLDC resistance is critical, such as components subjected to thermal cycling combined with mechanical loading.
2. TIG surfacing may be acceptable for applications with lower thermal cycling severity or where the cracking sensitivity is not a primary concern.
3. Grain refinement through process optimization (e.g., pulsed laser, oscillating beam) can further improve HTLDC resistance.
4. Heat input control is critical in TIG surfacing to minimize grain growth and precipitation.

The 52M alloy is a common stainless steel welding consumable used for surfacing applications in the chemical, petrochemical, and power generation industries. The findings of this study are directly relevant to the selection of surfacing processes for these applications.

Key Questions and Reflections

Several important questions arise from this study:

1. What is the effect of post-weld heat treatment on the HTLDC sensitivity of the surfacing layers? Would annealing or solution treatment reduce the Cr₂₃C₆ content and improve cracking resistance?
2. How does the substrate material affect the surfacing layer properties? The S304 substrate may dilute into the surfacing layer, affecting the Cr₂₃C₆ precipitation behavior.
3. What is the long-term stability of the surfacing layer properties under service conditions? Would prolonged thermal exposure at operating temperatures promote additional precipitation and cracking?
4. How does the surfacing layer thickness affect the HTLDC sensitivity? Thicker layers may have different thermal histories and microstructural characteristics.

The study focuses on a specific alloy (52M) and substrate (S304), and the findings may not be directly applicable to other alloy systems. However, the fundamental mechanisms identified (precipitation, chromium depletion, grain boundary cracking) are likely to be relevant to other austenitic stainless steel surfacing applications.

Study Insights and Implications

This research provides valuable insights into the microstructural mechanisms governing high-temperature low-ductility cracking in stainless steel surfacing layers. The comparison between laser and TIG surfacing clearly demonstrates that process selection significantly affects the cracking resistance of the surfacing layer. The finer grain structure and reduced precipitation in laser surfacing layers translate directly into improved HTLDC resistance, which is a critical property for components subjected to thermal cycling and mechanical loading. The study also highlights the importance of understanding the relationship between process parameters, microstructure, and service performance in surfacing applications. For engineers selecting surfacing processes for critical applications, this research provides a clear basis for preferring laser surfacing when HTLDC resistance is a primary concern, despite the potentially higher equipment costs associated with laser systems. The fundamental understanding of Cr₂₃C₆ precipitation and its role in cracking initiation can also guide alloy design and process optimization for future surfacing applications.