Effect of Rapid Cooling on Microstructure and Properties of Weld Overlay Deposits

Literature Overview and Research Context

This study examines the influence of rapid cooling rates on the microstructure evolution, mechanical properties, and service behavior of weld overlay deposits produced by plasma transferred arc (PTA) cladding and laser cladding processes. The research focuses on nickel-based alloy deposits (Inconel 625 and Hastelloy C276) applied to low-alloy steel substrates, which are common configurations in hydrogenation reactor linings and acid-resistant heat exchangers. The key experimental variable is the cooling rate, which is manipulated through adjustments to travel speed, heat input, and the use of chill plates or water quenching during the welding process. Cooling rates are quantified in the range of 5–500°C/s, covering both conventional slow-cooling conditions and rapid solidification regimes. This work is particularly relevant because the microstructure of weld overlay deposits is highly sensitive to thermal cycling, and understanding the cooling rate threshold at which detrimental phases form or beneficial phases are suppressed is essential for optimizing cladding process parameters.

Microstructural Evolution Under Different Cooling Rates

At low cooling rates (5–20°C/s), typical of conventional submerged arc welding or slow PTA cladding, the Inconel 625 deposit exhibits a fully austenitic microstructure with equiaxed γ grains and a moderate density of Laves phase (Ni2MoTi) precipitates. The Laves phase, which forms at grain boundaries and can cause intergranular cracking, is present at approximately 8–12 vol% at these cooling rates. As the cooling rate increases to 50–100°C/s, characteristic of high-speed PTA cladding, the Laves phase fraction drops to 3–5 vol%, and the γ grain size decreases from approximately 80 μm to 35 μm. At cooling rates exceeding 200°C/s, achievable through laser cladding or high-speed PTA, the deposit transitions to a fully single-phase austenitic structure with no detectable Laves phase, and the grain size is reduced to 10–15 μm. For Hastelloy C276 deposits, a similar trend is observed, but the critical cooling rate for eliminating the brittle Ni3Mo phase is approximately 150°C/s, compared to 100°C/s for Inconel 625.

Mechanical Properties and Corrosion Behavior

The mechanical properties of the deposits show a complex relationship with cooling rate. Tensile strength increases with cooling rate due to grain refinement, rising from approximately 950 MPa to 1100 MPa for Inconel 625 as the cooling rate increases from 10°C/s to 400°C/s. However, elongation decreases slightly at very high cooling rates (above 300°C/s) due to the formation of fine dendritic structures that limit dislocation mobility. The most significant improvement is in corrosion resistance. Intergranular corrosion testing (ASTM A263 6b method) shows that deposits cooled at rates below 50°C/s exhibit severe intergranular attack after 48 hours of exposure, while deposits cooled above 200°C/s show no detectable intergranular corrosion even after 168 hours. This improvement is directly attributable to the elimination of the Laves phase, which is a Mo-depleted, Cr-depleted brittle intermetallic that acts as a preferential corrosion initiation site.

Process Optimization and Engineering Recommendations

The study establishes clear process windows for achieving optimal deposit properties. For PTA cladding of Inconel 625, a travel speed of 150–200 mm/min with a heat input of 2.5–3.5 kJ/mm is recommended to achieve cooling rates above 150°C/s. For laser cladding, a laser power of 3–5 kW with a scanning speed of 20–40 mm/min and powder feed rate of 8–12 g/min produces cooling rates in the 300–500°C/s range. The use of a copper chill plate beneath the substrate during welding can increase the cooling rate by 40–60% without altering the welding parameters, which is a practical and cost-effective approach for production environments. However, engineers must be cautious about the residual stresses introduced by rapid cooling, as cooling rates above 300°C/s can generate tensile residual stresses exceeding 400 MPa in the substrate, potentially leading to distortion or cracking in thin-walled components. Post-weld stress relief at 700°C for 2 hours is recommended when rapid cooling methods are employed, provided the stress relief temperature does not exceed the solidus temperature of the deposit.

Study Insights and Reflections

The central message of this study is that cooling rate is not merely a process parameter but a design variable that directly determines the metallurgical quality of weld overlay deposits. In many fabrication shops, the focus is placed on achieving the correct dilution ratio and bond strength, while cooling rate is treated as an incidental outcome of the welding setup. This study argues that cooling rate should be elevated to the same level of importance as dilution control, particularly for nickel-based alloy claddings where the formation of brittle intermetallic phases can compromise both mechanical integrity and corrosion resistance. From a quality assurance perspective, cooling rate measurement during production welding is challenging and is not routinely performed. The study suggests that in-situ thermocouple monitoring at the weld pool boundary, combined with thermal modeling, can provide reliable cooling rate data for process qualification. Engineers involved in cladding specification and process development should integrate cooling rate requirements into their welding procedure specifications and qualification records, treating it as a critical process parameter alongside current, voltage, and travel speed.