Effect of Rapid Cooling on Microstructure and Properties of Cladding Deposits

Research Background and Significance

The cooling rate experienced by a weld overlay deposit during solidification and subsequent cooling is one of the most influential factors governing the final microstructure and mechanical properties of the cladding layer. In rapid cooling scenarios, such as those encountered when welding thin sections, using high travel speeds, or welding on high-thermal-conductivity substrates, the microstructure of the deposit can differ dramatically from that produced under slower cooling conditions. This study systematically investigates the effect of rapid cooling on the microstructure, hardness, and wear resistance of weld overlay deposits, providing valuable insights for process optimization in cladding applications.

Experimental Design and Cooling Rate Control

The study employs a systematic approach to control the cooling rate by varying welding parameters and substrate conditions. The cooling rate is estimated using the Rosenthal equation for a semi-infinite body and is measured directly using thermocouples embedded in the deposit at various depths. Three groups of specimens are produced: a slow-cooling group (cooling rate approximately 1–5 K/s), a moderate-cooling group (5–20 K/s), and a rapid-cooling group (20–80 K/s). The cooling rate is controlled by varying the travel speed, welding current, and the thermal mass of the backing plate. For the rapid-cooling group, a copper backing plate is used to extract heat rapidly, and the travel speed is increased to 25–35 cm/min.

Microstructural Evolution Under Rapid Cooling

Under slow cooling conditions, the deposit microstructure consists of coarse martensite with visible carbide precipitation at grain boundaries and within grains. The grain size is relatively large, with an average grain diameter of 30–50 μm. As the cooling rate increases to the moderate range, the martensite becomes finer, and the carbide precipitation shifts from grain boundaries to intragranular locations. The grain size decreases to 15–25 μm. Under rapid cooling conditions, the microstructure transforms to a fine acicular martensite with a high density of intragranular carbides and a significantly reduced grain size of 5–15 μm. In some cases, the rapid cooling rate is sufficient to suppress the formation of equilibrium carbides entirely, resulting in a supersaturated solid solution with retained austenite.

The presence of retained austenite under rapid cooling is a critical observation. Retained austenite is a soft, ductile phase that can transform to martensite during subsequent service loading, a phenomenon known as transformation-induced plasticity (TRIP). While TRIP can provide beneficial strain hardening under cyclic loading, it can also lead to dimensional instability and unexpected hardening that may cause cracking in subsequent welding passes. The volume fraction of retained austenite increases with cooling rate and can reach 15–25% under rapid cooling conditions, depending on the alloy composition.

Mechanical Properties and Wear Resistance

The hardness of the deposit increases with cooling rate, from approximately 40–45 HRC under slow cooling to 55–62 HRC under rapid cooling. This increase is attributed to the refinement of the martensite lath, the increased density of dislocations, and the formation of finer carbides. However, the ductility and impact energy decrease significantly with increasing cooling rate. The CVN impact energy at 20°C drops from 35–45 J under slow cooling to 5–15 J under rapid cooling, indicating a transition from a ductile to a brittle fracture mode.

The wear resistance, as measured by the taber abrasion test, increases with cooling rate up to a certain point and then plateaus or slightly decreases. The optimal wear resistance is achieved at a moderate-to-high cooling rate of 15–30 K/s, where the hardness is sufficiently high to resist abrasive wear but the microstructure still retains some ductility to accommodate plastic deformation. At very high cooling rates above 50 K/s, the excessive brittleness of the deposit can lead to microcracking during wear testing, which paradoxically reduces the wear resistance.

Engineering Implications and Process Optimization

The study provides clear guidance for engineers seeking to optimize the cooling rate in cladding applications. For applications requiring maximum wear resistance, such as grinding media and crusher components, a moderate-to-high cooling rate of 15–30 K/s is recommended, achieved through controlled travel speed and the use of a mild steel backing plate. For applications requiring toughness, such as pressure vessel repair and structural components, a slow cooling rate of 1–5 K/s is preferred, achieved through preheating, low travel speed, and the use of a high-thermal-mass backing plate. For applications requiring a balance of hardness and toughness, such as hydraulic valve seats and pump impellers, a moderate cooling rate of 5–15 K/s is optimal.

The study also highlights the importance of post-weld heat treatment in controlling the final properties of the deposit. A tempering treatment at 500–550°C for 2 hours can reduce the hardness from 60 HRC to 45–50 HRC while significantly improving the ductility and impact energy, effectively decoupling the hardness-ductility trade-off imposed by the welding process. This post-weld heat treatment is particularly important for rapid-cooled deposits that exhibit excessive brittleness.

Study Insights and Conclusions

The systematic investigation of cooling rate effects on cladding deposits reveals that the cooling rate is a powerful lever for tailoring the microstructure and properties of the overlay layer to meet specific service requirements. The key insight is that there is no single optimal cooling rate; rather, the optimal rate depends on the balance of properties required by the application. Engineers must carefully consider the service environment, loading conditions, and inspection requirements when selecting the welding parameters that determine the cooling rate. The use of a copper backing plate for rapid cooling, or a preheated mild steel backing plate for slow cooling, provides a practical and cost-effective means of controlling the cooling rate without requiring complex equipment modifications. The study also underscores the importance of post-weld heat treatment as a complementary tool for property optimization, particularly when the as-welded microstructure does not meet the required combination of hardness and toughness.