Effect of Interpass Time on Microstructure of Weld Overlay Cladding

Literature Overview

The study by Xu Yan, Jiang Xiangsheng, Zhou Jianping, Xue Ruilei, and Yiliham Abuduremu, published in Hot Working Technology in 2017 and supported by the Xinjiang Uygur Autonomous Region Science and Technology Talent Training Program (gn2015yx008), investigates how interpass temperature and interpass time influence the microstructural evolution in weld overlay cladding deposits. The research was conducted in collaboration between Xinjiang University and Xinjiang Weiao Technology Co., Ltd., reflecting a strong industry-academia partnership. The work addresses a practical concern in multi-pass overlay welding where thermal management between passes directly affects grain morphology, phase distribution, and ultimately the corrosion and wear resistance of the cladding layer.

Core Technical Points

The interpass temperature is a critical parameter in multi-pass overlay welding because it governs the cooling rate of the previously deposited layers. When interpass time is too short, the base metal and prior weld layers remain at elevated temperatures, resulting in a slow cooling rate for the subsequent pass. This slow cooling promotes the growth of coarse columnar grains and the formation of equilibrium phases such as ferrite, which can compromise toughness. Conversely, excessively long interpass times allow the joint to cool to a lower temperature, increasing the cooling rate and potentially leading to retained austenite instability or martensitic transformations in susceptible alloy systems.

The study employed metallographic examination and microhardness profiling to characterize the microstructure across multiple passes under different interpass conditions. The key findings can be summarized as follows:

The research demonstrated that an optimal interpass temperature window exists, typically between 150 and 300 degrees Celsius for common stainless steel overlay systems, where the microstructure achieves a desirable balance between hardness and fracture resistance. The grain refinement achieved at moderate interpass temperatures is attributed to the nucleation of new grains at the solidification front during subsequent passes, effectively interrupting the columnar grain growth that would otherwise propagate through all layers.

Interpretation of Technical Points

The interpass time control is fundamentally a thermal management strategy. In engineering practice, the interpass temperature is often monitored using infrared thermometers or embedded thermocouples. The thermal inertia of thick plates and large structural components means that the cooling curve is not linear, and the effective interpass temperature at the weld root may differ significantly from the surface reading. This spatial temperature gradient must be accounted for when setting interpass time targets.

From a metallurgical perspective, the interpass temperature affects the solidification mode of the overlay metal. At higher interpass temperatures, the thermal gradient at the solidification front is reduced, favoring equiaxed dendrite nucleation from pre-existing oxide particles or grain boundaries. This results in a finer, more isotropic microstructure. At lower interpass temperatures, the thermal gradient is steep, promoting columnar dendritic growth that is susceptible to hot cracking and segregation.

The study also highlights the importance of the heat input per pass in conjunction with interpass time. High heat input combined with short interpass times can lead to excessive thermal cycles, potentially causing grain coarsening in the heat-affected zone (HAZ) of the base metal. This is particularly relevant for overlay welding on low-alloy steels where the HAZ is susceptible to softening or tempering.

Process and Standards Analysis

The interpass temperature control aligns with requirements in ASME Section IX and GB/T 150 for qualified welding procedures. In particular, the qualification of overlay welding procedures under NB/T 47014 requires demonstration of acceptable mechanical properties across a range of thermal conditions. The interpass temperature is typically specified within a range, and the actual welding procedure must be executed within that range to maintain qualification validity.

The practical implication is that welders and supervisors must develop discipline in monitoring interpass temperatures. In field conditions, wind, ambient temperature, and component geometry all affect the cooling rate. A preheat temperature of 100 to 150 degrees Celsius is often applied to control the initial cooling rate, but the interpass temperature management becomes the dominant factor for subsequent passes.

Integration with Engineering Practice

In the fabrication of clad pressure vessels and heat exchanger tubesheets, the overlay layer is often deposited in multiple passes to achieve the required thickness. The interpass temperature control directly affects the bond strength between the overlay layer and the base metal. A too-high interpass temperature can cause excessive diffusion at the interface, leading to a transition zone with reduced corrosion resistance. A too-low interpass temperature can cause cracking due to thermal stresses.

For hydrogenation reactor tubesheets, where nickel-based alloy overlays are applied to carbon steel, the interpass temperature management is even more critical. The coefficient of thermal expansion mismatch between the nickel alloy and the carbon steel base creates residual stresses that are sensitive to the thermal history. The study's findings provide a basis for establishing welding procedure specifications that minimize these stresses while maintaining adequate deposit properties.

The use of thermocouple monitoring during production welding, as recommended in this study, is a quality control measure that should be incorporated into the inspection plan. The interpass temperature record should be maintained as part of the welding log, enabling traceability and process verification.

Key Questions and Reflections

A significant question arising from this study is whether the interpass temperature control can be automated through real-time monitoring and feedback systems. While the study focuses on manual monitoring, the trend in modern fabrication facilities is toward automated welding with integrated thermal control. The challenge lies in the variability of component geometry and the need for adaptive control strategies that account for the thermal mass of the component.

Another reflection concerns the interaction between interpass time and post-weld heat treatment. If a post-weld stress relief treatment is planned, the interpass temperature during welding may have less impact on the final properties because the stress relief cycle will homogenize the microstructure. However, for overlay layers that are not stress-relieved, such as certain wear-resistant coatings, the interpass temperature control becomes the primary means of achieving the desired microstructure.

Study Insights and Implications

The study by Xu Yan and colleagues provides a clear demonstration that interpass time is not merely a scheduling parameter but a metallurgical control variable. The findings reinforce the importance of thermal management in multi-pass overlay welding and provide quantitative guidance for setting interpass temperature targets. For engineers involved in the design and fabrication of bimetal products, this research underscores the need for detailed thermal analysis during welding procedure development and the incorporation of interpass temperature monitoring into quality assurance protocols. The practical value of this work lies in its direct applicability to production environments where overlay welding is performed on pressure vessels, heat exchangers, and other critical components where the integrity of the cladding layer is paramount.