NTPC Transactions on Energy Research

Modified 9Cr-1Mo steel (T91) is widely used in power generation due to its superior high-temperature strength, extended creep rupture life, excellent oxidation and corrosion resistance, and good weldability. However, weld joints in T91 components are often the most failure-prone regions, making it essential to assess their behaviour for long-term reliability. Post-weld heat treatment (PWHT) plays a critical role by relieving residual stresses and tempering the martensitic structure in the fusion zone, thereby minimizing strength gradients across the weld. In field applications, verifying the effectiveness of PWHT requires dependable non-destructive evaluation (NDE) methods. While portable hardness testing is commonly employed, it often yields inconsistent results and may not reliably reflect the overall condition of the material.

An alternative approach involves measuring Magnetic Coercive Force (MCF), which correlates with magnetic hardness in ferromagnetic materials. This study aimed to investigate the influence of different PWHT conditions on the MCF of T91 welds. The experiments examined how MCF varied under a range of PWHT cycles, with soaking temperatures up to 790℃ and durations up to 120 minutes. Results indicated that higher soaking temperatures and longer durations led to a decrease in MCF values within the weld metal. This reduction aligned with lower hardness levels and increased carbide precipitation in the microstructure. Based on these results, a threshold for acceptable MCF values has been established. Exceeding this threshold can signal the need for corrective actions, enabling proactive maintenance and reducing the risk of premature failures or unexpected shutdowns in thermal power plants.