Technology development towards automation & remote fabrication of fuel pins made of modified 9Cr–1Mo steel is in progress for metallic fuels proposed to be used in future in India’s Fast Breeder Reactor (FBR) programme.

An integrated process for laser based pre-heating, Pulsed Laser Beam Welding (PLBW) & post weld laser heat treatment (PWLT) has been developed for fabrication of fuel pins made of modified 9Cr–1Mo steel. The process is automated for operation in hazardous environment and the procedure is qualified.

Laser-based pre-heating of laser beam welding with optics, applied to 9Cr–1Mo steel was studied. Laser beam welding of 9 Cr–1Mo steel is challenging due to martensitic hardening in combination with defects. Pre-tempering aims the reduction of the cooling rates and martensitic microstructure within the weld seam.

An oscillating defocused laser beam was guided over the surface for pre-heating by means of scanner optics. During pre-heating, the laser power, the scanning speed and the number of cycles were varied. Welding with 200W and 2 m/min with a focused laser beam was executed. Thus, the resulting temperature profile behind the ongoing laser beam and cooling time between 800°C–500°C was significantly extended.

Laser welding of the samples were carried at optimized pulsed laser parameters, welding speed to obtain the weld samples at required depth of penetration. Subsequently the samples were subjected to PWHT.

A laser beam with rectangular profile is used for local heat treatment of weld joints. With the increase of laser power and scanning times and the decrease of scanning speed, the heat treatment temperature at the back of the joint increases gradually.

The peak temperature under different parameters changes from 450 to 700 °C, at which tempering takes place. This is consistent with the microstructural evolution from original lath martensite of weld metal and heat-affected zone to tempered martensite after laser heat treatment.

The hardness between weld and softened zone is similar under the same parameters. Analysis of variance reveals that the hardness is mainly influenced by the peak temperature instead of heating time.

The weld hardness decreases gradually with the increase of the peak temperature, and hardness less than 500 VHN is obtained when that temperature is over 600 °C. The micro hardness distribution across fusion zone, heat affected zone was measured and hardness was found less than 450VHN after PWHT.

This article is shared by Mayank Rawat, R. Lavakumar, M. BalaParandhma Raju and T.V. Prabhu.