In many critical engineering applications, guiding rod and sleeve mechanisms are used for controlled positioning of various components to fulfil the intended applications. For enhanced life cycle of such mechanisms, either one or both of the mating contact surfaces are hard-faced for eliminating/minimizing self-welding as well as to reduce wear of the contact surfaces.

In most of the cases, the sleeve is a fixed component and the inner surface of the sleeve is hard-faced to enhance the life cycle of the mechanism.

Although technologies exist for hard-facing of relatively large inner diameter (ID) components, it is always a challenge to hard face a component with narrow ID. The available hard facing techniques in the industry are not suitable due to limitations posed by the smaller ID.

In this work, a hard facing procedure using weld overlay for components with narrow ID was developed and the hard-faced deposit was characterized. There are many hard facing alloys available worldwide.

Cobalt base alloy (Stellite) is widely used in the engineering industry and the required hardness is given by Cobalt base hard face deposit. However, owing to the formation of cobalt 60 radioactive isotope in the nuclear reactor operation, cobalt base hard facing alloy is not found suitable for nuclear core applications.

Nickel base Colmonoy is another hard facing alloy widely used to resist wear & tear and found suitable under hot sodium working environment in the nuclear reactor. Hence, Ni-based Colmonoy hard facing alloy was chosen as hard facing consumable for fast breeder reactor [FBR] components.

The hard facing technique will be applied to the Control Rod Sheath Assembly of Fast Breeder Test Reactor (FBTR), which is a critical component used for guiding the control rod during raising and lowering of reactor power.

The control rod sheath inner diameter is only 20.2 mm at the hard-faced guiding location. There is a physical contact between the control rod and control rod sheath during entire life of operation and has a relative translating motion between them during the actuation of control rod.

To eliminate the chances of self-welding as well as wear and tear at contact area of these components, control rod diameter is hard chrome plated and corresponding control rod sheath inner diameter is hard faced.

The hard-faced layer is required to have a thickness of 2.5 mm to 3 mm, surface hardness of 40-45 HRC with surface finish of 0.8μm. The development works include nickel base hard facing on austenitic stainless steel 316L using metal cored filler wire and conventional TIG welding with special miniature torch.

To understand the mechanical and metallurgical properties of this hard face deposits, experiments were carried out. The sample was fabricated at preheat temperatures of 450°C using conventional TIG process with nickel based hard facing consumable in metal cored wire form.

The deposited sample was tested through visual examination, liquid dye penetrant testing, hardness testing and ultrasonic testing. The surface hardness measurements were carried out on hard faced deposits using Rockwell hardness testing machine and micro hardness across coating thickness as well as along the coating surface were carried out using Vickers’s Hardness Tester.

The effect of preheat temperatures on hardness, microstructures and wear rate of the deposit are the topic of the discussions.

This article is shared by Aniket Shingewad, M. Krishnamoorthy, Utpal Borah, V. Praveen Kumar, B.S. Ramesh Babu, Hemant Kumar, Chittaranjan Das and S. Surendar Kumar.