Kamaraj M

The objective of the study was to carry out creep and creep crack growth studies on as received and aged steels with a view to develop a method for estimating remaining life of CrMoV steel components when a crack develops during service. In service, the material is subjected to elevated temperatures, which results in aging due to transformation of bainitic structure, nucleation and growth of carbides and changes in their morphology. This results in degradation in mechanical properties such as hardness, tensile strength and creep properties. Therefore, it was proposed to study the creep behaviour of as-received and aged steels and understand the role of microstructures in creep deformation and fracture of this steel and use this understanding to predict the remaining life of turbine components. The microstructural condition of the service exposed material was simulated by accelerated aging carried out at 873K (600°C) for 3648 hours which is equivalent to the material condition exposed at 813K (540°C) for 200,000h. Creep and creep crack growth (CCG) testing on as received (normalised and tempered) and aged 1Cr1Mo1/4V rotor forging steel has been carried out at 773, 813, 848 & 873K (500, 540, 575 and 600°C) at four different stress levels in each material condition in the range of 110 to 350 MPa. Results of 873K (600°C) only shall be discussed in this paper in detail. Single edge notch tension (SENT) type of specimens were used for creep crack growth testing.

The wear behaviour of a sintered Fe-based alloy reinforced with mechanically alloyed intermetallic has been studied using pin-on-disk wear testing machine in dry condition. A sintered iron alloy made by powder metallurgy was used as matrix. The intermetallics Fe3Al particles prepared by mechanically alloying (15 hours) have been used as reinforcement (5-10%). The processing of the composites includes mixing and cold compacting followed by sintering at 1120°C in Nitrogen atmosphere. The presence and distribution of intermetallic Fe3́Al phase on the surfaces were identified with optical microscopy and further confirmed by XRD. The influence of mechanically alloyed Fe3Al intermetallics additions on the wear behaviour was studied with different sliding conditions. The experimental results indicate that improvement in wear properties in MMCs compared to plain sintered steel as standard material. Wear tracks were characterised by SEM techniques. Studies reveal that unreinforced sintered steel shows lower wear resistance, while composites with mechanically alloyed Fe3Al are the most wear resistant. Copyright © 2004 by Society for Advancement of Heat Treatment & Surface Engineering (SAHTSE).

Significant efforts have been made over the years at Hindalco Smelter Renukoot to improve the efficiency of smelter operation by pushing the design limit of various components. Enhancing the life of copper bar conductor, which holds the anode assembly, is one such example. These bars are exposed to static load of the anode assembly and severe thermal conditions and due to bath proximity. Failure of these bars was seen due to cracks after few cycles, thus preventing repeated use of these bars. The causes of failure have been analyzed by conducting a combination of thermo-mechanical modeling and micro-structural analysis. A simple finite element model with experimentally measured temperatures as boundary conditions has been used for thermal stress analysis. The creep deformation has been identified as the primary mechanism. The point of onset of creep can be attributed to micro-structural deficiencies and high temperature exposure of the bar.

The sliding wear behaviour of nickel-based Colmonoy-6 and Anval B60 hardfacing alloys were investigated at room temperature under contact stresses of 1 and 3.5 MPa for sliding distances upto 9 km. Commercial, Colmonoy 6 bare rods were deposited by gas tungsten arc welding (GTAW) and Anval B60 powder by plasma transferred arc welding (PTAW) processes respectively on stainless steel substrate. The hardness measurements across the interface revealed that, over 2.5 mm from the interface, the hardness along the deposits, were found to be 606 and 625 HV for Anval B60 and Colmonoy 6 respectively for single pass. XRD studies showed the presence of borides and carbides along with γ-nickel matrix for both deposits. Pin-on-disc wear tests showed the weight loss gradually increased with increase in sliding distance but steeply with increase in contact stress in both cases. Increase in coefficient of friction with increase in contact stress was also observed on both the deposits. Copyright © 2004 by Society for Advancement of Heat Treatment & Surface Engineering (SAHTSE).