This thesis is a comparative study of the Thermo-Mechanical Fatigue (TMF) performance of different grades of SiMo nodular cast iron for heavy-duty diesel engine exhaust gas manifold applications. The TMF performance of the current SiMo variant used to manufacture exhaust manifolds - SiMo 5.10 (C-3.25Si-4.45Mo-0.76), is compared with that of the variants SiMo 4.05 (C-3.22Si-4.66Mo-0.56) and SiMoNi (C-3.3Si-4.5Mo-1Ni-1.3) by performing three out-of-phase (OP) TMF test series under partial constraint conditions. A benchmark TMF test series in the temperature range: 50 ˚C to 550 ˚C with a hold time of 30 s at 550 ˚C showed that SiMo 5.10 had relatively better performance due to development of lower mechanical crack driving forces compared to other variants. However, a long holding time of 600 s at 550 ˚C saw a larger decrease of average TMF lifetimes for SiMo 5.10 than that of SiMo 4.05 despite similar crack driving forces. An investigation of the stress relaxation during TMF of the two variants showed that the SiMo 4.05 performs better during long hold time due to better stress relaxation properties. The SiMoNi variant which is very brittle at low temperatures was found to fail by a fracture by overloading mechanism taking over quite early in the fatigue cycle; which is confirmed by examination of the fracture surfaces and numerical estimations. This also explained the low lifetimes and scatter in previously performed TMF tests under total constraint conditions. The TMF test series performed in the temperature range: 150 ˚C to 550 ˚C with a hold time of 30 s at 550 ˚C found that a heat-treatment seemed to reduce the TMF performance of the SiMo 5.10 variant. Metallographic investigations and hardness measurements of as-cast and heat-treated materials revealed that the distribution of the Mo-rich phase from the grain boundary regions into the matrix due to an annealing heat-treatment seemed to affect the TMF performance.

Railway transport plays a vital role in a country’s economic growth. Derailments result in loss of life, damage to rolling stock, service disruptions and harm the environment leading to the decrease in economic growth of a country. Improving train operating safety has been a high priority of the rail industry and the government. Train accidents occur as a result of many different causes; however, some are much more prevalent than others. One of the important causes for derailment is Rolling contact fatigue (RCF), which has gained more importance after the 1990’s due to the several derailment accidents caused by it. Rolling contact fatigue is a group of rail damages that manifest themselves on the surface or near surface regions of the rail. RCF arises due to the rolling/sliding contact stresses that occur between rail and wheel leading to severe plastic deformation of the rail head with RCF cracks resulting in high maintenance costs. Another consequence of plastic deformation is the formation of white etching layer (WEL), a martensitic layer created from the deformed pearlite while the rail is in service. Effect of WEL is to reduce the resistance of rail steel to crack initiation because of its brittle nature.
In recent years, WEL has generated considerable research interests on its behavior under RCF conditions. Though most of the research carried out on WEL focuses either on the thermal or mechanical effects on its formation, little research has been carried out on the combined thermo-mechanical effects on WEL formation. In the present work, an attempt has been made to study the formation of WEL by thermo-mechanical simulation using the GLEEBLE thermo-mechanical simulator. Cylindrical samples prepared from the unaffected portion of the Dutch rail steel R260Mn were subjected to simultaneous deformation of maximum effective compressive strain of 3% and heating to maximum temperatures ranging from 650°C to 930°C. The simulation was carried out by varying the maximum temperature and no. of cycles. The metallurgical characterization of simulated WEL was performed using optical microscopy and scanning electron microscopy. The micro-hardness of WEL was studied using Vickers micro-hardness tester. The chemical composition of Mn was studied using electron probe microanalysis to ascertain the effect of Mn on band like formation occurred in samples processed at a certain temperature.
It is observed that the simultaneous heating and deformation increases the amount of WEL formed when compared to simulation by only heating. Hardness is found to increase with increase in temperature. When the no. of cycles increases the amount of WEL formation also increases. Simulation at very high temperature results in homogenous formation of WEL with negligible amount of untransformed pearlite.

Wire Arc Additive Manufacturing (WAAM), one of the Additive Manufacturing (AM) deposition methods which employs the basic principles of Gas Metal Arc Welding (GMAW) welding technique was used to show that it is possible to build moulds for ceramic products using AISI 420 stainless steel due to its characteristics such as corrosion resistance, machinability, hardness and dimensional stability. The WAAM process also is efficient in terms of its deposition rate, reduced material wastage and high surface quality.

To investigate the feasibility of AM of AISI 420 stainless steel using WAAM, Response Surface Methodology (RSM - a predictive technique) was used to navigate within the input parameter range for process optimization. Bead-on-plate welding experiments were performed with a MIG welding robot on a structural steel (S355J2) as the substrate. In the tested range according to RSM analyses, the optimum weld condition was 261 A (Current), 29 V (Volts) and 0.59 m/min S (Travel Speed) with preheating at 200°C. However, this condition was found to be unsuitable for AM due to its low deposition speed, non - uniform building surface morphology and inter-run porosities when overlapping welds were deposited. Further analyses on the metallurgy of the WAAM AISI 420 stainless steel through Scanning Electron Microscope (SEM) revealed that the weld metal consisted of a martensite matrix and delta-ferrite at the grain boundaries. The Vickers Hardness of the weld metal was 514HV.

To achieve WAAM feasibility using AISI 420 stainless steel, modified RSM method was performed by expanding the input parameters and visually inspecting the bead for its shape, size and quality. The experiment adopted was called the Ramping Procedure wherein a single resulting weld bead can represent many input parameter combinations. The optimum condition for the input parameters were identified to be 200 A (Current), 18.5 V (Volts) and 1.00 m/min S (Travel Speed). With the optimal processing conditions, rectangular blocks or walls were modelled and designed in the Autodesk Powermill software and built to evaluate the feasibility of WAAM AISI 420 stainless steel. Samples were built without preheating and no surface defects and cracking were observed. Microstructural and hardness studies were then performed. Results show that the as-welded weld metal consisted of delta-ferrite present in a martensite matrix. The hardness of the weld metal was 623 HV.

The WAAM optimization procedure for AISI 420 stainless steel that has not been explored for AM processes due to its high sensitivity to welding thermal cycles that can lead to cracking. In this study, it has been successfully demonstrated that crack free AISI 420 stainless steel can be deposited with WAAM.