In the last decade aluminium started to be considered as an alternative to steel to produce car body panels, especially considering the strict demands to decrease fuel consumption which require vehicle weight reduction. In order to keep their leading role, steel companies have to produce stronger materials to reduce the thickness of steel sheets used in cars and are now considering non-conventional steel making processes. The purpose of this PhD research was to investigate the possibility of strengthening thin sheets of interstitial free steel (IF steel) by using carbon rich films deposited on the steel surface using Physical Vapour Deposition (PVD). These films then act as a carbon reservoir which upon heat treatment release carbon in the IF steel and strengthen it. Coated tensile coupons 200 μm thick were annealed at different temperatures under high vacuum. Tensile tests show that a 100 MPa increase in yield stress can be obtained after annealing at 430 ˚C for 1h in high vacuum. The effects of annealing environment, film thickness and prestrain on carbon diffusion were also investigated. It was shown that carbon diffusion from the film to the IF steel substrate is limited by the film transformation into cementite at temperatures equal or higher than 530 ˚C. All tensile curves showed a plastic instability known as Lüders plateau, which is undesirable as it results in surface markings on the deformed part. FEM analyses were performed to find ways to suppress the Lüders plateau, proving that increasing strain-hardening or having a graded instead of uniform carbon content through thickness can suppress or limit Lüdering. The possibility of creating a through thickness gradient of microstructure was investigated as it could suppress Lüdering and result in higher strength. For these tests, FeC coated coupons were induction heated to 820 ˚C followed by water quenching. After only 2 minutes of heat treatment the yield stress was increased by 250 MPa and the ultimate tensile strength reached 400 MPa. With an annealing of 4 minutes, the Lüders plateau was fully suppressed and the microstructure consisted in ferrite grains and TiC nanocarbides. This work demonstrates that FeC films can be effectively used to diffuse carbon into steel and that a significant increase in mechanical properties can be obtained after a heat treatment of only a few minutes.

Heterogeneous welded joints – type ferrite/austenite is inseparable part of structure for energy industry. Welding conditions and post weld heat treatment have a significant impact on the structural stability of welded joint. The structure determines resulting mechanical properties that determine lifetime of these joints. At the same time in microstructure changes in heat affected zone of the base material is diffusion of carbon through the weld interface. This work is focused on the analysis of structural stability of heterogeneous weld carbon/austenitic steel, which was formed carbon steel 22K (base material) and austenitic buttering layer EA 395/9 with a higher content of nickel. Evaluation of the structure was focused on the structure of weld metal (Böhler FOX SAS 2) and structure of the heat affected zone of base material. Metallographic evaluation was performed on the light microscope and scanning electron microscope equipped with the energy and wave dispersive analysis, which identified the contents of substitutional elements and carbon on the interface of carbon/austenitic steel. The results of metallographic analysis were compared with measurements of Vickers hardness and microhardness and calculations using Thermo-Calc software and software SVARY. The description and evaluation of the stability of the weld joints was determined by carbon content of the course interface using wave dispersive spectroscopy. The results of energy dispersive analysis were used to determine the type of carbides present at the interface of carbon/austenitic steel.