Ladislav Falat

Ageing Effects on Microstructure, Mechanical Properties, and Fracture Behaviour of 9Cr-1.5Mo-1Co-VNbBN Martensitic Steel Welded Joint for High Temperature Application

This paper deals with long-term ageing effects of 9Cr-1.5Mo-1Co-VNbBN (CB2) steel weldment on its impact toughness, creep rupture behaviour, and hardness in relation to microstructure and fracture characteristics. The weldment was studied in PWHT state and after isothermal expositions at 625°C for 10000 and 30000 hours. Microstructure evolution was studied using analytical scanning and transmission electron microscopy. Charpy V-notch impact toughness tests were performed for all heat-treated states with a notch location in distinct weld regions such as weld metal (WM), heat-affected zone (HAZ), and base material (BM). The overheated HAZ region exhibited the lowest impact toughness as a result of severe welding induced microstructure degradation. Creep tests were performed at 625°C in the stress range between 80 and 120 MPa. At the highest applied stress, creep fracture occurred in WM, whereas at lower stresses the failure position shifted towards fusion zone at WM/HAZ interface. The hardness profiles experienced significant scattering due to weld microstructural heterogeneity. The major fracture mechanisms involved transgranular quasi cleavage and intergranular creep cracking in impact and creep loading conditions, respectively.

Effect of Microsecond Pulse Laser Modification on Electromagnetic Properties of Grain Oriented Silicon Steel

A microsecond pulsed laser beam was used to local magnetic domain modification of electrical grain oriented silicon steel. It was carried out using three different laser pulse regimes: a single pulse laser regime, a multipulse laser regime and a multipulse laser regime with modulation of laser pulses. The laser processing variables were pulse energy and and number of pulses. The samples were tested for nanohardness and coercivity before and after laser treatment. Light optical microscopy, scanning electron microscopy and magnetic force microscopy were used to observe the cross-sectional profile, surface of the samples, and magnetic domain visualization, respectively. The local laser treatment of grain oriented silicon steel surface has been studied in terms of its influence on the magnetic domains and coercivity. It was found that laser-modified samples showed coercivity improvement in comparison to the non-treated samples. The most significant improvement in coercivity was obtained in the modulated multipulse regime and negligible improvement in the single pulse laser regime. Three main effects responsible for the observed improvement were identified, namely: magnetic domain refinement, influence of number of laser pulses and shape of laser HAZ profile. The present work highlights on differences in the magnetic domain structure, microstructure of the laser modified material and basic electromagnetic and mechanical properties. In present study, the pulse laser surface processing was presented as a useful energy efficient alternative to other techniques e.g. mechanical scribing, electrical discharge scribing, plasma jet scribing, etc. The refined magnetic domains in electrosteels are responsible for the observed low coercivity, which indicates perspective application of the investigated laser modified steels in the power transformer cores with lower core losses.

The Influence of Thermal Exposure and Hydrogen Charging on the Notch Tensile Properties and Fracture Behaviour of Dissimilar T91/TP316H Weldments

Abstract The effects of ageing and hydrogen charging on the notch tensile properties and fracture behaviour of individual heat-affected zones (HAZ) and Ni-based weld metal (Ni WM) of T91/TP316H weldments were investigated. After the post-weld heat treatment at 750 °C for 1 h the weldments were annealed at 600 °C for 1000 h and 5000 h, respectively. All heat-treated states were studied in condition without as well as with hydrogen charging. Thermal expositions led to additional precipitation and microstructure coarsening but their influence on tensile strength was insignificant. In contrast, remarkable plasticity decrease and the fracture mode transition from ductile dimple tearing to transgranular cleavage were observed. The combined effects of thermal exposure and hydrogen charging were more complex. Whereas the regions of Ni WM and TP316H HAZ did not show any significant change in strength, the hydrogen effect caused the strength increase in T91 HAZ. Although the hydrogen embrittling effects were clearly manifested by decreasing plasticity, their significance was getting smaller with increasing annealing duration. The fracture behaviour of thermally exposed and hydrogen charged regions exhibited mixed fracture modes including transgranular cleavage, intergranular dimple fracture and intergranular decohesion.

Abrasive wear resistance of modified X37CrMoV5-1 hot work tool steel after conventional and laser heat treatment

Abstract The effects of two conventional heat treatments and one innovative processing by laser surface remelting of modified X37CrMoV5-1 tool steel on its abrasion wear resistance were investigated. Conventional heat treatments consisted of quenching from 990 °C, individually followed by two different tempering treatments to achieve secondary hardness either at 520 °C or 560 °C. Laser surface remelting was performed using optimized parameters of continuous laser beam scanning mode. The results showed that the highest wear resistance was obtained for almost carbide-free, surface remelted microstructure with the highest hardness due to microstructural refinement and martensitic transformation hardening. The observed differences in wear resistance among individual material states are discussed in relation to their microstructures, hardness, and wear mechanisms characteristics.