Papers by Keyword: Microstructural Changes

Abstract: The martensitic stainless steel ASTM A743 CA6NM is typically used in the production of hydroelectric turbines due to its known high resistance to cavitation induced surface damages. Despite the fact the material presents a high resistance to cavitation, depending on the loading condition to which the turbine runner is subjected and on its geometry, fatigue cracks can develop, thus requiring repair by means of removing material around the crack, up to its complete elimination, and by depositing weld metal in the cavity followed by a grinding process, in order to recover the original runner geometry. Such a repair process is normally done on site, which means that it is not possible to carry out the post weld heat treatment necessary to bring the newly deposited weld metal and the base metal to the same microstructure encountered in the runner when it comes out of manufacture. In order to study the effect that a layered welding has on the base material and on the weld metal, this work aims at studying the microstructural changes that occur in the CA6NM stainless steel welded in multiple layers with a AWS 410 NiMo welding wire. In order to attain such an objective, several 410 NiMo weld beads were deposited in successive layers on the border of a 5mm thick sheet while the resulting temperature fields were monitored by a thermographic camera. After the welding process, the samples cut from the welded sheet were examined in the perpendicular direction of deposition and their resulting microstructures where analyzed and correlated with the temperature history recorded during the welding process. Hardness tests were also carried out.

Abstract: The present work analyzes the influence of an orthogonal machining process on the generation of nanocrystalline surface layers. Thereby, AISI 4140 is used as work piece material. Metallic parts with a severe nanocrystalline grain refinement in the near-surface area show many beneficial properties. Such surface layers considerably influence the friction and wear characteristics of the work piece in a subsequent usage as design elements working under tribological loads. The focus of this paper is an experimental analysis of a finishing orthogonal cutting operation, carried out with a broaching machine, to generate nanocrystalline surface layers. The influence of process and geometry parameters on the generation of nanocrystalline surfaces is investigated with the aim to massively decrease the grain size in the work piece surface layer. Parameters that are studied and taken into account in the manufacturing process are cutting edge radius rβ, depth of cut h and cutting velocity vc. The cutting edge radius rβ is modified by a drag finishing process. The generation of nanocrystalline surface layers is especially influenced by the design of the uncoated carbide cutting tools. Additionally, cutting force Fc and passive force Fp are determined by a 3-component dynamometer to calculate the relationship between specific cutting force kc and specific passive force kp. The temperature beneath the clearance face is detected by a fiber optic pyrometer. These measurement methods and devices are applied to detect the impact of the most relevant measurement values occurring during machining and causing a drastic reduction of grain size in the surface layer. The evaluation of the manufacturing process is carried out by detailed analyses of the microstructural conditions in the surface layer after processing using a Focused Ion Beam (FIB) system. These material characterizations provide information about the surface engineering concerning the microstructural changes in the surface layer of the work piece due to finishing orthogonal cutting processes.

Abstract: The purpose of this study is to analyze the effect of glow discharge nitriding on hydrogen degradation of two types of steels: two-phase austenitic-ferritic and single-phase austenitic. The nitriding process resulted in formation of surface layers composed of expanded austenite (S phase), and in the case of two-phase steel of expanded austenite and expanded ferrite. Microstructural changes occurring under the influence of hydrogen on steels without and with nitrided layers were investigated with the use of scanning (SEM) and transmission (TEM) electron microscopy techniques. It was shown that the density of cracks formed during cathodic hydrogen charging is higher on the surface of the non-nitrided steels compared to the nitrided steels after identical hydrogen charging process. Moreover in non nitrided steel hydrogenation leads to considerable increase of dislocation density, which results from the high concentration of hydrogen absorbed to the steel during its cathodic charging. This leads in turn to high stress concentration and local embrittlement giving rise to cracks formation. Conversely nitriding reduces the absorption of hydrogen and prevents structural changes resulting in hydrogen embrittlement. The conducted studies show that glow discharge nitriding can be used to increase resistance to hydrogen embrittlement of austenitic and austenitic ferritic stainless steels.

Abstract: Critical high temperature components of machines and structures are often subjected to complicated load and temperature histories. The closest laboratory simulation of service loading conditions involves creep under nonsteady temperatures and stresses. For example, the start up and shut down cycles can be well simulated by temperature variation by use of intermittent heating tests. Such approach is illustrated by recent experimental results on advanced high creep strength 9- 12%Cr ferritic-martensitic steels (P91, P92 and E911). A comparison between the creep characteristics of nonsteady and monotonously creep specimens has revealed no significant deterioration of the creep strength and fracture resistance of these steels in power-law (dislocation) creep.

Abstract: Iron-base superalloys are well known materials having excellent high temperature properties .They are used in turbo superchargers and turbine engines required for aerospace and power plants. In this investigation precipitation hardenable X5NiCrTi26-15 was used to study the influence of microstructural changes on the creep behavior at different conditions . Different creep cycles were applied for both base alloy and laser beam welded alloy (6kW CO2 ) namely at 600 , 625 and 650C at applied controlled creep stresses of 400 and 450 MPa . The base material sheet was used in as solution annealed state ( 30 min, 960 C, WQ ).The specimens were hardened in two steps (24h,760 C, FC and 16 h, 705 C,AC ) before being investigated . The microstructural changes due to grain boundary sliding, intergranual fracture perpendicular to the metal flow axis , and the type ,morphology of different secondary carbides were measured and discussed . To examine the changes in microstructure Philips EM 400 TEM with an acceleration voltage of 120KV, and SEM as well as light microscopy were used . It was found that , laser beam welded structure investigated after creep deformation at temperatures lower than 650C and at controlled stress of 400 and 450 MPa , showed a textured weld metal zone with dendrite having lower hardness combined with a higher creep resistance than that for base material .It was found also that creeping at 650C at the same stress values offsets any gain in creep resistance of welded joints as compared with that for the base material at the same conditions.