Rudolf Foret

Simulation of dissimilar weld joints of steel P91

Abstract Results of computer simulations of long term service exposure for weldments of the CSN 15 128/P91 and SK3STC/P91 steels are presented and compared with corresponding results of phase and composition experiments. The welded material P91 (EU designation: X10CrMoVNb 9–1) represents progressive chromium steel alloyed with molybdenum, vanadium, carbon, and nitrogen. The CSN 15 128 (13CrMoV 2–5) material is low alloy Cr–Mo–V steel. The SK3STC alloy (12CrMo 10–10) represents the consumable electrode material. The stability of the weldment microstructure is investigated at elevated temperatures (500–700° C). The simulation method is based on the Calphad approach complemented with the theory of multicomponent bulk diffusion, local conditions of phase equilibrium, and the assumption that diffusion is the process that controls the rate of phase transformation. Significant phase profiles, concentration profiles, and phase transformation processes in the diffusion affected zone are simulated, investigated, and compared with experimental results. The potentially deleterious carbon depleted region inside each weld joint is discussed. The method described can be used to predict microstructure instability in weld joints.

Structural stability of dissimilar weld joints of steel P91

Abstract The present paper addresses the issue of carbon and nitrogen redistribution in weld joints between creep resistant chromium steels. The redistribution of carbon and nitrogen was measured using the wave dispersive X-ray method. The conditions of the long term diffusion annealing process were: temperature 575, 625, and 700°C; and time 10, 100, 1000, and 5000 h. It was ascertained that carbon and nitrogen diffused from the low alloy steels to the high alloy steel P91. The carbon diffusion coefficients in the individual steels in 15128/P91 and SK3STC/P91 weld joints, the ratio between the thermodynamic activity coefficients of carbon in the joints, the diffusion interaction parameters &bgr;Cr C, and their temperature dependencies were estimated from carbon concentration curves. The present paper also contains a microstructure analysis of decarburised and carburised zones in the weld joints studied.

More sophisticated thermodynamic designs of welds between dissimilar steels

Abstract In the present paper, steels that are mostly used in technical practice as base materials, filler metals and consumable electrodes in the design of dissimilar weld joints are studied. These steels differ mainly in the content of Cr, Mo, V, W and N. It is explained and emphasised in the present paper that the effect of thermodynamics, diffusion and phase transformations on the stability of dissimilar weld joints that are exposed to temperatures of over ∼500°C cannot be underrated during operation. For all the materials under study the carbon activity at 600°C is given as calculated by the CALPHAD method, and a correlation is shown to exist between the carbon activity and the total chromium content. This correlation can be used as a most general alternative in the selection of filler metals or consumable electrodes in the design of dissimilar weld joints. The primary goal of the present paper is, however, to present a more sophisticated alternative based on a mutual comparison of the temperature dependence of the carbon activities of the base alloys, filler metals and consumable electrodes under consideration. This alternative is suitable for routine engineering purposes. Attention is also drawn to the currently most advanced alternative, which enables the prediction of the redistribution of elements and phases after prolonged exposure.

Tribological Performance of Ti–Si-Based in Situ Composites

ABSTRACT In this study, a series of Ti–Si-based in situ composites was manufactured by means of a common argon arc melting technique and tribologically evaluated using a sliding ball-on-disc tester under simulated body fluid lubrication. The composite microstructure, mechanical properties, and surface roughness were characterized using light and scanning electron microscopy (SEM), vertical scanning interferometry (VSI), X-ray diffraction (XRD) analysis, and hardness measurements. The evolution of coefficients of friction (COFs) and the appearance of contacting surfaces showed that two the principal wear mechanisms were mixed elastohydrodynamic lubrication (EHL), typically followed by abrasive wear. The mixed EHL was due to the combined effect of serum solution lubrication and surface irregularities, which were produced during the routine surface preparation of samples. The mixed EHL provided the absence of wear and low and stable COFs, which did not depend on the phase composition, microstructure, or hardness of Ti–Si-based alloys. However, in most cases, the change in contact geometry led to the transition from mixed EHL to conventional boundary lubrication, accompanied by increased and unstable friction, adhesive material transfer of metal to the ceramic counterbodies, and abrasive wear. In this respect, the low wear resistance and high adhesion affinity of the titanium matrix of Ti–Si-based alloys should be improved.

Evaluation of Weldability of Titanium Alloy Ti-6Al-4V and Aluminum Alloy 6061 Produced by Electron Beam Welding

Aluminum and titanium alloys are among the most important and the most frequently used construction materials due to their physical and mechanical properties. Especially in the automotive and aerospace industry these materials allow to reduce the weight of structure which leads to reducing fuel consumption and environment pollution. These materials are often used together which leads to problems with junction between these materials. In addition to the mechanical joints, there is an effort to produce quality welded joints. Series of works focused to welding of Al/Ti joints by conventional and nonconventional welding methods were published [1, 4, 5, 6, 7]. By reduction of dimensions of molten material is possible to reduce the amount of emerging intermetallic phases and welding defects. Electron beam welding appears as suitable method for welding Al/Ti joints because it allows production of very narrow welds. The benefit is also necessity to perform electron beam welding in vacuum which is required for decrease energy losses of incident beam and simultaneously prevents reaction of molten metal with ambient atmosphere. This paper is focused to determine of appropriate parameters for electron beam welding of heterogeneous welds of titanium alloy Ti-6Al-4V and aluminum alloy 6061. Metallographic evaluation, analysis of chemical and phase composition were performed on the test welds for purpose to describing present phases. On the selected welds was evaluated the influence of intermetallic phases on the mechanical properties. The obtained results will be used for further experiments focused to optimize the process of electron beam welding of Al/Ti alloys.

Microstructural Stability of Dissimilar Weld Joint of Creep-Resistant Steels with Increased Nitrogen Content at 500 – 900 °C

The structural changes taking place in the dissimilar laboratory welds of 6CrMoV 8-3-2 (T25), being under development, and modified X12CrMoVNb 10-1 (P91) nitrided steels annealed at temperatures of 500 – 900 °C have been subject of study. Carbon and nitrogen redistribution measurement by the EPMA method was complemented with detailed structural analysis aimed at the phase profiles and chemical composition of coexisting carbides and carbonitrides in particular regions of the weld joint. Results of experimental work were compared with thermodynamic and kinetic modeling using the DICTRA software. A very good agreement between the modeling and the experiments was observed.

Application of Ductile Fracture Criteria to Industrial Cutting Processes

Experimental and numerical studies of fracture prediction with Czech steel no.41 2050 are presented. Seven ductile fracture criteria were calibrated, applied to simulation of rod cutting and bolt head trimming operation and compared with experimental results of these processes, obtained in semi-industrial conditions in cooperation with our industrial partner. Movement of the cutting tool was stopped in predefined positions, so that a step-by-step mapping of subsequent stages of both processes were obtained at different specimens. By metallographic analysis of these, full view of plastic deformation and damage in the vicinity of the cutting region could be seen, and their comparison with the computational results was done.