Nikolaj Ganev

Residual stress measurement in alumina coatings

Abstract Residual stresses in plasma-sprayed Al 2 O 3 layers were investigated by means of X-ray diffraction with Cr and Cu radiation. No stress gradient was indicated in the surface layers of the alumina coatings within the range of penetration depth of the applied radiation. Different stress values were obtained by a high speed drilling method.

Residual Stresses in Plasma-Sprayed Coatings Al 2 O 3

Residual stresses affect significantly the quality and performance of thermally sprayed coatings. The residual stresses in alumina coatings were investigated by means of X-ray diffraction with Cr- and Cu-radiations. The sin2Ψ method could be applied to the evaluation of the stress fields. A very small stress gradient was indicated in surface layers of alumina coatings.

X-ray diffraction analysis of Zr-based alloys oxidized in water and lithiated water at 360°C

The results of X-ray diffraction analysis of three Zr-based alloys are presented in this contribution. The tubular samples of ZrlNb, Zry4 W and ZIRLO alloys were simultaneously oxidized for various exposures in both pressurized VVER 1000 imitating water and water with 70 ppm Li as LiOH at 360°C. The effect of Li-environment was observed for all the characteristics obtained by means of XRD, i.e. residual stresses σ, krystallite size D and microstrains ε of oxide layers. The change of all the characteristics correspond to changes of corrosion kinetics in Li-environment for two types of the alloys: Zry4 W and ZIRLO. The behavior of ZrlNb alloy was found to be different. No effect of Li-environment was observed on stresses σ in metal underlying of alloys investigated.

X-ray stress analysis of oxidized Zr-based alloys

The results of X-ray diffraction analysis of macroscopic stresses in oxide layers and substrate of zirconium alloys are presented in this contribution. Stresses were studied on the tubular specimens of three types of alloys:Zr1Nb, low tin Zircaloy-4 and Zr-Nb-Sn alloy (ZIRLO) which were simultaneously exposed for various time in four environments: 360 degree(s)C water, 360 degree(s)C water with 70 ppm Li as LiOH, steam at 400 degree(s)C and steam at 450 degree(s)C. The courses of compressive stresses (sigma) vs. oxide thickness are quantitatively different for two types of alloys in all of environments used. The courses for ZIRLO seem to be less dependent on corrosion conditions. In the range of the large oxide thicknesses (> 10 micrometers ) two cases were observed: (1) the value of stress falls down to zero, (2) the values stay on some levels which differ for different alloys and environments. Only tensile residual stresses were indicated within substrate of all samples investigated. Microstructure characteristics (crystallite size and lattice strains) were evaluated for oxide layers and the metal underlying.

Effect of Boriding Time on Microstructure and Residual Stresses in Borided Highly Alloyed X210CR12 Steel

Boriding of highly alloyed steels done with the aim of increasing their wear resistance faces several issues connected with the microstructure of the base material and restraints during the diffusion of boron. The aim of the performed analyses was to ascertain whether significant increase of boriding time can enhance the surface hardness, contribute to creation of more compact microstructure and even lead to beneficial state of residual stresses in the borided layer. Using combination of X-ray diffraction and electro-chemical polishing, residual stress depth distributions in few tens of micrometres thick borided layers were obtained.

Non-Destructive Inspection of Surface Integrity in Milled Turbine Blades of Inconel 738LC

Nickel super-alloys are widely used in aerospace as material for turbine blades. Unfortunately, their machining is difficult since mechanical hardening and, consequently, extreme tool wear occur. Casting can no longer meet the requirements for precision, hence, the castings are being ground or milled. In this contribution, a quality check of the milled surface by several surface integrity parameters is proposed with respect to the surface structural inhomogeneities caused by mutual effect of plastic deformations and thermal fields during the cutting process. Castings from Inconel 738LC were milled with cutting conditions chosen by Design-Of-Experiments method and the resulting surfaces were assessed by non-destructive X-ray diffraction methods in several areas corresponding to various cutter orientation and work-piece angle. Surface integrity was described by macroscopic residual stresses, microstrains, grain sizes and phase composition. Mostly, favourable compressive surface residual stresses were observed in the cutting direction, grain sizes were distinctively smaller when the tool axis was perpendicular to the machined surface.

Study of Residual Stress Surface Distribution on Laser Welded Steel Sheets

Advent of high power diode laser has substantially contributed to the popularity of laser welding in industry where its virtues such as low heat input and good weld strength are highly appreciated. However, one of the drawbacks of the laser welding is distortion of the welded bodies that is closely linked with the generation and/or redistribution of residual stresses in the vicinity of the weld. In this contribution, mapping of surface macroscopic residual stresses in two directions, i.e. parallel and perpendicular to the welds, were performed for two bodies. The first contained a weld created with the high power diode laser beam speed of 2 m/min and the second with the speed of 15 m/min. Our aim was to compare not only resulting fields of residual stresses, but also to perform qualitative assessment of the possible presence of crystallographic texture and gain a qualitative apprehension about the grain sizes in the vicinity of a laser weld joining two steel sheets. Larger distortion of the body with a laser weld is exhibited by the sample manufactured with approximately 8 times bigger speed of laser beam. This sample is in the immediate vicinity characterized by substantial compressive residual stresses in the direction perpendicular to the weld.

X-RAY DIFFRACTION STUDY OF ANISOTROPIC STATE OF RESIDUAL STRESS AFTER DOWN-CUT AND UP-CUT FACE GRINDING

Differences between up-cut and down-cut grinding are usually not considered since both modes are alternating during conventional face grinding. Nevertheless, there is a pronounced distinction in the fashion of material removal which could lead to unequal states of surface residual stress. By means of X-ray diffraction analysis, ground plates made from three types of steel were investigated in order to compute, and compare, both macroscopic and microscopic residual stress, and domains of coherent scattering. In respect to the main sources of residual stress generation, i.e., plastic and thermal deformation, machining process was carried out in two types of cooling environment. The results indicate significant influence of heat removal since differences between the two grinding modes are virtually nonexistent for liquid cooling, whereas dry grinding results in higher compressive normal residual stresses for down-cut mode in comparison to the up-cut.

A new approach for pre-stressing of rail-end-bolt holes

Bolted joint railroad is the subject matter of this paper. Rail joint elements are subjected to cyclic and impact loads as a result of the passage of trains, which causes the origination and growth of fatigue cracks occurring, in most cases, around the bolt holes. Fatigue failure around rail-end-bolt holes is particularly dangerous because it leads to derailment of trains and, consequently, to inevitable accidents. Moreover, the cracking at rail-ends, which starts from bolt hole surface, causes premature rails replacement. The presence of residual compressive hoop stresses around the bolted holes, which is achieved by prestressing of these holes, extends the fatigue life of bolted joint railroads. This article presents an innovative technology for pre-stressing of rail-end-bolt holes, implemented on a vertical machining centre of Revolver vertical (RV) type. Two consecutive operations are involved in the manufacturing technology process: formation of the hole by drilling, reaming and making of a chamfer through a new combined cutting tool; cold hole working by spherical motion cold working through a new tool equipment, which minimizes the axial force on the reverse stroke. The new technology introduces beneficial residual compressive stresses around the bolted holes thereby preventing the fatigue cracks growth and increasing the fatigue life of these openings.

X-ray diffraction study of residual stress distribution on surface of laser welded steel sheets

Generally, the manufacturing processes of machine component introduce residual stresses (RS) that have an essential influence on their behaviour during service life. The purpose of this study is to evaluate the residual stress distribution of specimens joined by using high power diode laser welding. Non-destructive methods for detection and measurement of RS have been increasingly used in the last few years. The paper outlines the capability of X-ray diffraction (XRD), which can be used for quantitative analysis of macro and micro level RS separately, to describe a state of residual stresses of high pressure welds. The results show from this paper that laser welding has application potential in the welding of high pressure steels.