Jiří Čapek

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.

Comparison of Different Methods of Residual Stress Determination of Cold-Rolled Austenitic-Ferritic, Austenitic and Ferritic Steels

The aim of this contribution was to compare four methods of residual stresses determination of single-phase and dual-phase steels after cold-rolling primarily using X-ray diffraction techniques. Firstly, without taking into account the preferred orientation (sinψ method), secondly from the geometry of four-point bending, thirdly without neglecting the texture (harmonic function method). And mainly, the new method, by calculating anisotropic elastic constants as a weighted average between single-crystal and X-ray elastic constants with weighting being done according to the relative intensities in the measured directions. The applicability of the new method of residual stresses determination in textured materials was proofed; however, the method needs further verification. Introduction The majority of practically used diffraction measurements methods and algorithms for residual stresses (RS) calculation assume the case of isotropic (non-textured) polycrystalline material. Due to the comparatively frequent existence of the preferred orientation (texture), it is more than desirable to have at disposal a method, procedure and even a computation algorithm for proper and correct RS determination. Currently, a universal method with the potential to properly evaluate RS in textured materials is, unfortunately, still missing and this issue is tackled either, in the worst scenario, by neglecting the texture (X-ray elastic constants (XEC) are used) or by choosing one of the usually proposed methods (i.e. calculation of anisotropic elastic constants (X-ray stress factors – XSF), e.g. harmonic function [1], crystallite group [2], strain pole figures [3] methods etc.). For this purpose, a new method was developed and used for determination of residual stresses without neglecting texture. The new method is based on the Dölle model [4, 5]. However, contrary to Dölle method, this method determines the XSF (Rij) as the weighted average between the single-crystal elastic constants (s33ij) and the XEC (rij) where weighing is performed according to the relative intensities I in the measured directions φ, ψ, see Eq. 1. Rij(hkl, φ, ψ) = I(hkl, φ, ψ) rij(hkl) + (1 I(hkl, φ, ψ)) s33ij, (1) where hkl are the Miller indices of the analysed planes. For texture limits, i.e. I = 0 and I = 1, the applicability and correctness of the method is automatically proofed. The general method uses function f(I), which depends on the texture. To simplify and according to previous experiments, the function f(I) could be approximated by a quadratic function (there is linear function in Eq. 1). Instead of other methods, this method allows to determine RS in materials with a very weak, Residual Stresses 2018 – ECRS-10 Materials Research Forum LLC Materials Research Proceedings 6 (2018) 145-150 doi: http://dx.doi.org/10.21741/9781945291890-23 146 strong and moderate texture too. The main disadvantage of this method results from the accuracy of orientation distribution function (ODF) calculation, it is the same case as for the harmonic function method. Experiment The tested samples of plate shape were made of AISI 420 (ferritic), AISI 304 (austenitic) and AISI 318LN (austenitic-ferritic or duplex) type of stainless steel. The samples were cold-rolled to a thickness of 1.5 mm with 0, 10, 20, 30, 40, and 50% reduction in thickness (deformation). Before deformation, the austenitic and ferritic with duplex samples were annealed in air laboratory furnace for 4 hours at 840°C and 7 hours at 650°C, respectively, in order to reduce residual stresses. According to the type of steel and thickness reduction, the tested samples were marked as F0–F50, A0–A50, D0–D50, and DF0–DF50 and DA0–DA50 for particular phases of duplex steel. The CoKα radiation and the XPert PRO MPD diffractometer were used to texture and phase analyses. Texture analysis was performed on the basis of the ODF calculated from experimental pole figures (PF), which were obtained from three diffraction lines {110}, {200}, {211} of ferrite phase (α-Fe) and {111}, {200}, {220} of austenite phase (γ-Fe) using MTEX software [6]. Due to overlapping of diffraction lines {110}α and {111}γ in duplex steel, {220}α and {311}γ were analysed instead of them. The ResMat software was used to determine the harmonic coefficients, which are necessary for XSF calculation using harmonic function method. The XPert PRO MPD diffractometer with manganese and chromium radiation was used to measure lattice deformations in austenite and ferrite phases, respectively, in the rolling direction. The diffraction angles 2θ were determined from the peaks of the diffraction lines Kα1 of planes {311} of the austenite and {211} of the ferrite phase. The diffraction lines Kα1 were fitted by the Pearson VII function and the Rachinger’s method was used for separation of the diffraction lines Kα1 and Kα2. The XEC were obtained in accordance with the Hill model [7] from single-crystal elastic constants, see Tab. 1. The Winholtz & Cohen method [2] was used for calculation of RS from lattice deformations. Tab. 1 Single-crystal elastic constants of ferrite and austenite [8] phase C11, GPa C12, GPa C44, GPa ferrite 231 134 116 austenite 198 125 122 Results and discussions The phase compositions of the tested samples are shown in Tab. 2 and Fig. 1. In the ferritic samples, only the ferrite phase was observed. Due to stronger plastic deformation, increasing amount of strain-induced martensite (α ́) was found in the austenitic samples with higher deformation. The two phases based on chromium (Cr23C6 + FeCr) were analysed in duplex samples. The trends from Fig. 1 result from the combination of strong plastic deformation (γ → α ́) with annealing (α → γ + Cr23C6 + FeCr) [9]. For these reasons, the amount of austenite phase is decreasing, the ferrite phase is constant and the chromium phases is increasing with higher deformation. The presented stresses σ (measured by X-ray diffraction) are superposition of the external stresses σN generated by the four-point bending, and the RS of surfaces areas of the tested samples after annealing σRS, i.e. σ = σN+ σRS. For the external stresses calculation, Young ́s modules of tested materials were measured using the ultrasonic pulse-echo method [10]. Values of σ’N represent the theoretical values of the measured/experimental value of σ. In ideal case, the values of σ should be equal to σ’N. Residual Stresses 2018 – ECRS-10 Materials Research Forum LLC Materials Research Proceedings 6 (2018) 145-150 doi: http://dx.doi.org/10.21741/9781945291890-23 147 The determined stresses σ relevant for the surface layers depending on external stresses can be seen in Figs. 2. Stresses calculated using common sinψ method differ from σ’N values due to omitting of presence of texture, i.e. without taking into account the potential non-linear regression of d(sinψ), and using the XEC instead of the XSF. Tab. 2 Phase compositions in wt.% of austenitic samples sample strain-induced martensite austenite A0 2.0 98.0 A10 1.7 98.3 A20 3.2 96.8 A30 2.5 97.5 A40 4.7 95.3 A50 4.2 95.8 Fig. 1 Phase compositions of duplex samples with 95% confidence interval The stresses determined by the harmonic function method have in the most cases higher values and errors in comparison with other methods. The main reason is presence of a sharp texture, see Fig. 3 and [11], which the harmonic functions are not able to describe correctly. Secondly, this method strongly depends on the accuracy of harmonic coefficients derived from calculation of ODF, which is closely related to microstructure, mainly grain size. The new developed method uses relative intensities of pole figures of the given {hkl} diffraction planes. In this method, the PF or ODF are not fitted by any functions, so it is suitable for materials with very strong and sharp textures. Nevertheless, same as for the harmonic function method, the accuracy strongly depends on the ODF calculation and quality of measured data. This is the main reason, why the values of stresses determined by the new method are not equal to the σ’N values. The good validity of the proposed method is for one-phase steel, where the function f(I) could be approximated by a quadratic function, see Figs. 2a and 2b. Due to the multi-phase interaction during deformation of austenitic and duplex samples, the accuracy of the method was confirmed for stresses up to 50% of yield strength, see Figs. 2c–h. The influence of multi-phase interaction is not a part of the proposed method, that is the reason why the validity for higher deformation was not proofed. For all cases, the RS values were determined by the proposal method with smaller experimental errors than other methods. deformation, % 0 10 20 30 40 50 w t., % 0 20 40 60 80 100 uf067-Fe FeCr + Cr23C6 uf067-Fe Residual Stresses 2018 – ECRS-10 Materials Research Forum LLC Materials Research Proceedings 6 (2018) 145-150 doi: http://dx.doi.org/10.21741/9781945291890-23 148 a) Sample F0 b) Sample F50 c) Sample A0 d) Sample A50 e) Sample DF0 f) Sample DF50 g) Sample DA0 h) Sample DA50 Fig. 2 Determined stresses σ depending on σ’N stress with 0 and 50% deformation uf067N, MPa 0 100 250 325 500 650 uf067 , M Pa 0 100 200 300 400 500 600 700 800 900 sinuf067uf067method new method harmonic function method uf067uf067 uf067N, MPa 0 100 250 325 500 650 uf067 , M Pa 0 100 200 300 400 500 600 700 800 900 sinuf067uf067method new method harmonic function method uf067uf067

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.

REAL STRUCTURE AND RESIDUAL STRESSES IN ADVANCED WELDS DETERMINED BY X-RAY AND NEUTRON DIFFRACTION

The paper outlines the capability of X-ray diffraction (XRD) for evaluation of real structure changes and residual stresses (RS) on cross-section of advanced thick welds due to the welding of ferromagnetic plates. The results of neutron diffraction describe a three-dimensional state of RS and also verify previous assumptions of RS redistribution as a result of the surface preparation for determination 2D maps measured by XRD.

COMPARISON OF PARAMETERS OF SURFACE INTEGRITY OF MACHINED DUPLEX AND AUSTENITE STAINLESS STEELS IN RELATION TO TOOL GEOMETRY

The goal of this contribution was to describe parameters of surface integrity of two machined materials; austenite and duplex stainless steel. Residual stresses and presence of straininduced martensite were studied as a function of the side rake angle. Residual stresses of surface and sub-surface layers were determined using X-ray diffraction techniques and hole-drilling method. By using X-ray diffraction, it is possible to determine residual stresses in each phase separately, in comparison with hole-drilling method. The presence of strain-induced martensite was investigated using Barkhausen noise and optical microscope.

INFLUENCE OF MACHINING TECHNOLOGIES ON VALUES OF RESIDUAL STRESSES OF OXIDE CUTTING CERAMICS

Currently, the intensive development of engineering ceramic and effort to replace sintered carbides as cutting materials are in progress. With the development of the sintering technology it is now possible to produce compact ceramic cutting samples with very good mechanical properties. The advantage of these materials is their easy accessibility and low purchase price. In this work, the influence of the finishing machine technology on the values of surface residual stresses of cutting ceramic samples Al2O3+TiC were studying. The samples were supplied by Moscow State University of Technology STANKIN. Measurements made in the X-ray diffraction laboratory at the Department of solid state engineering were performed for both the phases. The influence of the parameters of machining to residual stresses was studied and the resulting values were compared with each other.

X-Ray Diffraction and Barkhausen Noise Diagnostics of Thick Welds Prepared by Metal Active Gas and Laser Welding

Non-destructive methods for detection and measurement of residual stresses (RS) have been increasingly used in the last few years. The paper outlines the capability of Barkhausen noise analysis (BNA) for evaluation of real structure changes and RS on cross-section of welds due to welding of ferromagnetic plates compared with X-ray diffraction (XRD). The purpose of this study is to evaluate the RS distribution of specimens joined using by high power diode laser and metal active gas (MAG) welding that can be used for quantitative analysis of macro and micro level RS separately. The principal advantages of BNA over XRD as a tool for RS analysis and real structure characterisation are that it is mobile, faster with more facile carrying out and hence BNA is frequently used for continuous monitoring of RS in industrial processes.

2D Residual Stress and Hardness Maps in Steel Laser Weld with Filler Wire

Residual stresses (RS) and welding process represent a traditional partnership between an ever evolving, and vital, technological process and a quantity characterizing the result of this process. As the modern way of joining materials is shifting gradually from conventional welding into friction stir or laser welding, RS remain a crucial parameter which gives clues about the welds’ behavior under dynamic loads. In this contribution the joint created by laser welding with filler wire is described by 2D maps of RS and hardness.