Neutron diffraction determination of residual stresses in a duplex steel centrifuge bowl

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1 Neutron diffraction determination of residual stresses in a duplex steel centrifuge bowl G.Albertini (1,2),A.Giuliani (1,2), R.Lin Peng (3), A.Manescu (2,4,5), F.Rustichelli ( 2,4 ), A.Ponzetti (6) Dip. di Fisica e Ingegneria dei Materiali e del Territorio, Universita' Ancona (Italy) 2. INFN (Istituto Nazionale per la Fisica della Materia) - Ancona Unit (Italy) 3. NFL Studsvik, (Sweden) 4. Istituto di Scienze Fisiche, Universita' Ancona (Italy) National R&D Institute for Welding and Material Testing - ISIM Timisoara (Romania) 6. NUOVA M.A.I.P. SpA, Via Don Battistoni1, Jesi (AN), (Italy) Abstract Measurements of the strain and stress field in a centrifuge component were performed by using neutron diffraction. The centrifuge belongs to the most recent line of production of the Nuova Maip and is used for food processing and for agricultural applications. The component, a bowl of duplex steel, is one of the largest rotors produced by that firm. The measurements were performed before centrifugation, after 58 hours and after 258 hours of centrifugation to evaluate the residual strain and stress after fabrication and forming, and also to evaluate their evolution after centrifuging. The upper part of the rotating bowl is investigated, where the highest stress field during centrifugation is forecast. Before centrifugation, compressive residual stresses are found in the hoop direction. They decrease at increasing distance from the top surface. The behaviour of residual strains and stress at the investigated depths is not highly altered by the centrifuging process. 1. Introduction Neutron diffraction for the analysis of the strain and the stress is more and more widely used in the recent years. A great enhancement is given by the study of residual stresses and strains in materials of technological and industrial interest. However, the potentiality and usefulness of neutron techniques are not so largely know as it happens for other techniques. In particular, neutron and x-ray techniques are quite similar; moreover neutron beams are in general less absorbed in materials then x-ray thus allowing a penetration depth some order of magnitude higher then for conventional x-ray beams. But the neutron sources are not so easy to obtain as x-rays thus hindering diffusion of neutron techniques. Great efforts were performed by the neutronists community in order to divulgate neutron techniques to people belonging to different fields of the scientific and technical world and in particular to enterprises of large, medium and small size. Among this efforts, the European project TRAINSS (Training Industry in Stress Measurement) aimed to train industrial people to use neutron diffraction for the determination of the strain and stress. The TRAINSS project promoted the collaboration between research/academic partners and industrial partners thus producing neutron diffraction studies of materials and components of industrial interest. The following results are a part of the studies stimulated by the project. The NUOVA M.A.I.P. SpA enterprise, manufacturer of centrifuges, is interested in studying the stresses in centrifuges for food processing. In fact, operating safety is an important problem and a duty for them. In the case of centrifuges, the operating safety is connected with the bowl and rotor parts, their building materials, their supports and bearings, their balancing state within the manufacturing tolerance limit. 1

2 Safety highly depends on the residual strain and stress. They can be induced during the building process in operations like casting or forging, or during the operational stage, due to thermal conditions or mechanical loads. As long as the rotor is considered, safety depends on its strain-stress state and on its balancing, these two problems being interdependent. If a good balancing is achieved, the knowledge of strain-stress state is a key point to the solution of safety problems. In this respect an important aim is to avoid the increase of residual stress, as it adds to the stresses induced during operation.. An analysis of residual stresses before operation and after operation gives useful information in order to know the future behaviour of rotors during their running life, to point out the overcoming of elastic limit, and to identify the causes of stress: manufacturing or design problems. This analysis would be a valid method to estimate the manufacturing quality and the design quality and to improve the technical and economical characteristics of the product. For these reasons, a study of the stress field in centrifuges for food processing and for agricultural applications was undertaken by the Nuova Maip company (Italy) together with the University of Ancona (Italy) and the NFL Studsvik Neutron Laboratory (Sweden) [ALBERTINI,G. etc, 2001 and ALBERTINI,G. etc., in print].. Fig.1 The type of centrifuge chosen for the investigation. 2. Concept phase The geometrical characteristics of the machinery to be studied were chosen first. In fact the size and the shape of the centrifuges produced by the company can vary in a large range. The model, the sizes and the material chosen are those of the most recent line of production of the Nuova Maip firm. They are also some of its largest produced rotors. Fig.1 reports a picture of the model chosen for investigation. Fig.2 is a picture of the investigated rotor, which is the subject of our investigation. Its characteristics are: weight: 102 kg; diameter: 48 cm, height : 27 cm ; wall thickness: 1,7 cm. Its material is a duplex steel containing a BCC ferrite phase (α) and a FCC austenite phase (γ) in a ratio austenite /(ferrite +austenite ) of (50± 2) vol.%. The commercial name of the steel is F51 ASTM A182 (known also as EN or W nr.: ; as DIN: X2CrNiMo; as commercial name: 2205). The chemical composition of the steel (\% wt) is reported in table 1. The tensile strength is 620 MPa; the yield strength is 450 MPa 2

3 Table I. Chemical composition of F51 (%) C Mn P S Si Ni Cr Mo others , The next step was choosing the region of the sample to be investigated. To this end a finite element analysis evaluating the elastic stress during centrifugation was considered (fig.3). In the finite elements analysis the sample was assumed free from residual stresses induced by the mechanical and thermal treatments related to manufacturing and forming. In order to check this point and to experimentally evaluate the residual stress after centrifuging, two kinds of measurements were performed: measurement before centrifugation, in order to evaluate the residual stress after fabrication and forming and measurements after centrifugation at 6400 rpm, in order to evaluate the residual stress induced by centrifuging after 58 hours and after 200 hours further. In particular, the upper internal part, where the highest stress is theoretically forecast during centrifugation(fig.3), was investigated. Fig.2 The rotor of the centrifuge and the geometry of the measurements The residual stresses were evaluated by using the neutron diffraction. In fact from the Bragg law: n λ = 2 D hkl sin θ (where: λ is the wavelength of the neutron beam impinging on the sample: primary beam; 2 θ is the deviation angle of the scattered beam with respect to the primary beam; D hkl is the interplanar distance of the planes with h,k,l Miller indices; n is an integer number and corresponds to the order of diffraction), the interplanar distance D hkl can be evaluated and thus the strain ε= (D hkl - Do hkl )/ Do hkl if the unstrained interplanar distance Do hkl is known. The strain is measured in the direction of the scattering vector Q: Q = K sca K pri (where K sca and K pri are the wavevectors of the scattered and primary neutron beams respectively). By rotating the sample, different direction of Q with respect to the sample reference frame are explored and the strain ellipsoid can be obtained. From this latter the stress ellipsoid can be evaluated by using the continuum mechanics relationships [NOYAN, I.C., COHEN J.B. 1987]. Neutrons can easily penetrate the steel walls of the bowl, thus information can be obtained on the strain/ stress state of the inner part of the components in a non destructive way. 3

4 Fig.3 Results of a finite element analysis evaluating the elastic stress during centrifugation 3. Experimental Diffraction measurements were performed at the neutron source of NFL Studsvik (Sweden). Gauge points were aligned along three radial lines at different depths from the upper surface: 3.4 mm, 15.2 mm and 27.0 mm (shifted along the axial direction) (Fig. 4). Axial, radial and hoop strains (assumed as main stresses, due to the geometry of the sample) were measured at each gauge point. fig.4. The geometry of the studied region and positions of the gauge points.(sizes are in mm) The slits before the sample (3 mm wide and 10 mm high for the radial and axial stresses; 3 mm wide and 4 mm high for the hoop stresses) and the detector slit (3 mm wide) defined an approximate gauge volume of 3 x 3 x 10 mm 3 for the radial and axial directions and 3 x 3 x 4 mm 3 for the hoop direction. Three reflections were used for strain measurements: 220 and 311 from the austenite phase and 211 from the ferrite phase. They correspond to scattering angles 2θ at about 87, 108 and 97 respectively (neutron wavelength λ= nm nm before centrifugation and after 58 hours centrifugation;=λ= nm after the last centrifuging.). The 220 and 311 reflections show essentially same strains and stresses and thus only the measurement using 311 reflection are presented in the paper. For the hoop measurement, a normal set-up with the bowl standing upside down (with respect to Fig.2) on the diffractometer will cause strong beam absorption by the wall of the bowl that lies in the neutron 4

5 beam path. To facilitate the hoop measurement, special fixtures were made to fix the bowl at a tilting angle, obtained by rotating the bowl around an axis parallel to the hoop direction. This rotation moved the wall out of the neutron beam path and the first set of measurement was made at 3.4 and 15.2 mm from the top surface. Later when measurement at 27.0 mm was decided, the tilting angle had to be increased. Data coherence between the two set-ups were checked by repeating the measurement at the 15.2 mm distance (see fig.4). Within the experimental uncertainty range a good agreement was found in the obtained lattice spacings 3. Results The problem of the two unstrained lattice parameters ao of the austenite and ferrite phases from which the unstrained reference Do hkl interplanar distances can be evaluated, was faced by using different methods: Method 1- The interplanar distances were observed to vary in a similar way in the two phases as a function of the position of the gauge volume and of the direction of the scattering vector: The macrostress (which is the same in the two phases) was thus assumed to be more important than the microstress (compressive in one phase and tensile in the other). Before centrifugation the interplanar distances corresponding to the main directions of stress were found to be the same in a particular point at 3.4 mm depth (this fact occurs in the same position both in ferrite and in austenite phase). In that point the same hydrostatic stress was assumed to occur in the two phases. In order to evaluate that hydrostatic stress, the balance of stress normal to the surface [NOYAN, I.C., COHEN J.B. 1987] was imposed inside a surface S a across the sample (the considered surface is normal to the axial direction, at a depth of 3.4 mm): i σ ai S i = 0 where the sum ranges over the N gauge points along the straight line at 3.4 mm depth, σ ai is the macroscopic axial stress at the i th point and S i is the area of the circular crown in which the axial stress is assumed to be σ ai. The total macroscopic stresses is given in every point and for every direction (main direction of stress) by: σtot = 100 pa σa pf σf (where pa and pf are the per cent of austenite and ferrite phases; σa and σf are they stresses) From the two imposed conditions, i.e. same hydrostatic stress and balance of total stress, the two unknown unstrained lattice parameters ao were obtained. Method 2 A powder was obtained by grinding a piece of material coming from the same melt. The interplanar distances evaluated from the neutron diffraction data were assumed stress free. The diffraction peaks obtained from that powder, however, were very broad, indicating the occurrence of secondary stresses. Method 3 - A piece of material coming from the same melt was chemically treated in order to obtain a powder of the austenite phase. The interplanar distances evaluated from the neutron diffraction data were assumed stress free. The ao of the austenite phase was thus obtained. The ao of the ferrite phase was then evaluated by imposing the balance of stress at the surface crossing the sample under investigation.. 5

6 Fig.5 Residual strains (x 10 6 ) along the axial (circle), radial (x) and hoop (square) directions at different depths: 3.4 mm (top), 15.2 mm (middle), 27mm (bottom). blue: before centrifugation; black: after centrifugation (58 hours); red: after centrifugation (258 hours) 6

7 Fig.6 Residual stresses (MPa) along the axial (circle), radial (x) and hoop (square) directions at different depths: 3.4 mm (top), 15.2 mm (middle), 27mm (bottom). blue: before centrifugation; black: after centrifugation (58 hours); red: after centrifugation (258 hours) Typical error range is ±30 MPa. 7

8 Although the phase strains and stresses depend on the method used to obtain the unstrained lattice parameters ao, the total macroscopic stresses σtot do not depend on the particular method, once that the stress balance over a cross-section is imposed. The so obtained values of strain and stress are reported in fig.5 and 6, respectively 4.Conclusions and discussion Before centrifugation, compressive residual stresses are found in the hoop direction which decrease with increasing distance from the top surface. They are assumed to be due to the mechanical processes employed during manufacturing. After 58 and 258 hours of centrifugation in standard operational conditions, changes in the strain and stress fields are not appreciable, inside the experimental incertitude. That result is obtained by considering the total residual stress in the duplex steel. In fact that quantity is usually considered in theoretical calculations as, for instance, those of finite elements analysis. However, the problem is open on the evaluation of stress in the separate phases: ferrite and austenite. From the experimental point of view the main problem is the evaluation of the unstrained lattice parameters ao of the two phases, as the comparison with an unstrained pure phase of the same material could be misleading. In fact a pure phase or a duplex material are obtained via different thermal treatments and they can induce different unstrained lattice parameters. This paper also report a method (the method 3 in the results section) aiming to face the problem of the phase stress. 5.Acknowledgements The present study is a follow-up of the European project TRAINSS (contract.n.brrt-ct ). A part of the measurements in Studsvik was funded by the European Project for Access to Research Infrastructures Action of the Human Potential Program (contract HPRI- CT ). 6. References ALBERTINI,G. etc. Neutron diffraction measurement of residual stress in a centrifugal bowl of duplex steel. J. of Neutron Research, 2001, n.9, pages ALBERTINI,G. etc. Neutron diffraction determination of the residual stress in a centrifuge bowl after 250 hours centrifugation. Applied Physics A, in print NOYAN, I.C., COHEN J.B. Residual Stress Measurements by Diffraction and Interpretation. Material Research Engineering, Springer-Verlag,