Residual Stress Analysis in Surface Mechanical Attrition Treated (SMAT) Iron and Steel Component Materials by Magnetic Barkhausen Emission Technique

Transcription

1 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 12, DECEMBER Residual Stress Analysis in Surface Mechanical Attrition Treated (SMAT) Iron and Steel Component Materials by Magnetic Barkhausen Emission Technique Nishanth S. Prabhu 1;2, J. Joseyphus 2, T. S. N. Sankaranarayanan 3, B. Ravi Kumar 4, Amitava Mitra 4, and A. K. Panda 4 Sievert India Pvt Ltd, Mumbai, India National Institute of Technology, Tiruchirappalli, India CSIR-National Metallurgical Laboratory Madras Centre, Chennai, India CSIR-National Metallurgical Laboratory, Jamshedpur, India Surface plastic deformation processes like shot peening (SP) have been extensively used by engineering industries to introduce compressive stresses at the surface to achieve superior mechanical properties (especially fatigue and wear resistance). A new family of surface severe plastic deformation processes (S 2 PD) using high-energy balls, widely known as the Surface Mechanical Attrition Treatment (SMAT), is gaining prominence. The investigation addresses the analysis of residual stresses in SMAT-exposed pure iron and SA 516 Grade 70 pressure vessel steel components by Magnetic Barkhausen emission (MBE) technique which is a potential nondestructive evaluation tool. The stresses corresponding to optimized band filter frequencies in MBE technique exhibit good correlation with the X-ray-diffraction based residual stress measurement data. The stresses correlate at a filter frequency of khz in the case of pure iron and khz in the case of SA 516 Grade 70 pressure vessel steel. Depth profiling by chemical etching showed that the nature of stresses changed from compressive to tensile. Microhardness measurements revealed an exponential decrease of hardness with depth. Index Terms Magnetic Barkhausen emissions (MBE), SA 516 Grade 70, shot peening (SP), surface mechanical attrition treatment (SMAT), X-ray diffraction (XRD). I. INTRODUCTION T HE self-equilibrating internal stress in a free body with no external forces/constraints on the boundary is referred to as residual stress. Residual stresses may be generated or modified at every stage in the component life cycle from original material to final disposal. It is invariably present in any material during manufacturing, fabrication, installation and repairs. There are different sources for residual stress generation during the manufacturing processes. These may include sole or a combination of sources like: 1) inhomogeneous plastic deformation in different portions of the components due to mechanical loads or constraints; 2) inhomogeneous plastic deformation due to thermal loads; 3) volumetric changes and transformations plastically during solid state phase transformations; and 4) a mismatch in the coefficient of thermal expansion [1]. The residual stresses are classified into three types [2], [3]. Residual stresses of the first kind (Macro stresses) are nearly homogeneous across large areas (several grains) of a material and are in equilibrium over the bulk of the material. Residual stresses of the second kind (Micro stresses) are nearly homogeneous across microscopic areas (one grain, or part of a grain) of material and are equilibrated across a sufficient number of grains. Residual stresses of the third kind (submicro stresses) are homogeneous across submicroscopic areas of a Manuscript received February 20, 2012; revised April 26, 2012; accepted May 29, Date of publication June 08, 2012; date of current version November 20, Corresponding author: A. K. Panda ( akpanda@nmlindia.org). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TMAG material, say by some atomic distances within a grain and are equilibrated across small parts of a grain. Usually, a superposition of residual stresses of the first, second, and third kind determines the total residual stress acting at a particular point in a material or component. Thus, emphasis is given for the assessment of residual stresses either destructively or nondestructively so as to enable/take corrective measures. The latter technique is usually preferred over the former. Some of the known nondestructive evaluation (NDE) techniques include X-ray diffraction (XRD) and magnetic methods. The limitation of the X-ray technique is that the measurement depth is very low, up to a few micrometers [4]. The magnetic NDE methodology is simple, reliable and quick, based on the principle of magneto-elastic interaction between magnetic domain wall movements and elastic stresses in the material. The advantage of the technique is that it has a greater depth of penetration compared to X-ray residual analysis, but the method is limited only to ferromagnetic materials. In this technique, the magnetization-induced electrical pulses, which are produced by domain wall movements, appear as a burst type of signal, known as Barkhausen noise. Two important material characteristics that can affect the intensity of the magnetic Barkhausen emission (MBE) signal are microstructural developments and mechanical stresses. The MBE technique is responsive to microstructural changes like carbide precipitation in Cr Mo steels [5], [6] and cold rolled low carbon steel [7]. The elastic stresses can influence the way domains choose and lock into their easy direction of magnetization. The elastic properties of the metal interact with its domain structure thereby affecting the magnetic properties via magneto-elastic interaction. As a result of this interaction in materials with positive magnetic anisotropy (iron, most steels and cobalt), compressive stress decreases the intensity of Barkhausen emission while tensile /$ IEEE

2 4714 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 12, DECEMBER 2012 stresses increase it [8]. MBE has been found to be a potential technique for explaining plastic deformation in silicon steels [9], [10]. Such stresses through few nanostructured layers can be achieved using different techniques like shot peening (SP), hammer peening, laser shock treatment, surface rolling, and high-speed machining. However, the thickness of the nanostructured surface layer of residual stress can be simulated using a controlled methodology like surface mechanical attrition technique (SMAT) followed by selective layer removal [11], [12]. SMAT is one of the most promising severe surface plastic deformation technique. The present investigation is focussed on establishing a correlation between magnetic properties and stresses in iron and steel samples. The objective is to have a better quantitative approach towards the applicability of the MBE technique for measurement of residual stresses in industrial components. II. EXPERIMENTATION In the present investigation, two ferromagnetic materials, pure iron and SA 516 Grade 70 steel, were taken in the form of plates. Iron had a purity of 99.8% while SA 516 Grade 70 had the chemical composition (wt %) as C: 0.17, Mn: 1.08, Cr: 0.04, Ni: 0.02, Si: 0.34 and Fe: balance. Tensile specimens and plates were cut using an electric discharge machine (EDM). The specimens were later heat treated in a furnace at 400 C for 1 hour to relieve the stresses present in the sample. A Tinius Olsen (Model: H25KS) tensile testing machine was used to conduct tensile and compressive tests in two separate cycles. The tests were performed below the yield point values of 210 and 335 MPa determined for pure iron and SA 516 Grade 70, respectively, to measure the magnetic properties in the elastic region. The experiment was carried out at a strain rate of per s. The tests were conducted on the same samples each of pure iron and SA 516 Grade 70 by placing stoppers in order to avoid buckling of the samples. MBE measurement was carried out in situ during the tensile and compressive tests. An MBE probe holder was specially designed to ensure proper contact of the probe with the sample during tensile and compressive tests. The Barkhausen signals were picked-up at regular intervals of stress-strain tests to get the plots for variation in MBE voltage with stress. The magnetizing frequency and voltage were selected as 125 Hz and 4 V, respectively. Various filter frequencies were selected in the khz range. Surface mechanical attrition treatment (SMAT) was employed for exposing the samples to residual stress. SNC-1 machine consists of an electric motor operating at an oscillating frequency of 50 Hz which is coupled to two sample pots containing 316 stainless steel shots of 1.5 mm diameter. A typical pot is schematically shown in Fig. 1. The samples which were fixed on the interior of the lid were exposed to surface mechanical attrition treatment and hence compressive stresses were induced on the sample. In the present study, the maximum duration for SMAT exposure was chosen as 10 minutes. Pure iron and SA516 steel samples were taken in the pots of SMAT. The values of the MBE RMS voltage on the surface of SMAT exposed samples were recorded in the laboratory using Fig. 1. Schematic representation of SMAT process. Rollscan 300. XRD measurements were also taken by setting the tube voltage at 30 kv and tube current at 6 ma. The stress values corresponding to MBE voltages (subjected to different filter frequencies) were compared with XRD stress values. Subsequently, such measurements were also taken in the same samples after removal of successive layers through chemical etching using aquaregia for a controlled time interval. During such depth profiling, microhardness was also measured using a microhardness tester (Omniscan). All measurements were taken at a load of 0.2 kg and a dwell time of 10 s. III. RESULTS AND DISCUSSION A. Calibration of MBE With Stress As magnetic Barkhausen emission reveals complex changes in material s structure that originate in microstructural properties, as well as stress configurations, calibration of MBE with stresses is very important. The calibration curve of MBE voltage and stress was established at a strain rate of per s for pure iron and SA516 steel as shown in Fig. 2. The low strain rate was maintained so that the material spontaneously acquired equilibrium stress during tensile and compressive loading. The plots show that MBE voltage decreases during compressive loading while it increases during tensile loading. Compressive stress induces large pinning sites leading to detachment of domain walls. Consequently, average mean free path of moving domain wall decreases leading to a reduction in MBE voltage [13]. While in the case of positively magnetostrictive Fe and Fe-based (SA516-grade70) alloys, tensile loading favors magnetoelastic anisotropy with a rise in MBE output signal [14] [16]. The Barkhausen voltage saturated at 140 MPa in tension and 140 MPa in compression for iron which suggested that the irreversible magnetic domain wall movement is restricted at this point and hence there is no change in voltage beyond this point. It was observed that the magnetic saturation occurs at 67% of the yield point (YS: 210 MPa). The Barkhausen voltage saturated at 130 MPa in tension and 130 MPa in compression for SA 516 pressure vessel steel as shown in Fig. 2. It was observed that the magnetic saturation occurred at 38.8% of the yield point (YS: 335 MPa). B. MBE Parameters for Residual Stress Analysis The importance of the selection of the filter frequency range becomes crucial when stresses at a fixed depth have to be esti-

3 PRABHU et al.: RESIDUAL STRESS ANALYSIS IN SURFACE MECHANICAL ATTRITION TREATED IRON AND STEEL 4715 Fig. 2. Calibration curve of stress versus MBE voltage for pure iron and SA516-grade 70 steel obtained at a strain rate of per s. mated. For this investigation, the linear region of MBE variation with stress values in the range of 90 to 90 MPa was selected from data in Fig. 2 for both the samples. This stress zone is considered to be the zone of reversible domain movement obviously within the elastic region. The data corresponding to MBE values in Fig. 2 was subjected to a range of filter frequency bands viz khz, khz, khz and khz, as shown in Fig. 3. The bands have been selected so that the contribution of the magnetization process from bulk to surface via subsurface would be depleted at higher frequencies [17]. It is clear that by changing the filter frequency from khz to khz, the absolute value changes from to 102 with a small change in the slope from 0.21 to 0.11 as shown in Table I. Hence, the stresses determined will not be the same in the case of four frequency ranges as shown in Fig. 3(a). Fig. 3(b) shows that the absolute value changes from to 85 with a small change in the slope from 0.20 to 0.11 in the case of SA 516 steel. It is observed that wide filter frequency range of khz showed high MBE voltage (intercepts) as well as high sensitivity to stress variation. C. Correlation of MBE and XRD Stress Data The XRD technique is a well-established technique for residual stress measurement. The portability of MBE demands logical consideration of measured experimental XRD results. A methodology of data correlation techniques was followed for the selection of an optimum MBE band-pass filter frequency for pure iron and SA516 samples. Initially, the residual stress in SMAT-exposed samples was measured using the X-ray technique. MBE signal was also obtained from the samples. The signals were filtered at a frequency range of khz, khz, khz and khz to obtain MBE voltage. The corresponding stress values were obtained from data using MBE voltage values in Fig. 3 for both the materials. X-ray residual stresses and stresses from MBE calibrated plots were obtained after removal of successive metal layers. Over the same span of stress range, MBE filter frequency stress data and XRD stress values for five different SMAT samples exposed for time intervals of 2, 4, 6, 8, and 10 min, respectively, are shown in Fig. 4(a) and (b) for pure iron and SA 516-grade Fig. 3. MBE band pass frequency filtering signal in (a) pure iron and (b) SA516 grade 70 steel. TABLE I LINEAR FITTING COEFFICIENTS OF MBE FREQUENCY FILTER SIGNALS FOR PURE IRON AND SA516 GRADE STEEL 70 steel, respectively. It was observed that data points corresponding to a filter frequency of khz and khz were found to be close enough to the XRD results in the case of pure iron and steel samples, respectively. Depth profiling of the 10 minute SMA treated sample was done by layer removal through chemical etching using aquaregia. The variation of the depth from the surface up to 580 microns is especially interesting in view of the reported grain size of pure iron 50 microns [18]. This suggests transmigration through all the three kinds of residual stresses. In the case of pure iron, the nature of the variation of the stresses

4 4716 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 12, DECEMBER 2012 Fig. 5. Comparison of MBE stress and XRD stress at different depths in (a) pure iron and (b) SA516-grade 70 after layer removal using chemical etching process. Fig. 4. Correlation of MBE stresses obtained at different filter frequencies with XRD stresses in SMAT exposed (a) pure iron and (b) SA516-grade 70 steel generated after SMAT. with depth [see Fig. 5(a)] was found to be compressive at the surface having a magnitude of 342 MPa and becomes tensile at 280 micron depth. Stresses tend to become 0 at a depth of 580 microns, indicating that stresses had been completely removed from the pure iron sample. The XRD and MBE results correlated well which is shown in Fig. 5. In the case of a steel sample shown in Fig. 5(b), compressive stress of 445 MPa was measured at the surface which became tensile at 380 micron depth. Microhardness shown in Fig. 6 was measured at different depths (post etching treatment) using a microhardness testing machine. In the case of pure iron, the microhardness varied from 176 VHN 0.2 kg at the surface to 82.5 VHN 0.2 kg at 580 micron depth. SA 516 Grade 70 had a microhardness of 273 VHN 0.2 kg at the surface and VHN 0.2 kg at the same depth of 580 microns. The initial drop in microhardness was much more rapid in SA516, which revealed that stress variation was drastic in this material due to its inherent higher hardness compared to pure iron which is relatively softer. Recent investigations of C45 carbon structural steels reveal interesting stress and hardness variations on the MBE signal from electrochemically etched samples [19]. Fig. 6. Variation of microhardness with depth profile of pure iron and SA516 grade 70 samples. IV. CONCLUSION Residual stresses generated by SMAT were correlated to the calibrated stress values with respect to magnetic Barkhausen emission (MBE) voltages in pure iron and SA516 Grade 70 steel. Optimum values of MBE band filter frequencies corresponded to the calibrated stress values which were closer to those obtained using the XRD technique. Depth profiling of SMAT exposed samples revealed transition of compressive

5 PRABHU et al.: RESIDUAL STRESS ANALYSIS IN SURFACE MECHANICAL ATTRITION TREATED IRON AND STEEL 4717 to tensile stress, wherein the calibrated stress values from MBE measurements could be correlated to the observations from X-ray techniques. Compressive stresses became tensile at depths of 280 and 380 microns in pure iron and SA516-grade 70 steel, respectively. Mechanical hardness was also found to decrease with depth. The results give ample scope for MBE technique to measure residual stresses in different types of steels and components where surface compressive or tensile stresses are desirable. The effect of different diameters of steel balls and their durations on SMAT can give deeper insight into the subject. ACKNOWLEDGMENT The authors express their sincere gratitude to the Director of the CSIR-National Metallurgical laboratory (CSIR-N.M.L), Jamshedpur, India, for his kind permission to carry out and publish the work. REFERENCES [1] E. Macheranch and K. H. Kloos, Origin, measurements and evaluation of residual stresses, Residual Stresses Sci. Technol., DGM Inform, pp. 3 26, [2] P. J. Withers and H. K. D. H. Bhadeshia, Residual stress part 2 Nature and origins, Materials Sci. Technol., vol. 17, pp , [3] D. Dye, H. J. Stone, and R. C. Reed, Intergranular and interphase microstress, Curr. Opin. Solid State Mater. Sci., vol. 5, p. 31, [4] V. Vengrinovich, V. Tsukerman, and Y. Denkevich, Barkhausen effect based sensor and intellectual system for accurate stress measurements and monitoring in harsh environmental conditions, in Proc. 15th World Conf. Nondestructive Testing, Rome, Italy, Oct , [5] A. Mitra, Z. J. Chen, and D. C. Jiles, Nondestructive magnetic measurements in weld and base metals of service exposed CrMo steel, NDT & E, vol. 28, no. 1, p. 29, [6] A. Mitra, J. N. Mohapatra, J. Swaminathan, M. Ghosh, A. K. Panda, and R. N. Ghosh, Magnetic evaluation of creep in modified 9Cr-1Mo steel, Scripta Materialia, vol. 57, no. 9, pp , [7] H. Kikuchi, K. Ara, Y. Kamada, and S. Kobayashi, Effect of microstructural changes on Barkhausen noise properties and hysteresis loop in cold rolled low carbon steel, IEEE Trans. Magn., vol. 45, no. 6, p. 2744, Jun [8] D. C. Jiles and L. Suominen, Effects of surface stress on Barkhausen emissions: Model predictions and comparison with x-ray diffraction studies, IEEE Trans Magn., vol. 30, no. 6, p. 4924, Dec [9] M. J. Sablik, B. Augustyniak, M. Chmielewski, and F. J. G. Landgraf, IEEE Trans. Magn., vol. 44, no. 11, p. 3221, Nov [10] L. Piotrowski, B. Augustyniak, M. F. de Campos, and F. J. G. Landgraf, IEEE Trans Magn., vol. 44, no. 11, p , Nov [11] K. Lu and J. Lu, Surface nanocrystallization(snc) of metallic materials Presentation of the concept behind a new approach, J. Mater. Sci. Tech., vol. 15, no. 33, pp , [12] S. D. Lu, Z. B. Wang, and K. Lu, Enhanced chromizing kinetics of tool steel by means of surface mechanical attrition treatment, Materials Sci. Eng. A, vol. 527, pp , [13] M. Lindgren and T. Lepisto, On the stress vs. Barkhausen noise relation in a duplex stainless steel, NDT & E Int., vol. 37, no. 5, pp , [14] C. Jagadish, L. Clapham, and D. L. Atherton, Influence of uniaxial elastic stress on power spectrum and pulse height distribution of surface Barkhausen noise in pipeline steel, IEEE Trans. Magn., vol. 26, no. 3, p. 1160, Jun [15] D. C. Jiles, The effect of stress on magnetic Barkhausen activity in ferromagnetic steels, IEEE Trans. Magn., vol. 25, no. 5, p. 3455, May [16] L. Mierczak, D. C. Jiles, and G. Fantoni, A new method for evaluation of mechanical stress using the reciprocal amplitude of magnetic Barkhausen noise, IEEE Trans. Magn., vol. 47, no. 2, p. 459, Feb [17] V. Moorthy and B. A. Shaw, Magnetic Barkhausen emission measurements for evaluation of material properties in gears, NDT & E Int., vol. 33, no. 4, p. 317, [18] D. Rittel, G. Ravichandran, and A. Venket, Mat. Sci. Eng., vol. 432, p. 191, [19] P. Zerovnik, J. Grum, and G. Zerovnik, Determination of hardness and residual-stress variations in hardened surface layers with magnetic Barkhausen noise, IEEE Trans. Magn., vol. 46, no. 3, p. 899, Mar