A new magnetic NDE method in inconel 600 alloy

Transcription

1 International Journal of Applied Electromagnetics and Mechanics 19 (2004) IOS Press A new magnetic NDE method in inconel 600 alloy S. Takahashi, H. Sato, Y. Kamada, K. Ara and H. Kikuchi NDE & Science Research Center, Faculty of Engineering, Iwate University, Morioka, Japan Abstract. Cr concentration and the volume fraction in Inconel 600 alloy were accurately analyzed by the new magnetic method according to the magnetic properties of the ternary alloys. The depletion has a maximum at aging for 10 h at 973 K and diminishes with longer aging. Cr depletion below 10 wt. % was observed in the aging. After aging for 100 h the depletion remarkably recovers, but it definitely remains compared with the initial state. The depleted zone extends to 300 nm from the grain boundary. These results agree well with those of a previous analysis by scanning transmission electron microscope analysis. The new magnetic method, herein reported, for evaluation of chromium depletion has nondestructive evaluation characteristics. 1. Introduction Recent use of thermal treatments of steam generator tubing for pressurized water reactors led us to evaluate techniques for detecting a grain boundary Cr-depleted zone. Also of interest is stress corrosion cracking caused by segregation due to Cr depletion and carbide precipitation. There are several techniques for testing Cr depletion, namely, scanning transmission electron microscopy (STEM), the modified Huey test, the reactivation polarization technique, magnetic susceptibility measurement, Eddy current measurement and the polythionic acid technique. These techniques have advantages as well as disadvantages [1,2]. Airey et al. concluded that the modified Huey test and the reactivation polarization technique showed greater sensitivity than the other methods [3]. The Huey test, however, has several disadvantages, one of them being the long time required for its performance. Although the magnetic permeability technique requires only a short time, it is insensitive compared with the other methods. The STEM technique yields microscopic information on Cr depletion near the grain boundaries. However, it is not a nondestructive evaluation method and does not yield complete information on the whole sample. Thus, a new technique or method combining the positive characteristics of the Huey test and the STEM technique but lacking their negative aspects is needed for evaluation of Cr depletion. It is possible to detect the Cr-depleted zone magnetically by use of the dependence of Curie temperature, T c, and that of spontaneous magnetization, M s (T ), on the Cr content. The magnetic method makes it possible to classify the depleted zone into small parts according to their chromium concentration. Chromium depletion is characterized by different Cr concentrations at different values of T c. Spontaneous Corresponding author: Y. Kamada, hkiku@iwate-u.ac.jp /04/$ IOS Press and the authors. All rights reserved

2 4 S. Takahashi et al. / A new magnetic NDE method in inconel 600 alloy Table 1 Chemical composition of Inconel 600 alloy Element Ni Cr Fe Mn Si C Wt. % magnetization consists of many ferromagnetic phases with different values of T c. M s (T i ) at T i K can be expressed by i k=1 M s (T i )= v km k (T i ), (1) V where v k is the volume of Cr depletion with Cr concentration, c k,m k (T i ) is the spontaneous magnetization with Cr content, c k, measured at T i K, and V is the total volume of the sample. The summation is carried out below the Cr concentration whose Curie temperature is above the measured temperature. This magnetic method is very simple and applicable to the evaluation of Cr depletion in Ni-based alloys and steels such as Inconel alloys and Austenite stainless steels. In the present study we applied this new magnetic technique to Inconel 600 alloys to quantitatively obtain Cr concentration and its volume fraction with high accuracy in the vicinity of grain boundaries. Although the magnetism of the ternary alloys Ni 76+x Cr 16 x Fe 8 as well as that of Inconel 600 alloy were investigated more than seven decades ago [4], little is known about the magnetic properties of these alloys. Magnetic properties, especially M s (T ) and T c, are sensitive to Cr concentration, and knowledge of the relationship between magnetic properties and Cr content is important in this new magnetic method. The detailed values of M s (T ) and T c were experimentally measured in ternary alloys Ni 76+x Cr 16 x Fe 8 (0 x 7) as well as in Inconel 600 alloy in the present study. 2. Experimental procedure We prepared Inconel 600 alloy and its model alloys of Ni 76+x Cr 16 x Fe 8 (x =0, 1, 2, 3, 4, 5, 6, 7). The model alloys were prepared by arc-melting the raw materials four times to attain chemical homogeneity on a water-cooled copper hearth in an argon gas atmosphere at a pressure of approximately 93 kpa. The raw materials used for alloying in the present investigation were mass % Ni, 99.8 mass % Cr and mass % Fe. The commercial Inconel alloy used as a sample was of plate shape with a thickness of 2 mm. The chemical composition of Inconel 600 alloy is shown in Table 1. The samples were aged at 1273 K for 1 hour and cooled rapidly to room temperature. The Inconel 600 alloy was aged at a temperature of 973 K for 1, 10, and 100 h. This aging temperature is the same as that of Was et al. [5], facilitating comparison of the present results with their results. The magnetization curves were measured using a magnetic flux-meter with a superconducting quantum interface device (Quantum Design) at temperatures between 4.5 and 300 K in a magnetic field of 0 to 3T. Cr depletion was observed by magnetic force microscopy (MFM) at room temperature and the grain boundary of Inconel 600 was observed by an optical microscope. The grain size was approximately 70 µm. 3. Experimental results M s (0) and T c in the Cr-depletion phase were obtained as a function of the Cr content. Analysis was carried out using the SCR theory of spin fluctuation. The experimental results of temperature dependence

3 S. Takahashi et al. / A new magnetic NDE method in inconel 600 alloy 5 Fig. 1. Cr concentration dependence of (a) spontaneous magnetization, M s(0), and (b) Curie temperature, T c, in Ni 76+xCr 16 xfe 8 ternary alloys. of M s (T ) showed good agreement with the SCR theory of spin fluctuation in these ternary alloys as well as in Inconel 600 alloy; M s (T ) has a temperature dependence in the form of M s (T ) 2 = M s (0) 2 ηt 2 at low temperatures and M s (T ) 2 = ξ(tc 4/3 T 4/3 ) over a fairly wide temperature range below T c [6]. The values of M s (0) and T c were obtained for Ni 76+x Cr 16 x Fe 8 alloys. M s (0) shows a gentle increase with decreasing Cr content [see Fig. 1(a)]. T c shows a linear relation with Cr content below and above 14 wt.%, where a discontinuity is caused by the different magnetic structure [see Fig. 1(b)]. The reason for the discontinuity of T c is not clear. The temperature dependence of M s (T ) was measured in Ni 76+x Cr 16 x Fe 8 alloys. M s (T ) decreases with increasing temperature and disappears above T c. The temperature dependence of M s (T ), however, is different above and below 14 wt.%; it is represented as a similar curve above or below 14 wt. % (see Fig. 2). The difference is consistent with the discontinuity of T c at 14 wt. % Cr content [see Fig. 1(b)]. The magnetic properties of Cr depletion can be substituted for those of the model alloys. The magnetization curves were measured in Inconel 600 alloy after aging at 973 K for 10 h. T c is 110 K in the chemically homogeneous Inconel 600 alloy. The values of T c and M s (0) were analyzed using the SCR theory of spin fluctuation. The Cr-depleted zone was observed to be in a ferromagnetic state even above 110 K. Cr depletion below 10 wt. % concentration, for example, is ferromagnetic even at room temperature. The depleted zone has its own T c value depending on its Cr concentration [see Fig. 1(b)]. The value of M s (T i ) was obtained by extrapolating the straight part of the magnetization curves to the zero magnetic field at each temperature; the value of M s (T i ) is that of the depleted zone whose Curie temperature is above T i K. 4. Analysis of chromium depletion In Inconel alloy, Cr carbides of the form M 23 C 6 or M 7 C 3 precipitate at grain boundaries and are accompanied by Cr depletion in the surrounding matrix. The Cr-depleted zone can be classified according to the different values of T c by use of Eq. (1). M k (T i ) can be obtained from the values of M s (T ) and T c in the model alloys.

4 6 S. Takahashi et al. / A new magnetic NDE method in inconel 600 alloy Fig. 2. Temperature dependence of M s(t) in Ni 76+xCr 16 xfe 8 ternary alloys. A: x =0,B:x =1,C:x =2,D:x =3,E: x =4,F:x =5,G:x =6, and H: x =7. Fig. 3. Cr concentration and its volume rate before aging in Inconel 600 alloy. The volume of each Cr depletion, v I, due to the 973 K aging was obtained by comparing two values of v i that before and that after aging. We first examined the chemical uniformity of chromium in the initial state. The sample was not ideally uniform even after aging at 1273 K and lacked Cr uniformity. The volume of Cr depletion, v i, just after aging at 1273 K was obtained versus Cr concentration (see Fig. 3). The lack of Cr uniformity was mainly observed near the matrix concentration. The region below 9 wt. % Cr concentration clearly exists. The net volume of the Cr zone depleted by aging at 973 K should be obtained by subtracting the value before aging from that after aging. The analyzed result above 15 wt. % Cr can be eliminated for two reasons. The first reason is the large fluctuation near the matrix concentration before and after the 973 K aging. The net volume by aging includes a big fluctuation above 14 wt. % Cr concentration, since the

5 S. Takahashi et al. / A new magnetic NDE method in inconel 600 alloy 7 Fig. 4. Cr concentration and its net volume rate of Cr depletion in Inconel 600 alloy by aging at 973 K. (a) for 1 h (A) and 10 h(b). (b) 10 h (B), and 100 h (C). volume of Cr depletion before 973 K aging is too large compared with its change due to aging. The second reason is the discontinuous magnetic properties at 14 wt. % Cr [see Fig. 1(b)]. Cr depletion was analyzed after aging at 973 K for 1, 5, 10, 24 and 100 h. Cr depletion was confirmed in all the samples during aging at 973 K. The Cr-depleted zone increases with aging time and attains its maximum at 10 h [see Fig. 4 (a)]. The depleted zone shrinks with aging longer than 10 h [see Fig. 4 (b)]. The volume of the depleted zone showed a maximum near 13 wt. % Cr concentration. The maximum in the aging condition of 973 K for 10 h was observed in the previous investigation by Was et al. [5]. 5. Discussion As the present measurement was carried out between 110 and 300 K, the information below 10 wt. % Cr is insufficient. The present analysis of v 1 gives the mean value of Cr depletion whose concentration is assumed to be 8.8 wt. %, T c being 350 K. In practice, the measurement value of M s (300) contains the whole depletion when the Curie temperature is above 300 K or when the Cr concentration is below 10.2 wt. %. A Cr-depleted zone of 6 wt. % was observed in the vicinity of the grain boundary by STEM analysis [5]. The Cr-depleted zone with a concentration between 10 and 6 wt. % extends about 50 nm from the grain boundary. The value of v i includes an inaccuracy also caused by M 1 (T i. The ambiguity of M 1 (T i ) would exert only a slight influence on the analysis of v i. This could be avoided by the measurement of M s (T i ) at temperatures between 300 and 500 K. Cr concentration of the depleted zone is the local value at the carbide and matrix interface, which starts at a minimum of 6 wt. % and approaches a value close to the matrix content. The distribution of Cr concentration is a function of time and the ternary phase diagram. Cr depletion was observed by the MFM technique at room temperature in Inconel 600 alloy after aging at 873 K for 10 h (see Fig. 5). A ferromagnetic zone was observed along the grain boundary, the Cr concentration being below 10.2 wt. %, at a Curie temperature above 300 K. The depletion spreads in the Gauss distribution. Was et al. analyzed Cr concentration across a grain boundary in a sample thermally treated at 973 K by STEM analysis, and their results agree well with the chemical analysis in the depleted zone [5]. The

6 8 S. Takahashi et al. / A new magnetic NDE method in inconel 600 alloy Fig. 5. Cr depletion observed by the MFM technique at room temperature. The sample is Inconel 600 alloy aged at 873 K for 10 h. thermal treatment of the present study is the same as that of Was et al. We can estimate the width of the depleted zone in the vicinity of the grain boundary on the supposition that all the Cr depletion occurs along the grain boundary. We assume that the grain forms a sphere with a radius of r. The width of Cr depletion d k with Cr concentration c k is given as d k = rv k 3V. (2) The grain size in the present sample was observed by an an optical microscopy as well as by the MFM technique. The mean diameter of the grain was 70 µm in Inconel 600 alloy. The volume ratio v k /V is obtained by Eq. (1). The width d k is obtained as a function of Cr concentration. The Cr-depleted zone extends about 300 nm and the region with Cr concentration below 13 wt. % is within 100 nm. The present magnetic method for analysis of Cr depletion in Inconel 600 alloy has several advantages. It is highly sensitive to Cr concentration, this high sensitivity resulting from the wide range of T c depending on Cr concentration. Its second advantage is the information yielded on the mean value of the depleted zone in the whole sample, and this characteristic is conversely disadvantageous. The method allows nondestructive evaluation and is independent of the sample size, its shape and its surface roughness. The fourth advantage of this method is its simplicity of operation and time efficiency. References [1] G.J. Theus, Corrosion 33 (1977), [2] R.C. Scarberry, S.C. Pearman and J.R. Crum, Precipitation reaction in Inconel 600 and Their Effect on Corrosion Behavior, Corrosion 32 (1976), [3] G.P. Airey, A.R. Vaia, N. Pessall and R.G. Aspden, Detecting Grain-Boundary Chromium Depletion in Inconel 600, J. Metals (1981), [4] P. Chevenard, Magnetic properties of Ni-Cr-Fe alloys, Rev. Metall. (Paris) 25 (1928), [5] G.S. Was, H.H. Tischner and R.M. Latanision, Metall. The Influence of Thermal Treatment on the Chemistry and Structure of Grain Boundaries in Inconel 600, Trans. A 12 (1981), [6] H. Sasakura, K. Suzuki and Y. Masuda, Curie Temperature in Itinerant Electron Ferromagnetic Ni3Al system, J. Phys. Soc. Japan 53 (1984),