DEVELOPMENT OF A NON-DESTRUCTIVE METHOD TO IDENTIFY DIFFERENT GRADES OF STAINLESS STEEL

Transcription

1 DEVELOPMENT OF A NON-DESTRUCTIVE METHOD TO IDENTIFY DIFFERENT GRADES OF STAINLESS STEEL Meor Yusoff Meor Sulaiman Malaysian Institute for Nuclear Technology Research (MINT), Bangi, Kajang, Selangor ABSTRACT One of the non-destructive methods used for the identification and verification of metals is by the energy-dispersive X-ray fluorescence (EDXRF) technique. EDXRF analysis provides several important advantages such as simultaneous determination of the elements present, enable to analyse a very wide concentration range, fast analysis with no tedious sample preparation. The paper shows how this technique is developed and applied in the identification and verification of different grades of stainless steels. Comparison of the results obtained from this analysis with certified reference standards show very small differences between them. ABSTRAK Salah satu teknik ujian tanpa musnah dalam menentukan dan mengesahkan logam-logam ialah dengan teknik pendefeloran sinar-x tenaga tersisih (EDXRF). Analisis EDXRF memberikan beberapa kelebihan saperti penentuan serentak unsur-unsur yang ada, membolihkan analisaan dalam julat kepekatan yang sangat lebar, analisis yang cepat tanpa penyediaan sampel yang remeh. Kertaskerja ini menunujukkan bagaimana teknik ini dibangun and digunakan dalam pengesahan keluli kaliskarat dari pelbagai gred. Perbandingan antara nilai yang diperolehi dengan nilai yang ditetapkan bagi sampel piawai rujukan yang diiktiraf menunjukan perbezaan yang rendah antara mereka. Keywords: EDXRF, stainless steel, manganese, chromium, molybdenum, copper. 67

2 INTRODUCTION Stainless steel can be classified into the 300 and 400 series. Each of these series can be further reclassified into different grades (Tiwari et al, 2001). Stainless steel 300 series and in particular the SS 304 and SS 316 grades are the most widely used metal. The SS 304 is used in sanitary, cryogenic applications as well as tank structural parts and processing equipments. While SS 316 is more resistance to corrosive and it is used in applications like handling of hot organic and fatty acids, boat rails and hardware and facades of buildings near the ocean. One of the important method of identifying type and grade of stainless steel is from their elemental content. Generally in stainless steel samples besides Fe, other alloying elements like Mn, Cr, Mo and Ni are also present to improve the strength and properties of the metal. A typical content of the alloying elements in different stainless steel grade is as that shown in Table 1 below (Goldstein, 2002). As can be seen from the above Table 1, the identity of stainless steel grade can be established if the content of the alloying elements is known. A fast and non-destructive procedure for determining the different grades of stainless steel is required especially in quality control procedures. Table 1: Alloying elements composition of different stainless steel grade Elements % element content in different grades SS301 SS304 SS316 Manganese Chromium Molybdenum Nickel Energy dispersive X-ray fluorescence (EDXRF) is a modern technique developed for fast and non-destructive analysis. It has its beginnings in 1913 when H.G.J. Mosley shows the possibility of using X-ray as an analytical tool. When X-rays bombard a metal, characteristic X-rays that associated with the elements in the metal will be emitted as a result of the absorption process. As the characteristics energy of each element differs from one to another, this energy could serve as the finger print for elemental identification. The technique had its breakthrough with the development of lithium drifted silicon detector and computer in the 1970s (Bertin, 1975). EDXRF is both non-destructive as well as a simultaneous elemental analyzer. Traditional EDXRF equipment can analyze a wide range of elements, from sodium (atomic number =11) to uranium (atomic number =92). Modern equipment with better windowless detector can analyze lighter elements from as low as beryllium (atomic number =4). In determining the identity of the different stainless steel grades, EDXRF has the advantage of analyzing the alloying metals chromium, manganese, nickel and molybdenum simultaneously thus avoiding the long time required for analyzing one element at a time (Yokhin and Tisdale, 1993). Another major advantage of this technique is higher accuracy of its results from the repeated analysis that needs to be done. 68

3 INTENSITY (counts) JOURNAL Of NUCLEAR And Related TECHNOLOGIES, Volume 1, No.1, June 2004 EXPERIMENTAL METHOD EDXRF analysis was carried out using Baird Ex-3000 at MINT. Stainless steel standards and samples are analyzed direct without any prior treatment. Bars and large pieces of stainless steels are placed on to the sample position whilst small and delicate samples are placed in a sample cup. The sample cup is enclosed with a X-ray penetrative prolene plastic film. X-ray tube serves as the source for X-ray beams in EDXRF and this will results to the development of an absorption process. The transition of a higher energy level electron to its lower level will produce characteristics X-ray that will be channeled to the detector. The presence of a multichannel analyzer card in the computer enables the identification of the elements present through their energy spectrum (Baird, 1985). The percentage of element present in the metal was determined by a regression quantitative procedure, an in-house developed procedure using certified reference standards of metals with the same matrix as the sample. Six low alloy stainless steel standards (BCS ) with different Cr, Mn, Ni and Mo concentrations were used for the plotting of the element calibration graphs. RESULTS AND DISCUSSION The different stainless steel grades can be identified by the content of its associated elements. A typical EDXRF spectrum of a SS316 sample is as that shown in Fig Energy (kev) Fig.1: EDXRF spectrum of SS316 sample 69

4 The EDXRF spectrum shows characteristics energy peaks belonging to the K lines attributed by the presence of Cr, Mn, Fe, Ni and Mo. Also, the spectrum shows that unlike the other minor elements, Fe has two strong peaks arises from K and K. These two peaks tend to interfere with the characteristics peaks of the minor elements belonging to Cr and Mn. Identification of the different stainless steel grades requires an accurate quantitative analysis of the minor elements. This was done through the development of a procedure based on the regression method. The regression method involves the plotting of calibration graphs for each of the minor element using standard reference materials of different concentrations. Before this procedure can be developed, a correct counting technique for all the minor elements had to be established by identifying the nature and range of the K peak. Ni and Mo have peaks that are relatively well resolved without any interference from the other elements. Hence the counts under these peaks can be counted by using the gross method (Baird, 1985). Cr and Mn on the other hand have their K peaks interfere and overlap with the Fe K and K peaks respectively. This will result to an error in the quantitative analysis especially as Fe is the dominant element present in stainless steel. The peaks had to be resolved with the low and high energy peaks being identified. Digital filter counting method was applied as this will resolved and removed the counts attributed from the Fe K lines. Table 3 below shows the low, high peak energies chosen with the counting used for the measurement of these minor elements. Element (atomic number) Table 3: Counting parameters for quantitative analysis K,L,M lines Absorption energy (KeV) Low peak energy (ev) High peak energy (ev) Counting method Cr (24) K Gross Mn (25) K Digital Filter Ni (28) K Digital Filter Mo (42) K Gross Both reference materials and samples were exposed to the primary radiation of the X-ray tube and the resulting flourescent radiation that is characteristics of the constituents of samples was measured. The energy spectral intensity data for standards and samples were processed by leastsquare regression analysis with statistcal evaluation carried out after repeating each analysis 5 times. The resulting regression equation is of the form (Baird, 1985); C i = A o + A i I i (1) where; I i is the radiation intensity of i-element (kilocounts per second) A i is the slope A o is the intercept C i is the concentration of the i-element However in the analysis it was found that results obtained by using this mathematical model does not give an accurate result on the elemental content of the sample. This may be due to the matrix effect phenomenon, a common error in X-ray analysis. X-ray is not only generated by 70

5 the source but also it can be produced from other atoms if the characteristics energies are greater than the absorption energies required (Yokhin and Tisdale, 1993). Table 4 below shows the absorption and characteristics energies of Fe, Cr, Mn, Ni and Mo. Table 4: Absorption and characteristics energies of Fe, Cr, Mn, Ni and Mo Elements Absorption energy (KeV) K 1 Characteristic X-ray (KeV) Cr Mn Fe Ni Mo Characteristic X-rays from Mo and Ni are higher than the minimum absorption energy required by Cr, Mn and Fe. This will eventually resulted into more absorption processes to take place for these elements. Hence, resulting to the enhancement in intensities of the Cr, Mn and Fe elements (Bertin, 1975). Nevertheless, the characteristics X-ray will be absorbed if its energy is smaller than the minimum absorption energy of the atom. This will eventually result to the reduce intensity. Thus in an X-ray analysis the composition of the sample and reference standards must be of the matrix and elemental content (Tiawri et al, 2001). The used of mathematical modeling technique had also help in minimizing the matrix effect. A modified Lucas-Tooth and Price model (Equation 2) shown below was used to reduce the matrix effect on the quantitative analysis of stainless steel (Baird, 1985). C i = A o + A i I i + ( A ij C j ) (2) where i and j are analyte and matrix element respectively. This model accounts for inter element influence and peak overlap such as those experience by Cr and Mn. The calculated concentration obtained from this model was then plotted against the certified concentrations of the reference standards. Figures 2 to 5 shows the calibration graphs obtained for Cr, Ni, Mn and Mo respectively. Straight-line calibration graphs were obtained for all the elements with the correlation coefficients values ranges from to (Table 2). This tend to show that the used of digital filter counting technique and modified Lucas-Tooth and Price model is effective in reducing the overlapping peaks and matrix effect errors. The accuracy of the developed quantitative procedure was then tested by analysis and comparing its results with the certified value. British Chemical Standard low alloy stainless steel BCS 406 was used for this purpose. Table 3 shows the differences of the certified and analysis values. Table 2: Correlation coefficient values of calibration graphs of minor elements in stainless steel Element Correlation coefficient, R 2 Cr Mn Ni Mo

6 Calculated concentration (%) calculated concentration (%) JOURNAL Of NUCLEAR And Related TECHNOLOGIES, Volume 1, No.1, June R 2 = concentration (%) Fig. 2: Calibration graph for Mo in steel R 2 = Concentration (%) Fig. 3: Calibration graph of Cr in steel 72

7 Calculated concentration (%) Calculated concentration (%) JOURNAL Of NUCLEAR And Related TECHNOLOGIES, Volume 1, No.1, June R 2 = Concentration (%) Fig. 4: Calibration graph for Ni in steel R 2 = Concentration (%) Fig. 5: Calibration graph for Mn in steel 73

8 Table 3: Differences between acquired and certified value of BCS 406 stainless steel reference standard Trace Certified Value EDXRF Value Discrepancy, % d Elements (%) (BCS 406) (%) d Cr Mn Ni Mo The above result shows the high accuracy of the developed quantitative procedure and a significantly low difference when compared to the certified reference standard value. Conclusion A highly accurate quantitative procedure for determining of different grades of stainless steel was developed using the EDXRF technique. The technique also enables to analyze samples nondestructively and fast. This will help in improving the identification, sorting and verification process for the metal. References Tiwari M.K., et al., 2001, Analysis of stainless steel sample by EDXRF spectrometry, Bull. Mater. Sci., 24(6), p Goldstein S.J. and Sivils L.D., 2002, A non-destructive XRF method for analysis of metal alloy wire samples, Advances in X-ray Analysis, 45, p , ICDD, Philadelphia. Bertin E.P., 1975, Principles and practices of X-ray spectrometric analysis, 2 nd Ed., Plenum Press, New York. Yokhin B. and Tisdale R.C., (1993), High-sensitivity energy-dispersive XRF technology Part 1: Overview of XRF technique, Am. Lab., 36G. Baird, 1985, Operation manual of Ex-3000 EDXRF spectrometer, Baird Corporation, Boston. British Chemical Standard, 1980, Low alloy steel BCS406 certificate of analysis, Marlborough, England. 74