Measurement of nitrogen depth profile in steel

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NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 266 (2008) 1455–1459 www.elsevier.com/locate/nimb

Measurement of nitrogen depth profile in steel J.F. Curado, N. Added *, M.A. Rizzutto, M.H. Tabacniks IFUSP – Travessa R da rua do Mata˜o 187, Cidade Universita´ria, CEP 05508-970 Sa˜o Paulo, SP, Brazil Received 21 September 2007; received in revised form 21 December 2007 Available online 20 January 2008

Abstract The characterization of nitridated steel samples, in special the depth profile of nitrogen, aims to help improving the quality of the surface and to increase the durability of the steel pieces. In this work we used ERDA and NRA to determinate the profile of nitrogen in different sets of stainless steel samples. An incident beam of 35Cl of 56 MeV was used for ERDA analysis of a first set of samples. Results indicated an homogeneous distribution for most of the identified elements, with atomic nitrogen concentrations around 2% in the analyzed depth range (0.2 lm) and the presence of thin films on the surface (about 50  1015 at/cm2), one of C and the other of iron oxide. In a second set of samples, 4.43 MeV gamma rays produced from 15N(H,ac)12C reaction, using an external proton beam of 1.3 MeV, were used to quantify nitrogen concentration. N concentrations of about 0.47% were obtained comparing the gamma production rate of the samples with a referenced material (Stainless steel CRM298 – 0.236% of N in mass) irradiated in the same conditions. Also PIXE analyses were done on both sets of samples in order to identify main elements in the matrix. Ó 2008 Elsevier B.V. All rights reserved. PACS: 25.40.Ny; 25.70.Bc; 29.40.Cs; 81.05.Bx; 82.80.Ej Keywords: ERDA; NRA; PIXE; Trace elements

1. Introduction

2. Experimental

Due to the different applications, the study of steels has been widely carried out measuring the composition of alloys and studying the depth profiles of elements incorporated during its manufacture. The addition of nitrogen to the surface, one of most popular, causes the effect of precipitation hardening. This effect improves ductility of the material changing mechanical properties such as resistance to abrasion or toughness. Because these characteristics change depending on the depth profile of nitrogen, it is important to establish experimental procedures that can verify this profile in a quick mode, like ion beam analysis techniques [1–3]. In this work we aim to obtain the depth profile for total natural nitrogen by measuring the profile of the less abundant isotope (15N).

2.1. Samples

*

Corresponding author. Tel.: +55 11 3091 6824; fax: +55 11 3031 2742. E-mail address: [email protected] (N. Added).

0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.12.111

Two different sets of samples were used. The first (MET samples) consisted of small steel plates of 10  8 mm2 produced using different cooling processes. Initially, samples were exposed under a nitrogen atmosphere with temperature of 1473 K and a pressure of 0.25 MPa during 6 h. After the nitridation, one set of the samples was cooled in oil and cleaned in a ultrasound bath, followed by a thermal treatment at 1273 K, during 1 h in an atmosphere of argon at 0.15 MPa. The second set of samples was cooled in open air intending to produce a sample with a smaller microstructure (Sample 3). Two of them were obtained from the same primary sample cooled in oil, but were cut at different depths. The first one (Sample 1), is a piece from the surface of the material and the second one (Sample 2), a cut 8 mm deep.

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The second set (CEFET Samples) were samples of stainless steel nitridated in plasma at several pressures of nitrogen.

of 15N(p,ac)12C reaction were detected using a NaI scintillator (7% energy resolution @ 1.173 MeV) mounted at 0°.

2.2. Experimental setup

3. Results and discussion

A DE–E ERDA analysis setup [4] of the 8UD Tandem accelerator of the Pelletron Laboratory of Institute of Physics at University of Sa˜o Paulo (IFUSP) was used with an incident beam of 56 MeV CL7+ ions. The ionization chamber was positioned at 40° with respect to the beam direction and the angle between the normal to the surface of the sample and the incident beam was 60°. The ionization chamber used a 220 lg/cm2 polycarbonate beam entrance window and was filled with P10 (90% argon and 10% methane) at a pressure of 18.5 Torr. PIXE measurements were done at the Laboratory for Ion Beam Material Analysis (LAMFI) of IFUSP. MET samples were irradiated in vacuum with a 2.4 MeV proton beam (target angle of 45°). Two X-ray detectors were used: Detector L (147 eV @FWHM for 55 Fe), located 90° in respect to the beam, with a Mylar filter of 200 lm for the high energy X-rays; and detector K (138 eV @FWHM for 55Fe), located at 45° in relation to the beam, with a plain Beryllium filter of 50 lm. CEFET samples were irradiated in air (external beam setup). A 1.3 MeV beam probed the samples and a XR100 detector (150 eV @FWHM for 55Fe) was mounted at 120° in respect to the beam direction. A XRD study was performed at the Crystallography Laboratory at IFUSP. For the measurements, radiation of Cu Ka (k = 0.15418 nm) with a graphite monochromator was used in a symmetrical geometry h–2h, using a current of 20 mA at 40 kV. For NRA measurements, the samples were irradiated with a 1.3 MeV external proton beam and the 4.43 MeV gamma rays from the resonances

In the typical two dimensional spectrum obtained by the DE–E ERDA technique shown in the Fig. 1 is possible to identify light elements like oxygen, carbon and nitrogen in the heavy matrix (Fe). In this spectrum we can identify the presence of thin films on the surface, by the higher number of counts in the high energy region of the carbon and oxygen spectra. Thicknesses of these films and the elementary concentrations in each layer were obtained adjusting single energy spectrum by SIMNRA [5] predictions for our experimental conditions. According to these simulations, the first layer is composed only by carbon atoms and the second layer is composed by a thin iron oxide film (Fe2O3). The relative atomic concentrations of each layer as well as the thickness of each film are shown in Table 1. For nitrogen, a simulated concentration value of 2% at. was obtained for the selected region in the energy spectrum, which corresponds to a depth of 0.2 lm (assuming a stainless steel matrix). The depth of this selected region was limited by the experimental parameters of our setup, like the range of the 56 MeV 35Cl beam in steel (a few microns), the thickness of the window and the gas pressure in the ionization chamber. MET samples were also analyzed by PIXE. In the spectra, shown in Fig. 2, we can clearly identify the elements V, Cr and Fe as the main components of the alloy. In this analysis it was possible to observe small differences in the relative contributions of the elements Co, Ni and Cu, as shown in Table 2. These relative concentrations were calculated from the areas for the Ka peaks adjusted using

Fig. 1. Two dimensional spectrum DE X residual energy obtained using a 35Cl beam (Elab = 56 MeV) to probe samples and a ionization chamber (polycarbonate window of 220 lg/cm2 and 18.5 Torr of P10) to collect the recoils. It is indicated in the figure the regions and elements used in analysis.

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Table 1 Relative atomic concentration and thicknesses of layers for two analyzed samples using ERDA technique Layer

Elem

Sample 1

Sample 2

Layer 1

Comp Thickness

C

1.00 ± 0.13 (20 ± 3)1015 at/cm2

1.00 ± 0.10 (30 ± 4)1015 at/cm2

Layer 2

Comp

O Fe

0.600 ± 0.045 0.400 ± 0.013 (50 ± 5)1015 at/cm2

0.600 ± 0.038 0.400 ± 0.014 (60 ± 5)1015 at/cm2

C N O Fe

0.01190 ± 0.00092 0.01570 ± 0.00089 0.00730 ± 0.00062 0.9651 ± 0.0031 >Range

0.01830 ± 0.00093 0.01820 ± 0.00075 0.00735 ± 0.00051 0.9562 ± 0.0022 >Range

Thickness Layer 3

Comp

Thickness

Uncertainties were evaluated by usual statistical and propagation methods for the integrated area for each energy spectrum.

QXAS code [6]. We assumed that these concentrations do not vary significantly for the range of proton in steel. Finally, the XRD analysis (Fig. 3) shows a small difference in the structure of the two sets of MET samples. As it can be seen in the figure, Sample 3 has an additional small peak in the spectrum that indicates the presence of a crystalline Ni in the structure of this alloy. The three most prominent peaks shown in all spectra corresponds to the 434L stainless steel (Fe–Cr alloy) lines according to JCPDS libraries [7]. Extracting the size of crystallites from these data it was possible to observe a difference in the particle size for the samples: Sample 2 has bigger particles than Samples 1 and 3 which are samples from the surface of the primary material. In the study of CEFET samples a NRA analysis with an external beam setup was used to probe samples. For the selected 15N(p,ac)12C reaction [8], there are several resonances in the cross section for the production of gamma rays from the first excited level of carbon (4.43 MeV). The most important are the resonances at 429 keV (r = 300 mb and dE = 0.12 keV) and 897 keV (r = 800 mb and dE = 1.7 keV). The energy of accelerated beam and the distance of sample to exit window were adjusted to

Table 2 Relative atomic concentration of analyzed MET samples using PIXE technique

V Cr Mn Fe Co Ni Cu

Sample 1

Sample 2

Sample 3

0.0414 ± 0.0089 15.651 ± 0.085 0.423 ± 0.038 83.82 ± 0.28 – – 0.0640 ± 0.0059

0.0522 ± 0.0075 15.594 ± 0.083 0.229 ± 0.033 83.94 ± 0.28 0.211 ± 0.026 0.0321 ± 0.0089 0.0443 ± 0.0052

0.0824 ± 0.0087 15.19 ± 0.10 0.428 ± 0.036 84.24 ± 0.41 – 0.061 ± 0.010 –

have a 1.3 MeV proton beam on the surface of samples, defining the maximum depth to be analyzed (4.6 lm), related to the 429 keV resonance. In this experiment, the nitrogen concentration of the samples was evaluated in relation to measurements with a standard referenced steel CRM298. In order to evaluate the blank contribution for the gamma ray spectra, a pure Fe foil was irradiated in the same conditions. The NRA spectra are shown in Fig. 4 where it can be observed the characteristic curve for the de-excitation of carbon (4.43 MeV). The calculated concentrations of nitrogen for CEFET samples subtracting

Fig. 2. PIXE spectra obtained for MET samples using a 2.4 MeV proton beam in vacuum conditions. The peaks used in analysis are labeled with respective X-ray definition. Peaks around channel 600–700 are related to sum effect in the acquisition system.

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Fig. 3. XRD spectrum obtained for MET samples. For the measurements radiation of Cu Ka (k = 0.15418 nm) with monochromator of graphite was used in a symmetrical geometry h–2h, using a current of 20 mA and a tension of 40 kV.

Fig. 4. Gamma spectra for CEFET measurements (standard, iron and CEFET 1). In these measurements, a 1.3 MeV proton beam (at the surface of sample) was used to probe the samples in air.

blank contribution and comparing with referenced material are shown in Table 3. PIXE analyses of the CEFET samples indicate the same alloy (Fe, Cr, Ni) for all samples Table 3 Normalization between integrated area for 4.43 MeV region and respective N% in weight using a CRM298 standard sample (stainless steel) as reference Sample

Normalized area (4.43 MeV)

N% (weight)

Reference CRM298 CEFET 1 CEFET 2 CEFET 3 CEFET 4 CEFET 5 CEFET 6

1.000 ± 0.018 1.78 ± 0.66 1.80 ± 0.67 1.68 ± 0.64 1.81 ± 0.65 1.34 ± 0.65 1.71 ± 0.70

0.236 ± 0.004 0.47 ± 0.17 0.47 ± 0.18 0.44 ± 0.17 0.48 ± 0.17 0.35 ± 0.17 0.45 ± 0.19

Blank contribution was estimated using a pure Fe foil. Uncertainties were evaluated by usual statistical and propagation methods for the integrated area.

and it was also used for total charge normalization. CEFET samples present an averaged value of 0.4% (weight) for the nitrogen contribution for the depth analyzed. 4. Conclusion The use of different techniques allows identifying the matrix composition and the structure of different sets of samples. We measured nitrogen concentration (0.2 up to 2%) for depth profile ranging from tenths of micron to few microns. In special, DE-ERDA analysis also showed the presence of thin films at the surface of the samples related to a process of oxidation. Acknowledgements M.A.R. and N.A. acknowledge FAPESP and CNPq for financial support.

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