Corrosion-resistant analogue of Hadfield steel

Materials Science and Engineering A 420 (2006) 47–54

Corrosion-resistant analogue of Hadfield steel V.G. Gavriljuk a,∗ , A.I. Tyshchenko a , O.N. Razumov a , Yu.N. Petrov a , B.D. Shanina b , H. Berns c a

G.V. Kurdyumov Institute for Metal Physics, UA-03680 Kiev, Ukraine b Ruhr University Bochum, D-44780 Bochum, Germany c Institute for Semiconductor Physics, 03028 Kiev, Ukraine

Received 1 June 2005; received in revised form 1 January 2006; accepted 1 January 2006

Abstract The concept of alloying austenitic steels with carbon + nitrogen is used for the development of a corrosion-resistant austenitic CrMn steel having an impact wear resistance close to that of the Hadfield steel. A higher stabilization of the austenitic phase by C + N, as compared to carbon or nitrogen alone, is substantiated by ab initio calculation of the electron structure, measurements of the concentration of free electrons and calculations of the phase equilibrium. Based on these results, the compositions (mass%) Cr18Mn18C0.34N0.61 and Cr18Mn18C0.49N0.58 were melted and tested along with Hadfield steel Mn12C1.2. Mechanical tests have shown that, as compared to the Hadfield steel, the experimental steels possess a higher strength, plasticity, hardness and the same resistance to impact wear. TEM studies of the surface layer after impact treatment revealed a mixture of the amorphous phase, nanocrystals and fine-twinned austenite. At the same time, using M¨ossbauer spectroscopy of conversion electrons, the ferromagnetic ordering was found in the surface layer of up to 10 ␮m in depth, which is the sign of the strain-induced martensitic phase. The hypothesis of a transition from the low-spin to the high-spin state of the iron atoms within the thin twins in austenite was proposed in order to interpret the discrepancy between TEM and M¨ossbauer studies. Potentiodynamic measurements and immersion tests show that the CrMnCN steels possess a significantly higher pitting potential and resistance to general corrosion in comparison with Hadfield steel. © 2006 Elsevier B.V. All rights reserved. Keywords: Austenitic steel; Carbon + Nitrogen; Electron structure; Phase equilibrium; Strength; Corrosion; Wear

1. Introduction The austenitic wear-resistant manganese–carbon steel containing (mass%) 1.0–1.2 C and 12–14 Mn was developed by Sir Robert Hadfield at the beginning of the 20th century [1]. Its superior resistance to impact wear was attributed to rapid work hardening, and different mechanisms were proposed for that: strain-induced ␥ → ␣ or ␥ → ␧ transformations (e.g. [2–4]), mechanical twinning [5], interaction of dislocations with carbon atoms in solid solution [6,7], etc. These features of the structure and mechanical properties of the cold-worked Hadfield steel were mainly clarified before the eighties. Subsequent studies did not bring any new knowledge concerning mechanisms of high impact wear resistance of Hadfield steel and were devoted to its alloying with carbide-forming elements (e.g. [8]), development of composites “WC-Hadfield steel” [9], confirmation of a

∗

Corresponding author. Tel.: +380 44 4243310; fax: +380 44 4243310. E-mail address: [email protected] (V.G. Gavriljuk).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.01.066

decisive role of deformation twins in the work hardening based on the data obtained on poly- and single crystals (e.g. [10–12]). A low corrosion resistance is an essential shortcoming of Hadfield steel. Attempts to improve it by alloying with chromium were not successful because the precipitation of chromium carbide led to a deteriorating wear resistance without any positive effect on corrosive properties. The substitution of carbon by nitrogen is known to provide a good combination of strength and corrosion resistance (see e.g. [13]), however, an extremely high pressure of gaseous nitrogen would be needed to reach a nitrogen content equivalent to that of carbon in Hadfield steel. Earlier [14], the concept of a cost-effective stainless austenitic steel with C + N was proposed, which is characterised by an increased thermodynamic stability n spite of a high content of interstitials. It is the aim of the present study to use this concept in an attempt to develop a corrosion-resistant analogue of Hadfield steel. The paper contains the results of: (a) theoretical calculations and experimental studies of the electronic structure, (b)

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Table 1 Chemical composition (mass%) Steel

C

N

Cr

Mn

Ni

Si

V

Mo

C+N

C/N

CN0.96 CN1.07

0.344 0.489

0.614 0.578

18.1 18.9

19.0 19.0

0.34 0.39

0.26 0.40

0.06 0.07

0.06 0.08

0.95 1.07

0.56 0.85

thermodynamic calculations of phase equilibria, (c) studies of structural changes in the surface layer after impact wear treatment and (d) standard mechanical and corrosion tests. 2. Experimental Two C + N steels containing 0.96 and 1.07 mass% (C + N) with the C/N ratio of 0.56 and 0.85, respectively (see Table 1), were melted in an induction furnace, electro-slag-remelted in air of normal pressure and subsequently hot worked to rods. The standard Hadfield steel MnC containing 12 mass% Mn and 1.2 mass% C was stemmed from hot worked stock and taken for reference. Calculations of the electronic structure (the energy bands and the total electron energy per cell or per atom) were carried out using the modern computational program package WIEN2k elaborated by a European scientific group [15]. The theoretical calculations are based on the Kohn–Hohenberg–Sham density functional theory [16,17]. Conduction electron spin resonance (CESR) was measured to estimate the concentration of free electrons in the ␥ solid solution. An electron paramagnetic resonance (EPR) spectrometer with a frequency ν = 9.3 GHz, a microwave field power P = 10 dB, an amplification coefficient f = 2 × 103 , a modulating field with the modulation amplitude Hm = 2 × 10−4 T and the frequency νm = 105 Hz was used for the measurements in the temperature range of 4.2–300 K. The sample thickness of 2 × 10−2 mm was small enough to neglect the skin effect at T > 100 K. The phase equilibria were calculated using the ThermoCalc programme in order to determine the temperature-concentration field of single austenitic phase in CrMnCN alloys. Impact wear treatment was carried out using special equipment at the K¨oppern Company, Hattingen, Germany. Two wear plates of the studied steels were mounted on both ends of a rotor arm and impacted by mineral particles of greywacke of hardness 760 HV0.1 and grid size 11 mm × 8 mm, which fell parallel to the vertical rotor axis. They were hit by the plates rotating under an impact angle of 90◦ at a velocity of 26 m/s (1200 rotations per minute). The weight loss of the two plates was measured after each 1000 impacts and cleansing in an ultrasonic bath of alcohol. After 12,000 impacts (about 6000 per plate), samples of 3 mm in diameter were taken from the most densely impacted area and thinned mechanically from the back side to about 200 ␮m. A change in the constitution of the surface layer after wear treatment was studied by backscattering M¨ossbauer spectrometry, X-ray diffraction and transmission electron microscopy (TEM). A M¨ossbauer spectrometer WISSEL and a source of ␥ quanta 57 Co in the Cr matrix with activity of 50 mCi were used for the studies. Measurements were made in the conversion

electron scattering mode so that the information was obtained from a layer of about 150 nm in depth. The equipment used was characterised by the line-width of the source of 0.22 mm/s. The occurrence of ferromagnetic ␣-martensite could be estimated due to Zeemann splitting of the nuclear levels, which results in the appearance of the Zeemann sextet in the spectrum. The distribution of hyperfine fields was calculated using the programme DISTRI developed by Prof. Rusakov, Moscow State University, Russia. X-ray diffraction patterns were obtained using the “HUBER” diffractometer with a two-circle Θ − 2Θ goniometer. Transmission electron microscopy (TEM) was used to study structural changes under impact treatment. The impact wear surface of thin discs was cleaned by ion milling in a GATAN Ion Duo Mill 600 at a milling rate of 2 ␮m/h for 0.5 h. Thereafter it was covered with a Teflon foil, 50 ␮m thick, in order to protect it from electropolishing in a solution of 5% HClO4 + 95% of the ice-cold acetic acid (density 1.5 g/cm3 ), while the back side of the disc was thinned for TEM observation of the very surface. In order to get information on the structure below the wear surface, we determined the rate of electropolishing using a reference sample of 100 ␮m in thickness, which was electropolished until a hole was obtained. Each disk was electropolished from both sides for some controlled time and, thereafter, the side of the wear surface was covered with Teflon protection foil and the back side was electropolished until a hole was obtained. Tensile tests were performed at different temperatures and strain rates. The hardness profiles after the impact treatment were measured in a section orthogonal to the surface. ISO-V impact test were carried out to reveal an embrittlement by deepfreezing. The corrosion resistance was evaluated by immersion test and by current density/potential tests in different aqueous media. 3. Results and discussion 3.1. Electron structure Fig. 1 shows the density of the electron states, EF ± kT, at T = 300 K for fcc iron and the binary solid solutions Fe–C, Fe–N and Fe–(C + N) with an i/Fe ratio of 6.25%. It is seen that carbon decreases whereas nitrogen and carbon + nitrogen increase the state density in the vicinity of the Fermi level. These results suggest an increase in the concentration of conduction electrons due to nitrogen and carbon + nitrogen in the iron austenite, which was confirmed by the measurement of conduction electron spin resonance (CESR) in austenitic steels [18–20]. The calculated data of full electron energy per cell and per one atom in the cell are presented in Table 2. One can conclude from these data that the thermodynamic stability of the

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Table 2 Full electron energy per cell and per one atom of the cell in Fe–C, Fe–N and Fe–C + N solid solutions Composition

Estruct (Ry × cell−1 )

Egain (Ry × cell−1 )

Estruct (eV × atom−1 )

Egain (eV × atom−1 )

Fe Fe + 3 at.%N Fe + 3 at.%C Fe + 6 at.%N Fe + 6 at.%C Fe + 3 at.%C + 3 at.%N

−14.756 −15.616 −15.469 −15.632 −15.797 −15.814

0.00 −0.8600 −0.7132 −0.8763 −1.0414 −1.0577

−6.2712 −6.4356 −6.3751 −6.2529 −6.3189 −6.3255

0.00 −0.1644 −0.1039 −0.0183 −0.0477 −0.0543

fcc phase in the binary iron-based solid solution is increased if 3 at.% of carbon are substituted by 3 at.% of nitrogen. If the fraction of interstitials increases up to 6 at.%, the Fe–C solid solution becomes more stable. The highest stability is obtained for a Fe–C–N solid solution with 6 at.% (C + N). Thus, alloying with carbon + nitrogen is expected to stabilize the ␥ phase in austenitic steels, as well. The results of CESR measurements on steels CN0.96 and CN1.07 are presented in Fig. 2 along with data obtained earlier for austenitic steels alloyed with carbon, nitrogen or carbon + nitrogen (see e.g. [20,21]). As compared to carbon, nitrogen increases the concentration of conduction electrons in CrMnNi steels up to some limit at about 2.5 at.%, correspondingly at ∼0.65 mass%. Alloying with carbon + nitrogen raised the concentration of conduction electrons further and shifted its maximum to higher contents of interstitials (about 3.3 at.%). Along with the data on the full electron energy characterising the thermodynamic stability of the fcc phase, the results of CESR measurements, showing the enhancement of the metallic character of interatomic bonds due to the combined alloying with carbon + nitrogen, suggest that austenitic C + N steels are

Fig. 1. Density of electron states at the Fermi level over the range of EF ± kT, T = 300 K.

promising in terms of their stability to precipitations and in terms of mechanical properties, particularly plasticity and toughness. 3.2. Phase equilibria Results of calculations of the phase equilibria for the Cr18Mn18 basic compositions alloyed with carbon and carbon + nitrogen at different C/N ratios are presented in Fig. 3. No single austenitic phase exists for the Cr18Mn18C composition at any temperature and carbon content (Fig. 3a). The substitution of carbon by carbon + nitrogen stabilizes the austenitic phase (Fig. 3b and c). A decrease of the C fraction in the C + N content is more favourable for stabilization of the ␥-phase. 3.3. Mechanical properties The results of tension tests are shown in Fig. 4 and Table 3. Both C + N steels possess a strength and plasticity which are significantly higher as compared to Hadfield steel. Correspondingly, the product of yield strength and relative reduction in area, Rp0.2 × Z as well as the absorbed energy of deformation, Ws , exceed the values for Hadfield steel. The impact toughness of steels CN0.96 and CN1.07 was studied at different temperatures in order to estimate the possible temperature range of their practical usage (Fig. 5). The results

Fig. 2. Concentration of free electrons at the Fermi level (conduction electrons) measured using CESR for different austenitic steels alloyed with carbon and/or nitrogen. Arrows show the increase in free electron concentration for CrMn compositions by an exchange of nitrogen by carbon + nitrogen. The data for steels CN0.96 and CN1.07 are obtained in the present study. The earlier data from studies [20,21] are presented for the comparison.

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Fig. 3. Phase equilibria in the systems Cr18Mn18C (a), Cr18Mn18(C + N) at C/N = 0.5 (b) and Cr18Mn18(C + N) at C/N = 1.0 (c) as calculated by the ThermoCalc programme.

of tests show that the ductile-to-brittle transition occurs at lower temperatures in steel CN0.96, which is obviously due to the smaller fraction of carbon in the C + N mixture. No additional phases were observed after low temperature impact tests.

A crucial test for development of Hadfield-type steel is the wear resistance under impact loading. The data presented in Fig. 6 prove that all three steels, CN0.96, CN1.07 and MnC, have nearly the same impact wear resistance. It is interesting to compare the change in hardness of the steels after impact loading. The hardness profile within the surface layer is shown in Fig. 7 to a depth of several millimeters. The hardness of the Hadfield

Table 3 Mechanical properties of the studied steels in comparison with Hadfield steel (MnC), RT after solution treatment at Ts Steel

(◦ C)

Ts Rp0.2 (MPa) Rm (MPa) A5 (%) Z (%) Rp0.2 × Z/104 Ws (J/cm3 ) HV30 Fig. 4. Stress–strain curves of the studied steels. Tension at RT with the strain velocity of 4.2 × 10−4 s−1 for steels CN0.96 and CN1.07 and 3.3 × 10−4 s−1 for the Hadfield steel.

CN1.07

CN0.96

MnC

1100 604 1075 73.5 52 3.14 694 285

1100 600 1020 73.5 68.7 4.12 676 280

1050 370 829 46 33 1.22 330 195

The strain rate is equal to 4.2 × 10−4 s−1 for steels CN0.96 and CN1.07 and 3.3 × 10−4 s−1 for the Hadfield steel.

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steel MnC after impact treatment is highest at the very surface. With an increasing distance from the surface up to 0.5 mm the hardness of three steels is being equalized. In the deeper layers, where the effect of impact treatment disappears, the steels CN0.96 and CN1.07 reveal a higher hardness in comparison with the Hadfield steel. Taking into account that the Hadfield steel has the smallest strength and hardness in the solution treated state, one can conclude that its surface acquires the strongest hardening by impact treatment. This hardening cannot be just attributed to the effect of cold work because, as follows from Fig. 4, the cold work hardening of steel MnC is not higher than that of CN steels. Some additional reasons must exist for the impact-induced surface hardening. Fig. 5. Ductile-to-brittle transition in steels CN0.96 and CN1.07.

3.4. Structure of the surface layer after impact treatment The impact-induced change in the structure of the surface was studied using X-ray diffraction, backscattering M¨ossbauer spectroscopy and transmission electron microscopy. Fig. 8 shows a section of the X-ray diffraction pattern taken from the very surface of the impact-treated steel CN0.96 and after subsequent removal of a surface layer by electropolishing. The diffraction pattern of the surface contains reflections of silicon oxide SiO2 , which stemmed from the greywacke particles, and gradually disappear after electropolishing. Except for austenite, we did not succeed to detect any other phases by means of X-ray diffraction. A M¨ossbauer spectrum in the mode of conversion electrons obtained from the surface layer (about 150 nm) of the impacttreated steel CN0.96 is presented in Fig. 9. The spectrum consists of a paramagnetic component belonging to the austenitic phase and a ferromagnetic part typical for a martensitic structure. Qualitatively the same spectra were obtained from the surface of the two other steels. The probability of hyperfine fields, P(H), as shown in the insert, reveals the intensive component Fe0 with Hi of about 33 T belonging to iron atoms having no atoms of chromium, man-

Fig. 6. Mass loss during impact wear caused by mineral particles of greywacke.

Fig. 7. Profile of microhardness after impact treatment.

Fig. 8. X-ray diffraction of steel CN0.96 after the impact treatment and electropolishing of its impact-treated surface.

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Fig. 9. M¨ossbauer spectrum obtained from the impact-treated surface of steel CN0.96 in the mode of conversion electrons.

ganese and interstitials C(N) as nearest neighbours and some low intensive components from iron atoms with atoms of alloying elements as neighbours. Taking into account such complicated chemical compositions and hard cold work, a detailed interpretation would be rather difficult and it is not our task within the frame of this study. Based on the data of P(H) and using the mean square procedure, we interpreted the ferromagnetic component as consisting of the component Fe0 of pure iron (hyperfine field H0 ≈ 33 T, isomer shift δ ≈ 0.01 mm/s, quadruple interaction ε ≈ −0.01 mm/s), the iron atoms having one (Fe1 ) or two (Fe2 ) interstitial atoms as nearest neighbours (hyperfine field H1 ≈ 30 T and H2 ≈ 27 T, isomer shift δ1 ≈ 0.1 mm/s and δ2 ≈ 0.2 mm/s, quadruple interaction ε1 ≈ −0.04 mm/s and ε2 ≈ −0.1 mm/s) and some component Fecl from iron atoms in clusters of Cr(Mn) and C(N) atoms (H3 ≈ 13.4 T, δ ≈ 0.6 mm/s and ε ≈ −1.0 mm/s). The large width of lines, of about 0.4 mm/s for the first component and 0.8–2 mm/s for the second and third components, accounts for uncertainties of such rough approx-

Fig. 10. Structure of the surface in steel CN0.96 after impact treatment (after removing 1 mm layer by ion milling in a GATAN Ion Duo Mill 600): amorphous state (a), pseudo-martensitic twin structure (b), key diagram for the diffraction pattern of 11b (c).

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Table 4 Fractions (%) of paramagnetic and ferromagnetic phases according to M¨ossbauer studies of steels CN0.96, CN1.07 and MnC Steel

CN0.96 CN1.07 MnC

Paramagnetic phase

20.7 18.8 23.8

Ferromagnetic phase P1 (Fe0 )

P2

P3 (Fecl )

30.5 49.1 44.05

39.5 (Fe1 ) 24.0 (Fe2 ) 20.2 (Fe2 )

9.3 8.1 11.95

imation. It is worth noting that, in spite of the rough fitting of the spectra, one can distinguish the components Fe1 and Fe2 , which are typical for nitrogen and carbon martensites, respectively (see, e.g. [22,23]). The fractions of ferromagnetic and paramagnetic components obtained by fitting of M¨ossbauer spectra are presented in Table 4. Traces of Zeemann’s sextets are observed even at a depth of 10 ␮m while after further electropolishing the spectrum consists only of the paramagnetic austenitic component. TEM observations of the surface after the impact treatment reveal a complicated picture characterised by two types of submicrostructure (see Fig. 10): an amorphous state (a) and a strongly deformed twinned crystalline structure similar to martensite (b), whereas the diffraction pattern corresponds to twinned austenite, not the martensite (c). This result is in contradiction with M¨ossbauer data, because even a strong deformation cannot make the austenite ferromagnetic. We propose the following interpretation of this non-trivial phenomenon. In accordance with the Kurdyumov–Sachs orientation relationship, the atomic distribution in the martensitic bcc plane (1 1 1) is consistent with that in the austenitic plane (1 1 0), which is demonstrated in Fig. 10c. Taking into account the high density of twins and their small thickness (about 1–5 nm), one can imagine that the bcc-like coordination of atoms at the twin boundary transforms the low-spin state of iron atoms in paramagnetic austenite to the high-spin state entailing magnetic ordering within the twins. Such a transformation will be accompanied by Zeemann’s effect with a corresponding magnetic splitting in M¨ossbauer spectra (Fig. 9).

Fig. 12. Twins and dislocations in the structure of the impact-treated steel CN0.96 observed at 45 ␮m below the surface. Along with the twins, amorphous islands were also observed.

An amorphous and nanocrystalline twin structure was also observed at a distance of 5 and 15 ␮m below the surface (Fig. 11, compare [24]). After a removal of 45 ␮m, the structure was characterised by the occurrence of macrotwins and a high density of dislocations (Fig. 12), although one could also find some amorphous areas. 3.5. Corrosive properties The corrosion tests included potentiodynamic measurements in a 3% NaCl aqueous solution and the estimation of resistance to general corrosion in H2 SO3 aqueous solution of pH 2.0. The results of potentiodynamic measurements are presented in Fig. 13. Both steels, CN0.95 and CN1.07, have a higher pitting potential than Hadfield steel. The smaller fraction of carbon in steel CN0.96 results in a higher pitting potential. According to the weight loss measurements during 120 h of immersion tests, both experimental steels are not prone to

Fig. 11. Structure of the impact-treated steel CN0.96 observed at 5 ␮m (a) and steel CN1.07 at 15 ␮m (b) below the surface.

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made that the atomic order at twin boundaries is close to that in martensite and this is a reason for a transition from the low spin to the high spin iron state, which results in ferromagnetic order detected by M¨ossbauer spectroscopy. 6. The Cr18Mn18(C + N) steels are characterised by a high resistance to pitting and general corrosion. Acknowledgements This study was carried out within the frame of a project of the Science and Technology Center in Ukraine No. 3026 and a project of the German Research Foundation (DFG) No. BE 1022/17. The authors thank the young scientific researchers I.M. Boynitska and G.S. Mogilny for their help with X-ray diffraction studies. Fig. 13. Potentiodynamic curves for steels CN0.96, CN1.07 and MnC in a 3% NaCl aqueous solution.

general corrosion at the given experimental conditions, whereas the Hadfield steel lost its weight with the average rate of about 3.069 g/(m2 h). Thus, alloying with chromium in combination with carbon + nitrogen provides a good corrosion resistance of the austenitic steel in spite of the high manganese content. 4. Conclusions 1. A corrosion-resistant analogue of Hadfield steel is proposed based on the studies of the electron structure, thermodynamic calculations, structural studies and tests of mechanical and corrosive properties. 2. According to ab initio calculations, the alloying of iron with carbon + nitrogen increases the density of electron states at the Fermi level, which is confirmed by experimental data revealing an increased concentration of conduction electrons. A decrease in the full electron energy of the fcc iron by carbon + nitrogen suggests a higher thermodynamic stability of the Fe–C–N composition compared to FeC or FeN alloys. 3. The calculation of phase equilibria provides evidence of a wide single austenitic field in the T–C diagram of Cr18Mn18 steel alloyed with carbon + nitrogen. A decrease of the carbon-to-nitrogen ratio allows a higher interstitial concentration and wider temperature limits of the austenitic state. 4. Mechanical tests show that austenitic CrMnCN steels possess a higher yield and ultimate strength, relative elongation, reduction in area and hardness as compared to Hadfield steel. They reveal an appropriate impact toughness and are comparable with Hadfield steel in respect to impact wear resistance. 5. Structural studies have shown that the surface layer of impacttreated steels contains a mixture of the amorphous and nanocrystalline structures to a depth of 45 ␮m. The top layer of about 10 ␮m is characterised by ferromagnetism. A TEM study reveals thin twins (1–5 nm) in austenite while the impact-induced martensite is not found. The suggestion is

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