Development of nitrided layer during nitriding of steel Jerzy Ratajski, Roman Olik

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1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Development of nitrided layer during nitriding of steel Jerzy Ratajski, Roman Olik Institute of Mechatronics, Nanotechnology and Vacuum Technique, Koszalin University of Technology, Poland Keywords: Compound zone; Diffusion zone; Hardness profiles Abstract The present work is devoted to research the influence of micro-structural evolution of the compound zone (iron (carbo)nitrides zone) upon development of hardness profiles in diffusion zone. A different phase structure of iron (carbo)nitrides zone on steel as compared to iron, further changing with the process, may result in upsetting the quasi-equilibrium of nitrogen concentration in the iron (carbo)nitrides zone/diffusion zone interface and may as a result have impact on the kinetics of this layer s growth. Aimed at solving this problem there was a research carried out to evaluate influence of (carbo)nitrides zone, with intentionally created diametrically different phase composition, on hardness profiles in the diffusion zone. Based on the research conducted, it was shown that the evolution of phase structure of the compound zone contributes significantly, regardless of nitrogen potential and temperature, to the formation of the diffusion zone and in particular to its effective thickness. It makes this complex picture of nitrided case development on steel even more intricated. Introduction The case developed during gaseous nitriding of steel consists of a surface zone of iron (carbo)nitrides, the compound zone and a diffusion zone underneath. The compound zone for pure 3 / 2 iron developing at a high value of the nitriding potential K p p 1 is in accordance with N NH 3 / H 2 Lehrer's diagram [1]: at the surface the phase developed, while the phase ' lies directly adjacent to the substrate. It is assumed that there is local equilibrium at the / ' interface and at the '/substrate interface [2]. Consequently, the growth kinetics of the diffusion zone is controlled by temperature only. In the case of alloy and carbon steels, the sequence of phases in the compound zone evolves with nitriding time [3,4]. The subdivision in well-defined and ' subzones is replaced by a mixture of phases, particularly in the lower part of the compound zone. On the basis systematic studies regarding the phase constitution of the compound zone on carbon steels [5-7], it was concluded that a direct nucleation of the phase on the diffusion zone was explained from the small solubility of carbon (< 0.2 %wt.) in the ' phase in contrast to the large solubility for carbon in the phase. Later it was shown that cementite can easily be converted into phase, due to a crystallographic resemblance of these phases (orthorhombic and hexagonal, respectively) [8-10]. The combination of this easy Ɵ conversion and the stabilisation of phase (with respect to ' phase) by the presence of carbon is thought to be responsible for the generally observed occurrence of phase in the lower part of the compound zone [11]. The complex relation between nitriding parameters and the composition and phase constitution of the compound zone hinders tailoring of the properties of the nitrided case and controlling the growth kinetics of the diffusion zone. Apart from a few remarks [12, 13], this aspect has so far not been discussed in the literature. The present work is devoted to research the influence of micro- 1 The nitriding potential is directly proportional to the nitrogen activity in the gas mixture. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-21/02/16,04:54:32)

2 1026 Advances in Materials and Processing Technologies structural evolution of the compound zone on a few commercial steels upon development of hardness profiles in diffusion zone. Specimen preparation 3 Specimens of mm were cut from a slab. The nominal compositions of these steels and their heat treatment prior to nitriding are summarised in Table 1. The surface roughness of the specimens after polishing was R a = 0.3 m. Gas nitriding Nitriding processes were carried out in laboratory furnace due both, possibility of immediate change of nitrogen potential in a retort vessel as well as due to possibility of precise setting of its value. Process temperature was T=803 K (530 0 C), what is usually applied in case of low- and mediumalloy steel nitriding. Values of potential were set the way (Table 2) to obtain different phase structure of (carbo)nitrides zone in isothermal processes, in the same time. During 16 h of the process a fixed potential of K N =10, was applied according to Lehrere s system in the field of phase, or two level variation of potential. In the second option of processes, during the first stage of 8 h there was nitriding atmosphere with potential K N =10 applied and in the second stage, which lasted also 8 h, potential was lowered to value of K N =0,45, being in the field of phase '. Nitrided layer characterisation Nitrided layer characterisation included: i. Measurement of the thickness of the compound zone by light optical microscopy (LOM) ii. iii. iv. and X-ray diffraction (XRD); phase analysis of the compound zone by LOM 2 and XRD; determination of the total interstitial content of nitrogen and carbon from the lattice parameters (a,c) of the hexagonal ε phase (XRD); determination of the profiles hardness. ad i: Compound zone thickness The compound zone thickness was determined with light optical microscopy and X-ray diffraction methods. For X-ray diffraction, the thickness of the compound zone was calculated from the attenuation of the diffracted intensity of the substrate, applying Beer s absorption law: ln I 0 / I sin g /1/ 2 where: g- effective layer thickness, I o - matrix line intensity for a specimen without a compound zone, I α - matrix line intensity for a specimen with a compound layer, - linear absorption coefficient of the compound layer, - Bragg s angle. 2 Phase analysis of the compound layer by LOM was performed in M. A. J. Somers laboratory, Technical University of Denmark, Lyngby

3 Advanced Materials Research Vols ad ii: Phase analysis. The phase constitution was investigated with XRD phase analysis combined with LOM. The volume fractions of the (carbo)nitride phases in the compound zone were determined as a function of depth by performance of XRD after successive layer removals (by careful mechanical polishing). Corrected intensities [14] of the main diffraction lines of the analysed phases ( 101 -ε 3, 200 -γ and 110 -Fe) formed the basis for calculation of the nitride contents (ε and γ ). For the applied CoK radiation a penetration depth over 20 μm [15] was obtained within the examined 2 range (2 = o ). Hence, the determined phase contents are the diffracted intensity weighted averages for the actual layer thickness (remaining after polishing). A Neophot 32 microscope was used for light optical microscopy analysis (LOM) of cross sections of the nitrided samples. Before embedding in Bakelite, the surfaces to be investigated were protected with an electrodeposited nickel layer to prevent rounding off at the edges during polishing. The cross sections were etched in 1 % Nital modified by 0.1 vol. % hydrochloric acid to reveal phase boundaries between the constituent phases in the compound layer and the substrate. Additionally the samples were etched in 1% Nital for revealing the microstructure in the substrate. ad iii: Total interstitial atom content in the ε phase The relations for lattice parameter dependence on total interstitial atom content [16] were used for the determination of the total interstitial atom content in the ε phase from the a and c lattice parameters. The lattice parameters a and c were determined from the peak positions of the diffraction lines for lattice planes of types {10l} (cf. Ref.9,17). ad iv: Profiles of hardness Determination of the profiles hardness was determined by Vickers method (HV0.5). Each value of hardness is average from 5 measurements. ad v: Effective thickness Effective thickness were determined on basis of the hardness profiles. It is distance from the compound zone/diffusion zone interface to the place of hardness: 600, 500 or 400 HV respectively. Results and discussion As it was presented in paper [18], for the 4340 steel the 2 (Fe 2 (N,C) 1-x ) phase is accompanied by the (Fe 4 N 1-x ) phase in the substrate-adjacent part of the compound zone. The volume content of the 2 phase in the lower part of the compound zone increases on prolonged nitriding (Fig. 1), which can be explained from a redistribution of carbon initially present in carbides. As can be seen in Fig 1 a maximum content of phase occurs at some distance from the layer/substrate interface. The region in the compound zone where the γ phase dominates, separates two subzones consisting of the ε phase, 1 (Fe 2 N 1-x ) and 2 (Fe 2 (N,C) 1-x )). Since the flux of nitrogen through the γ phase is limited (due to the small homogeneity range of phase), the 2 phase (adjacent to the substrate) grows mainly by the incorporation of carbon atoms from the substrate and the redistribution of nitrogen atoms present before the development of a, more or less, continuous zone. As a 3 {hkl} indices for phase are given with respect to the hcp sublattice of Fe atoms.

4 1028 Advances in Materials and Processing Technologies consequence, a relatively flat N+C distribution and shallow gradients for nitrogen and carbon occur in the 2 zone (Fig.1). The formation of ' phase zone in the inner part of the compound zone may upset the balance of concentration between diffusion zone and compound zone and may as a consequence influence upon the kinetics of the diffusion zone growth. Aimed at solving this problem there was a research carried out to evaluate influence of (carbo)nitrides zone (compound zone), with intentionally created diametrically different phase composition, on hardness profiles in the diffusion zone. Applying of presented procedure (Tab. 2) of nitrogen potential variation caused quick phase change in the compound zone. Two-phase layer ' developed after 8 h (Fig.2a), after just 0,5 h of the second stage has transferred into single-phase layer consisting of nitride ' (Fig. 2b). As a result in the single stage process (K N =10, t=16 h) development of the diffusion zone occurred in the presence of (ccarbo)nitrides zone ' (Fig. 3), while in the two-stage process (1 st -stage: K N =10, t=8 h, 2 nd -stage: K N =0,45, t=8 h) during almost the whole 2 nd -stage (Fig. 2 b,c,d) growth of the diffusion zone was accompanied by superficial zone made of ' nitride. Hardness profile has been determined in nitrided cases developed in these processes, what was the base for further determination of the effective thickness. Obtained results (Fig. 4) present formation of different growth kinetics of the diffusion zone in both process options. In the two-stage process, in which during the 2 nd -stage growth of the diffusion zone was accompanied by single-phase (carbo)nitrides zone made of ' nitride, hardness on the zone s cross section were lower than in single stage process, in which two-phase zone of (carbo)nitrides ' has been developed. These differences, especially evident in the effective thickness of nitrided case, appertain all surveyed grades of steel. The above results from higher homogeneity of the phase (small difference of nitrogen concentration in this case, in practice not depending on temperature) comparing to phase. This causes generated zone of ' phase to become an effective obstacle on the way of nitrogen flow to diffusion zone. As a result nitrogen concentration in the diffusion zone on the border with (carbo)nitrides zone will depend on phase structure of (carbo)nitrides zone. As a consequence of that there is lower kinetics of the diffusion zone growth, characterised by its the effective thickness in the presence of development of compact subzone of ' phase in (carbo)nitrides zone. Summary Up until now, as it was in the case of iron, it has been assumed that for the steel between diffusion and compound zone, a quasi-equilibrium of nitrogen concentration is being established. The above assumption narrows the scope of the effective control over the kinetics of diffusion zone growth merely to accelerating the phenomena on the gas-metal border, i.e. to shortening the time of maximum nitrogen achievement in diffusion zone on the border with compound zone. As a result of the research presented in the article, this widely accepted assumption has been questioned. On the basis of these research, it was shown that, other to iron, structure and phase composition of the compound zone on steel as well as structural changes taking place in this layer during the process cause the quasi-equilibrium of concentration of nitrogen between diffusion and compound zone to be non-existent. That evolution of surface phase structure of compound zone contribute significantly, regardless to nitrogen potential and temperature, to the formation of the diffusion zone and in particular it s the effective thickness. The above process is making the development of control system software with regard to optimal kinetics of layer growth more and more challenging.

5 Advanced Materials Research Vols References 1. Lehrer, E.: Z. Elektochem. 26 (1930) , 2. Somers M. A. J. and E. J. Mittemeijer: Metall. Trans. A, 1995, 26 A, 57-74, 3. Rozendaal, H. C. F.; Colijn, P. F.; Mittemeijer, E. J.: Surface Eng. 1 (1985) 30-43, 4. Langenhan, B.; Spies, H. J.: Härt-Tech. Mitt. 47 (1992) , 5. Zyśk, J.; Tacikowski, J.; Kasprzycka, E.: Härt. Tech. Mitt. 34 (1979) , 6. Prenosil, B.: Härt-Tech. Mitt. 20 (1965) 41-49, 7. Schwerdtfeger, K.: Trans. AIME 245 (1969) , 8. Mittemeijer, E. J.; Straver, W. T. M.; van der Schaaf, P. J.; van der Hoeven, J. A.: Scripta Metall. 14 (1980), , 9. Somers, M. A. J.; Colijn, P. F.; Sloof, W. G.; Mittemeijer, E. J.: Z. Metallkde. 81 (1990) Du, H.; Somers, M. A. J.; Agren, J.: Metall. Trans. A 31 (2000) Mittemeijer, E. J.; Rozendaal, H. C. F.; Colijn, P. F.; van der Schaaf, P. J.; and Furnée, R. Th.: Proc. of the Conf. on Heat Treatment, 1981, The Metal Society, London, (1983) , 12. Lightfoot, B. J.; Jack, D. H.: Proc. of the conf. on Heat Treatment 73, London, (1973) 59-65, 13. Ratajski, J.; Olik, R.; Tacikowski, J.: Proc. 4 th ASM Heat Treatment and Surface Engineering Conference in Europe, Eds. D. Firrao and E.J. Mittemeijer (1998) , 14. Martowickaja, N. N.: MiTOM 2, (1983) Delhez, R.; Keijser, Th. H.; Mittemeijer, E. J.: Surface Eng., 3 (1987) Firrao, D.; De Benedetti, B. Rosso, M.: Metall. Ital., 71 (1979) Somers, M. A. J.; Kooi, B.J.; Małdziński, l.; Mittemeijer, E. J.; van der Horst, A. A.; van der Kraan, A. M.; van der Pers, N. M.: Acta Mater. 45 (1997) Ratajski, J,; Tacikowski, J,; Somers, M. M. A. J.: Surface Engineering., 3 (2003)

6 1030 Advances in Materials and Processing Technologies Fig. 1: The final phase composition of (carbo)nitrides zone after 3 and 10 h of single stage process of steel 4340 at K N =3.25 and T=853 K (580 0 C): a) depth distributions of and ' phases in the compound zone, b) total interstitial content (N+C) as a function of depth, c) optical micrographs of the cross-sections of the compound zone ' appears dark grey, 2 - appears light grey.

7 Advanced Materials Research Vols Fig. 2 Change of phase composition of (carbo)nitrides zone and its thickness as a result of nitrogen potential change in the 2-stage process processes B,C, D and E (table 2) presented by mean of X- ray diffraction pattern (a) and by bar graphs (b).

8 1032 Advances in Materials and Processing Technologies Fig. 3. The final phase composition of (carbo)nitrides layer after 16 h of single-stage process process A (Table 2) presented by mean of X-ray diffraction pattern (a) and quantitative volume fraction of phases ( and ' ) as well as the layer thickness bar graph (b).

9 Advanced Materials Research Vols Fig. 4. The final hardness profiles in the diffusion zone after the 16 h of single-stage process - process A (blue) and after 16 h of two-stage process process E (black) (Table 2).

10 1034 Advances in Materials and Processing Technologies Table 1. Chemical composition of the materials investigated (in wt-%) and heat treatment prior to nitriding. Grade of steel Heat treatment prior to nitriding Composition wt.-% C Si Cr Mn Mo Ni Ti 18HGT (PN) annealing 1 h / 1223 K 26H2MF (PN) normalizing 1163 K C22 (DIN) air cooled (AISI) hardening 1118 K, 40NiCrMo6* (DIN) oil quenched, tempering 893 K. *Cr 1.15 wt.-%, Ni 1.55 wt.-% , , Tab. 2. Parameters of processes. Process Mark Stage I Temp. [K] Time [h] Nitrogen potential [atm. -1/2 ] Stage II Temp. [K] Time [h] Nitrogen potential [atm. -1/2 ] A B 8 10 C D E

11 Advances in Materials and Processing Technologies / Development of Nitrided Layer during Nitriding of Steel / DOI References [2] Somers M. A. J. and E. J. Mittemeijer: Metall. Trans. A, 1995, 26 A, 57-74, /BF [8] Mittemeijer, E. J.; Straver, W. T. M.; van der Schaaf, P. J.; van der Hoeven, J. A.: Scripta Metall. 14 (1980), , / (80) [10] Du, H.; Somers, M. A. J.; Agren, J.: Metall. Trans. A 31 (2000) /s [11] Mittemeijer, E. J.; Rozendaal, H. C. F.; Colijn, P. F.; van der Schaaf, P. J.; and Furnée, R. h.: Proc. of the Conf. on Heat Treatment, 1981, The Metal Society, London, (1983) , /BF [17] Somers, M. A. J.; Kooi, B.J.; Małdziński, l.; Mittemeijer, E. J.; van der Horst, A. A.; van der raan, A. M.; van der Pers, N. M.: Acta Mater. 45 (1997) /S (96) [18] Ratajski, J,; Tacikowski, J,; Somers, M. M. A. J.: Surface Engineering., 3 (2003) ig. 1: The final phase composition of (carbo)nitrides zone after 3 and 10 h of single stage process /