Residual stresses in the surface layer of M2 steel after conventional and low pressure (‘NITROVAC 79’) nitriding processes

Surface and Coatings Technology 124 (2000) 19–24 www.elsevier.nl/locate/surfcoat

Residual stresses in the surface layer of M2 steel after conventional and low pressure (‘NITROVAC 79’) nitriding processes Zbigniew Gawron´ski Institute of Materials Engineering and Chipless Technologies of the Technical University of Lło´dz´, 90-924 Lło´dz´ ul. Stefanowskiego 1Poland Received 24 March 1999; accepted 16 October 1999

Abstract The distribution of residual stresses in the surface layer of M2 steel after conventional and low pressure (‘NITROVAC 79’) nitriding is reported and discussed in this paper. The stresses depend on the structure of the nitrided layer, which result from the applied nitriding parameters. The optimum structure has been obtained after low pressure nitriding at an ammonia partial pressure p=2×103 Pa. Cyclic annealing after nitriding at temperatures below the nitriding temperature did not cause any significant changes in residual stress distributions. Annealing temperatures higher than that used in nitriding caused a marked increase of residual stresses in the surface layer of the steel. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Cyclic annealing; Nitriding; Residual stress

1. Introduction Most surface treatments are designed to produce a specified state of residual stress which can have a very significant influence on the mechanical and tribological properties of tools and machine parts. For that reason, it is an important task to define the values and distribution of residual stresses both from the scientific and application points of view. Technologies which essentially change the state of surface stress are thermochemical ones, especially nitriding. Papers which have been published hitherto (e.g. Refs. [1–5]) concentrated mainly on investigations of residual stresses obtained in conventional gas nitriding processes. Similar studies of residual stresses in the c∞ layer after ion nitriding are given in Ref. [6 ]. A wide range of studies on problems connected with low pressure nitriding (mostly the NITROVAC 79 process) [7] have been conducted in the Institute of Material Engineering and Chipless Technologies of the Technical University of Lło´dz´. NITROVAC gives practically unlimited possibilities to repeatably form different phase compositions and structures of nitrided layers on a wide range of alloying steels. The essence of the process lies in the ability to continuously control (within the full range of concentrations) the nitrogen quantity supplied to the surface of nitrided parts. This is done by con-

trolling the absolute pressure of the partially dissociated ammonia gas in the retort of the pit furnace. The hardened surface layers manufactured by the NITROVAC technology are characterised by properties similar to those for bath nitriding or ion nitriding, whilst eliminating inconveniences connected with these processes [8]. The structure of nitrided layers produced by the NITROVAC method is extremely beneficial because of the absence of brittle nitride precipitates in the surface layer. Additionally, hydrogen interaction with the steel substrate is strongly decreased during the vacuum nitriding process. That is why NITROVAC can be the final process applied and no grinding processes are needed to remove brittle zones. All these features enable the NITROVAC technology to be applied for the hardening of highly loaded parts such as gears, cams, camshafts, crankshafts, clutches, parts of pumps and rolls, as well as machining tool, dies and punches for forming and forging. The advantages of the NITROVAC technology are given below: $ high hardness of treated surfaces (up to 1400 HV ), $ good ductility of hard layers, $ high fatigue strength of treated parts, $ high resistance against contact stresses, $ dimensional stability of treated parts, $ low cost of the process, and

0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 62 0 - 9

Z. Gawron´ski / Surface and Coatings Technology 124 (2000) 19–24

20

Fig. 1. Difractogram from the surfaces of nitrided specimens.

technology which is environmentally friendly. The investigations which refer to the comparison of residual stress distributions in surface layers of M2 high speed (HS) steel after conventional and low pressure nitriding are presented in this paper. Taking into account that the nitriding processes are applied to cutting tools working at periodically changing temperatures, a trial to estimate the influence of temperature on the residual stresses has been undertaken. $

2. Experimental 2.1. Materials investigated Specimens in the form of plates (120×20×2.5 mm) were prepared from M2 steel. The specimens were exposed to the following heat treatment: austenizing temperature=1483 K; cooling media=oil; tempering temperature=843 K; and tempering time=1 h. The hardness after bulk heat treatment was assessed by the Rockwell method in the C-scale. The average result for all specimens was 63±0.5 HRC. The low pressure nitriding was carried out at ammo-

nia partial pressures: p =2×103 Pa and p = 1 2 18×103 Pa, for two series of samples; a third series underwent conventional nitriding. The parameters of the nitriding processes were: temperature=833 K; time=6 h; dissociation degree ~30% (in all the above mentioned cases). Some low pressure nitrided samples (process pressure p=2×103 Pa) were additionally subjected to cyclic vacuum annealing (5×30 min with periodical cooling to 373 K ) at temperatures 673, 773 and 873 K. The selection of the parameters of the low pressure nitriding was made as a result of experiments which had been done previously by the author [12]. They were used to obtain different types of the microstructures of the surface layers. The structure, thickness of diffusion zones and residual stresses were investigated in every group of specimens.

2.2. Structural investigations Phase composition of the diffusive layers obtained was investigated on a D500 Siemens diffractometer using ˚ ). The results are given CoKa radiation (l=1.79021 A

Table 1 The thickness of nitride layers and the inner nitriding zone Type of nitriding

NITROVAC p=2×103 Pa NITROVAC p=18×103 Pa Conventional

Nitriding time (h)

6 6 6

Nitriding temperature ( K )

843 843 843

Type and thickness of the layer (mm) Inner nitriding zone c∞

e+c∞

~100 ~125 ~100

~8 ~17

2–3 -

Z. Gawron´ski / Surface and Coatings Technology 124 (2000) 19–24

in Fig. 1. The metallographic structure was also examined. The results are given in Table 1 and in Figs. 2 and 3. 2.3. Microhardness test Microhardness tests were done using the Vickers test method with an applied load of 1 N on a SOPOLEM apparatus (made in France). The results (average from five measurements) are given in Fig. 4. 2.4. Residual stresses examination Three specimens from each series underwent the Waisman–Phillips test [9]. In accordance with the author

21

of Ref. [10], this method gives the distribution of residual stresses in surface layer much better than the commonly used X-ray analysis (sin2Y ) method. The Waisman–Phillips method is based on measuring the deflection of flat specimens which deform during releasing the stresses in consequence of electroeaching of consecutive material layers. Young’s module of the phase present in nitrided layers (e+c∞) as well as that of the substrate need to be known to calculate the residual stresses. These values are given in Ref. [11]: Ee=181.5 GPa, E =211.0 GPa, and Ee=206.0 GPa c∞ respectively. The results of the investigations are given in Figs. 5 and 6.

Fig. 2. Microstructure of heat treated and nitrided M2 steel: (a) and (b) low pressure; (c) conventional.

Fig. 3. The appearance (by scanning microscope) of the surface of specimens made of M2 steel: (a) and (b) after low pressure nitriding; (c) conventional nitriding.

22

Z. Gawron´ski / Surface and Coatings Technology 124 (2000) 19–24

Fig. 4. Distribution of average microhardness in the surface layer of low pressure and conventional nitrided M2 HS steel.

Fig. 5. Distribution of residual stresses in the surface layer of low pressure and conventional nitrided M2 HS steel.

Z. Gawron´ski / Surface and Coatings Technology 124 (2000) 19–24

23

Fig. 6. Distribution of residual stresses in the surface layer of low pressure nitrided M2 HS steel at p=2×103 Pa, cyclically annealed at temperatures of 673, 773 and 873 K.

3. Discussion The results of microstructural investigations show that the low pressure nitriding NITROVAC, as with ion nitriding, allows the formation of different structures, optionally. Surface layers without porous and brittle nitrides (which are in most cases detrimental ) can be obtained with special parameter settings for low pressure nitriding. Changing the ratio of nitrogen atoms absorbed at the surface to the quantity of their adsorption (diffusion into the material ) enables the structure of the surface layer to be controlled. This can be achieved by changing the partial pressure of nitrogen. In conventional nitriding, the adsorption of atomic nitrogen is too large compared to the absorptional ability of the material, which causes the accumulation of overbound atomic nitrogen on the surface and, in consequence, the creation of thick nitride layers (e+c∞) [7,8,12]. The distribution of average microhardness in nitrided layers, for particular treatment variants, seems to be very interesting. An increase in ammonia partial pressure gives a small increase in maximum hardness (from 1130 to 1180 HV ). The interesting effect is, however, that the maximum of the hardness profile appears deeper in the nitrided layer ( Fig. 4). Analysing Fig. 5 (distribution of residual stresses) it can be noted that the nitriding pressures have a significant influence on the stress character and values. The maximum compressive stress (−842 MPa) occurred at p=2×103 Pa near the surface

of the specimen. The course of residual stress distribution curve changes at about 20 mm at a pressure of 18×103 Pa. The values of compressive stress are much lower in the first case and is about −300 MPa close to the surface. This value increases and reaches the maximum (−680 MPa) at a distance of 18 mm. The distribution curve of residual stresses has a similar character to that for conventional nitriding, but the stresses have a significantly lower value: −122 MPa. The maximum value of compressive stresses (−603 MPa) is obtained at a depth of 18 mm. The conclusion is that the structural formation of nitrided layers determines the values and distribution of residual stresses. At a partial ammonia pressure p=2×103 Pa, the rigid layer of c∞ nitrides (2–3 mm thin) and below that an inner nitriding zone reaching 100 mm ( Fig. 2a), have been obtained by low pressure nitriding. In the case of ammonia partial pressure p=18×103 Pa the ‘white layer’ of nitrides e+c∞ is obtained (about 8 mm thick) and below that we have the inner nitriding zone to about 125 mm ( Fig. 2b). The e nitride in the surface layer significantly lowers the maximum value of compressive stresses (in comparison to the first treatment variant). This phenomenon can be better observed in the case of conventional nitriding when the ‘white layer’ of e+c∞ nitrides (about 17 mm thin) was obtained ( Fig. 2c). Scanning microscope investigations have shown that this layer is much more porous in the case of the second treatment variant ( Fig. 3). The conclusion is that the nitrides (e+c∞) nitride layer, obtained at specified treatment parameters, sig-

24

Z. Gawron´ski / Surface and Coatings Technology 124 (2000) 19–24

nificantly lowers the value of compressive stresses on the specimen surface. It was noted that with an increase of ammonia partial pressure, the porosity of this layer increases. This has a significant influence on stress levels on the specimen surface. The low pressure nitriding NITROVAC of M2 steel, at partial ammonium pressure p=2×103 Pa assures an optimal microstructure and residual stress distribution from the point of view of the friction couples applications. Cyclic annealing at temperatures of 673 and 773 K does not cause significant changes in residual stress distribution of nitrided specimens ( Fig. 6). Neither does it change the structure and chemical composition of the specimens. However, an important change of residual stress occurs after cyclic annealing at 873 K (a temperature 40 K higher than used for nitriding): the compressive stresses in the surface decrease from −850MPa to −250MPa.

4. Conclusions

1. The NITROVAC low pressure nitriding process allows the formation of nitrided layers on M2 steel in a controlled manner. 2. The residual stresses distribution in the nitrided layer depends on its structural formation, and thus on process parameters.

3. The optimal structure is obtained at an ammonia partial pressure p=2×103 Pa and consists of a very thin (2–3 mm) sublayer of rigid c∞ nitride and an inner nitriding layer. 4. Cyclic annealing, after the low pressure nitriding, does not cause any significant changes in the residual stress distribution unless it takes place at temperatures below the nitriding temperature. Annealing at temperatures higher than the nitriding temperature leads to a significant decrease of residual stresses in the surface layers.

References [1] H.C.F. Rozendaal, P.F. Colijn, E.J. Mittemeijer, Surf. Eng. 1 (1985) 30. [2] H. Oettel, B. Ehrentraut, HTM 40 (1985) 183. [3] U. Kreft, F. Hoffmann, T. Hirsch, P. Mayr, in: V. Hank ( Ed.), Residual Stress, Deustche Gesellschaft fu¨r Metallkunde, Oberu¨sel, 1993, p. 115. [4] M.A.J. Somers, E.J. Mittemeijer, Met. Trans. 21A (1990) 189. [5] M.A.J. Somers, E.J. Mittemeijer, Met. Trans. 21A (1990) 901. [6 ] J. Szawłowski, M. Psoda, Inz= . Mat. XVIII (1997) 100 (in Polish, abstract in English). [7] Z. Has´, Europa¨ische Patentenschrift Nr. 0034761. [8] Z. Has´, P. Kula, Proc. II Int. Conf. on Carburizing and Nitriding with Atmospheres, Cleveland, Ohio, 6–8 December (1995) 227–231. [9] J. Waisman, A. Phillips, Proc. Soc. Exp. Stress Analysis XI (2) (1952). [10] S. Janowski, H. Oettel, MOCiP 112–114 (1991) 2. [11] W. Schro¨ter, A. Spengler, HTM 51 (6) (1996) 356. [12] Z. Gawron´ski, Inz= . Mat. 1 (1997) 27 (in Polish, abstarct in English).