Researches referring to the plasma nitriding of some alloyed construction steels.

Deac, Cristian ; Bibu, Marius ; Nemes, Toderita 等

Abstract: Due to the many technical-economical advantages it offers, compared with classical heat treatment processes, plasma nitriding considerably enlarged its range of industrial applications in the past few years. The purpose of using plasma nitriding is to provide the most advantageous conditions of machinability and reliability, by modifying the chemical composition, the structure and internal induced stresses. Nitrogen diffusion in the base material crystal lattice determines the formation, in the part's superficial layer, of compounds that determine an increase of wear and corrosion resistance, and the improvement of tribological properties. The paper presents some researches regarding the process kinetics and the hardness of layers obtained after the plasma nitriding treatment of alloy steels: 39MoAlCr15, 42MoCr11, 18MnCr11 and 40Cr10.

Key words: heat tratment, alloy steel, process kinetics

1. INTRODUCTION

Plasma nitriding as surface thermochemical treatment is applied to the metallic products in order to bring them into certain structural states, to a certain chemical composition and to certain levels of internal induced stresses. These states correspond to properties associations that are prescribed to them according to processing conditions until their finite form and their putting into service. The application purpose is that of providing to the parts the most advantageous conditions of functioning and reliability. Typically, plasma nitriding is applied for achieving a high surface hardness (Eberhart, 1990).

An important aspect concerning the use of plasma nitriding is the sensible employment of metallic materials in products (parts, tools, blanks), this being one of the most important issues regarding the experimental research and technology in most of the countries. The efficient use of metals and their alloys represents a major objective for the productive area as well for the customers, which implies taking advantage of all technological characteristics and exploiting them.

Due to the many technical-economical advantages it offers, compared with classic processes of thermal treatments, plasma nitriding considerably enlarged its range of industrial applications in what concerns the metallic materials (steel, pig iron, non-ferrous alloy, agglomerated cake) the type, form and destination of parts (Cartis, 1988). Plasma nitriding is especially used for complex parts subjected to intense wear, fatigue, contact pressure, shocks, and corrosion in humid environments (Vermesan & Deac, 1992). This method can be applied to small or medium-sized parts with simple or complex geometry as well as very heavy parts (shafts, gear wheels etc.), with dimensions reaching 11 meters in length and 3--3.5 meters in diameter (Vermesan & Deac, 1992). The use and spreading of the plasma nitriding technology requires the prior knowledge of possible results (structure, hardness, layer width etc.) regarding the steels' behaviour after applying this surface thermochemical treatment (Agius, 1993, Baker, 1990).

In this context, the authors present in this paper the studies and experimental researches regarding the forming and hardness kinetics of layers obtained after a plasma nitriding thermochemical treatment on wide range alloy steels, used in mechanical engineering such as: 39MoAlCr15, 42MoCr11, 18MnCr11 and 40Cr10.

2. EXPERIMENTAL RESULTS

The process took place for all the cases in a plasma nitriding installation INI--30, and the suspension of samples was realised with a device that allows a symmetrical placement inside the installation of the test samples as well of the gauge parts that require temperature measurements. The gas used was pure ammonia.

The samples have been realised as bars with different dimensions according to the material type, the test samples taking the form of disks.

After the preliminary thermal treatments in all the cases the parts have been set right plan-parallel (Ra = 0,025 mm), and then degreased with westrosol. As well they have been cleaned through a cathode spraying (at the beginning of each process corresponding to the tested materials) at 100 V voltage, the pressure of 40 Pa for approximately 15 minutes.

After the nitriding have been prepared the metalo-graphic samples for macroscopical investigations, microscopical analyses and proofs of micro-hardness for determining the depth of the nitrided layer. Before the metalographic attack the samples have been modified on plane surfaces on a depth of 0,5 mm for removing the nitrided layer, and using a charge of 0,2 daN was measured the HV macrohardness. A. In the first experimental case the material used for the test samples was a 39MoAlCr15 steel bar ([empty set] 60 x 200mm) having the chemical composition: 0.42 %C; 0.51 %Mn; 0.28 %Si; 1.69 %Cr; 0.28 %Mo; over 0.5 %Al. The samples had the shape of disks with the dimensions [empty set] 60 x 20 mm, that were afterwards hardened (quenching followed by a high tempering at 550 [degrees]C). For the tests, temperatures of 480 [degrees]C, 510 [degrees]C, 540 [degrees]C have been chosen and for each temperature there were treated 5 samples, the processing time being of: 1; 2; 4; 6; 8 and 16 hours, respectively. The working voltage was of 700 V, the working pressure 240--320 Pa. The mentioned parameters were continually monitored and registered on the installation's control panel.

B. In the next experiment samples made of 18MnCr11 steel (0,21 %C; 0,44 %Mn; 1,04 %Cr and 0,32 %Si) have been used. The test samples have been cut from a bar ([empty set] 70 x 250 mm), also in the shape of disks with dimensions of [empty set] 70 x 25 mm.

After cutting, the material has been subjected to a heat treatment (quenching followed by a high tempering at 550 [degrees]C). The resulted microstructure presented partially spheroidized carbides and a certain quantity of free ferrite.

The tests have been made at temperatures of 500 [degrees]C and 550[degrees]C and at pressures correlated accordingly with these temperatures (values between 125--750 Pa). For each temperature-pressure pair of values the processing time was varied to: 1; 2; 3; 4; 6 and 8 hours, in each case being treated 5 samples.

Figures 1 and 2 present the microstructures (x 400) in the section of plasma nitrided steel samples 18MnCr11 (8h / 500 [degrees]C and 8h / 550 [degrees]C, respectively).

C. For the third experiment the test sample material was a bar with dimensions of [empty set] 65 x 150 mm made of 42MoCr11 steel (0,39 %C, 0,59 %Mn, 0,28 %Si, 1,04 %Cr and 0,15 %Mo). Afterwards, the material (disks of [empty set] 65 x 15 mm) has been subjected to a heat treatment consisting of quenching followed by high tempering at 550 [degrees]C.

The tests have been made at three different temperatures -450 [degrees]C, 500 [degrees]C and 550 [degrees]C. At each temperature, treatment periods of: 1; 2; 4; 8 and 16 hours, respectively have been used, in each case being again tested 5 samples.

D. The metallic samples subjected to the last experiment have been made of a 40Cr10 steel having the following chemical composition: 0,41 %C; 0,64 %Mn; 1,05 %Cr; 0,28 %Si.

The test samples, disks with dimensions of [empty set] 60 x 10 mm (made of a bar of [empty set] 60 x 10 mm) have been divided into two different lots. In the first lot were test samples in an annealed state. In the second lot there were test samples in a hardened state (quenching followed by high tempering at 550 [degrees]C). In the annealed state, the structure was formed of polyhedral grains of ferrite and pearlite in approximately equal quantities, and in the case of the hardened state the structure consisted of globular sorbite.

3. CONCLUSIONS

* After analysing the microstructure of the layer of combination resulted after the plasma nitriding of 39MoAlCr15 steel, it was concluded the continuous combination area with limit of separation towards the basis material appears in none of the treatment cycles applied.

Still, the appearance of acicular nitrides and an accentuation of grain limits was noticed. The quantity of globular carbides was reduced in the neighbourhood of the surface.

* After analysing the microstructure of the plasma nitrided layer of the 18MnCr11 steel, it could be noticed the existence of a combination layer delimited by the diffusion layer. The depth of this layer varies between 2 [micro]m (1h / 500 [degrees]C) and 9 [micro]m (8h / 550 [degrees]C), respectively.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

* In the case of the 42MoCr11 steel, analysing the combination layer microstructure it was noticed the emersion of the continuous combination layer with limit of separation towards the basis metal. The depth of this layer varies between 1 [micro]m (1h / 450 [degrees]C) and 12 [micro]m (16h / 550 [degrees]C); the metal structure remained unchanged, with globular carbides.

* With regard to the 40Cr10 steel, the metallographic investigation revealed the existence of two different layers:

* the white layer with a depth in the order of microns and made of [gamma] mono-phase nitride of type Fe4N (that has very good properties of wear and viscosity).

* the diffusion layer, right after the white layer, with dimensions between 0,1-0,4 mm.

Unlike the mono-phase nitride [epsilon] (Fe2--3N or Fe2--3CxNy) obtained at bath nitriding or carbonitriding and which is glance pitched and porous, the [gamma] phase offers very good properties of wear and viscosity. In certain situations of complex stress, when the presence of a white layer is not wanted, this one can be totally suppressed, by acting on the nitrogen content.

* The macroscopic and microscopic analyses made obvious for all the plasma nitriding regimes and for all types of materials the absence of superficial faults.

The hardness of the diffusion layer depends a lot on the nature and concentration in alloying constituents, as well on nature and quantity of nitrides precipitated on the granules. Such a powerful precipitation of nitrides leads to a more fragile diffusion layer, a weaker viscosity and implicitly a lower resistance at fatigue.

In the case of ionic nitriding the fragility can be eliminated by using a free carbon working environment, which is the main element in the precipitating of nitrides or carbonitrides on the grains.

4. REFERENCES

Agius B. et al. (1990) Surfaces, interfaces and films (in French), Dunod Publishing House, Paris

Baker M.A. (1993) Surfaces and Interfaces Analysis, London.

Cartis, I.G (1988), Thermochemical treatments (in Romanian), Facla Publishing House, Timisoara

Eberhart J.P. (1990) Structural and chemical analysis of materials structurale et chimique des materiaux (in French), Dunod Publishing House, Paris

Vermesan, G. & Deac V. (1992) The technological bases of ionic nitriding (in Romanian), University of Sibiu Publishing House.