Experiments regarding some modern steel treatment methods.

Bibu, Marius ; Deac, Cristian ; Petrescu, Valentin 等

Abstract: Plasma nitrosulphuring and plasma carburising as new processes in the field of thermochemical treatments determined an increase in the interest in them by laying the scientific foundations of physical-chemical phenomena of metallurgical nature that take place during their development. They are important especially for parts that are subjected to complex and intense wear, fatigue, contact pressure, shocks and even corrosion. The paper presents an analysis of microhardness and wear behaviour for some steel types used in mechanical engineering: 41MoCr11, 18MnCr11, 39MoAlCr15 and OLC15, treated by nitrosulphuring and plasma carburising. The results have been compared to those achieved for parts of the same steel types subjected to plasma nitriding.

Key words: heat treatment, alloy steel, process kinetics

1. INTRODUCTION

The increase and diversification of semi-manufactured and finite parts, as well taking a maximum advantage of the metals and alloys, imposed a continuous development of thermochemical technology treatments.

Beneath plasma nitriding, other thermochemical treatments in plasma also went through a major development: nitrocarburising, nitrocarbosulphuring and plasma carburising. All these processes are based on the diffusion phenomenon of non-metals (nitrogen, carbon, sulphur) in the base material crystal lattice. This way, in the superficial layer of the treated parts compounds are formed that determine an increase of the resistance at wear, fatigue, corrosion and even an improvement of the tribological properties (Wang, 1988; Agius, 1990).

The ecological problems that appear concerning the environmental protection determined an increase in interest for plasma treatment technologies, these processes being a serious alternative for the conventional ones (Bell & Staines, 1983; Czanderna, 1993). Besides the non-polluting character, the treatment environment, plasma, provides superior performances of the superficial layers and an increased power efficiency.

[FIGURE 1 OMITTED]

2. EXPERIMENTAL INSTALLATION

The basic structure is that of a 30 kW (INI -30) plasma nitriding installation, with a bin of [PSI] 600 x 800 mm, which has been modified and completed with a series of contiguous subassemblies to allow the study of changes of the ferrite and austenite compounds ([T.sub.max] = 1000 [degrees]C).

The installation, presented in figure 1, consists of following components: a treatment room comprising the heating system, heat screens and the charge suspension attachment, the gas feeding system that allows the entrance and simultaneous control of five types of gases or steams, the sulphurous hydrogen generator, the exhaustion system, the variable voltage regulator with potential transformer and control panel.

For the purpose of the presented research, the installation was modified as follows. The length of the charge sustaining drains has been increased from 185 mm to 400 mm, the 16 mm needle stem was replaced with a 16 x 1,5 mm stainless steel bar. The additional heating is realized with two kantal filaments of 2 mm diameter, each having the length of 7,8 m, spiral disposed on a 270 mm circumference and sustained by 16 ceramic legs. The system provides a work volume of 200 x 250 mm.

For reducing the losses by radiation, around the spiral subassembly a set of four thermal screens made of stainless steel of 0.8 mm have been installed. Each of the five gas lines is adjusted for the type of gas/vapours that flow through it. The sulphured hydrogen is produced by the hydrogen passing over the sulphur vapours at 400 [degrees]C.

3. PLASMA NITROSULPHURING

The sulphur, once inserted in the superficial layer, determines a reduction of the friction coefficient together with an increase of wear and sticking resistance (until now good experimental results have been obtained only by sulphuring in salt baths (Gregory, 1975). The working gas in which the treatment took place was a mixture of ammonia, ethylic alcohol and carbon sulphide in different proportions. The optimal composition has been established at (Baker, 1993):

([C.sub.2][H.sub.5]OH+[CS.sub.2])/N[H.sub.3] = 1/3 , for a ratio [C.sub.2][H.sub.5]OH/[CS.sub.2] = 2/1 (1)

4. EXPERIMENTAL RESULTS

In order to test the changes of the tribological properties of the superficial layer obtained through various thermochemical treatments, following treatment types and regimes were tested:

a.--plasma nitriding (550 [degrees]C / 3,5 torr / 8h);

b.--plasma nitrosulphuring (550 [degrees]C / 3,5 torr / 8h);

c.--5h plasma nitriding followed by 3h plasma nitrosulphuring (550 [degrees]C / 3,5 torr).

The results of subjecting alloy steel types like 41MoCr11 and 39MoAlCr15 to these thermochemical treatments are presented in figures 2, 3 and 4.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

5. CONCLUSIONS

After studying the sulphur and nitrogen concentrations profile it was noticed that the sulphur is concentrated only in the superficial layer of the test samples, on a depth of 8 - 20 [micro]m. Regarding nitrogen, this chemical element has higher concentrations in depths between 15 -40 [micro]m.

In the case of 39MoAlCr15 steel it was noticed that the sulphur stops the nitrogen diffusion, the hardness obtained in the depth area of 20 - 150 [micro] m being inferior to the one achieved through the plasma nitriding process.

Much better results from the superficial hardness point of view have been obtained by plasma nitrosulphuring process and especially by plasma nitriding and nitrosulphuring with the 41MoCr11 steel. Without registering a considerable decrease of the hardness characteristic in the diffusion layer, an increase of it with approximately 30% was realised in the combination layer of the metallic samples.

Concerning the carburising steel 18MnCr10, the microhardness curves obtained by nitrosulphuring were systematically under those of plasma nitriding. In all three cases, a significant increase of the combination layer depth was noticed for the sulphuretted hydrogen in the luminescent discharge.

As a general conclusion on the plasma nitrosulphuring process, can be noticed the superiority of the structural and physical-mechanical characteristics of metallic layers obtained through this process compared with the samples subdued to the other practical experiments.

6. REFERENCES

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

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

Bell T., Staines A.M. (1983) High Temperature Technology, p.209.

Czanderna A.W (1991) Ion Spectroscopies for Surface Analysis, Application of Surface Analysis Methods to Environmental/Material Interaction, The Electrochemical Society, Pennington, NY.

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

Gregory J.C. (1975) Heat Treatment of Metals, London, p.55.

Wang S.Q. (1988) The First Int.Conf. on Plasma Surface Engineering, Garmisch-Partenkirchen, Germany, p.105.