**6. Conclusion**

334 Corrosion Resistance

The highest hardness for 8N specimen is considered to be responsible for best wear resistance property among the treated specimens used in this investigation. However the findings suggest that nitriding, hybrid, nitrocarburizing and carburizing the austenitic stainless steel at 4500C using a fluidized bed furnace can improve surface hardness and wear resistance of austenitic stainless steel. It is to be noted that at the initial stage of sliding, all the specimens in Fig. 11 gave accelerated weight loss and then leveling off after certain period. It is presumed that at the initial stage of sliding, the 600g load of the sliding mate material was encountered by the asperities of substrate surface, which effectively caused high load sliding and thus more wear loss. The eventual dropping off may be related to smoothening of the asperities at the wear surface, which produced more contact area for the

The work hardening effect may also cause this tendency together with possibilities of surface oxide or carbide/nitride formation at a certain period of sliding, thus leading to an equilibrium condition of constant wear rate. However, no evidence is available to explain

Corrosion tests using the electrochemical technique demonstrated that the precipitation free carburized and nitrided layers have very good corrosion resistance in the corrosive

The most subtantial improvement in properties of austenitic stainless steels by the hybrid process lies in corrosion resistance as evaluated by electrochemical testing (Li & Bell, 2004). Fig. 12 shows the anodic polarization curves measured for several specimens in 3.0% NaCl solution. As expected, both individual nitriding and carburizing reduce the current density of the steel in the anodic region, indicating improved corrosion resistance. After the hybrid treatment, the anodic polarization curve is shifted towards lower current density by several orders of magnitude as compared to that for the untreated and individually nitrided and carburised steel. This registers an improvement in corrosion resistance by several orders of magnitude and signifies the excellent corrosion resistance of the hybrid treated surface. The much enhanced corrosion resistance observed for the hybrid treated surfaces may be attributed to the extremely large supersaturation of the upper part of the nitrogen-enriched layer with both nitrogen and carbon (see Fig. 9). This would contribute to the observed higher hardness and better corrosion resistance as compared to those achieved by

The treatment conditions are the same as those in Fig. 7. The electrochemical test results for Hybrid-NCT, Nitriding-NT, Carburizing-CT were described in Fig. 12. The NT and CT showing that the current density of treated stainless steel were decreased in the anodic region which indicating positive effect regarding the improvement of corrosion resistance compared to the substrate. After Hybrid-NCT treatment, the anodic polarization curved is shifted towards lower current density which explain that the corrosion rate was decreased and the polarization current measurement gave 0.00003 mA/cm2 and demonstrate an improvement in corrosion resistance as compared to that untreated and individually nitrided and carburized steel, while passivation current of NCT is the lowest followed by CT, NT and untreated respectively. This trend also similar to the maximum potential passivation behaviour since the

sliding load of 600g and hence reduced or constant wear rate.

the exact reasons of these wear phenomena.

individual nitriding and carburizing.

**5.4 Corrosion properties** 

environments.

The thermochemical treatments of AISI 316L stainless steel in a fluidized bed process at 450 C demonstrate that it is possible to produce hard layer of an expanded austenite phase without precipitation of chromium carbide/nitride. For nitriding and carburizing treatments the expanded layers consisted of a single layer γN or γC phase while specimens treated by nitrocarburizing or hybrid process gave dual layers consisting of γN at the surface and γC ahead of γN. The layer produced in fluidized bed process is not uniform in thickness under the same treatment conditions. The nitriding treatment produced 8.35 µm

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## **7. References**


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**15** 

*1Egypt 2Germany* 

**Corrosion Performance and Tribological** 

A.M. Abd El-Rahman1,2, F.M. El-Hossary1, F. Prokert2,

N.Z. Negm1, M.T. Pham2 and E. Richter2

*2Institut für Ionenstrahlphysik und Materialforschung,* 

*1Physics Department, Faculty of Science, South Valley University, Sohag Branch, Sohag,* 

*Helmholtz-Zentrum Dresden-Rossendorf,* 

**Properties of Carbonitrided 304 Stainless Steel** 

In general, the solid solution austenitic phase (γ) with high chromium content (12 % - 20 %) is responsible about the excellent corrosion performance of austenitic alloys. This advantage allows these alloys to use in biomedical, food and chemical, pulp and paper chemical, petrochemical, heat exchange and nuclear power plant industries [1-4]. However, most of these applications are suffering from their relatively low hardness and poor tribological

Various surface modification technologies such as nitriding, carburizing and nitrocarburizing are used to improve the mechanical and tribological properties of austenitic stainless steels [5-12]. In most cases an increase in surface hardness is accompanied by a decrease in corrosion resistance [13]. The decrease in the corrosion resistance is caused by heavy precipitations of chromium carbide and chromium nitride on the grain boundaries, which are surrounded by chromium-depleted zones [14]. More investigations are succeeded to maintain and sometimes to improve the corrosion resistance of stainless steels after nitriding [15-16]. It is well known that the formation of nitrogen supersaturated solid solution phase without CrN precipitations should maintain the good corrosion resistance of

In this paper we present the effect of N2 to C2H2 gas pressure ratio on the corrosion performance and tribological properties of AISI 304 austenitic stainless steel after rf plasma

The samples were treated at a fixed input plasma power of 450 W and for a processing time of 10 min. The gas pressure related to N2/C2H2 ratio was varied from 100% N2 to 100% C2H2. The pressure was increased from an atypical base pressure of 1.3x10-2 mbar to a total gas pressure

**1. Introduction** 

properties.

stainless steel [5, 17].

**2. Experimental work** 

carbonitriding at a relatively low pressure.

