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In this study boriding and nitriding treatments were carried out on 55CrMoV4 low alloyed steel. The thermochemical treatments were carried out in solid medium by the powder technique at 900℃ for 4 hours for boriding treatment and at a temperature of 550℃ for 12 hours for nitriding treatment. The phases analysis of the boride and nitrite layers formed on the surface was carried out by optical microscopy (OM), and X-ray diffraction (XRD). The results of the surface analysis show that the boride and nitride layers a presence of FeB, Fe2B, CrN, Fe3N and Fe4N compounds. The thickness of boride layers and nitride layers was found to be 55 and 12 µm, respectively. Microhardness of boride and nitride layers are between 800 HV0.2 and 1200 HV0.2. Corrosion tests by immersion in a 1M HCl solution have shown the beneficial effect of boriding and nitriding treatments on treated steels. Increase in corrosion resistances was observed after nitriding and boriding treatment steel 55CrMoV4 was around 6 times.

In the present investigation, some electrochemical properties of dual phase X70 steels with different martensite morphologies which have undergone boriding were studied. To obtain a variety of martensite morphologies, Direct Quenching (DQ), Intermediate Quenching (IQ) and Step Quenching (SQ) heat treatments were applied at an intercritical annealing temperature (IAT) of 760℃. The treatment (DQ) allowed the formation of fine martensite evenly distributed in the ferrite matrix. (IQ) treatment showed the formation of martensite along the ferrite / ferrite grain boundaries. In contrast, treatment (SQ) induced the formation of a banded morphology of martensite and ferrite. The realization of borides on X70 (DP) steel was carried out in a powder mixture containing 5% of B4C as source of boron, 5% of NaBF4 as activator and 90% of SiC as diluent at 950℃ for a period of time from 4 h. The corrosion behavior of X70 (DP) steel has been explored by the Tafel extrapolation method in a 5 wt. % H2SO4 solution. The corrosion resistance of steel which has undergone boriding (BDP) is higher than that of steel which has not undergone it (DP).

Low carbon steel substrates were surfacehardened by carburizing in solid medium, and then thin layers of chromium weredeposited by electrolytic way. After chromium deposition, the samples wereexposed to isothermal annealing at temperatures between 500 and 1100 °C for 1and 2 hours duration. The obtained thin layers were characterized by X-raysdiffraction, optical microscopy and Vickers micro-hardness.The obtained results showed that chromium layersare transformed into chromium carbide by metastable phases of transition. At500 °C annealing temperature thin films retain their monophasic character Crα,and they almost keep the same hardness values. When temperature range isbetween 700 and 900 °C the incipient phase appears and is increased while thechromium mother phases decrease. After annealing at 1100 °C, the chromium layeris completely converted into chromium carbides that would be responsible forthe increase of the hardness and adhesion layer / substrate.