Structural Modifications of Superficial Layer of C45 Steel Samples Through WT20 and WZr8 Depositions

The paper presents technological aspects regarding the modification of mechanical chracteristics in the superficial layer of C45 steel samples, through thin layers deposition using WT20 and WZr8 electrodes. Deposition of thin layers was made through electrical discharge method in impulse. The obtained samples were microstructural analyzed, at various magnitudes, on an VegaTescan electronic microscope. Also, measurements of mechanical chracteristics were made through indentation, highlighting the improved values after layers deposition with the 2 electrods.


Introduction
The steel presents the posibility of allloying with the most various chemical elements, resulting highly improved properties [1,2].
The carbon steels contain beside iron and carbon, other alloying elements. Other chemical elements are considered impurities. The exception from this rule are some elements, which are imposed by the technological process of elaboration and casting (manganese, silicon, aluminium).
The regular steel carbon is manufactured curently and used without heat treatments at civil constructions, metallic constructions and machines facturing.
The quality carbon steel is used in machine manufacturing, usually heat treated and has guaranteed chemical composition and mechanical characteristics.
The superior carbon steel is a quality steel in which the content of sulfur and phosphorus is limited at below 0.035% each and conditions are imposed regarding the structure (size of austenitic grain, depth of hardening) and the maximum content in non-metallic inclusions.
Low alloyed steels are steels which contain alloying elements willingly introduced in mininum quantities which influence the physical-chemical properties and mechanical characteristics.
The alloyed steels are the medium and high alloyed and are used only heat treated. The alloyed steels have the sum of alloying elements concentrations higher than 10% or one of this concentration higher than the following limits: Si 6%; Mn 6%; Cr 6%; Ni 4.5%; Mo 1%; W 4%; Co 1%; V 1%.
The steels are delivered to the beneficiaries in the form of cast parts or laminated: blooms, slabs, wires, billets, sinkers, round steel, square, hexagonal, flat, profiles, thin sheets, plats, strips [8].

Deposition of thin layers through electrical discharge in impuse method
The deposition of layers through electrical discharge on Fe-C alloys are based on electroerosion phenomena and polar transfer of electrode material to the metallic parts. Through the proximity of the electrode to the part, at the critical distance of puncture, electrical discharge through impulse is triggered [3,4,7]. Due to polar effect, the transfer of electrode material to the part assures the forming of the superficial layer, with highly determined physical-chemical properties. As a result of the material transfer and thermal modification from the discharge area, the superficial layer of the part is modifying the structure and the chemical composition. The characteristics of this layer can variate in high limits according to the electrode material, the environment composition between electrod and part, the parameters of impulse discharging. The superficial alloying, realized through layers deposition with electrical discharge method, can have the following purposes [5,6,7]:

Determination of chemical composition through spectral quantitative analysis
The analyze of chemical composition was made on prepared samples through polishing, using an Foundry Masters optical spectrometer, type 01J0013. With WasLab software and calibration programs, has been obtained a analysis bulletin, which present the values determined by the apparatus.

Superficial processing through electric discharge with compact electrodes
As hardness measured values after the improving treatment are low, on the normalized samples thin layers were applied using Elitron 22A apparatus, through electrical discharge method. For the experimental attempts, 2 tungsten electrodes were used, having different compositions (table 3).

The structure analysis of superficial layers
After the WT20 and WZr8 layers deposition, were made SEM photos at 100x, 200x, 500x, 1000x magnitudes (figures 1, 2), [9]. From metallographic photos achieved through electronic microscopy, at different magnitudes, it can be observed that in the case of WZr8 deposition, the layer is more uniform, and in case of WT20, the surface formed structure are smoother.

Measurements of mechanical characteristics at the superficial layer level
The determination of mechanical characteristics was made on CETR-UMT2 microtribometer.
Hardeness and Young modulus modifications were analized. The analized samples had small dimensions (according with the requirements of apparatus manufacturer) and were prepared in advance (without oxides or other substances resulting from previous machining). The achieved layers through depositions with WT20 and WZr8 electrodes, were presented with different structures and their hardness average is improved against the obtained values for the basic material (C45 steel). In the case of Young modulus there is a 25% increase in measured values for deposited layers (WT20 and WZr8) compared to the base material (steel C45).