حمید ناظمی

In this paper, 50CrV4 steel wire with a diameter of 4 mm were subjected to pure Zn electroplating. The effect of shot peening before and baking after electroplating on tensile performance of steel was investigated. Bombardment was done before plating for 20 minutes with shots 0.5 mm and 58 RC. Baking was performeIn this paper, 50CrV4 steel wire with a diameter of 4 mm were subjected to pure Zn electroplating. The effect of shot peening before and baking after electroplating on tensile performance of steel was investigated. Bombardment was done before plating for 20 minutes with shots 0.5 mm and 58 RC. Baking was performed after plating for 24 hours at 200°C. Finally, the samples were subjected to slow strain rate test, microhardness test and SEM imaging. The results showed that the effect of baking on improving the mechanical life of the sample under tension alone is greater than the shot peening. The mean time to failure of plated and baked samples was 2.5 hours, but for shot and plated samples was 2.1 hours. The simultaneous effect of pre-plating shot peening and post-plating baking led to further improvement of the tensile performance of the wire and the mean failure time reached 3.55 hours in the slow strain rate test. The embrittlement susceptibility index is now reduced from 0.76 for only plated samples to 0.13 for shot peened and baked samples, which shows a very good improvement. The effect of shot peening on the tensile properties of steel can be attributed to the formation of a residual compressive stress layer on the substrate surface, which leads to a reduction in the growth rate of microcracks due to tensile stresses under working conditions. The results also showed that due to the baking, continuous microcracks are created in the coating / substrate interface, which provide suitable paths for hydrogen to leave the substrate. after plating for 24 hours at 200°C. Finally, the samples were subjected to slow strain rate test, microhardness test and SEM imaging. The results showed that the effect of baking on improving the mechanical life of the sample under tension alone is greater than the shot peening. The mean time to failure of plated and baked samples was 2.5 hours, but for shot and plated samples was 2.1 hours. The simultaneous effect of pre-plating shot peening and post-plating baking led to further improvement of the tensile performance of the wire and the mean failure time reached 3.55 hours in the slow strain rate test. The embrittlement susceptibility index is now reduced from 0.76 for only plated samples to 0.13 for shot peened and baked samples, which shows a very good improvement. The effect of shot peening on the tensile properties of steel can be attributed to the formation of a residual compressive stress layer on the substrate surface, which leads to a reduction in the growth rate of microcracks due to tensile stresses under working conditions.

In this paper, the effect of tungsten element on microstructure and mechanical properties of Fe-C-Ni hard coating was investigated. Two hard coating electrodes were made with 10 and 30 gr of tungsten powder. The microstructure of the welding metals included fine carbides in the area of ​​needle martensite and residual austenite. Electron microscopy studies showed that there were very fine cracks in the weld metal martensitic phase with 10 gr of tungsten but these microscopic cracks were not found in weld metal with 30 gr of tungsten. The results of the EDS analysis showed that the amount of soluble tungsten element in the austenite phase of both welding metals is high. This amount in weld metal with 30 gr of tungsten was about 3.66% higher than the weld metal with 10 gr of tungsten. The results of the XRD analysis showed that the phases present in the weld metal with 10 gr of tungsten included martensite, austenite and W2C carbide, but in the weld metal with 30 gr of tungsten in addition to these phases also iron oxides were observed. The results of hardness test showed that the average hardness of weld metal with 10 gr of tungsten is 42.5 RC and the average hardness of welding metal with 30 gr of tungsten is 49.6 RC.

In the present research, the influences of explosive welding parameters on the corrosion behavior of the three layer explosive welds of the tubes Al 5083 / Al 1050 / stcr18Ni9Ti in the salt environment was investigated. The selected parameters included the stand-off distance and the thickness of the explosive charge. Therefore, two samples A and B were welded with stand-off distances of 6 and 6.75 mm respectively. The results showed that with increasing the standoff distance, the straight interface transited to the wavy interface. The results showed that, with increases in the standoff distance, the hardness near the interface was increased. The results of the electrochemical impedance spectroscopy showed that the number n in sample A is lower than that of sample B and as a result, the corrosion current in sample A is higher which leads to the reduction of the charge transfer resistance. The increase in the transferred impact kinetic energy in the sample with higher stand-off distance and explosive charge thickness was also effective on the increase in the corrosion.

In this research, corrosion behavior of explosively bonded 304 stainless steel-Ck45 carbon steel plates was investigated. Explosive welding carried out in constant explosive ratio with 4 and 5 mm standoff distances. The results showed that by standoff distance increasing, the shape of interface transited to the wavy vortex type, and locally melted zones formed. Due to shock hardening, the hardness increased in near of the interfaces, and maximum hardness was related to the sample with 5 mm standoff distance. Polarization and impedance tests results showed that maximum corrosion rate was related to the sample with maximum standoff distance. Due to the formation of locally melted zones and increasing in impact kinetic energy in the sample with maximum standoff distance, corrosion rate was increased.

In this paper, the hydrogen embrittlement of the high strength Cr-Si spring steel with after Zn-12%Ni electroplating, have been evaluated. Slow strain rate test with 3.34 × 10-6 sec-1 strain rate was performed, and the hydrogen embrittlement index of the different samples was calculated. The effect of baking after plating and also the effect of applying a nickel interlayer by electroless method, is examined by using the weibull statistical model. The results showed that alloying Zn with nickel considerably reduce the diffusion of hydrogen on the substrate steel structure. So that it increases the average failure time of samples from 4.9 hours for pure Zn to 8.7 hours for Zn-Ni. By applying nickel interlayer, the failure time increased to 11.15 hour. Baking after the electroplating of Zn-Ni samples without interlayer, leaded to an increase in the failure time to 10.15 hours. While baking in samples containing nickel interlayer, showed the opposite effect, reducing the failure time to 8.15 hours.

The use of the Zn alloy coatings with iron group elements such as Fe and Co is developing increasingly. Zn-Co alloy coatings have 0.4 - 2 wt% cobalt, but the Fe content of Zn-Fe alloy coatings would be up to 40 wt%. In this research, the reasons of high difference in amount of alloying element in the deposits in the same electrochemical conditions have been investigated. Zn-Co and Zn-Fe alloys deposited galvanostatically using acidic electrolytes in 25ºC for 10 minutes. The selected current densities were in the range of 4- 10 A/dm2. Composition of deposits was determined using EDAX analysis. Cathodic current efficiency and partial current density of Zn, Fe and Co were obtained using Faraday relations. Also, the thickness of coatings was determined for calculating of the deposition rate in Zn-Fe and Zn-Co alloys. The results indicate that Zn partial current density in Zn-Co deposition is more than in Zn-Fe. But Fe partial current density in Zn-Fe deposition is much more than Co in Zn-Co. Therefore, in the same current density, the presence of Fe in Zn-Fe alloy deposit would be much more than the presence of Co in Zn-Co deposit.

One of the methods in production of composite materials with different chemical composition is the centrifugal casting method. In this research the centrifugal casting method has been used for making the alloy of Al-15%Si. With this method، reduction in volume of silicon particles on the surface zone of the samples is 25%. As a consequence the alloy hardness is increased and this alloy becomes applicable to resistance in suitable wearing. In this direction obtained results in wearing tests characterized that the formation of silicon particles in the distance of 20 mm from the surface causes the increase in the hardness of samples in the rate of 95 HV and consequently the increase to resistance in the wearing in these zones. In addition this microhardness shows one reduction almost linear with respect to the increase of distance from the surface. Also the studies on the wearing surfaces with the use of electronic microscope shows that the wearing surfaces containing of silicon particles separated and also the tribologic layers are attached together. These two cases both indicate the resistance to wearing of the produced samples by this method.