مجله مهندسی متالورژی
پیاپی 45 (بهار 1391)

In this research, the effect of double tempering on impact properties and micro hardness of ferrite & martensite on a dual phase steel with chemical composition of C 0. 17%-Si 0. 41%-Mn 1. 24% was investigated. For dual phase ferrite-martensite microstructure formation, intercritical annealing at 780 °C for 45 min and quenching in water was selected. After that, double tempering at 150, 250,350 and 450 °C for 30 min and quenching in still air was performed. Impact testing was carried at room temperature and sub-zero temperatures. Micro & macro hardness testing and microstructure evaluation by optical and scanning electron microscope were done. Investigation on impact properties shows that the impact energy & lateral expansion at different testing temperatures will increase gradually due to higher tempering temperature. However, there is remarkable increase of impact properties when samples are tempered at 450 °C. The results indicate that hardness of ferrite and martensite decrease with increasing the tempering temperature. Although hardness of ferrite is decreasing gradually, there is remarkable decrease of martensite hardness at temperature over 250 °C.

Chemical precipitation or electrolysis is a method to produce high purity copper powder. Producing fine powder of copper was one of the purposes of this research. Effect of parameters such as current density, sulfuric acid concentration, copper ion concentration and chlorine ion concentration on producing copper powder by this method, were studied in this research. Other parameters such as temperature, electrodes surface area, agitation speed, anode-cathode distance, anode and cathode surface roughness were kept constant while considered parameters were varied as; current density 0/25-0/35 A/cm2, sulfuric acid concentration 140-180 g/L, copper ion concentration 2-8 g/L and chlorine ion concentration 0- 60 ppm. To optimize these parameters, irregular factorial design of experiments was used and 15 tests were carried out. Produced powder was characterized by X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM). After surveying the results, optimized parameters were concluded and a mathematical model was presented.

Nickel free nano structure austenitic stainless steels(HNSS) have been extensively investigated lately due to their excellent properties. In particular, nitrogen as an alloying element improve mechanical and corrosion properties and nitrogen makes it possible to reduce the nickel content in stainless steels and as a result, offers additional advantages such as biomaterial application and cost savings. High pressure techniques have been developed for fabrication of high nitrogen stainless steel, but these procedures are typically very expensive. Alternatively, powder metallurgy can be employed in order to increase the nitrogen content in steels. Recently, it has been shown that nitrogen concentration in iron and steel powders results in a highly refined crystalline microstructure. The powder were characterized by means of X-ray diffraction(XRD) and scanning electrone microscop (SEM) with an energy dispersive X-ray microanalysis system(EDAX). The result showed that maximum nitrogen levels of 1.52 wt pct nitrogen were achieved after for 80 hours in addition, the grain size structure continually decreased and reached 10 nm.

In the present study, the influence of annealing heat treatment and aging treatment after annealing process on the superelastic behavior of 40 and 60% cold worked Ni-rich NiTi shape memory alloy have been investigated. Results showed that at the presence of the fine grains due to the recrystallization process, superelastic behavior improved; however, some strains about 0.2% remain in the strain-stress curves of the alloy. With the formation of recrystallized fine grains, due to annealing heat treatment as well as Ni4Ti3 precipitates, due to the aging treatment after the annealing process, the best superelasticity behavior achieved. It is observed that the variation of grains size result in variation of precipitates distribution within microstructure. This, therefore, leads to variation of transformation temperatures. Differential scanning calorimetry (DSC) analysis was also used to evolution the transformations temperatures and behavior of the samples. Consequently, it is observed that, with variation of grains sizes during annealing process and with aging treatment after that, transformation temperatures can be controlled for the different thermal applications.

Repetitive corrugation and straightening (RCS) is one of sever plastic deformation (SPD) methods that is used for imposing a high magnitude of strain to the sheets and grain refining of the microstructure. In this paper the effect of RCS on the microstructure of commercial aluminum alloy 5083 sheets was investigated. For this purpose, initially sheets with the dimension of 1×15×15mm were annealed at various temperatures then corrugated with a pair of sine wave dies and straightened with a pair of flat dies. The microstructure evolutions were studied by optical microscopy and the changes of mechanical properties were investigated by hardness test as a function of the number of process passes. The results indicated that the RCS process effectively reduced the grain size of Al-5083 alloy sheets annealed at the 400-450 °C temperature. The rotation of specimens between passes resulted in higher hardness in comparison with the specimens without rotation. With increasing the process passes the hardness of the sheets became more uniform.

In this research, the 5-mm-thick copper plates were friction stir welded at a constant rotation rate of 600rpm with various welding speeds. Optical microscopy, tensile and microhardness test were used to study of microstructure and mechanical properties of the joints. The results show that by increasing the welding speed, the grain size nugget zone first decreased and then increased. The average hardness of nugget zone increased first and then decreased. The ultimate tensile strength and elongation of the joints decreased with increasing the welding speed due to decreasing the material plastic deformation during welding process and formation of cavity defects in nugget zone. The defect free joints were produced at lower welding speeds, and the fracture locations were in base metal. With increasing the welding speed, the fracture locations were in nugget zone.

In this research, the effects of plasma nitriding parameters including temperature and time on the microstructures and fatigue strengths of quenched and tempered DIN 1. 2210 cold work tool steel were investigated. The microstructures of the base material and nitrided layer were examined in details by optical microscopy and X-ray diffractions. Micro-hardness measurements were used to determine surface hardness and case depth. A rotating bending fatigue test machine was employed to obtain the fatigue strengths. The results indicated that plasma nitriding considerably increases the micro-hardness and fatigue strength depending on the case depth. The maximum fatigue strength was attained for Plasma nitriding of samples at 550 °C for 6 hours which increased the fatigue life Fe4N and ε: Fe3N: ׳ by about 67%. It was obtained that, the compound layer of the treated samples consisted of γ Fe4N to ε: Fe3N increased by increasing treatment temperature.: ׳ phases which Proportion of γ.

In order to improvement of wear resistance of 1.2344 hot work tool steel, Vanadium Carbo-nitride V(N,C) was applied by duplex treatment which involved gas Nitro-carborizing followed by Thermo Reactive Diffusion (TRD) process. The coating layer was investigated by X-Ray diffraction method and SEM microscopy. Hardness profile from surface to the substrate was obtained by Micro-hardness test. Wear rate and coefficient of friction of the V(N,C) coating in comparison with Hardened, Nitrocarborized, and Vanadium carbide (VC) coated samples was evaluated by Pin-on-Disk wear test machine during 700m displacement under 140N normal load. Wear mechanism was investigated by SEM microscopy of wear traces and debris with secondary electron (BSE), back scatter mode, and EDS analyses. Results indicated that duplex process provides a gradual hardness leads to supportive subsurface for the super hard V(N,C) layer. The V(N,C) showed minimum wear rate and friction coefficient due to oxidation of vanadium at high temperature in contact region. The predominant wear mechanism was recognized as a micro-polishing of oxidized layer.

Electro slag remelting (ESR) process which is used for production of steels, Ni-based super alloys and Titanium alloys in industrial scale provides a comparable quality for producing the ingots prepared by the conventional methods such as vacuum arc remelting (VAR) process. The ESR process can refine and improve the ingot composition and produce the high quality and homogeneous structure without solidification defects in comparable to the expensive VAR process, so that these ingots can be directly forged or rolled. The ESR process is also capable to reduce the production cost due to simplicity and flexibility of the equipments. Refractory casing or slag prevent molten metal from atmospheric impurity, limit spattering and create the ionic path for stability the flow discharge, that it cause to achieve a more homogeneous and sound part. In this paper, the principles of ESR process for producing Titanium and its alloys, the melting parameters and slag composition of these alloys have been studied. In addition, the refinement of various gaseous inclusions and mechanical properties of these alloys have been investigated.