فهرست مطالب hamid reza karimi zarchi

In recent years, more attention has been paid to the development of advanced micro-compositic high strength steels. Multiphase steels are a new family of these steels. It is possible to create the micro-composite phases containing bainite, martensite and retained austenite in their microstructures by selecting the appropriate chemical composition and heat treatment cycles in order to achieve the optimum engineering properties. In the present research work, high silicon steel sheets were selected and then heat treated to achieve triple phase steels. The heat treatment cycles included austenitizing at 900oC for 5 min, quick transferring to 350oC salt bath, holding for different times between 5 seconds to 5 hours and finally quenching in water in order to achieve the triple phase microstructures. The mechanical properties and microstructural investigations were carried out by using macro-hardness tester, tensile test, optical and scanning electron microscopes, respectively. The observation of resulted microstructures showed the various amounts of bainite, martensite and retained austenite phases have been formed in the specimens that are responsible for sharp and uncommon changes in tensile and macrohardness properties in comparison with those of third generation of advanced high strength steels. The results indicated the ultimate tensile strength was increased from 1850 to 2052MPa for the specimens treated from 5s to 50s and then decreased to 1386 MPa in 300s and again increased to 1460MPa for 1 hour specimens. These changes in properties were analyzed on the basis of the volume fractions and different mechanical behavior of bainite, martensite and retained austenite phases. The best condition for achieving the highest toughness which can be calculated by the product of tensile strength and elongation (25.9 GPa%) was obtained in the specimens heat treated in 350oC salt bath for 200 seconds.

The purpose of this research is to investigate microstructural features and mechanical properties of MO40 low alloy steel under dual-phase ferritic-bainitic in comparison with those of full bainitic conditions. To do so, various heat treatment cycles including Austenitizing the samples at 860°C for 50 minutes, step quenching in primary 620°C salt bath for different duration times of 1 to 10 minutes, quenching in secondary 350°C salt bath for 30 minutes and then water quenching to room temperature in order to develop dual-phase ferritic-bainitic microstructures in the samples. The isothermal heat treatment in primary 620°C salt bath for various duration times was used in order to develop different volume fractions of ferritic phase in the microstructures, then the secondary 350°C salt bath was used for subsequent transformation of remaining prior austenite to bainite. Tensile test, micro and macro hardness testers, optical metallography and FE-SEM equipped with EDS were used for investigation of variations of mechanical properties and microstructural features. The experimental results show that there is a non-linear and abnormal relationship between the mechanical properties of dual-phase ferritic-bainitic samples and variations of ferrite and bainite volume fractions. The mechanical properties of dual-phase ferritic-bainitic samples involving 8-12 vol% grain boundary ferrite in the vicinity of bainite is considerably higher than other dual phase ferritic-bainitic samples and even full bainitic ones. This improvement in mechanical properties of dual-phase ferritic-bainitic samples involving 8-12 vol% ferrite is originating from interaction of bainite crystals on grain boundary ferrite.

Nickel-based alloys are largely used in aerospace, power generation and petrochemical industries. Machining of these alloys is usually accompanied with high cutting force and temperature and low surface quality. In order to avoid these limitations, small values of cutting parameters are used in machining of nickel-based alloys that significantly decrease the productivity. In this paper, the effects of cutting speed, feed rate and depth of cut on the surface roughness in dry turning of nickel-based super alloy Inconel 600 by using carbide tool in finishing range have been experimentally investigated. Full factorial design of experiments was used and totally 48 tests were done. Results of the analysis of variance showed that feed rate 69%, depth of cut 6% and cutting speed 2% affects on surface roughness of Inconel 600. In addition to measuring the surface roughness, chip removal rate was calculated for each test and then, for achieving to minimum surface roughness and maximum chip removal rate at the same time, the optimal machining parameters have been achieved by using neural network and imperialist competitive optimization algorithm. By using this algorithm, several levels of optimum cutting parameters are set and a process planner according to the required surface roughness and material removal rate can use these cutting parameters.

Machining of hard steels has it’s own problems. According to the recent advances in implementing of new cutting tools, the machining of hard steels with operations such as turning and milling is possible and it can replaced with some of grinding operations. Turning of workpieces with 45 HRC or upper hardness, is said hard turning. The aim of this article is the investigation of the effect of workpiece hardness and cutting speed and feed rate parameters on surface roughness in hard turning of cold work tool steel X210Cr12 or SPK in dry condition. For achieving this goal, the workpieces of X210Cr12 steel where hardened with different heat treatments cycles such that their hardnesses lie in the hard turning range. Then the workpieces were machined with different cutting parameters using CBN tool and the resulted surface roughness were compared. Experimental tests designed with full factorial method and totally 36 tests have been done. According to obtained results of experimental tests and analysis of variance, the effect of feed rate and workpiece hardness on surface roughness was 90.4% and 8.3%, respectively. The effect of cutting speed on surface roughness is negligible. Increasing the feed rate results in the upper surface roughness. Increasing the workpieces hardness to 50 HRC, decreases surface roughness and increasing workpieces hardness from 50 to 65 HRC, increased surface roughness.

In this research, aluminizing packs containing NaF and NH4Cl activators were used for aluminizing of pure titanium samples at 1100 ºC. The results showed that the layers of TiAl3, TiAl2, γ-TiAl, α2-Ti3Al, and α-solid solution are formed on titanium samples in both cases. However, the weight gain and the thickness of TiAl3 layer in treated samples with NH4Cl activator are greater than those when NaF activator is used. Using a thermodynamic model showed that the partial pressures of aluminum-containing gases when NH4Cl is used are much greater and accordingly, the experimental results were justified.