فهرست مطالب

Iron and steel society of Iran - Volume:16 Issue: 1, 2019
  • Volume:16 Issue: 1, 2019
  • 60 صفحه،
  • تاریخ انتشار: 1398/04/10
  • تعداد عناوین: 7
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  • A. Ghorabaei, M. Nili Ahmadabadi * Pages 1-8

    One of the most promising ways to produce a grain-refined microstructure in some steel materials is the thermomechanical processing route of subcritical recrystallization annealing of a cold-deformed martensite structure. In the present study, the microstructural evolutions and the associated recrystallization kinetics under various subcritical annealing heat treatment conditions are explored in an API X120 grade, advanced, High-Strength, Low-Alloy (HSLA) steel with an initial cold-deformed martensite microstructure. The steel sheet was the subject of a conventional cold rolling process for moderate true strain of 60% followed by isothermal recrystallization for different temperature-time combinations. Optical microscopy and scanning electron microscopy were used to characterize the microstructural evolutions, and the recrystallization kinetics was evaluated by hardness measurements with the aid of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) relationship. The experimental results indicated that annealing at 948 K (675 °C) for 18 h is the optimum condition to achieve a grain-refined ferrite microstructure with an averaged grain size of 5.2 µm. The slow kinetics of recrystallization was also revealed by JMAK model as the Avrami exponent was calculated around one for all of the experiments. These observations are rationalized in part by the possible formation of microalloying elements carbides during the annealing process in association with the existence of the inhomogeneously deformed initial microstructure. This results in the appearance of a continuous regime for the recrystallization nucleation along with the sluggish movement of recrystallization fronts.

    Keywords: Advanced HSLA steels, Thermomechanical processing, Martensite recrystallization kinetics, Grain refinement
  • M. Yoozbashi *, S. Yazdani Pages 9-13
    Electroless Ni-P (EN) plating, as an important group of metallic coatings, employed in a wide range of industrial applications. The current work aims to investigate the effects of pretreatment process before EN plating on fatigue behavior of SAE 1045 steel. The specimens of rotating bending fatigue test were prepared from the steel in two series. A group of samples used in as-polished condition as a base metal and the next group of samples was degreased by Aston and then NaOH 150 gr/lit at room temperature. Subsequently, pickling carried out in an aqueous solution of HCl 30 Vol.% for 3 min. Finnaly, the etching of samples done for 30 seconds in an aqueous solution of HCl 20 Vol.% at 50 °C. In the following, rotating bending fatigue tests carried out on samples. The fatigue fracture surfaces examined by scanning electron microscopy (SEM). Fatigue test results showed an increase in fatigue strength of pretreated samples compared to that of base metal. Reducing the roughness of the surface caused by the pretreatment operation is responsible for this behavior. According to SEM observations, the pretreatment process influences the crack initiation stage, which eventually results in a better performance in fatigue strength.
    Keywords: Pretreatment, Electroless, fatigue, SAE 1045
  • M. Mohamadi, M. Tavoosi *, M. Dehnavi, M. Loghman Pages 14-22
    The development of ultrafine bainitic structure in AISI 431 stainless steel was the goal of this study. For this purpose, the AISI 431 specimens were austenitized at 1100 °C for 60 min followed by low-temperature austempering treatment at different temperatures and times. Austempered samples were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and tension test. According to achieved results, the microstructure of AISI 431 steel after austempering treatment consists of ultrafine bainitic ferrite plates and retained austenite with two morphologies of micrometer-block and fine film. At austempering temperature up to 300 oC, the micrometer-blocky morphology of austenite was vanished completely from the microstructure. The maximum values of strength and ductility of about 1664 MPa and 9.5 % were achieved in austempered sample at 350°C for 24 h. By enhancement the austempering temperature and time, the strength and elongation percent were reduced simultaneously as a result of change in shape and size of bainite phase.
    Keywords: Ultrafine bainite, Austempering, AISI 431 steel
  • M. Alizadeh *, S. Hosseini, A. Ghasemi Pages 23-30
    In this study, the effects of various heat treatments on microstructure, mechanical and magnetic properties of HSLA-100 steel were evaluated. The heat treatments consisted of austenitizing at 900°C for 60 minutes, then quenching by different cooling rates via furnace, air, oil and water; and quenched specimens were aged at 600°C for one hour. Optical and field emission scanning electron microscopes were used for the characterization of the microstructure. A vibrating sample magnetometer, an inductance meter and a susceptometer were used for the characterization of the magnetic properties. Mechanical properties of the specimens were also studied using hardness, tensile strength and impact toughness methods. The fracture surfaces of charpy specimens were examined using scanning electron microscope. The results showed that martensite phase was spread in the all of the cooling rate. The results of magnetic coercivity force indicated that coarse Cu precipitates and martensite-austenite (MA) constituents hinder the domain wall motion and behavior in effective magnetic susceptibility. Moreover, the non-magnetic copper precipitates reduced magnetic permeability. The hardness value of the aged specimens is the same as that of the quenched steels (approximately 300 VHN30). In addition, yield strength as high as 900 MPa was achieved in aged specimens. The highest impact toughness value at -84℃ (approximately 200 J) was obtained in as-received and aged specimens.
    Keywords: Aging, Coercivity force, Heat treatment, HSLA-100 steel, Mechanical properties
  • I. Ebrahimzadeh *, K. Amini, F. Gharavi, Z. Ghorbani Pages 31-40
    The deep cryogenic treatment is a complementary operation that is done on a variety of tool steels aimed at improving their abrasion resistance and hardness. In the case of the H13 hot-work steel, which is widely used at high temperatures as a hot-deformation tool, we need to determine the efficiency of subzero treatment on it at the working temperature. In this regard, this paper is focused on effect of deep cryogenic treatment on the wear behavior of H13 hot-work steel at the working temperature. We compared two types of quench-tempered and quenched-subzero-tempered samples in this study. The microstructure of the samples was determined by scanning electron microscopy and their structure was determined by X-ray diffraction device after the cryogenic treatment. The wear test was performed at work temperature of 600 °C (die temperature in the iron-based alloys forging process). The results indicated that the highest hardness rate has occurred in the quenched-subzero-tempered conditions, which amount is about 26% higher than that of the quenched-tempered in the oil. Applying quenched-subzero-tempered operations has reduced the percentage of residual austenite by 10%. Also, the fine, dispersed, and uniform sediments in this sample are more observed than the quenched-tempered samples. The improved weight lose were respectively achieved by 50% and 44% at temperatures of 25 and 600 °C. The evaluation of the wear surfaces indicates that the abrasion dominant mechanism is close-fitting and tribochemical.
    Keywords: Deep cryogenic treatment, High-temperature abrasion, AISI H13 hot-work steel, Residual austenite, Hardness
  • M. Rabani, M. Zandrahimi *, H. Ebrahimifar Pages 41-50
    Aluminide coatings are widely used as a protective coating material due to their high corrosion and oxidation resistance properties. In this research, an aluminide coating was fabricated through aluminizing on UNS S30815 austenitic stainless steel using pack cementation method at 950 °C for 5 h. The isothermal oxidation was exerted on uncoated and aluminide coated steels for 200 h at 1050 ºC. Also cyclic oxidation was applied for 50 cycles at 1050 ºC on coated and uncoated steels. Surface morphology and cross section of coated and oxidized samples were characterized by means of scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). X-Ray diffraction (XRD) was used to identify the formed phases in the surface layer of as-coated and oxidized specimens. As-coated UNS S30815 consisted of Al5Fe2, FeAl3 and Al2O3 phases. The results of the isothermal oxidation showed that the coated steel had lower weight gain (2.9 mg.cm-2) after 200 h of oxidation in comparison with uncoated one (7 mg.cm-2). Also the results of cyclic oxidation showed that the coated specimens had good resistance to thermal cycles.
    Keywords: UNS S30815 austenitic stainless steel, Aluminizing, pack cementation, Oxidation
  • M. Safari *, J. Joudaki Pages 51-57
    Resistance Spot Welding (RSW) is a powerful tool for overlap joint of thin sheets. The welding procedure contributes to affecting the electrical, thermal and mechanical properties of the sheets. In this article, resistance spot welding of ultrathin IF steel sheets (0.67 mm thickness) have been studied. The IF steel sheets were welded by different welding currents (65, 70, 80, 90 and 100 A). The microstructure of the samples was observed using an optical microscope and the tensile test has been carried out to determine the joint strength and fracture mode. The microstructural observations showed that the grain size increase in the fusion zone by increasing the welding current and the heat-affected zone experienced recrystallization and small equiaxed grains were formed in the heat-affected zone. The base metal keeps its high hardness value from the work-hardening induced by the previous rolling process. Best joint strength was obtained at 80 A welding current (2600 N). The fracture mode changes from partial interfacial failure (at lower welding current) to the pullout with tearing of the sheets (at higher welding current).
    Keywords: Resistance Spot Welding Process, Joint strength, Failure Mode, IF steels