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Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel

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Journal of Materials Science & Technology 2012年第4期28卷 336-342页

作者： Khodamorad Abbaszadeh Hassan Saghafian shahram kheirandish School of Metallurgy and Materials Engineering Iran University of Science and Technology Tehran *** Iran

The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910℃ for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 ℃ and the remaining samples were quenched in salt bath at 425 ℃ for various holding times. All samples were tempered at 200 ℃ for 2 h. Microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）. Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch （CVN） impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior.

关键词： Upper bainite Lower bainite Microstructure D6AC Mechanical properties

Formation kinetics of austenite in pearlitic ductile iron

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International Journal of Minerals,Metallurgy and Materials 2012年第6期19卷 506-510页

作者： Payam Abdollahi Alborz Amirsadeghi shahram kheirandish Shamsoddin Mirdamadi Department of Materials Science and Engineering Iran University of Science and TechnologyTehran ***Iran

This work evaluated the isothermal transformation of austenite in unalloyed pearlitic ductile iron and drew the isothermal phase diagram of austenitization in the ductile iron, Austenite forms at grain boundaries and then grows up to graphite regions during austenitiza- tion. The formation kinetics of austenite complies with the Avrami equation, in which the parameter （n） ranges from 4.71 to 4.99. The start time and finish time of transformation can be calculated at each temperature using the Avrami equation.

关键词： cast iron ductility pearlite austenitization kinetics isothermal transformations

Investigation on the homogenization treatment and element segregation on the microstructure of a γ/γ′-cobalt-based superalloy

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International Journal of Minerals,Metallurgy and Materials 2019年第2期26卷 222-233页

作者： Saeed Aliakbari Sani Hossein Arabi shahram kheirandish Golamreza Ebrahimi School of Materials and Metallurgical Engineering Iran University of Science and Technology ((IUST)) School of Materials and Metallurgical Engineering Iran University of Science and Technology (IUST) Materials and Polymers Engineering Department Faculty of Engineering Hakim Sabzevari University

The aim of the present study was to investigate the effect of element segregation on the microstructure and γ′ phase in a γ/γ′ cobalt-based superalloy. Several samples were prepared from a cast alloy and homogenized at 1300°C for different times, with a maximum of 24 h. A microstructural study of the cast alloy using wavelength-dispersive spectroscopic analysis revealed that elements such as Al, Ti, and Ni segregated mostly within interdendritic regions, whereas W atoms were segregated within dendrite cores. With an increase in homogenization time, segregation decreased and the initial dendritic structure was eliminated. Field-emission scanning electron microscopy micrographs showed that the γ′ phases in the cores and interdendritic regions of the as-cast alloy were 392 and 124 nm, respectively. The size difference of γ′ was found to be due to the different segregation behaviors of constituent elements during solidification. After homogenization, particularly after 16 h, segregation decreased; thus, the size, chemical composition, and hardness of the precipitated γ′ phase was mostly uniform throughout the samples.

关键词： superalloy segregation homogenization microstructure γ′phase

Effect of Heat Treatment on Microstructure of Modified Cast AISI D3 Cold Work Tool Steel

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Journal of Iron and Steel Research International 2010年第9期17卷 40-45,52页

作者： shahram kheirandish Hasan Saghafian Jalal Hedjazi Mohammad Momeni School of Metallurgy and Materials Science Engineering Iran University of Science and Technology Center of Excellence for Advanced Materials and Processing Iran University ofScience and Technology

The effects of heat treatment on the microstructure of modified cast high chromium high carbon AISI D3 steel were studied. The modified AISI D3 steel was developed by replacing part of chromium with niobium and titanium, in which chromium carbide was partially replaced with MC carbides. The cast samples produced by investment casting were heat treated under different conditions. The microstructures of the samples were studied by optical microscope and scanning electron microscope equipped with EDS analyzer. To determine the optimized homogenizing process, the effects of homogenizing treatment on the microstructure and the morphology of carbides were also studied. The results show that the MC carbides are not changed in shape and amounts during high temperature homogenizing, whereas the M3C carbides are reduced in amount and become semiglobular in shape, and the homogeneity of microstructure is improved.

关键词： D3 cold work tool steel cast Nb Ti microstructure

Tempering Behavior of 4.5Cr-2W-0.25V Steel

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Journal of Iron and Steel Research International 2010年第10期17卷 57-62页

作者： Mohsen Asadi Asadabad shahram kheirandish Abdul Javad Novinrooz Department of Materials Science and Metallurgical Engineering Iran University of Science and Technology Advanced Materials Group Materials Research School Center of Excellence for Advanced Materials and Processing

The tempering behavior of a Cr-W-V steel was investigated in this research. This new alloy with the com- position of Fe-4.5Cr-2W-0. 25V-0. 1C was austenitized at 1 000 ℃ for 30 rain and tempered at 600 and 700 ℃ for dif- ferent time up to 100 h. An OM analysis of the microstructure of air cooled and water quenched specimens before tempering showed that although under both conditions fully martensitic matrix formed, finer structure had formed in the water quenched specimens. The XRD and TEM results showed that the most stable carbides formed during tempering of the steel were M23C6 and M7C3, respectively. Other carbides such as M3C and M2C, formed in the first stages of tempering, and stable MC were also observed. The results showed that when the tempering time, tempera- ture and cooling rate were increased, mass percent of extracted precipitates was increased. In addition, the formation rate of the stable carbides such as M23 Cs and dissolution rate of the metastable carbides such as M3C and M2C were increased.

关键词： Cr-W-V steel carbide tempering TEM XRD