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Ebrahimi G.R.,Kharazmi University | Keshmiri H.,Esfarayen Steel Complex | Momeni A.,Amirkabir University of Technology
Ironmaking and Steelmaking | Year: 2011

In order to understand the best regime of heat treatment for 15Cr-4Ni-0.08C martensitic stainless steel, different heat treatment procedures involving solution treatment and tempering were investigated. The solution treatment was performed at temperatures of 900-1100°C for different soaking times of 30, 60 and 120 min followed by air cooling. Optical and scanning electron microscopy observations revealed that grain growth commenced by dissolution of pre-existing chromium carbides at ∼ 1000°C after 60 min soaking. Therefore, this regime was considered as the best solution treatment procedure. The tempering treatment was performed on the solution treated samples at temperatures of 250-600°C for 2 h followed by air cooling. Standard tensile test and Charpy impact tests were performed on the tempered specimens and showed that two secondary hardening steps at 450 and 550°C contribute in ductility loss and increment of strength and hardness. The fractography observations corroborated the mechanical testing results. © 2011 Institute of Materials, Minerals and Mining. Source


Momeni A.,Amirkabir University of Technology | Dehghani K.,Amirkabir University of Technology | Ebrahimi G.R.,Kharazmi University | Keshmiri H.,Esfarayen Steel Complex
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2010

The hot deformation behavior of AISI 410 martensitic stainless steel was investigated by conducting hot compression tests between 1173 K (900 °C) and 1423 K (1150 °C) and between strain rates of 0.001 s-1 to 1 s-1. The hyperbolic sine function described the relation well between flow stress at a given strain and the Zener-Hollomon parameter (Z). The variation of flow stress with deformation temperature gave the average value of apparent activation energy as 448 kJ/mol. The strain and stress corresponding to two important points associated with flow curve (i.e., peak strain and the onset of steady-state flow) were related to the Z parameter using power-law equations. A model also was proposed based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation to estimate the fractional softening of dynamic recrystallization at any given strain. This model can be used readily for the prediction of flow stress. The values of n and k, material constants in the JMAK equation, were determined for the studied material. The strains regarding the peak and the onset of steady-state flow were formulated in term of applied strain rate and the constants of the JMAK equation. A good agreement was found between the predicted strains and those obtained by the experimental work. © 2010 The Minerals, Metals & Materials Society and ASM International. Source


Momeni A.,Amirkabir University of Technology | Dehghani K.,Amirkabir University of Technology | Keshmiri H.,Esfarayen Steel Complex | Ebrahimi G.R.,Kharazmi University
Materials Science and Engineering A | Year: 2010

Hot deformation behavior of the superaustenitic stainless steel type 1.4563 was investigated by conducting hot compression tests at the temperatures of 900-1050 °C and at strain rates in the range of 0.001-1 s-1. The microstructural changes were then characterized using optical and scanning electron microscopy as well as energy dispersive X-ray (EDX) microanalyses. The results showed that hot deformation at low temperatures, i.e. 900-950 °C, and at low and medium strain rates, i.e. 0.001-0.1 s-1, can lead to the formation of wormlike precipitates on grain boundaries resulting in the restriction or even inhibition of dynamic recrystallization. At higher strain rates or higher temperatures when respectively the time was too short or the driving force for dynamic precipitation was rather low, dynamic recrystallization occurred readily. Further, at low strain rates and high temperatures, where the occurrence of dynamic precipitation is difficult, there was no sign of particles. In this case, the interactions between solute atoms and mobile dislocations resulted in tiny serrations in the flow curves instead. The EDX analyses indicated that the chemical composition of the observed precipitates was (Cr, Fe, Mo)23C6. © 2009 Elsevier B.V. All rights reserved. Source


Mirzaee M.,Islamic Azad University at Mashhad | Momeni A.,Hamedan University of Technology | Keshmiri H.,Esfarayen Steel Complex | Razavinejad R.,Niroo Pajooh Shargh nps.co
Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science | Year: 2014

The effects of Ti and Nb on the microstructure of cast K100 tool steel were studied by optical and scanning electron microscopes. The amounts of Ti as 0.3, 0.7, and 1 wt pct and Nb as 0.2 and 1 wt pct were added to the studied steel. The addition of 0.3 wt pct Ti did not result in a considerable change in the size of carbides and prior austenite grain size. However, microstructure of K100 with 0.7 and 1 wt pct Ti was considerably modified (about 55 pct) and a uniform grain size was obtained at different positions (bottom, middle, and top) of the ingot. With addition of 0.2 and 1 wt pct Nb, microstructure was modified and a more uniform grain size was obtained all over the ingot. The average modification of microstructure in the bottom, middle, and top of the ingot was about 22 pct. Both Ti and Nb could effectively decrease the segregation of Cr and C from the bottom (high cooling rate positions) to the top of the ingots (low cooling rate positions). The homogeneity of chemical composition increased with increasing Nb or Ti. In alloy with 0.7 to 1 wt pct Ti, the average size of prior austenite grains was finer than alloys with 0.2 to 1 wt pct Nb. Therefore, Ti was found more capable than Nb in the modification of microstructure and decreasing the segregation of Cr and C in cast K100 tool steel. © 2014, The Minerals, Metals & Materials Society and ASM International. Source


Farnoush H.,Amirkabir University of Technology | Momeni A.,Amirkabir University of Technology | Dehghani K.,Amirkabir University of Technology | Mohandesi J.A.,Amirkabir University of Technology | Keshmiri H.,Esfarayen Steel Complex
Materials and Design | Year: 2010

High temperature behavior of 2205 duplex stainless steel was studied by considering behavior of each constituent phase. The specimens were subjected to hot compression tests at temperatures of 800-1100 °C and strain rates ranging from 0.001 to 1 s-1 at intervals of an order of magnitude. The flow stress analysis showed that hot working empirical constants are different at low and high temperatures. The strain rate sensitivity m was determined and found to change from 0.12 to 0.21 for a temperature rise from 800 °C to 1100 °C. The apparent activation energy Q was calculated as 554 and 310 kJ/mol for low and high temperature, respectively. The validity of constitutive equation of hyperbolic sine function was studied and stress exponent, n, was assessed to be 4.2. Assuming the hyperbolic sine function for determination of strain rate and application of the rule of mixture, the interaction coefficients of δ-ferrite, P, and austenite, R, were estimated at different hot working regimes. It was found that the interaction coefficients are functions of Zener-Hollomon parameter Z and obey the formulas P = 1.4Z-0.08 and R = 0.76Z0.005. Therefore, it was concluded that at low Z values δ-ferrite almost accommodates strain and dynamic recovery is the prominent restoration process which may even inhibit dynamic recrystallization in austenite. Otherwise, at high Z, austenite controls the deformation mechanism of material and dynamic recrystallization leads in finer microstructure. © 2009 Elsevier Ltd. All rights reserved. Source

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