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Kinetics of Austenite Layer Growth on the Surface of Fe-23Cr-2.4Mo Ferritic Stainless Steel During Solution Nitriding
Kh. Farjam Hajiagha, A. R. Akbari, R. Mohammadzadeh,
Volume 36, Issue 2 (9-2017)
Abstract

In this study, the kinetics of austenite layer growth on the surface of Fe-23Cr-2.4Mo ferritic stainless steel during solution nitriding and the effects of nitrogen adding on microstructure and hardness of the steel have been investigated. Steel plates of 2 mm thick were solution-nitrided at 1200˚C under nitrogen pressure of 0.25 MPa for 2, 3, 6, 9, 12 hours. Microstructure, the thickness of austenite layer and the hardnes of the nitrided samples, were investigated by using optical microscope, X-ray Diffraction (XRD) and microhardness measurements. The results showed that during solution nitriding, nitrogen diffuses through the lattice and grain boundaries and transforms ferrite to austenite phase, with average nitrogen diffusion coefficient of 6.54×10-5 mm2s-1. The thickness of the austenite layer formed on the samples surfaces increased proportional to the square root of the nitriding time, so that after 12 hours niriding, the whole thickness of the ferritic sample with hardness of 262 HV0.1 transformed to austenite with hardness of 420 HV0.1.


The Effect of Nano-magnesia Addition on the Properties of Magnesite-hercynite Refractory Bricks Made with the Use of Convertor Mud
H. Ahmadi, S. Otroj, M. R. Nilforushan, A. Dehghani Varnamkhasti,
Volume 36, Issue 2 (9-2017)
Abstract

In this study, the composition of magnesium aluminate spinle and the converter mud were used as raw materials to in-situ formation of hercynite phase in magnesite-hercynite refractory bricks. The pressed samples were sintered at 1400 and 1500℃ and then, the phase composition of bricks was evaluated after firing at 1400℃. Besides, the effect of nano-magnesia particles addition on the properties of magnesia-hercynite refractory bricks was examined. Hence, the physical peroperties, thermal shock resistance and microstructure of refractory bricks were evaluated. The phase composition results showed that hercynite is well-formed in the refractory matrix, which leads to bonding formation and its increase between magnesia particles. The evaluation of results indicated that the addition of nano-magnesia particles can reduce the porosity of brick via increasing particles packing. In this relation, 1 wt. % nano-magnesia addition was determined as optimum content. Further addition of nano-magnesia leads to increasing of porosity via agglomeration of particles. Due to the high surface area of used nano-magnesia particles, the adequate sintering of refractory brick containing nano-magnesia take places at 1400℃. This leads to increasing of particles bonding and then, increasing mechanical strength, but it can not affect the thermal shock resistance of refractory bricks. The microstructural evaluations showed the lower porosity and further particles bonding with addition of nano-magnesia optimum content.
 


The Effect of Temperature on the Fracture Surface Morphology and Ductility of La55Al25Ni5Cu10Co5 BMG
M. T. Asadi Khanouki, R. Tavakoli , H. Aashuri,
Volume 38, Issue 2 (9-2019)
Abstract
In this research, the effect of temperature on the mean size of fracture surface features, as well as the relation between fracture surface morphologies and ductility of a La-based BMG as a relatively brittle alloy, was systematically investigated. After producing the alloy, three-point bending experiments, over a wide range of temperatures, were conducted on the samples; then the fracture surfaces were analyzed using scanning electron microscopy. The results demonstrated that the width of stable crack growth region (ΔW) was increased upon ductility (δp). Conversely, the mean size of the features on both stable (Ds) and fast (Df) crack growth regions and also, shear offset width (ΔL) were found to decrease with increasing ductility. In this case, the shear band instability was reduced, and the plastic strain could be more homogeneously distributed on the shear bands. The similarity of ΔL and Ds values suggested that the formation of vein pattern was caused by steak-slip behavior and multiple-step sliding inside the shear band through the fluid meniscus instability mechanism. Furthermore, the results obtained from correlation between ductility and fracture surface morphologies in the BMG indicated that the size of features was reduced with increasing ductility.

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