Visualizing shear bands in 3-D using axisymmetric sample: An experimental study

In this study a qualitative description of the occurrence of shear bands produced by a sudden impact on an axisymmetric specimen made of medium carbon steel 0.45% C is given. A simple experiment was developed aimed at producing a pinch shear stress in the front side of the test sample in order to visualize shear bands in 3-D. Curve fitting using MATLAB was employed based on the points taken from the images of the front section of the test sample. The predictions of the curve fitting suggests a hyperbolic section leading to the conclusion that within the sample there is a double cone region of material where the shear band region is located on its outer surface. The formation of the shear band is explained by the fact that the interaction of the stress wave front with the free surface of the test sample produces reflection waves that attenuate the incoming stress wave inwards leading to a stress gradient in the plane of the front side of the specimen that causes shear localization. Also, the progressively increasing cross sectional area of the test sample causes the expansion of the wave front, which also results in a stress gradient in the normal direction of the front side of the specimen. So the formation of shear bands depends not only on the impact momentum and strain rates but also on the sample\u27s geometry. \ua9 2015 The Author

Optimizing liquid phase sintering of ferrous powder metallurgical materials

The properties of powder metallurgical, PM, components are dependent on that the dilemma of full density and suitable microstructure is resolved. Whilst it is often possible to achieve full density, it may or may not coincide with a suitable microstructure. Therefore, this thesis has addressed key aspects in the PM processing chain in the viewpoint of achieving a suitable microstructure after sintering. These key aspects have included Properties of Ferrous Powder, Green Body Consolidation, Sintering and Post-sintering Heat Treatments. To do this two ferrous alloy systems have been considered. A pre-alloyed gas atomized high speed steel, HSS, system that has either been loose powder sintered or sintered after a forming process called starch consolidation. Starch consolidation concerns the manufacture of a porous green body by mixing an aqueous-slurry consisting of spherical powder, starch, dispersant and thickener. After drying, these green bodies have been liquid phase sintered in different atmospheres to manipulate the microstructure locally at the surface or globally. The other considered system has concerned die-pressed grey iron powder mixtures consisting of lubricant coated iron powder and ferrosilicon as well as liquid forming additive in powder form for some mixtures. Common to both systems is that the sintering temperature needs to be accurate in order to achieve a successful microstructure. Furthermore, if the heating rate and the cooling rate are controlled for the grey iron powder mixture system, a microstructure with bainite and nodular graphite leads to yield and ultimate tensile strength of 500 and 600 MPa, respectively. For the HSS system conventional heat treatments were not attempted, but the experimental results and thermodynamic modelling results show that the carbon content in the matrix needs to be around 0.6 wt-% C if the desired microstructure of martensite and carbide is to be obtained. This work has demonstrated through model experiments and X-ray photoelectron analysis that the nitrogen pressure, p, in the atmosphere and the bulk nitrogen content CN is related through CN = constant√p if the surface oxide has been reduced first

Tailoring the surface microstructure of starch consolidated vanadium-rich high speed steel powder

The nitrogen sintering of vanadium rich high speed steel has been applied for tailoring of surface microstructure. The approach has been facilitated by thermodynamics modelling of phase equilibria, novel shaping methods like starch consolidation and various manipulations of the sintering atmosphere. It is demonstrated that elevated nitrogen sintering densification occurs when an open pore structure interacts with nitrogen slightly below the nitrogen-reduced solidus temperature. The compact can then first be pre-sintered to around 85% relative density in vacuum, and thereafter be infiltrated with nitrogen at the nitrogen-reduced solidus temperature and finally be sintered to near full density at a temperature of 10% liquid phase formation in nitrogen. The end result is a microstructure with an elevated MX transformation in the surface, whose depth depends on the carbon balance of the material