The effect of austenitising and tempering practice on the microstructure and mechanical properties of two martensitic stainless steels was examined with the aim of supplying heat treatment guidelines to the consumer or fabricator that, if followed, would result in a martensitic structure with minimal retained austenite, evenly dispersed carbides and a hardness of between 610 HV and 740 HV (hardness on the Vickers scale) after quenching and tempering. The steels examined during the course of this examination conform in composition to medium-carbon AISI type 420 martensitic stainless steel, except for the addition of 0.13% vanadium and 0.62% molybdenum to one of the alloys. The effect of various austenitising and tempering heat treatments was examined. Steel samples were austenitised at temperatures between 1000°C and 1200°C, followed by quenching in oil. The as-quenched microstructures were found to range from almost fully martensitic structures to martensite with up to 35% retained austenite after quenching, with varying amounts of carbide precipitates. The influence of tempering, double tempering, and sub-zero treatment was investigated. Optical and scanning electron microscopy was used to characterise the as-quenched microstructures, and X-ray diffraction analysis was employed to identify the carbide present in the as-quenched structures and to quantify the retained austenite contents. Hardness tests were performed to determine the effect of heat treatment on mechanical properties. As-quenched hardness values ranged from 700 HV to 270 HV, depending on the amount of retained austenite. Thermodynamic predictions (using the CALPHAD™ model) were used to explain these microstructures based on the solubility of the carbide particles in the matrix at various austenitising temperatures. The carbide particles were found to be mainly in the form of M7C3 at elevated temperatures, transforming to M23C6 on cooling. / Dissertation (MSc)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted

Experiments on the production of porous metallic materials were performed on Fe-base and Al-base alloys. The method involves dissolution of gases in the liquid state and solidification at various cooling rates. The alloy compositions were selected to induce solidification of primary particles intended to control the pore distribution. For the Fe-base alloys, nitrogen was introduced into the melt by dissolution of chromium nitride powder. Fe-Cr-Mn-Si-C alloys featuring M7C3 carbide particles were selected. For the Al-base alloys, hydrogen gas was dissolved into the melt by decomposition of water vapor. Al-Ti and Al-Fe alloys featuring primary Al3Ti and Al3Fe intermetallic particles, respectively, were considered. In the Fe-base alloys, a homogeneous distribution of gas pores through the specimens’ volume was obtained at high cooling rate (water quenching) and after introduction of external nucleating agents. In the case of the Al-base alloys, a good pore distribution was observed at all cooling rates and without addition of nucleating agents. Calculations of the variation of nitrogen (respectively hydrogen) solubility based on Wagner interaction parameters suggest that pore nucleation and growth occur during precipitation of the primary particles (M7C3 carbides, Al3Ti or Al3Fe intermetallics), due to composition changes in the melt and resultant supersaturation with gas atoms. Microscopic analyses revealed that the primary particles control the pore growth in the melt and act as barriers between adjacent pores, thereby preventing pore coalescence and promoting a fine pore distribution. Uniaxial compression testing of the porous Al-Ti and Al-Fe materials showed the typical compressive behavior of cellular metals. Further work is needed to improve the quality and reproducibility of the porous structures which can possibly be used in energy absorption or load-bearing applications. As a corollary result of the quenching of hypereutectic Fe-Cr-Mn-Si-C alloys in the experiments of synthesis of porous metals, a homogeneous featureless structure was observed in some parts of the samples, instead of the equilibrium structure of M7C3 and eutectic phases. Subsequent investigations on rapid solidification of Fe-base alloys at various alloy compositions and cooling rates led to the formation of a single-phase structure for the composition Fe-8Cr-6Mn-5Mo-5Si-3.2C (wt.%), at relatively low cooling rates (≈103 K/s) and for large sample dimensions (2.85 mm). The single phase, which is likely to be the hcp ɛ-phase, was found to decompose into a finely distributed structure of bainite and carbides at ≈600 °C. The annealed structure showed very high hardness values (850 to 1200 HV), which could be exploited in the development of high-strength Fe-base materials. / QC 20100809

High chromium cast iron (HCCI) is considered as one of the most useful wear resistance materials and their usage are widely spread in industry. The wear resistance and mechanical properties of HCCI mainly depend on type, size, number, morphology of hard carbides and the matrix structure (γ or α). The Hypereutectic HCCI with large volume fractions of hard carbides is preferred to apply in wear applications. However, the coarser and larger primary M7C3 carbides will be precipitated during the solidification of the hypereutectic alloy and these will have a negative influence on the wear resistance. In this thesis, the Ti-added hypereutectic HCCI with a main composition of Fe-17mass%Cr-4mass%C is quantitatively studied based on the type, size distribution, composition and morphology of hard carbides and martensite units. A 11.2μm border size is suggested to classify the primary M7C3 carbides and eutectic M7C3 carbides. Thereafter, the change of the solidification structure and especially the refinement of carbides (M7C3 and TiC) size by changing the cooling rates and Ti addition is determined and discussed. Furthermore, the mechanical properties of hypereutectic HCCI related to the solidification structure are discussed. Mechanical properties of HCCI can normally be improved by a heat treatment process. The size distribution and the volume fraction of carbides (M7C3 and TiC) as well as the matrix structure (martensite) were examined by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Especially for the matrix structure, EBSD is a useful tool to classify the fcc (γ) and bcc (α) phases. In conclusion, low holding temperatures close to the eutectic temperature and long holding times are the best heat treatment strategies in order to improve wear resistance and hardness of Ti-alloyed hypereutectic HCCI. / <p>QC 20121130</p>

High chromium cast iron (HCCI) is considered as one of the most useful wear resistance materials and their usage are widely spread in industry. The mechanical properties of HCCI mainly depend on type, size, number, morphology of hard carbides and the matrix structure (γ or α). The hypereutectic HCCI with large volume fractions of hard carbides is preferred to apply in wear applications. However, the coarser and larger primary M7C3 carbides will be precipitated during the solidification of the hypereutectic alloy and these will have a negative influence on the wear resistance. In this thesis, the Ti-alloyed hypereutectic HCCI with a main composition of Fe-17mass%Cr-4mass%C is studied based on the experimental results and calculation results. The type, size distribution, composition and morphology of hard carbides and martensite units are discussed quantitatively. For a as-cast condition, a 11.2μm border size is suggested to classify the primary M7C3 carbides and eutectic M7C3 carbides. Thereafter, the change of the solidification structure and especially the refinement of carbides (M7C3 and TiC) size by changing the cooling rates and Ti addition is determined and discussed. Furthermore, the mechanical properties of hypereutectic HCCI related to the solidification structure are discussed. Mechanical properties of HCCI can normally be improved by a heat treatment process. The size distribution and the volume fraction of carbides (M7C3 and TiC) as well as the matrix structure (martensite) were examined by means of scanning electron microscopy (SEM), in-situ observation by using Confocal Laser Scanning Microscope (CLSM), Transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). Especially for the matrix structure and secondary M7C3 carbides, EBSD and CLSM are useful tools to classify the fcc (γ) and bcc (α) phases and to study the dynamic behavior of secondary M7C3 carbides. In conclusion, low holding temperatures close to the eutectic temperature and long holding times are the best heat treatment strategies in order to improve wear resistance and hardness of Ti-alloyed hypereutectic HCCI. Finally, the maximum carbides size is estimated by using statistics of extreme values (SEV) method in order to complete the size distribution results. Meanwhile, the characteristic of different carbides types will be summarized and classified based on the shape factor. / <p>QC 20130913</p>