TY - BOOK

T1 - Chemical changes of TiC particles in a tool steel matrix alloy during hot isostatic pressing

AU - Pörnbacher, Josef

N1 - embargoed until 05-03-2025

PY - 2020

Y1 - 2020

N2 - The goal of this work is the investigation of chemical and structural changes of TiC particles in a tool steel matrix alloy during solid-state consolidation by means of hot isostatic pressing and subsequent steps of heat treatment. Only little information is available in literature regarding the chemical stability of TiC particles in an alloyed steel matrix during HIPing at a typical temperature of 1140 °C. This demands a detailed investigation of chemical reactions between the involved phases during HIPing. For this reason the current work focusses on changes regarding the chemical composition and structure of TiC particles in different size ranges in a steel alloy during consolidation. A challenge here was the evaluation of different powder mixing techniques and their influence on TiC particle size in the final microstructure of the metal matrix composite. The methodological approach is (1) design of an alloyed steel matrix that is hardenable similar to a high speed steel and additionally shows a low reactivity towards TiC, (2) to select suitable powders (e.g. powder size distribu-tion) and a powder metallurgical process that allows for the incorporation of TiC powders with different particle sizes and (3) investigation of test samples’ microstructures with methods capable of resolving changes in TiC structure and composition such as SEM, TEM and HEXRD. The according results are (1) contrary to currently available literature, TiC particles were found to be thermodynamically instable during solid-state HIP consolidation and subsequent heat treatment, (2) TiC partially transformed into a mixed MC carbide which additionally contained V, Mo and W (3) transformation rate increases with absolute TiC particle surface, (4) increasing mixed carbide formation rates lead to a lower concentration of matrix elements in these carbides and (5) previously suggested solid-state formation mechanisms in other MMCs seem unsuitable to explain the observed phenomena. Consequently, this work yields a new theory based on nucleation of a mixed carbide onto TiC surfaces and subsequent growth. Conclusions are (1) when developing PM MMCs with steel matrix and TiC hard phase, Ti-based mixed carbide phases need to be expected even when solid state consolidation methods are applied, (2) previous investigations might have been unable to identify TiC transformation due to low transformation rates caused by the use of coarse TiC particles, unappropriated heat treatment exacerbating the differentiation with matrix carbides, or the use of investigation methods with insufficient resolutions and (3) the proposed mixed carbide formation mechanism fits the observed phenomena.

AB - The goal of this work is the investigation of chemical and structural changes of TiC particles in a tool steel matrix alloy during solid-state consolidation by means of hot isostatic pressing and subsequent steps of heat treatment. Only little information is available in literature regarding the chemical stability of TiC particles in an alloyed steel matrix during HIPing at a typical temperature of 1140 °C. This demands a detailed investigation of chemical reactions between the involved phases during HIPing. For this reason the current work focusses on changes regarding the chemical composition and structure of TiC particles in different size ranges in a steel alloy during consolidation. A challenge here was the evaluation of different powder mixing techniques and their influence on TiC particle size in the final microstructure of the metal matrix composite. The methodological approach is (1) design of an alloyed steel matrix that is hardenable similar to a high speed steel and additionally shows a low reactivity towards TiC, (2) to select suitable powders (e.g. powder size distribu-tion) and a powder metallurgical process that allows for the incorporation of TiC powders with different particle sizes and (3) investigation of test samples’ microstructures with methods capable of resolving changes in TiC structure and composition such as SEM, TEM and HEXRD. The according results are (1) contrary to currently available literature, TiC particles were found to be thermodynamically instable during solid-state HIP consolidation and subsequent heat treatment, (2) TiC partially transformed into a mixed MC carbide which additionally contained V, Mo and W (3) transformation rate increases with absolute TiC particle surface, (4) increasing mixed carbide formation rates lead to a lower concentration of matrix elements in these carbides and (5) previously suggested solid-state formation mechanisms in other MMCs seem unsuitable to explain the observed phenomena. Consequently, this work yields a new theory based on nucleation of a mixed carbide onto TiC surfaces and subsequent growth. Conclusions are (1) when developing PM MMCs with steel matrix and TiC hard phase, Ti-based mixed carbide phases need to be expected even when solid state consolidation methods are applied, (2) previous investigations might have been unable to identify TiC transformation due to low transformation rates caused by the use of coarse TiC particles, unappropriated heat treatment exacerbating the differentiation with matrix carbides, or the use of investigation methods with insufficient resolutions and (3) the proposed mixed carbide formation mechanism fits the observed phenomena.

KW - metal matrix composite

KW - titanium carbide

KW - hot isostatic pressing

KW - Verbundwerkstoff

KW - Metallmatrix

KW - TiC

KW - Heißisostatisches Pressen

M3 - Doctoral Thesis

ER -