Simulation and Experimental Based Hardenability Evaluation of Chromium Alloyed Powder Metal Steels

Abstract: Powder metallurgy is a branch of metal forming technology where metal powders are used to manufacture parts and components. It is a flexible and economical technique for manufacturing complicated shapes. This present work focuses on press and sinter technology and forms a part of Höganäs’s efforts of modelling hardenability through quenching. It aims to reduce the number of experimental trials for optimising heat treatment. Hardenability is a measure of how much martensite can be formed during heat treatment, thereby making steels hard, tough and impart strength. The presence of alloying elements like carbon, manganese, chromium, molybdenum, and nickel affects the hardenability of the steel and improves performance like fatigue strength and corrosion resistance. These elements influence the critical cooling rate necessary to form martensite during heat treatment. Component geometry also influences hardenability. Depending on the surface area available to cool, and volume of component, cooling rates may locally be different thereby resulting in an inhomogeneous structure. The work focuses particularly on two grades of powders manufactured by Höganäs AB - Astaloy® CrA and Astaloy® CrS which are evaluated for their hardenability. The aim of this work is to take cooling conditions observed in the actual furnace, use them to predict the amount of martensite present and the martensite start temperature and then compare it with experimental results thereby linking experiment to simulations. For the experimental part, dilatometry was used. Quenching data is obtained from the furnace along with heat capacity of the component and are used as input in Abaqus, which gives us the cooling rates for the component in the furnace. This data is then utilised as an input to dilatometry, where the samples are representative of sections of component. After dilatometry, vital information like martensite start temperature is recorded and metallography is performed, where phase fraction is obtained. Hardness measurements are also performed to verify the phases present. Simulation tools like JMatPro and Thermo-Calc are employed to obtain data for correlation. An extensive study on the difference between them are also studied and presented. The data from simulation and actual experiment is compared and, Ms evaluated from JMatPro and Thermo-Calc for CrA shows a deviation of 12°C. For CrS samples, a higher deviation is observed, with JMatPro showing deviation of 44°C and Thermo-Calc, 52°C in respect to the measured values. For CrA, we observe a fully martensitic structure for the higher carbon samples, including ones alloyed with Ni. For samples with lower carbon, metallographic investigation results in an unclear picture as to if the structure observed is bainite or martensite. CrS samples are mostly martensitic with some bainite present. CrS samples alloyed with Ni and Cu show the least amount of bainite present. The phase fractions predicted by JMatPro show good agreement with results from metallography. Data from microhardness confirms the presence of phases present. Samples with low carbon are softest but show a great improvement in hardness when alloyed. Overall, simulations and actual experimental values are seen to be in good agreement, thereby establishing a strong foundation for future work, where actual components can be evaluated. Quenching conditions observed in the furnace are validated through this work.