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Carbides in martensitic mediumcarbon low alloyed tool steels studiedwith small angle scatteringtechniques, electron microscopy andatom probe tomography

Time: Wed 2023-03-29 10.00

Location: F3 , Lindstedtsvägn 26

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Language: English

Subject area: Materials Science and Engineering

Doctoral student: Erik Claesson , Egenskaper

Opponent: Assoc. Professor Erik Offerman, TU Delft

Supervisor: Professor Peter Hedström, Strukturer, Egenskaper; Dr. Hans Magnusson, Swerim AB

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Medium carbon low alloyed tool steels are used today in various areas toshape plastics, nonferrous metals, and steels, and they are crucial in themanufacturing industry. To be effective, tool steels must be strong andtough, and have high wear resistance and temperature stability. To achievethe desired properties, materials are alloyed so that secondary phaseparticles precipitate during processing, especially during the tempering ofmartensitic steels. However, the hardening contribution relates to the size,volume fraction and number density of precipitates, controlled by alloycomposition and heat-treating parameters. It is therefore essential tounderstand how, where, and when the particles nucleate and how theprecipitation sequence and kinetics are affected by alloying additions,tempering temperature, and time.This work is aimed to study carbide precipitation in two commercial lowalloytool steel using small-angle neutron and X-ray scattering. To supportthese methods, samples were characterized with transmission electronmicroscopy (TEM) and atom probe tomography (APT). With a combinationof high-resolution techniques, it was possible to establish the precipitationsequence in these steels. It was also possible with various small anglescattering techniques to determine the evolution of volume fraction andnumber density of precipitates as a function of tempering parameters.First, small angle neutron scattering (SANS) was used, which is an excellentmethod for bulk quantification of small precipitates in steel. It was possiblewith SANS to broadly study the precipitation process depending onannealing temperature and time. However, it is difficult with regular smallangle scattering (SAS) to distinguish particle types with overlapping sizedistributions. To possibly separate the scattering signal from differentcarbide types, measurements were carried out with polarized small angleneutron scattering (SANSPOL) and anomalous small-angle X-ray scattering(ASAXS). With ASAXS it was possible to isolate the signal frommolybdenum-rich carbides from other types of carbides.With SANSPOL, it was possible to follow the enrichment of alloy elementsin cementite. The appearance of cementite can be described as an iron-richcore with a chromium-enriched shell. The partitioning of substitutionalelements affects the stability of cementite and the alloy carbides. It was alsoiiipossible with SANSPOL, during heating, to follow the initial precipitationof particles.Results from different experimental methods have been compared withprecipitation simulations using thermodynamics-based precipitationmodeling. Equilibrium calculations indicates the possible stability ofdifferent precipitates, and the kinetics are captured with tools such as TCPRISMAto simulate structure evolution during tempering. Hardnessmeasurements were made to correlate structure evolution to mechanicalproperties.