Carbides in martensitic medium carbon low alloyed tool steels studied with small angle scattering techniques, electron microscopy and atom probe tomography

Abstract: Medium carbon low alloyed tool steels are used today in various areas to shape plastics, nonferrous metals, and steels, and they are crucial in the manufacturing industry. To be effective, tool steels must be strong and tough, 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 of martensitic steels. However, the hardening contribution relates to the size,volume fraction and number density of precipitates, controlled by alloy composition and heat-treating parameters. It is therefore essential to understand how, where, and when the particles nucleate and how the precipitation sequence and kinetics are affected by alloying additions,tempering temperature, and time.This work is aimed to study carbide precipitation in two commercial low-alloy tool steel using small-angle neutron and X-ray scattering. To support these methods, samples were characterized with transmission electronmicroscopy (TEM) and atom probe tomography (APT). With a combination of high-resolution techniques, it was possible to establish the precipitation sequence in these steels. It was also possible with various small angle scattering techniques to determine the evolution of volume fraction and number density of precipitates as a function of tempering parameters.First, small angle neutron scattering (SANS) was used, which is an excellent method for bulk quantification of small precipitates in steel. It was possible with SANS to broadly study the precipitation process depending on annealing temperature and time. However, it is difficult with regular small angle scattering (SAS) to distinguish particle types with overlapping size distributions. To possibly separate the scattering signal from different carbide types, measurements were carried out with polarized small angle neutron scattering (SANSPOL) and anomalous small-angle X-ray scattering(ASAXS). With ASAXS it was possible to isolate the signal from molybdenum-rich carbides from other types of carbides.With SANSPOL, it was possible to follow the enrichment of alloy elements in cementite. The appearance of cementite can be described as an iron-richcore with a chromium-enriched shell. The partitioning of substitutional elements affects the stability of cementite and the alloy carbides. It was also possible with SANSPOL, during heating, to follow the initial precipitation of particles.Results from different experimental methods have been compared with precipitation simulations using thermodynamics-based precipitationmodeling. Equilibrium calculations indicates the possible stability ofdifferent precipitates, and the kinetics are captured with tools such as TCPRISMA to simulate structure evolution during tempering. Hardness measurements were made to correlate structure evolution to mechanical properties.