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.

Verktygsstål används idag inom en mängd olika områden för att förädla plaster, metaller och stål, och är således viktiga inom tillverkningsindustrin. Utmärkande egenskaper för verktygsstål är kombinationen av styrka,seghet, nötningsbeständighet och värmebeständighet vid applikationen. För att nå dessa egenskaper legeras materialen så att partiklar av sekundära faser skiljs ut under tillverkningen, framför allt under anlöpning av martensitiska stål. Vid anlöpning genomgår materialet en isotermvärmebehandling vid500–600°C under ca 1–4 timmar. Detta är oftast tillräckligt för att nå den optimala kombinationen av små partiklar (1–10 nm) med hög nummertäthet som effektivt härdar materialet. Det optimala härdningsbidraget varierar dock beroende på legeringssammansättning och anlöpningsparametrar. Det är således viktigt att förstå hur, var och när partiklarna kärnbildas, och hur partikelutskiljningen påverkas av legeringstillsatser och anlöpningstemperatur och tid. Detta är viktig information som kan användasför en mer effektiv legeringsutveckling.

Målet med detta projekt har varit att studera partikelutskiljning i två kommersiella låglegerade verktygsstål med hjälp av lågvinkelspridning av neutroner (SANS) och röntgenstrålar (SAXS). För att underlätta utvärderingen av spridningsdata har prover även utvärderats i transmissionselektronmikroskop och med atomsondstomografi. Olika varianter av lågvinkelspridning har använts, både ex-situ och in-situ, för att utvärdera möjligheten studera utskiljning i kommersiella verkstygstål. Till en början utvärderades prover med lågvinkelspridning av neutroner(SANS), med vilka det var möjligt att i stora drag studera utskiljningsförloppet beroende på anlöpningstemperatur och tid. Dock fanns det osäkerheter kring resultaten kopplat till överlappande storleksfördelningar av olika karbidtyper. För att bättre kunna separera spridningssignalen från olika karbidvarianter genomfördes mätningar med ”anomal lågvinkel röntgenspridning” (ASAXS). Med vilket det var möjligtatt isolera signalen från molybdenrika karbider från övriga typer av karbider.Med lågvinkelspridning av polariserade neutroner (SANSPOL) var det möjligt att följa anrikningen av legeringselement i cementit karbider, vars utseende kan beskrivas som en järnrik kärna med ett kromanrikat skal, vilket tros påverka både stabiliteten av cementit och utskiljningsförloppet av legeringskarbider. Det var även möjligt att med SANSPOL, under uppvärmning, följa den initiala utskiljningen av partiklar. Resultaten jämfördes med jämvikts- och utskiljningsberäkningar med Thermo-Calcoch TC-PRISMA. Mekaniska egenskaper korrelerades med hjälp avhårdhetsmätningar.

Precipitation of carbides after tempering of a medium carbon low alloyed Cr-Mo-V tool steel at 600 degrees C was studied with scanning precession electron diffraction (SPED) in a transmission electron microscope (TEM). The precipitation sequence was evaluated by mapping the carbide distribution on carbon extraction replicas prepared from samples tempered for different durations of up to 24 h. The SPED results were supplemented by equilibrium calculations and energy dispersive x-ray spectroscopy (EDX) measurements in a TEM. It was found that e-Fe2C precipitates within martensite laths during quenching via auto-tempering. During reheating to tempering temperature e-Fe2C was dissolved and replaced by cementite, 0-M3C, which predominately form on martensite boundaries. It was further found that the small carbides in the early stage of tempering have predominantly a cubic MC structure, even if the V/Mo-ratio of the studied steel was only 0.12, and it is known from literature that V and Mo form cubic MC or hexagonal M2C carbides, respectively. Later during tempering more stable carbides, such as M7C3 and M23C6, also form, and it was concluded that the M7C3 form both by separate nucleation and precipitation on the cementite/matrix interface. The latter phenomenon was seen as particles with a core of cementite and a shell of M7C3 after 24 h of tempering.

Two industrially processed low-alloyed martensitic tool steel alloys with compositions Fe-0.3C-1.1Si-0.81Mn-1.5Cr-1.4Ni-1.1Mo-0.13V and Fe-0.3C-1.1Si-0.81Mn-1.4Cr-0.7Ni-0.8Mo-0.14V (wt.%) were characterized using small-angle neutron scattering (SANS), scanning electron microscopy (SEM), Scanning transmission electron microscopy (STEM), and atom probe tomography (APT). The combination of methods enables an understanding of the complex precipitation sequences that occur in these materials during the processing. Nb-rich primary carbides form at hot working, while Fe-rich auto-tempering carbides precipitate upon quenching, and cementite carbides grow during tempering when Mo-rich secondary carbides also nucleate and grow. The number density of Mo-rich carbides increases with tempering time, and after 24 h, it is two to three orders of magnitude higher than the Fe-rich carbides. A high number density of Mo-rich carbides is important to strengthen these low-alloyed tool steels through precipitation hardening. The results indicate that the Mo-rich secondary carbide precipitates are initially of MC character, whilst later they start to appear as M2C. This change of the secondary carbides is diffusion driven and is therefore mainly seen for longer tempering times at the higher tempering temperature of 600 ◦C.

The magnetic scattering of iron carbides in low-alloy tool steel was investigated ex-situ by polarized small angle neutron scattering measurements after tempering the steel at 550 °C and 600 °C. Magnetic features could be detected in the as-quenched sample resulting in a negative interference term, believed to be either θ-Fe3C, η-Fe2C, or ε-Fe2-3C. During tempering the evolution of cementite could be studied by the variation of the interference term and in γ-ratio, which is the ratio of the magnetic to nuclear scattering length density contrast. From scanning transmission electron microscopy (STEM) and atom probe tomography, it is evident that cementite (θ-Fe3C) is present directly when reaching the tempering temperature of either 550 °C or 600 °C. At longer tempering times, cementite gets enriched with substitutional elements like chromium and manganese, forming an enriched shell on the cementite particles. STEM and energy dispersive x-ray spectrometry show that the chemical composition of small cementite particles approaches that of Cr-rich M7C3 carbides after 24 h at 600 °C. It is also seen that small non-magnetic particles precipitate during tempering and these correspond well with molybdenum and vanadium-rich carbides.

The strength of tempered martensite in low alloyed tool steel depends on the precipitation of secondary carbides. Tempering parameters such as time and temperature together with heating rate to tempering temperature will influence the precipitation sequence and these critical process parameters must be controlled to achieve ultimate precipitation strengthening. In this work precipitation was studied in a low alloyed tool steel using in-situ polarized small angle scattering (SANSPOL) during heating and isothermal holding. It was possible with SANSPOL to study the initial precipitation of secondary Mo- and V-rich carbides. It was found that Mo- and V-rich carbides form above 500 °C, simultaneously to the formation of cementite, which started to form already at lower temperature. It was furthermore found that the cementite particles can be described as having a core-shell structure, the Mo-V-rich carbides small and were approximated as spherical. Precipitation simulations performed using TC-PRSIMA were finally performed to compare with the experimental results, and the simulations were found to describe the experimental results well. 

Precipitation of carbides after tempering at 600 °C of a medium carbon low alloyed tool steel was studied with scanning precession electron diffraction (SPED). The precipitation sequence was evaluated by mapping the carbide distribution on carbon extraction replicas. This gave detailed information about formed precipitates which could be identified based on electron diffraction. The experimental work was compared to equilibrium calculations and energy dispersive x-ray spectroscopy (EDX). It was found that precipitates ε-Fe2C within laths during quenching via auto-tempering. During reheating to tempering temperature ε-Fe2C was dissolved and replaced by cementite, θ-M3C, which predominately form on martensitic lath boundaries. Vanadium and molybdenum are known from literature to form cubic MC or hexagonal M2C carbides, respectively. In this work it was found that the small carbides in the early stage of tempering have predominantly a cubic MC structure, even if the V/Mo-ratio the studied steel was only 0.12. Later during tempering more stable carbides, such as M7C3 and M23C6, also form, and it was concluded that the M7C3 form both by separate nucleation and in-situ transformation on cementite. The latter phenomenon was seen as particles with a core of cementite and shell of M7C3 after 24 h at 600 °C of tempering. 

Anomalous small angle X-ray scattering was used to study Mo (Mo) rich secondary carbides during tempering of medium carbon low alloyed tool steel. The scattering contrast of Mo-rich carbides varies systematically close to the absorption edge of Mo. No anomalous effect could be seen for the as-quenched sample. However, a clear effect could be seen for all tempered conditions. By studying the energy dependency of scattering invariant, Mo-rich carbides are believed to have a MC structure in the early stages of tempering. The  -ratio is decreasing with time which is believed to be related to the formation M2C carbides. This is more pronounced for samples tempered at 600 °C as compared to 550 °C. The volume fraction was calculated using the ASAXS gradient method. At 550 °C the volume fraction of (Mo,V)C was increasing up to 24 h of tempering to 1.7·10-3. At 600 °C tempering a maximum in volume fraction was reached already after 1 h with 1.6·10-3 fraction. The size and number density of carbides are more stable at 550 °C compared to 600 °C.