Carbide Precipitation during Processing of Two Low-Alloyed Martensitic Tool Steels with 0.11 and 0.17 V/Mo Ratios Studied by Neutron Scattering, Electron Microscopy and Atom Probe

Abstract

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.

Evolution of iron carbides during tempering of low-alloy tool steel studied with polarized small angle neutron scattering, electron microscopy and atom probe

“…AT-carbides believed to be ε-carbides, together with (Ti, V, Nb)C, and large Mo 6 C were found in the as-quenched material. The latter, Mo 6 C, is too large to be in the detected length scale (DLS) of the SANSPOL experiment [30]. However, both AT-carbides and (Ti, Nb, V)C highlighted in Fig.…”

“…Samples were austenitized and quenched to room temperature (RT), and subsequently tempered at and 600 • C for up to 24 h with intermediate steps of 0 h, 0.5 h, 1 h, and h. The 0 h samples represent heating to tempering temperature and directly quenching to RT. The samples were put into the preheated furnace, and it took about 1 h for the material to reach the tempering temperature of 550 and 600 • C. Details about the manufacturing conditions have been explained in previous work [30].…”

“…The polished surfaces were etched in 2% Nital solution for about 5 s. A 20 nm thin carbon film was sputtered onto the etched surface using a Gatan Precision Etching and Coating System. The carbon film was removed by immersing the sample surface in 10% Nital solution [30]. The EDS analysis was performed using line scans to study the chemical distribution around cementite particles after different tempering times, 0, 1, and 24 h at 550 • C and 600 • C.…”

“…The laser pulse energy was 0.30 nJ, and the sample temperature was 50 K. The IVAS 3.6 software (Cameca) was used for data analysis. Samples for APT analysis were cut into thin strips and electropolished into needles shaped samples [30].…”

“…The scattering intensities, perpendicular to H, for the two polarization states and interference terms for samples tempered for 0 h, 1 h, and The interference term and the corresponding γ-profile as a function of tempering time depend on the evolution in chemical composition, magnetic properties, and the average dimension of the formed precipitates. From the STEM-EDS and APT results, it is known that the ATcarbides consisted of Fe and C, with no dissolution of other alloying elements [30]. These AT-carbides are believed to be replaced by cementite when heating up to a tempering temperature, of 550 • C or 600 • C. With further heating, cementite particles get increasingly enriched in substitutional elements like Cr and Mn while Si is rejected.…”