Partitioning Of Substitutional Elements Research Articles

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

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-Fe 3 C, η-Fe 2 C, or ε-Fe 2-3 C. 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-Fe 3 C) 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 M 7 C 3 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. • SANSPOL was used to separate the signal between cementite and alloy carbides. • The evolution of magnetic properties was recorded with SANSPOL. • The evolution of cementite during tempering was captured with APT and TEM. • Partitioning of substitutional elements in cementite could lead to in-situ transformation.

Intercritically Austenitized Quenched and Tempered Ductile Iron

Ductile iron with a matrix of ferrite plus martensite was produced using a ductile iron alloyed with ∼0.7 wt% manganese and ∼0.5 wt% nickel. Three different volume percentages of martensite (16, 24 and 37 vol %) were formed by austenitizing in the intercritical region (ferrite + austenite) followed by quenching in a polymeric solution to room temperature. The materials were evaluated in the untempered condition and after tempering at 400C (752F) and 500C (932F) for one hour. This paper reports the effects of volume percent martensite and tempering conditions on microstructure and tensile properties. Semi-quantitative chemistry and microhardness of ferrite and martensite were also determined as a function of martensite volume percent. The results indicated that the carbon available to form the austenite during intercritical austenitizing came mainly from the carbon in the matrix and that partitioning of substitutional elements is limited. The results also showed that ultimate and yield strength and hardness increased and elongation decreased as the volume fraction of martensite increased. Tempering significantly increased the elongation with only a small decrease in the strengths.

Understanding of the Bainite Transformation in a Nano-Structured Bainitic Steel

A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 200°C for 10 days to form a nano-scale bainitic microstructure consisting of nanobainitic ferrite laths with high dislocation density and retained austenite films. The crystallographic analysis using TEM and EBSD revealed that the bainitic ferrite laths are close to the Nishiyama-Wassermann orientation relationship with the parent austenite. There was only one type of packet identified in a given transformed austenite grain. Each packet consisted of two different blocks having variants with the same habit plane, but different crystallographic orientations. The presence of fine C-rich clusters and Fe-C carbides with a wide range of compositions in bainitic ferrite was revealed by Three-dimensional Atom Probe Tomography (APT). The high carbon content of bainitic ferrite compared to the para-equilibrium level of carbon in ferrite, absence of segregation of carbon to the austenite/bainitic ferrite interface and absence of partitioning of substitutional elements between the retained austenite and bainitic ferrite were also found using APT.