Element partitioning in low-carbon Si2Mn2CrMoVNb TRIP-assisted steel in intercritical temperature range

The present paper is aimed at the study of the kinetics of Mn, Si, Cr partitioning in 0.2wt%C-Si2Mn2CrMoVNb TRIP-assisted steel under the annealing at 770 oC and 830 oC to be within the intercritical temperature range. The work was fulfilled using SEM, EDX, dilatometry, and hardness measurements. It was found that under heating a redistribution of the alloying elements between ferrite and austenite took place. Specifically, silicon partitioned to ferrite while chromium diffused to austenite with distribution coefficient values of 1.12-1.21 (KSi) and 0.75-0.86 (KCr). Manganese was found to partition to a much greater extent resulting in a distribution coefficient of KMn=0.38-0.50 and 2.6 times higher concentration in austenite as compared to ferrite. As annealing temperature raised from 770 oC to 830 oC the elemental partitioning was accelerated, followed by the decrease in manganese content in austenite (by 1.44 time) and ferrite (by 1.34 time) caused by an increase in austenite volume fraction. Silicon featured uneven distribution within ferrite to be accumulated at the “martensite/ferrite” interface and near ferrite grain boundaries, while manganese was concentrated in MC carbides. The recommendation for annealing holding was formulated based on elemental partitioning kinetics.

Тепловые эффекты при кристаллизации сплавов с неограниченной и ограниченной растворимостью компонентов

The processes of crystallization of two-component alloys with unrestricted and limited solubility of the constituent components were investigated by the method of differential high-resolution scanning calorimetry. It was found that in all cases a jump in the heat evolution of crystallization that is noticeable in magnitude and speed is recorded directly below the liquidus line. Traditional approaches in describing the development of crystallization processes in the intercritical temperature range do not provide an explanation for the observed phenomena. It is assumed that the observed effects are associated with the formation of a large number of local microvolumes (concentration fluctuations) enriched in the leading crystallization component. Their appearance precedes the beginning of processes of spontaneous formation of a large number of crystals in a noticeable volume of the liquid phase.

Intercritical Quench Hardening of Structural Pipeline Steels

The paper considers the effect of low-carbon pipeline steels initial condition on their mechanical properties and structure after quench hardening at different temperatures in the intercritical temperature range Ac1–Ac3 (ITR) followed by tempering at 600 °C. If the prior heat treatment is annealing or normalizing , which produces the ferrite-pearlite structure, then quenching from temperatures just above Ac1 causes very strong embrittlement due to the formation of a high-carbon austenite film at ferrite/pearlite boundaries. Increasing quenching temperature in the intercritical range increases impact toughness and ductile fracture fraction for both types of prior treatment, though normalizing provides higher impact toughness than annealing. On the contrary, if the prior heat treatment is quench hardening, then the highest impact toughness is observed when the second quenching temperature lies a little above Ac1. Impact toughness and ductile fraction for preliminarily quenched samples gradually decreases along with the increase in austenitization temperature in the intercritical range.