The Effects of Heating Rate on Austenite Grain Growth in a Ti-modified SAE 8620 Steel with Controlled Niobium Additions

Abstract

The effects of heating rate on austenite grain growth and precipitate distribution in Ti-modified SAE 8620 steels with Nb additions of 0.02, 0.06 and 0.1 wt% were evaluated with pseudo-carburizing, i.e. without a carburizing gas, heat treatments characteristic of high temperature vacuum carburizing. Laboratory plates were produced to simulate conventional hot-rolling and controlled-rolling processes. Specimens were heated at rates between 10 and 145°C min Ϫ1 to 1 050 and 1 100°C, held at the desired austenitizing (carburizing) temperature for 60 min, and immediately quenched in iced-water. Austenite grain structures developed at the austenitizing temperatures were evaluated with light optical metallography, and precipitate dispersions were evaluated using extraction replicas in the transmission electron microscope. Abnormal grain growth was observed in all samples processed at the highest heating rate to 1 050°C, but was suppressed at the lower heating rates with additions of 0.06 and 0.1Nb. Suppression of abnormal grain growth was correlated with the development of a critical distribution of fine NbC precipitates, stable at the austenitizing temperature. The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed.

Effect of Annealing Before Cold Forging on the Behavior of Abnormal Grain Growth during Carburizing

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In addition to the steel composition, forging conditions also greatly influence the distribution of precipitate particles and γ grains. According to Fujimatsu et al,18) abnormal grain growth tends to occur in a region where the shear strain introduced by cold forging is high (hereinafter referred to as macro shear band).…”

The influence of niobium content on austenite grain growth in microalloyed steels

“…It is generally accepted that the grain boundary migration velocity (v) is proportional to the driving force (Seok et al, 2014;Maalekian et al, 2012;Yue et al, 2010;Olasolo et al, 2011;Alogab et al, 2007;Hodgson and Gibbs, 1992;Nanba et al, 1992), an approximation justified at the relatively small driving forces involved in grain growth. This velocity is given by the expression (Stumpf, 2010) [1]…”

“…Effective grain growth control is reported to be achieved through addition of precipitate-forming elements, such as Nb that, slow down the grain boundary migration through pinning and solute drag mechanisms (Yu et al, 2010;Olasolo et al, 2011;Alogab et al, 2007). Much work has been carried out on austenite grain lsize control by the addition of precipitate-forming elements that have a strong affinity for interstitial elements, such as carbon and nitrogen, which form the dispersed pinning particles to inhibit the austenite grain growth (Alogab et al 2007;Hodgson and Gibbs, 1992;Nanba et al, 2003;Flores and Martinez, 1997;Rollett, Srolovitz and Anderson, 1989). Austenite grain growth can be described using a conventional or a modelling approach.…”

Optimizing Gear Performance by Alloy Modification of Carburizing Steels

“…Many studies have been indicating that prior austenite grain size control in carburizing steel can be effectively achieved by using niobium microalloying in combination with other microalloys such as titanium, aluminum and nitrogen [13][14][15][16][17][18][19][20]. The developed concepts have been used to generally refine and homogenize the grain size under standard case carburizing conditions.…”