Austenite Grain Growth and Precipitate Evolution in a Carburizing Steel with Combined Niobium and Molybdenum Additions
- PDF / 3,109,589 Bytes
- 21 Pages / 593.972 x 792 pts Page_size
- 17 Downloads / 255 Views
TRODUCTION & BACKGROUND
MICROALLOY additions are increasingly employed for the control of microstructure to achieve desired properties in automotive, structural, and engineering steels. In manufacturing processes that require reheating, e.g., slab and ingot reheating after casting and high-temperature carburizing, the effect of microalloying additions such as niobium (Nb) and titanium (Ti) on austenite grain size at elevated temperatures is of importance as abnormal coarsening of the austenite grain structure can result in the general degradation of fatigue performance and toughness.[1] The coarsening of microalloy precipitates in austenite is thought to be the controlling factor in the onset of abnormal grain growth, and solute drag is assumed to be of secondary significance.[2–4] Thus, microalloyed steel variants have been developed which employ fine Nb- and/or Ti–rich carbonitrides to suppress grain growth and provide resistance to austenite grain coarsening.[2,4–9] This study investigates the effectiveness of both molybdenum (Mo) CHARLES M. ENLOE, Materials Engineer, is with General Motors LLC, GM Product Development«Materials Engineering, Warren, MI 48090. Contact e-mail: [email protected] KIP O. FINDLEY, Associate Professor, and JOHN G. SPEER, John Henry Moore Distinguished Professor, Director, are with the George S. Ansell Department of Metallurgical and Materials Engineering, Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Manuscript submitted January 16, 2015. Article published online August 27, 2015 5308—VOLUME 46A, NOVEMBER 2015
additions and thermal processing to increase the coarsening resistance of Nb-rich precipitate distributions in austenite and delay the associated onset of abnormal grain growth. A. Grain Refinement with Microalloy Additions The mechanism of grain boundary pinning was first quantified by Zener with the classic expression shown in Eq. [1][3] r R¼n ; ½1 f where R is the average radius of a shrinking grain pinned by a two-dimensional precipitate array of volume fraction, f, and average radius, r. Equation [1] demonstrates that austenite grain size can be stabilized by second phase precipitates when the volume fraction of precipitates is sufficiently high and their size sufficiently small. In Zener’s model n = 4/3, but more sophisticated derivations incorporating grain size uniformity have produced the pre-factor relation shown in Eq. [2].[3] p 3 2 n¼ ½2 6 2 Z The size advantage parameter, Z, a measure of grain size uniformity, is defined as the ratio of the largest grain radius to average grain radius and typically ranges from Z = 2 to Z = 2. Equations [1] and [2] demonstrate METALLURGICAL AND MATERIALS TRANSACTIONS A
that greater grain size uniformity is desirable for resistance to the onset of abnormal grain growth at a given r/f value. The r/f value is a time-dependent function of the precipitate size distribution evolution, and abnormal grain coarsening resistance is, therefore, believed to be limited by the coarsening of
Data Loading...
