Effects of second-phase particles on coarsening of austenite in 0.15 Pct carbon steels
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INTRODUCTION
THE strength and toughness
of high-strength, low-alloy structural steels with ferrite-pearlite structures are greatly influenced by ferrite grain size. Fine ferrite grains are beneficial for both yield strength and toughness. El'2'3J Refining the prior austenite grain size is a prerequisite for fine ferrite grains, because of ferritic nucleation on austenite grain boundaries. Austenite grain growth can be inhibited by micro-alloying additions which cause the appearance of second-phase particles in austenite. Initially, grain refining was done with the addition of aluminum and nitrogen through the formation of aluminum nitride precipitates. [4] Later, other additives such as niobium, vanadium, or titanium were also shown to produce austenite grain refinement. 15-9] But the particles responsible in this case are carbonitrides of vanadium, niobium, and titanium. The presence of these second-phase particles dramatically changes the grain-coarsening characteristics due to their pinning effect on the austenite grain boundaries. The pinning mechanism between the grain boundary and the second-phase particle is largely due to the reduction of the grain boundary area and hence energy when the grain boundary intersects the second phase particles. [1~ Any movement of the grain boundary away from the particle would result in a local energy increase and have a drag effect on the migrating boundary. The binding force between particle and boundary is greater than is available by thermal activation, ln] Energy changes accompanying the migration of a pinned boundary[12] have shown that the criterion for unpinning may be expressed as: r* =
6R0f~3 7r \ - 2 - -
2 ) -1 [1]
where r* is the critical particle radius, i.e., the maximum MD. MOHAR ALl BEPARI, formerly with the Department of Metallurgy, The University of Sheffield, Sheffield, England, is Associate Professor, Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh. Manuscript submitted September 11, 1987. METALLURGICALTRANSACTIONS A
size of the particle that will effectively counteract the driving force, f is the volume fraction of the particles in the microstructure, R 0 is the matrix grain radius (average), and Z is the ratio of the radii of the growing grain and the matrix grains. For most polygonal structures, experimental w o r k [13A4'15] suggests that a Z value of 1.5 would-be reasonable. In Eq. [ 1] the critical particle radius increases as either the volume fraction of particles or matrix grain size increases. Increasing the grain size heterogeneity decreases the critical particle radius. At some critical particle size, the driving force for grain growth equals the pinning force. If the particle radius increases further, the driving force exceeds the pinning force. Thus, fine particles are more effective than coarse particles in restricting grain growth. For effective pinning the specific alloy addition must be selected to produce fine particles. A fine initial particle size is desirable to prol
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