The effects of non-metallic inclusions in nucleating acicular ferrite in steels during cooling from a weld or cooling from an austenitic temperature are reviewed. The influence of the acicular ferrite (AF) structure on mechanical properties of steels such as strength and toughness is briefly mentioned. The different factors affecting the formation of acicular ferrite, such as the soluble content of alloying elements in steel, cooling rate from austenitizing temperature, austenite grain size and inclusion characteristics in steel, are discussed. The mechanisms of acicular ferrite formation on non-metallic inclusions, such as reduction of interfacial energy, mismatch strain between the inclusion and ferrite/austenite, thermal strains at the inclusions and changes in matrix composition near the inclusions are also discussed. Finally, the effects of inclusion characteristics, such as size, number and composition are described and their effectiveness in nucleating acicular ferrite is discussed.

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On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

The conditions for acicular ferrite formation and the overall transformation behaviour of mild and low-alloy steel weld metals are discussed in terms of deoxidation and alloying practice, solidification microstructure, and operational conditions. Selected weld metal microstructure and composition data are critically reviewed, and new semi-empirical correlations for predictions of the final mechanical properties of the weldment under various welding conditions are presented. The results of this review provide a systematic basis for future research which is required if a sound theoretical treatment of weld metal microstructure development is to be obtained.

Microstructural development in mild and low-alloy steel weld metals

A review of precipitation behaviour in AISI type 316 stainless steel

This research work studies the effect of boron contents on the hot flow behavior of boron microalloyed steels. For this purpose, uniaxial hotcompression tests were carried out in a low carbon steel microalloyed with four different amounts of boron over a wide range of temperatures (950, 1000, 1050 and 1100 ◦ C) and constant true strain rates (10 −3 ,1 0 −2 and 10 −1 s −1 ). Experimental results revealed that both peak stress and peak strain tend to decrease as boron content increases, which indicates that boron additions have a solid solution softening effect. Likewise, the flow curves show a delaying effect on the kinetics of dynamic recrystallization (DRX) when increasing boron content. Deformed microstructures show a finer austenitic grain size in the steel with higher boron content (grain refinement effect). Results are discussed in terms of boron segregation towards austenitic grain boundaries during plastic deformation, which increases the movement of dislocations, enhances the grain boundary cohesion and modificates the grain boundary structure. © 2007 Elsevier B.V. All rights reserved.

Hot flow behavior of boron microalloyed steels

Fe-12 Mn alloys undergo failure by catastrophic intergranular fracture when tested at low temperature in the as-austenitized condition, a consideration which prevents their use for structural applications at cryogenic temperatures. The present research was undertaken to identify modifications in alloy composition or heat treatment which would suppress this embrittlement. Chemical and microstructural analyses were made on the prior austenite grain boundaries within the alloy in its embrittled state. These studies failed to reveal a chemical or microstructural source for the brittleness, suggesting that intergranular brittleness is inherent to the alloy in the as-austenitized condition. The addition of 0.002 to 0.01 wt pct boron successfully prevented intergranular fracture, leading to a spectacular improvement in the low temperature impact toughness of the alloy. Autoradiographic studies suggest that boron segregates to the austenite grain boundaries during annealing at temperatures near 1000 °C. The cryogenic toughness of a Fe-12Mn-0.002B alloy could be further improved by suitable tempering treatments. However, the alloy embrittled if inappropriate tempering temperatures were used. This temper embrittlement was concom-itant with the dissolution of boron from the prior austenite grain boundaries, which reestablishes the intergranular fracture mode.

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The use of a boron addition to prevent intergranular embrittlement in Fe-12Mn

The effect of boron concentration on the surface and grain-boundary energies of AISI 316 steel at temperatures in the range 950–1250°C has been examined. Four ingots were used with B contents of 0.001 and 0.006 or 0.0065 wt.-% and C contents of 0.12 and 0.05 or 0.06 wt.-%. The grain-boundary energies were decreased by 12–39% on increasing the B concentration but the effect on the surface energies was less clear. Grain-boundary grooves were formed by a surface diffusion process at 950–1150°C but no consistent effect of B on the diffusion coefficient was apparent.

M. G. Nicholas

Papers

Surface and grain-boundary energies of AISI 316 stainless steel in the presence of boron