Formation of Controlled Adiabatic Shear Bands in AISI 4340 High Strength Steel.

摘要

Adiabatic shear banding is one of the predominant failure modes of ultrahigh strength (UHS) steels under high rates of deformation. Though these bands have been previously studied, the specific factors governing when, how, or if a certain material fails by shear band localization are still relatively unknown. This article examines these questions in terms of the microstructure using a controlled shear strain. Four microstructures of equal hardness (Rc52) with different carbide distributions were produced in a VAR 4340 steel by varying the normalizing temperature (one hour at 845 C, 925 C, 1010 C, or 1090 C). A short duration reaustenitizing treatment was subsequently used to produce similar prior austenite grain sizes; this was followed by a 200 C temper. The controlled shear strain was introduced by the use of a hat-shaped specimen loaded in a split Hopkinson bar rig, producing a shear zone at local strain rates up to 5 x 0.00005. The use of mechanical stops to arrest the deformation process allowed the development of the shear bands to be studied under controlled stress conditions. Transmission electron microscopy (TEM) studies revealed a microcrystalline structure within the shear band with a crystalline size of 8nm to 20nm. A gradual change from the microcrystalline structure within the band to lath martensite was observed. Tensile unloading cracks were observed in the sheared regions. A normalizing temperature of 925 C produced the microstructure with the greatest resistance to unstable shearing.