Effects of Silicon on Structures and Properties of AISI 4320Low Alloy Steel.

摘要

The effects of 2percent silicon addition on the structures and properties in AISI 4320steel were studied;transmission electron microscopy was extensively used to characterize the microstructures. In as-quenched and tempered martensite,silicon additions increase the temperature range in which epsilon carbide exists,delay the cementite precipitation and retained austenite decomposition to higher temperatures. The silicon addition increases the yield and ultimate tensile strength,but deteriorates the Charpy impact toughness except in the tempering range 200-300exp 0 C. This is mainly due to more internal twinning structure and more coarse or continuous carbide films precipitation along the martensite lath boundaries. Tempered martensite embrittlement is raised from around 200-300exp 0 C to around 350-550exp 0 C. Different mechanisms are proposed for normal silicon and high silicon steels. In isothermal transformation bainite structures,silicon additions increase the utlimate tensile strength,but decrease the yield strength due to large amounts of retained austenite entrapped between bainitic ferrites. Silicon addition retard the carbide precipitation rate,suppress the carbide precipitation inside the ferrite laths and promotes coarser carbide precipitation along the bainitic ferrite boundaries and so strongly deteriorates the impact toughness property. (ERA citation 01:013609)