An Analysis on the Inhomogeneous Microstructure in Crack Healing Area

Abstract

Three hypo-eutectoid steels were chosen to study the inhomogenoues microstructure in crack healing area. Internal cracks were made in 20, 20MnMo and 45 steel samples. The precracks in 20 and 20MnMo steel can mostly be healed under the conditions of hot plastic deformation in which the reduction of hot pressing were 35.6% and 44.9% respectively, and the healing area is a ferrite band. The process of crack healing in crack tips in 45 steel could be finished in a very short time during upsetting, but there exists inhomogeneity in healing area which include more ferrite, fine grain size and many micro-voids. The reason why ferrite aggregated in healing area is that large deformation and high strain energy at high temperature and low strain rate around crack produce pro-eutectoid ferrite grain precipitating in hypo-eutectoid steels. Due to numerous voids restraining the migration of grain boundaries, the grain growth in a healing area becomes a slow process in the crack tips in 45 steel. This results in the grain size in crack tips healing area much finer than that in the steel matrix in 45 steel even after a long time of heat treatment at high temperature.

Evolution of Internal Crack in BCC Fe under Compressive Loading

“…It was suggested that the thermal or mechanical treatment provides the driving force to minimise the surface energy resulting in the crack healing [6]. The healing of internal pre-cracks obtained using plate impact technology was studied in a vacuum at an elevated temperature [7,8], and the inhomogeneous microstructure in crack healing area was analysed [9]. It has been shown that the healing of fine cracks (100 nm) begins immediately after heating then proceeds according to the vacancy diffusion mechanism in the surface layer about 10 μm thick [10].…”

A study on crack healing in 1045 steel

“…Han et al [4] investigated the crack healing in 20MnMo steel. Wei et al [5] studied the crack healing behaviour in plain carbon steel at elevated temperature.…”

Modelling of the evolution of crack of nanoscale in iron

“…Real internal pre-cracks were obtained in pure copper and steel using plate impact technology [8]. The healing of these cracks was studied in a vacuum at an elevated temperature [9][10][11], and the inhomogeneous microstructure in crack healing area was also analysed [12]. It has been shown that the healing of fine cracks (100 nm) begins immediately after heating then proceeds according to the vacancy diffusion mechanism in the surface layer about 10 μm thick [13].…”