Cleavage-like fracture at low temperatures in an 18Mn-18Cr-0.5N austenitic steel.

Abstract

The low temperature embrittlement in an 18Mn-18Cr-0.5N retaining ring steel has been investigated by means of tension and Charpy impact tests. The effects of grain size, strain rate and pre-deformation were examined.The ductile to brittle transition (DBT) behaviour associated with the cleavage-like fracture has been found, where the fractured plane was regarded as {111}. The traces of intense planar plastic flow which may be related with twinning, slip band, or epsilon martensite were observed on the surface of plastically deformed specimen. When the pre-deformation was applied to the specimen at room temperature, the fracture stress at 77 K was slightly increased but the fracture mode was still cleavage-like. The DBT temperature (DBTT) is scarcely influenced by grain size, although the fractured surface unit is considered as austenite grain size. This is discussed based on the experimental data on the grain size dependences of flow stress and fracture stress. When the strain rate in tension test is increased up to 2.7×10-1/s, the fracture mode at 77 K has been found to change from cleavage-like to dimple. In this case, the plastic deformation occurs very locally and heavily, so as to induce the necking of the specimen.