Abstract
The effects of prior austenite grain (PAG) refinement on the mechanical properties of bainitic/martensitic steels not only come from itself, but also have more complex effects by affecting the substructure formed by coherent transformation. In this study, the samples of a low-alloy steel were water quenched from different austenitizing temperatures and the bainitic/martensitic microstructures with different PAG sizes were obtained. Electron back-scattered diffraction was used to characterize the microstructure and different types of boundaries were identified and quantitatively analyzed. The tensile tests and series temperature Charpy impact tests of different heat treatment were also carried out and comprehensively analyzed with microstructure characterization works. The results show that the uniform refinement of prior austenite grain can increases the density of packet boundary and block boundary, which leads to microstructure refinement with higher density of high-angle grain boundaries with misorientation >45°. The contribution of this microstructure refinement to toughness is significant, but its contributions to strength and elongation are relatively limited. Compared to uniform refined PAG, if the PAGs are mixed crystal, the density of block boundary will be reduced, which leads to a lower density of the high-angle boundary with misorientation >45° and the positive effects of microstructure refinement on toughness improvement are weakened. The observation of fracture surface of impact specimens indicates that refining the PAG can delay the tendency of brittle fracture with the decrease in test temperature, and even in the case of brittle fracture, the cleavage facet of the fracture surface is relatively smaller. This result also verifies that PAG refinement can effectively improve toughness by inhibiting cleavage fracture.
Highlights
It is well known that the strength and toughness of iron and steel materials can be simultaneously improved by grain refinement
A prior austenite grain (PAG) is divided to several packets, which are a group of laths with almost same habit plane
There is no doubt that microstructure refinement is good for the mechanical properties of bainitic/martensitic steels
Summary
It is well known that the strength and toughness of iron and steel materials can be simultaneously improved by grain refinement. For most low-alloy highstrength steels, whose microstructure is bainite or martensite, the concept of grain is more complicated, because bainite/martensite has multi-level crystallographic structures. Grain in bainite/martensite can be prior austenite grain (PAG), packet, block/sub-block or lath [1]. There is a hierarchical relationship among these crystallographic units. A PAG is divided to several packets, which are a group of laths with almost same habit plane. A packet is sub-divided by blocks which contain a group of laths. Lath is the smallest crystal unit which has almost the same orientation
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