The need to know the toughness limits at transition temperature

Abstract

Charpy’s impact test has been enriching our knowledge of the toughness properties of steels since 1901 [8], and anyone working in engineering or involved in engineering education is likely to have encountered it. In Hungarian literature, the phenomenon of transition temperatures is almost always mentioned in the impact test, and these are represented in spectacular ‘S’ diagrams. Most of their examples are ‘plotted’ theoretical curves without actual measurements and the literature sources available do not cover this subject in full depth.
The ductile-to-brittle transition is a commonly observed phenomenon in metals and steels, corresponding to the change in fracture behaviour of metals from ductile (stable) fracture at high temperatures to brittle (unstable) fracture at low temperatures. The change in fracture mode usually occurs over a range of temperatures centered on a specific temperature, known as the Ductile–Brittle Transition Temperature or DBTT in English literature. In Hungarian literature, the term TTKV (TT: Transition Temperature) has become known [1].
Different metals with different lattice structures behave differently. Metals with a body-centered cubic (BCC) crystal structure typically display ductile-to-brittle transition properties, as they do not exhibit close-packed planes that allow for easy migration of dislocations and therefore the movement of dislocations in these materials requires thermal activation. At low temperatures, more drastic mechanisms, such as bond breaking, are triggered by the applied stress [1].
For nuclear structures, welded joints and pressure vessels, it is important to know the transition temperature of the materials incorporated in the structure, because the design is scaled to the base material. Therefore, the need to know the toughness limits of materials arose very early [2]. This article has been written to give an overview of the transition temperature and its modelling in international literature by presenting our own measurements.