The Effect of Cold Work and Fracture Surface Splitting on the Charpy Impact Toughness of Quenched and Tempered Steels

Quenched and tempered (QT) steels are commonly used in the manufacture of transportable pressure vessels in Australia. During the weld fabrication process sections of the head and shell are cold formed (2-5% strain) prior to welding. The effect of this plastic strain on the impact toughness of the base (parent) plate is commonly overlooked because mandatory postweld heat treatment (PWHT) is likely to counteract any effects of cold work. However, since the PWHT of QT steels for transportable pressure vessels is currently under review in Australia, it is pertinent to consider the effect of cold work on impact toughness. This paper specifically reports on the effect of 3.5% total strain (∼3.2% plastic strain) on the impact energy values of 12 mm 700 grade QT steel. This steel, although not currently classified as a pressure vessel steel, is being considered as a potential steel for transportable pressure vessels based on its superior impact and fracture toughness to 700PV grade (currently used for transportable pressure vessels). It was found that a permanent strain of 3.2% significantly reduced the impact toughness of the 12 mm 700 grade steel, and that PWHT (545°C) resulted in recovery of the impact energy to a level similar to that for PWHT of non-cold worked steel. This paper also reports on the role of splitting on the impact energy values of 700PV grade (11 and 20 mm) and 700 grade (12 mm) steels. Reports in the literature and the current work suggest that steels with a tendency to split record impact energy values significantly lower than similar steels that do not exhibit splitting. However, when other variables are kept constant, an increased incidence of splitting of the Charpy fracture surface was found to be associated with higher impact energy values. It is concluded that variable degrees of splitting is a major contributor to the well-known scatter in impact test results.