Effects of hydrogen on the mechanical properties of ASTM A 203 D nuclear structural steel

The influence of hydrogen on the yield and fracture bahviour of ASTM A 203 D nuclear structural steel has been investigated. The steel is normalised and tempered condition has been hydrogenated to 1.5 and 4.6 ppm of hydrogen concentration and subsequently tested in tension and slow notch bending in the temperature range, 77 K to 300 K, employing a cross head speed of 8.33 x 10-6 m/s. It has been observed that hydrogen charging does not affect either the tensile strength or the ductility of unnotched tensile specimens at temperatures, 150 K to 300 K. However, the uniaxial yield stress has been found to increase with hydrogen charging at all the test temperatures used; the increase in the yield stress at a given temperature being more at higher hydrogen content (4.6 ppm). The bend angle at fracture varied with test temperatures in a manner analogous to the typical impact energy transition curve of bcc iron, consisting of an upper shelf, a lower shelf and a ductile-brittle transition region. It has been found that charged-in hydrogen lowers the upper shelf values and increases the ductile-brittle transition temperature (DBTT) of bend angle-temperature curves, the effect being more with higher hydrogen content. Fracture modes of unchanged bend specimens are ductile (microvoid coalescence) above DBTT brittle (cleavage) below DBTT. However, failure of hydrogen-charged specimens above DBTT is always by mixed mole of quasi-cleavage and microvoid coalescence, while that below DBTT is by cleavage. At attempt has been made to explain the observed results with the help of existing theories of the effect of hydrogen on the pasticity and fracture of iron and steel. 14 refs. (author)