Effect of Ferrite Grain Size on Tensile Behaviour of a Hydrogenated Low Alloy Steel
A low alloy steel containing 0.10C, 0.25Si, 0.87Mn, 0.56Cr, 0.47Ni, 0.21Mo, 0.023S and 0.01P (mass %) was reduced by cold rolling to 1.6 mm in thickness and recrystallised at 700ºC for 24, 72 and 120 h to get ferrite grains of 8, 21.5 and 32.5 μm respectively along with a random distribution of some spheroidal carbide particles and a few inclusions. Tensile specimens prepared from these sheets were cathodically charged in 1 N NaOH, 0.1 N H<sub>2</sub>SO<sub>4</sub> and 1.4 N H<sub>2</sub>SO<sub>4</sub> solutions for periods varying from 2 to 24 h, with the current density of 50 mA/cm<sup>2</sup>. Tensile tests were carried out with the cross-head velocity of 1.2 mm/min and fracture surfaces were examined in SEM.<br> It has been found that not only the charging time in an electrolyte but also the electrolyte itself exert influences on the tensile properties and fracture characteristics of the steel. Increase in ferrite grain size has enhanced toughening as well as embrittling (beyond a certain limit of hydrogen content) of the steel under the charging conditions, and no remarkable damage effect of hydrogen has ever been noted at grain boundaries. Influence of ferrite grain size on tensile properties and fracture characteristics appears to be due to the change in hydrogen concentration inside the grains as the grain boundary area per unit volume changes with grain size since the hydrogen trap density at grain boundaries follows a certain ratio to the hydrogen concentration inside grains.<br> Charging with 1.4 N H<sub>2</sub>SO<sub>4</sub> solution embrittles the steel even with a low hydrogen content by causing an irreversible damage and ferrite grain size seems to have little effect on this.
- ISIJ international
ISIJ international 39(4), 371-379, 1999-04-15
The Iron and Steel Institute of Japan