Embrittlement of 50-60 kg/mm² High Strength Steels and their Welded Joints by Heating in Hydrogen at High Temperatures and High Pressures

Abstract

50-60 kg/mm² class four high strength steels and their welded joints were heated for various time at various temperatures 300-550°C in 200 kg/cm² hydrogen, and the changes of their mechanical properties and structures were studied at room temperaure. The process of embrittlement in longtime testing and embrittlement limits of these steeles caused by heating in hydrogen at high temperatures and high pressures were discussed.
The results obtained were as follows.
(1) Steels which were heated at high temperatures and high hydrogen pressures became brittle and remarkably decreased the mechanical strength and the ductility, besides both hardness and carbon content decreased, many inner cracks initiated and fissured at grain boundaries. Then this grain boundaries fissuring gave rise to deterioration of materials.
(2) An addition of carbide forming elements, Mo and Cr, and the heat treatment as normalizing and tempering increase the resistanse to hydrogen attack and raise embrittlement limits.
(3) The welded joints are easy to become brittle in the heat-affected zone, because of the coarse grain size, therefore they decrease more severely the resistanse to hydrogen attack than the base steels.
(4) At a constant temperature and pressure, it requires an incubation time (t) to become brittle and a plot of In t vs. reciprocal of heating absolute tempereture holds a straight line relationship.
(5) According to this straight line relationship, it is possible to extrapolate a critical temperature of embrittlement limits at a long time about each meterial.
(6) The activation energies of embrittlement on the kinetics of hydrogen attack were calculated, but they were fairely larger than each activation energy for diffusion of hydrogen and carbon through iron. Therefore, the process of hydrogen attack will not be the hydrogen and carbon diffusion-controlled behavior. Also, it is considered that the embrittlement process is related to the temperature dependence of the movement of dislocations and vacancies in the prepicitation and crack growth process of the methane gas in carbide-hydrogen reactions. But it will be necessary to make more detailed experiments to explain the mechanism of it.