UHV Thesis

Hi,
Some answers:
1. Hydrogen enters steel when the steel is a liquid, such as during melting and refining.
2. Several gases are removed from steel under vacuum. Nitrogen is also removed under vacuum. Typicall to remove H2 down to 2.5ppm or less requires a the steel to be vacuumed at less than 5mb for greater than 10-15 minutes depending on the amount of steel being treated and the type of degasser being used.
3. Hydrogen is present in steel normally between 0.5 ppm (parts per million) and 10ppm in the liquid phase. Mostly comes from moisture in the iron/scrap steel and from vapour during melting. Most industrial steel furnaces are water cooled. The type of steel determines the amount to Hydrogen which can exist in it without causing cracking. Hydrogen can also be removed by heating the steel to around 600-700 deg C and holding for between 12 and 500 hours depending on the acceptable level of hydrogen and the section size of the material being treated.
4. No sure. How good is your knowledge of crystal structures of metals? Up to 2ppm it is dissolved.over 2ppm it generally exists as a gas.

My knoledge is quite good, i'm a physicist ; but i haven't stell found some publications or books that is clear about this topic

1. Just to add to what has been said already, the presence of impurities such as sulphur (ie. H2S) increase the chances of hydrogen diffusion because it prevents the formation of molecular hydrogen. Therefore, the atomic hydrogen is at the metallic surface longer.
4. Hydrogen exists as an interstitial because of the drastic size difference between atoms. In BCC iron it most likely occupies the octahedral sites (I'll let the hawks of exactitude correct me if I'm wrong). FCC structures generally have lower hydrogen concentrations which I can't explain except possibly there being less vacant space in the structure. Hydrogen affects the higher strength steels more and is especially harmful within the martinsite phases.
Seth
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