Hydrogen in Steels

Hydrogen in Steels  Hydrogen (H) (atomic number 1 and atomic weight 1.008) is a colourless gas. It has a density of 0.0899 gm/litre. Melting point of H is – 259.2 deg C and boiling point is -252.8 deg C. The phase diagram of the Fe-H  is given at Fig 1. Fig 1 Fe-H phase diagram  H in steels is considered as an undesirable impurity which is quite harmful in certain applications. It is always a source of various problems within steel production because of its generally detrimental effects on processing characteristics and service performance of steel products. Just a few parts per million of H dissolved in steel is sufficient to do the harm. Hence where necessary, it should be avoided or removed as required. Source of hydrogen There are multiple sources identified for H to enter into steel by any of several routes. In the primary steel making furnace, source of H is water which enter the furnace through wet scrap, flux materials, ferro alloys and refractory materials which are not fully dried. Water dissociates on contact with liquid steel and produces H which is absorbed by the steel bath. This H generally get removed by the purging action of the carbon (C) boil, but some can remain in the steel. Contact between the liquid steel and moisture in refractory materials  of the steel teeming ladles and/or humid air can cause pick up of H by the liquid steel. The hydrophilic calcium oxide (CaO) in the slag and decomposition of refractory binders which are required for sufficient thermal shock resistance, also account for H entering into the steel. Dissociation of water vapour (equation given below) contained in furnace gases generated during the steel making practices employing the combustion of hydrocarbon fuels produces H...