Debasis High Temp Hydrogen Attack - accompanied with decarburization - depends on the partial pressure of hydrogen and with temperature for different materials ( Refer Nelson curve limit for this) If the partial pressure of hydrogen is high or temperature is high, then depending on the carbon content in plain carbon, low alloy steels, decarburization will occur. If the pressure of hydrogen (pure) and temperature of operation or service is lower, which meets to Nelson curve limits, then chances will be lower. If the temperature is higher or partial pressure is higher, decarburization will be pronounced or faster depending on the carbon content in the material used. Degree of decarburization and the drop in strength for a particular high temperature hydrogen service has to be evaluated carefully - by NDT and in-situ metallography (replica technique) to decide and assess thelikely future decaruburation / high temp hydrogen attack trend if allowed to continure. For NDT, UT attenutation is the best approach with a pre-calibrated samle of the same characteristics as original suppled material for comparing on a periodic basis and evaluate the decarburization trend from attenuation signal loss. Low alloy steels, CR-Mo varieties, generally, need longer time and higher pressure and higher temperatue for this to occur. Trust this will help you C.V.Srinivasan Nishi Engineers Pvt Ltd India July 28,2007 E-mail: nishi@vsl.com >In principle High Temp. Hydrogen Attack ( HTHA) led to >decarburisation . Any comment about LTHA causing severe >Decarburisation ? >regads