Debashish Basically - i think - you have clubbed the three independent transformation phenomenon into one single question. 885 deg F is a long term process related embrtittlement. after 30,000 to 50,000 hours continuous exposure at that temperature to this specific failure in Cr-Mo alloy steels. Low Intermediate PWHT is done from convenience by users where such temperature is only practical to attain (for heat exchangers). Dehydrogenation is done from preparing for good weld repairs in Cr-Mo exchangers. (again the tubes which have been exposed to hydrogen service for a long time of 30,000-50,000 hrs at temperature range of 330-450 deg centigrade) First : 885 deg F - 480 deg cent Temper Embrittlement occurs and is referred is a long term effect - of not less than 30,000-50,000 hrs of continuous exposure a this temperature & service. Morphological transformation and phase effect is different in this. Usually, at this temperature of 885 deg F a hard acicular Cr-nitride preciptiates to embritttle the Cr-Mo alloy steel after 30-000-50,000 hours service in hydrogen service operating at 450 deg centigrade +. Please do not confuse this morpohological transformation with the low temperature intermediate PWHT done at 450-500 deg centigrade for 2-3 hours. Just be heating or exposuing for 2-3 hours at 450-49%5C80 deg Centigrade a Cr-Mo tube will not fail by temperature embrittlement Second : The low temperature PWHT is an alternate practice applied to Cr-Mo tube to tube sheet joints - as per code practice - for locations which cannot be heated to temperature range of 700-720 deg centigrde or where regular PWHT temperature of 700-720 deg cenitigrade cannot be attained. Regular PWHT for Cr-Mo alloy is in the range of 700-720 deg Centigrade. Third : Dehyrodgenation is another alternate approach for longservice use Cr-Mo tube to tube sheet joints which develop failures in hydrogen service. This is done mainly for Cr-Mo tube heat exchangers - in hydrogen service - for preparing for effecting weld repairs in heat exchangers susceptible to have developed in tube to tube sheet joints and not with the purpose of avoiding delayed hydrogen embrittlement. Dehyorgenation is done not for just 2-3 hours - repeat not just for 2-3 hours - but usually for 10-12 hours to make sure that the Cr-Mo tube to tube sheet is free from service hydrogen inclusions. Trust this answers your querry C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai India May 16 E-mail: nishi@vsnl.com >Specially for Cr-Mo Ferritic Steels like P22, P5, P91, after >welding ,it is a standard practice to offer a POST HEAT at >around 400-450C for 2 -4 hrs ( Japannese often called it Low >Temperature Intermediate Postweld HeatTreatment)as a measure >to dehydrogenate to HAZ specially which may have picked up >atmospheric hydrogen during welding at temperatures when HAZ >is having austenitic phase. Thus this is significant and >required to eliminate chances of delayed craking/ Hydrogen >Embrittlement. > >But it is also known that these steels can be subjected to >Temper Embrittlenment (885F)when subjected to around >400-500C. > >AS such how the issue of Temper Embrittlement can be >negotiated here ? regards