Debasish Yes you are correct NACE / API Refinery committee had lowered limiting hardness to 200 BHN (18 HRc). This is based on global experience of users in steels and low alloy steels subject ed to hydrogen service at higher partial pressure of hydrogen and lower temperature range of 230-250 deg cent. Earlier NACE / API Refinery Committee recommendation was to limit 22 HRc ( 240 BHN) for steels and low alloy steels in high pressure hydrogen service for a temperature range of 230-250 deg centigrade. From experience of users, users found this to be border limit where an error in measuring hardness with site indenting hardness teters gave problems to some users in refinery, fertilizer, chemical aand petro chemical industries. For Ex: a 5% error in measurement would give hardnes value of 252 BHN ( close to 24 HRc). In order to eliminate the error in measurements in hardness cintirbuting to failures- either due to instrument or operator error or due to job location, NACE / API Refinery committee debated and have agreed for the lower limit of 200 BHN ( 18 HRc) in recent years. This is based on global experience shared in Refinery committee for hydrogen damage in carbon and low alloy steels. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai 20 India June 13 E-mail:nishi@vsnl.com >For Low Temperature Hydrogen Damage as Sulphide Stress >Corrosion Cracking (SSCC) for carbon and low alloy steel >NACE latest has recommended a limiting hardness of 200BHN >(13 HRc) whereas earlier specs. was not more than 22 HRc to >become resistant to SSCC. The question remains how this >specific 9-10 HRC decrease in hardness minimises the risk of >SSCC? Is it experience based based on global survey or does >their any specific technical reasonings behind the same? >Please share.