Oil field always limit Ni content. Why? 2

[pktong]

I know that oil field always limit the metal Ni content to a very low limit.Why?

Also they limit the metal hardness to below 237HB. Why?

 

[Carburize]

The controls you mention all originate from a document called NACE MR 01 75 in which the then knowledge and experience of ways to control hydrogen cracking caused by the presence of sour gas (hydrogen sulfide) was gathered together.
The usual oil field alloy steels such as 4130, 4140 etc were observed to suffer from cracking problems if the hardness exceeded 24Rc but no cracking if they were less than 22Rc. The 22Rc number was chosed to represent the limit for sour service and this is interpreted as 237BHN via the standard ASTM E 140 conversion table for alloy steels.
Similarly if these alloy steels contained nickel levels in excess of 1% cracking problems were observed and the nickel level maximum was included.
The NACE document has grown over the years adding new alloys and new materials and new controls to go with them including in some cases specific heat treatments which they must receive in order to be acceptable.