Insane Hydraulics

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How to Use Hydraulic Pumps For Bluing Steel

Gunsmiths have used bluing for decades to give steel parts that characterstic dark finish. Nowadays, many different types of cold and hot bluing techniques exist - some can be performed at home using a DIY kit, others require special equipment and a professional hand.

It appears that oil hydraulics can also make a contribution to this proud craft! On these pictures you can see parts from a Sauer Danfoss series 90 closed loop pump. The peculiarity here is the gorgeous black finish ALL the internal metal parts got as a result of extreme overheating and some weird reaction with the hydraulic fluid. Unfortunately, I have no information about the type of oil used in this particular hydraulic system, but I can tell you that the black coating is very uniform and very resistant to abrasion.

I believe this bluing technique falls under the category of "requiring special sophisticated equipment"...