c-g-designs

BARRELS CARBONITRIDATION BY MELTED SALT BATH IMMERSION.

Some good reasons for not submitting Stainless Steel barrels to this treatment..

            The subject of the carbonitridation treatment (Melonite) of the SS rifle barrels is actually often discussed on the forums with opinions often subjectives and not controlleable. I come here to express my views on the subject :

            The process of superficially hardening steels by melted salt baths (Melonite, QPQ, Tennifer...) is derived fron the ‘Durferrit’ process created in the early 1930s by the German chemical firm Degussa, widely used  by Germany  in WW2 to allow  for their armament industry the use of low carbon content, low  alloyed steels  who were,by this process  made able to be superficially hardened , up to a value of 60-62HRC and at thicknesses of  0,1mm and more, depending of the initial carbon content of the alloy.

            The treatment consist to enrich in carbon the superficial layer  of the steel part to make it hardenable.

            The QPQ process (Quench,Polish,Quench) add an intermediate polishing and a passivation.

Negative points :

            1°) The carbonitrurisation is not recommended for Stainless Steels having a Chromium content over 12%. The 416R hass a 13% Chromium content !!.

            2°) If the surface hardness is increased to a thickness expressed in microns, the core hardness remain those of the original metal, tempered at a temperature of 600°C..

            3°) The new ferritic structure of the hardened coat reduce the stainless steel corrosion resistance. This is treated by the final passivation.

            4°) The carbonitridation increase the affected coat carbon content,  one can however question the value of the process on a steel of such a low carbon content value (0,12%). Normal case-hardening steels are over 0 ,20%.

            5°) The immersion in a salt bath at some 570-600°C provoke an instant temperature rise of the metal . Even if some pre-heating is done , such a thermal shock can have consequenceson the barrel geometry.

            Conclusion ;

            It is undeniable that carbonitridation give to low-alloyed carbon steels and irons a great superficial hardness and a certain improvement of the friction coefficient. But those improvements can only be limited in the case of stainless steels with very low carbon percentage and high Chromium content.

            Remain the passivation (the second ‘Q’ of the denomination) who give a nice mat black finish. But its long-time aspect remain an interrogation.

            It also remain some interrogations that those who offer this treatment never replied :

            --How is made the barrel immersion in the 600° salt bath : flat horizontal ,at an angle  or in vertical immersion ?.

            --How one can state that the salt action is the same in the bore than on the outside surfaces, the only place where hardness can be measured ?.

            -As barrel throat wear under two associated causes : heat  and moreover abrasion caused by projection at high speed of powder   and primer compound residues. How long will resist to the second the very thin hardened layer when the substrate remain of the natural 600°C tempered metal hardness ??.

R.G .C.

05/2017