c-g-designs

PLEA FOR THE USE OF CrMo FOR BARREL MAKING.

            The actual great favor of barrels made from corrosion resistant stels (commonly named ‘Stainless’) led me to to speak  of barrels made from Chromium/Molybdenum steel, also still used but are now  losing favor despite their superior qualities.

            I think I should take their defense, because they possess numerous qualities, including longer life expectancy..    

            The ‘custom’ barrelmakers are the most devoted propagandists  of  the use of the Martensitic stainless steel grade 416R+S, particularily because of its machine-ability.   

            This machineability is obtained by addition of sulfur In the alloy creating porosity that  ensure breaking and easy evacuation of the swarf and extend  cutting tools life. 

            The 416R+S is one of the less ‘corrosion resistant of all the stainless steels.

            But, who says sulfur,pressure and temperature  also says thermo-chemical reactions and the 416 is quoted  for its tendancy to cave corrosion who led some steel producers (the french Ugitech amongst them) to recommend not to use it for ‘pressure vessels’ applications. This internal corrosion can sometimes be noticed by the apparition of very small corrosion spots on the external surface in hot  moisty or saline conditions.           

            The grade 420 is best suited for stainless barrelmaking but it does not have  the same machineability, up to the point that some gunplumbers refuse to chamber and thread them on the pretext of their lack of being hard on cutting tools. Although necessitating more skill than even the 4140, this attitude reflect more the lack of competence than anything else.        

            Before coming to the CrMo steels, I would establish the hereunder comparison between the different steels used in barrelmaking:

1°) The 416R+S, as delivered in annealed condition has the following characteristics:

                        Carbon: 0,12%

                        Chromium: 13%

                        Sulfur: 0,15-0,20%

                        Brinell Hardness : 170HB.

                        Rm: 630MPa.

                        Modul of elasticity: 200GPa.

                        Thermal Conductivity: 25W/m-k.                                                                                                                

2°) Whilethe 420, in same delivery condition is:

                        Carbon: 0,20%

                        Chromium: 13%

                        Manganese: 1%

                        Brinell Hardness: 205HB.

                        Rm: 725MPa.

                        Module of elasticity: 200GPa.

                        Thermal Conductivity:: 25W/m-k.

3°) And the classic 42CrMo4 (AISI4140) is:

                        Carbon: 0,40%

                        Chromium: 1%

                        Molybdenium: 0,50%

                         Brinnell Hardness: 225 to 241HB

                        Rm: 1000MPa.

                        Module of elasticity: 205GPa.                                                                                       Thermal Conductivity: 43W/m-K.

            Over the mechanical properties differences, it is to notice 2 important factors who sensibly differ for the 2 steel categories:

            1°) Friction coefficient Steel416 / Copper= 0,20.

                  Friction coefficient Steel 42CrMo4-4140 / Copper= 0,12.

            Those values are averages from different sources. They can be affected by different factors such as (surfaces polish, coppering, composition of the bullet jacket,etc,etc.. The high Chromium content of the 416 and 420 is certainly the cause of the higher friction coefficient?.

            2°) Thermal Conductivity Steel 416-420=  25W/m-k. 

                 Thermal Conductivity 42CrMo4-AISI4140= 43W/m-K.

                        This is a factor who can well not be neglectible, not really by the heat dissipation capacity, but for its faster linear propagation making the barrel temperature rise more even?.

            It is certain the superior mechanical properties of the CrMo makes the button rifling more difficult? This is probably an element of the position of the barrelmakers who for most are using this rifling method?.

            The CrMo  accept an as good polish as the stainless of the 400 grades, but it is not to lose sight on the fact that a too fine polishing can have a negative effect as, then, it  increase the friction coefficient by  ‘adherence’.

            I persist to consider that the erosion happening at rifling leade level is less caused by heat than by  the powder  combustion  and primer compound  residues abrasion. Projection of those solid hard particles a t this level and at  very high  speed cause an effect similar to abrasive blasting.

Due to their higher hardness (225HB mini as to 175HB), CrMo steel resist better to this abrasion.

            A metallurgical process named nitridation consisting to heat at 550-600°C steels in an atmosphere rich in nitrogen. Nitrogen is superficially absorbed by the steel, leaving a deposit of extremely hard nitrides.          

            All steels are not susceptible to accept  nitridation, but the more suitable to nitridation are the lightly alloyed carbon steels such as those used in barrelmaking. The gaz generated by the combustion of nitro-cellulose powders are rich in nitrogen and their combustion temperature is very high, particularily at the rifling leade level. The nitrides so deposited are extremely hard, but also brittle.

            This self-hardening is diminished in the case of the 416 because of its initial lower  carbon and con only be, although only partially be obtained by the specific  salt bath nitriding , and I already had the opportunity to express what I think of this process applied to match  rifle barrels. 

            One can object  the carbon steels tendancy to corrosion , but  the actual surface treatments (Cerakote, etc..etc) perfectly fullfill their role in that matter.

            R.G.C

            06/2017