Some data, thoughts & questions on stainless steel alloys for silencer tubes.
Most cans I see made from stainless steel use either 304 or 316 for the tube. This is understandable since they are readily available in seamless tubing and they have good corrosion resistance. The main caveat I see for these alloys is they do not have great yield strength, in particular as things heat up.
Below are some 0.2% yield strengths for the alloys from this source.http://www.aksteel.com/pdf/markets_products/stainless/austenitic/304_304L_Data_Bulletin.pdf304 SS
0.2% yield strength from Table 2
70 F – 35 k PSI
400 F- 23 k PSI
800 F - 17 k PSI
1000 F – 14 k PSI
1200 F – 13 k PSI
1400 F – 11 k PSIhttp://www.aksteel.com/pdf/markets_products/stainless/austenitic/316_316L_Data_Bulletin.pdf316 SS
0.2% yield strength from Table 2
70 F – 42 k PSI
400 F- 35 k PSI
800 F - 28k PSI
1000 F – 24 k PSI
1200 F – 21 k PSI
1400 F – 18 k PSI
For some perspective 1000 F is a very dull red heat and is a temperature that can be reached with a silencer running 223/308 in fast semi auto (3 mag dumps?).
This chart gives a good visual representation of temperature.
From the yield strength data 316 looks to be marginally better than 304 (24 k vs 14 k PSI @ 1000 F). What are the opinions on 316 being stronger than 304 in suppressors? Is there a sufficient difference in strength to be useful?
There are better stainless alloys available for suppressor use. The one I see most often is 17-4, a precipitation hardening steel. I have used this alloy for a lot of projects including baffles on a form 1 build.
There is a range of hardness and strength with this alloy depending on the heat treatment used.
The strongest and hardest grade is obtained by heating annealed 17-4 to 900 F for 1 hour then letting it cool in still air. This provides a hardness of ~43 Rc and a yield strength of 170-190k PSI. This is referred to as H900 17-4. The least strong and hard grade is prepared by heating to 1150 F for 4 hours. This provides a 35Rc and a yield strength of 150k PSI and is known as H1150.
There is essentially no warping from the heat treating process but there is a very small shrinkage in the range of 0.0005 to .002 inch/inch for the H900 & H1150 respectively. This type of heat treating can be done using a small pottery kiln.
This alloy has very good corrosion resistance, comparable to 304 SS, but where it really performs is in yield strength compared to the 300 series SS.
Here are the yield strength ratings for this alloy from this source.http://www.aksteel.com/pdf/markets_products/stainless/precipitation/17-4_PH_Stainless_Steel_PDB_201404.pdf17-4 SS
0.2% yield strength From Tables 5, 7 & 8
70 F - 190k PSI (H900 heat treated) 150k PSI (H1150 heat treated)
750 F – 115k PSI (H1150 heat treated)
1000 F – 108k PSI (H1150 heat treated)
1200 F – 71k PSI (H1150 heat treated)
1400 F – 42k PSI (H1150 heat treated)
As you can see this alloy has 2-4 times the yield strength of 316 SS at all temperatures and 4-6 times that of 304 SS.
This leads to the question of the gains that can be had in using 17-4 instead of the 300 series stainless steels.
If a typical 1.5in OD rifle silencer made for 223 or 308 pressures has a wall thickness of 0.065in using 316 SS how thin can a similar can be made using 17-4 SS?
The yield strength of 17-4 would suggest a 0.035 wall to be at least strong as a 0.065 wall 316 SS can. This would cut the weight of the tube in half. Using 17-4 for other components would suggest a similar savings in weight.
I’m not a mechanical engineer (I’m a chemist) so perhaps I’ve oversimplified the analysis using just yield strengths. I would like to hear from those with a background in building silencers their thoughts on alloy choices and in particular the advantage of using an alloy like 17-4 over the standard 300 series.