Hello, ...I used the AISI 630 for this plugs, this material have high yield stress and more machinability, more of the 300 family stainless steel, obviously the 630 is not weldable .... and is adapt for loose pieces
You are doing great work and I have enjoyed your photos to see its progress
Fabio, please be patient with me while I get a little technical here and share some of my metallurgical engineering expertise, since this is a good opportunity to pay you back for all the help that you have given me in the past
To everyone else here, I apologize in advance and you can simply scroll down to the last paragraph or skip this post entirely
In 1995 the American Iron and Steel Institute (AISI) system for designating stainless steel grades in the USA was discontinued and replaced by the Unified Numbering System (UNS) for Metals, which both the American Society of Testing Materials (ASTM) and the Society of Automotive Engineers (SAE) have standardized.
Likewise, AISI 630 is an older designation that unfortunately is still used by some material suppliers today (again, it was discontinued 20 years ago). Its current standard designation is UNS S17400. It is also commonly called 17-4 PH since it nominally contains 17% chromium, 4% nickel, and 4% copper and is precipitation heat treated to change its properties for different applications. It belongs to the family of precipitation hardening (PH) stainless steels and is typically supplied in the solution annealed heat treated condition (which I expect is what you have).
This material's mechanical properties will significantly change with its heat treatment where it can be precipitation hardened after solution annealing at 900F (480C) to 1400F (760C), in steps of 100F (38C), which are designated as H900, H1000, etc. The H900 condition will provide the highest tensile strength and hardness but with minimal ductility and formability, while the H1400 will provide lower tensile strength and hardness but with higher ductility and formability. The solution annealed condition will provide the lowest tensile strength and hardness but the maximum ductility and formability that the stainless steel has to offer.
It's common to fabricate components using this material in the solution annealed condition, then to heat treat it afterwards for achieve its desired properties for the specific application. However, it can also be left in the solution annealed condition after fabrication, which I expect to be just fine with your exhaust system.
When buying this type of material it's always useful to ask for the Materials Test Report (MTR) which will include its heat treatment condition so that you better know how to work with it. For example, if you get a batch of this stainless steel that was supplied in the H900 heat treated condition, you would probably be screaming at the supplier that they sold you bad material because it would bend in your press when you cold formed it and would end up cracking (breaking) since H900 does not have good ductility or formability
All of the PH stainless steels are prone to work hardening since they go through metallurgical phase transformations when cold formed, which would account for the dimensional instability you've already noticed.
This grade of stainless steel is weldable, but requires special attention. I have used it for many applications, for example, fabricating large industrial compressor impellers, and welded them successfully. If you have a need to weld this material, please send me a PM and I'd be glad to assist you further.
I wish I could be your workshop helper to enjoy this project with you. Please keep posting more photos