Advice to keep stainless looking like stainless after forging

[sea trout]

Hello y'all and I hope the title makes sense.
My son and I have forged a lot of mild steel into blades. Horse shoes, rail road spikes and automobile suspension parts. We forge some good lookin blades and grind em into lookin real good! But they always rust out in weather and after use if not cleaned and oiled etc... And that's fine.
But we want to make a boat blade, to have on board or on person to cut rope and such.
I have a piece of stainless...(or so I was told). It's a rod about a yard long, bigger around than my thumb, female threaded on both ends a few inches in.
What do we need to do to forge that into a blade, grind it, and it maintain its stainless properties?
Sorry if my choice of words are incorrect, basically how do we finish this blade and it stay on a boat and be least likely to rust?
Thanks for any input! Sorry I don't know much about different types of steel and their properties or even how to word what I ask.
This has to be done soon. My son was gifted something very VERY special from a fisherman/lure maker in California and we just thought of this Idea to gift back and want our best shot at having good steel for life on a boat.
Thanks for any input!!!

 

[Anvil Head]

Please understand that I am not trying to discourage you from trying something a bit new, but your post sounds a little urgent or at least timely. I'll preface this with I wish I'd had someone tell me some of this earlier in my smithing endeavors. Would have saved me a lot of trial/error/grief. I can only speak from my own experiences and research. I am by no means an authority. It's totally up to you what you do with this information.

Not knowing what alloy of stainless the steel is makes a huge difference. Lot of SS alloys are not designed/formulated for cutting or edge holding. Making a knife for yourself and making one for someone else is a different breed of cat as well. We can convince ourselves that, well, that's good enough for me. However you let one blade get out that does not perform as it should and you are marked for life as less than average knifemaker (word travels faster than pew gossip on Sunday morning). You don't want to "gift", trade, or sell an inferior or poor quality product - not inferring here how it's constructed, but in terms of material quality and performance.

Your material description indicates a "low" carbon SS (3XX) series that will not make a good cutting blade no matter how you work it. It was alloyed for machine-ability and tensile strength, not hardenability or edge retention.
Might get a serviceable oyster shucker out of it if you get the geometry right but will most likely make a "one cut" wonder of a knife blade.

Then there is the issue of temperature control while working SS steels if you wish to maintain blade desirable characteristics. Blade worthy SS's have a very narrow temp range of forgability. Most have a very complex recipe of thermal cycling to reach even moderate results. This takes rather sophisticated equipment with accurate temp control, not something done at an open forge. Hammer impact outside those temp margins will cause molecular structure issues you cannot see. Some SS's will crumble like cornbread, others will develop micro-fractures that manifest later, and some will just refuse to cooperate no matter what. Another issue is open air oxidation scaling on SS. Very hard to remove if you desire the shiny SS look.

I tried to stay away from getting overly technical about SS, but if you want to delve into the real science and physics of working with these steels, plenty of good long-read publications available. Just better enjoy reading late into the night and next day and the next.
In short (?), SS is not the average bladesmith's choice for forging blades - just not a shade tree steel.

All that being said, you can fire her up and start whacking. Make some decent oyster shucker/pry bars and have fun.

 

[sea trout]

Thanks and I realize there's so much more to it.
You're right we don't want to send him a one cut wonder or an oyster shucker.

 

[Anvil Head]

You can repurpose the SS by flattening out and using for fittings - ie. guards and pommels. Just make sure you work it at a bright red to orange so it doesn't crack or crumble.