12mm stainless rod might do the job. you might have to give us some kind of sketch so we can do the experiment. the strongest source material I can think of is the shaft material from a shock absorber like one one those from a Van rear door. In fact they might be just the go, or a small car shockie. I have yet to see one bend.

Thanks guys, I knew this was the right place to ask

I want to build a super special windsurfing fin. This will be roughly designed the same way a sail is, with a stiff mast and a floppy 'sail'. It's not as simple as that but you get the idea.

The trouble is that this material has to be reasonably skinny for an efficient foil, while still being able to support the full load of a windsurfer at full throttle (20 odd kgs, 15cm from the root).

Would such a material be able to be worked with normal power tools?

Using a "Stainless Steel" welding rod is a great idea for welding unknown steels. A better rod might be a Cast Iron welding rod that is 100% Nickel. If you buy some of these rods, sit down (better yet, lie down on the floor) before you are told the cost of these rods. Ask around your local area at various machine shops or engine rebuilders and see if they will sell you just one rod.
4340 can be worked with the usual tools found at home. If you drill/tap into the material, a good lubricant to try is sump oil from your car. The more miles the oil has "done", the better it works. As the oil gets older/used, it picks up sulphur from the fuel or whatever and the sulphur makes it work better for tapping threads and drilling. I collected this little gem from somewhere on the web or in one of my engineering magazines (Australian Model Engineer perhaps?) If you want one of the best lubricants, buy a tin of "Trefolex", its also a bit "exie" but well worth it.
Try not to heat the high tensile steels if possible. After saying that, you can heat the bar and forge it with a heavy hammer and a solid anvil. The pounding on the steel will help overcome the softening of the steel from being heated. ie, the steel will (often) retain its initial strength when it has cooled down. The forging shapes and consolidates the "grain" of the steel and can make it stronger if the heat is kept to less than "yellow" heat. The best way is to make it and see how it works. It is essential that you dont hit the steel when the red heat has dissapeared. Working (hitting/pounding) the steel at less that red hot will fracture the material.
Another very good steel to use would be spring steel. This is easy to obtain from any spring manufacturer as a round rod. It is supplied in the annealed state and can be heated and forged easily. After the final shaping, it can be heat-treated to produce maximum strength and stiffness. If you use spring steel, contact me and I will describe how to heat-treat it for you. Better yet, I will post the info in the hints page tonite and everyone will have access to the info.
When tapping 4340, a slightly greater thread clearance can be used to help facilitate the tapping. I would recommend a thread engagement of 75%. If you are grinding with an angle grinder, any wheel suitable for metal will work perfectly. I prefer the "Penguin" brand sold at the "Wharehouse" or similar cheap outlets. Refer to my comments in the tools hints page I wrote. If the 4340 is too weak (soft) after forging, welding, heating, try heating to a red heat and dropping the part into a tin of oil.

Kody