303 vs 416 stainless for valves

[petertha]

I've made a few prototype valves out of 416 SS & very happy with how it machines. I just now realized I need to order more material for the real thing. Doing a bit of research on the forum I see 303 seems to be the favored alloy for valves on several respected engine builds. I presumed it was because of better corrosion resistance despite slightly lower machine-ability rating. But when I got looking at the actual corrosion difference between the two, I'm having difficulty seeing the big difference beyond the somewhat generic recommendations. In particular, have a look at my annotated corrosion rate vs Chromium. They seem quite similar to one another.

Has anyone made valved out of 416 & encountered adverse issues they had not experienced with 303?
Maybe 303 is just easier to obtain for folks?
I'll be running a methanol engine so corrosion will be more of a factor than gasoline all things equal, so leaning towards 303 just for that reason alone. More just wondering what your experience has been.

 

[Lloyd-ss]

That's a real can of worms with selection of stainless steels. I think everybody has their favorites that they often use without giving it much thought, probably from experiences many decades ago (that's me, anyway.).

I have never used 416, but have always used 303 for a general purpose free machining stainless. But with a yield of 35ksi and uts of 90ksi, it is not that strong.
304 is even weaker at 31ksi y and 73ksi uts, plus, it work hardens real bad if you are timid about machining it.

Both 303 and 304 have moderate corrosion resistance. I think it is important to note that most stainless steel fasteners you are made from 303 or 304 stainless and are therefore fairly weak. Corrosion resistant high strength stainless fasteners are much stronger, but also much more expensive.

For a corrosion resistant stainless my first choice is 316. It is much stronger at 60ksi yield and 89ksi uts. Its corrosion resistance is very good, and its service temperature is also much higher than 303. It has a reputation of being difficult to machine, but with sharp tools or new inserts, and depth of cut that is enough to get into the metal, it machines well. I find that it gives a superior surface finish than 303.

For stuff that is delicate, but needs strength, I always go to 17-4ph (sometimes refered to as 630). In the as purchased annealed condition it has strengths of 145ksi yield and 160ksi uts. Right up there with 1144 stress proof alloy steel and the high strength titanium. It is readily available, but is a little expensive, but it does have good corrosion resistance too. Unfortunately, its service temperature might be too low for valves, but I am not sure.

With my limited knowledge of stainless steels, I think type 316 is a good choice for valves: strength, corrosion resistance, availability, and service temperature. But please, wait for a second opinion!
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Just a side noted. One of the first times I was really aware of the strength of 17-4ph was when I was turning a small gizmo on the lathe from a 1/4" rod. I was parting the piece off to reverse it in the lathe and used the lazy way of just plunging in a triangular insert. I stopped the machine with about a 1mm dia left and went to bend/break it off. I remember it taking about twice as much strength as I was expecting, and then, it didn't bend, it snapped. The kind that stings your fingers a little.
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AND ONE MORE ITEM
If you want the stainless steel bible, download the Carpenter stainless bluebook. Its an 87 page pdf and has more info than you need, but its a great reference book.
Lloyd

 

[DJP]

I second your observations on stainless steels. We had large sailboats for many decades and 316 was the best for corrosion resistance and strength. Machining required a slow turning speed and heavy continuous cut. Any slippage of the cutter and the workpiece with harden.

The resistance to corrosion makes stainless my favourite material for anything that has to stay outdoors. I'm not sure of the stainless grade used in the food processing industry but there is always a supply of flange bolts, tubing and odd pieces at our local scrap yard. Perhaps that's why I like SS. It's not expensive at the scrap yard.

 

[Entropy455]

416 is a heat-treatable martensitic stainless steel. Works great for steam turbine blades (when hardened). 422 and 440C are also commonly used in steam valve applications. Note that not all 400 series stainless are the same. Some tolerate through-hardening with very little temper (440C), whereas others require additional temper, and benefit from case-hardening/nitriding (422).

Extreme-duty exhaust valves can hit 1500 degrees F. These temperatures will temper most 400 series stainless. This is why high-end exhaust valves are constructed with a 300 series austenitic valve head, joined (welded) to a 400 series martensitic stem. The austenite holds up to the heat, and martensite holds up to the wear. Sodium can be added to the core of the stem to assist in cooling the valve head.

When you weld 300 series to 400 series, you'll need to heat-treat the assembly after welding. The same thermal process that hardens and tempers the 400 stem, will heal the weld damage in the heat-affected zone of 300 - and will also correct any over-hardening in the heat-affected zone of the 400 from welding.

In short, 300 series tolerates the heat, but doesn't wear well. 400 series tolerates the wear, but not the heat.

 

[Entropy455]

And do not weld (ever) 303 stainless. 303 has sulfur to make it free-machining, and will crack before your eyes if welded.

 

[petertha]

Thanks for replies thus far. These are for delicate fiddly tolerance intake/exhaust valves so machine-ability & minimized work hardening effects are the big driver issues. I have some 303 coming so I'll make some practice valves & report back with any findings. I've seen them used in other successful model engines so i know it must work. Having seen the insides of several badly stored methanol engines (thats right Dad, I'm talking about you LOL) I'm amazed at how the valves looked. I wouldn't say corrosion to the degree the bearings typically see with puddles of fuel sucking in moisture. More the heat & burned oil goo. Amazingly they clean up quite well with solvents & mild buffer wheels.

 

[velocette]

Nimonic 80a used for the valves for a Velocette Venom over 60 years ago is still available. Well proven when a Velocette Venom did 2400 miles in 24 hours.