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Valve Seating

Posted by frustrated 
Valve Seating
May 17, 2017 08:40AM
Hi Guys,

I'm drawing up a cylinder head meant to run at high pressure and temperature. In order to minimize clearance volume, and to allow both upwards and downwards opening poppet valves, I'd like to bore both poppet valve seats into a single plate maybe 1/4" thick. Obviously, this plate is going to have to handle temperatures in the 800 to 1000 degree range (doesn't hurt to assume steam temperature might occasionally get too high in an experimental superheated boiler). Also, obviously, the poppet valves are going to produce a certain amount of "slamming" regardless of how well the cam is made. I have a few ideas in mind but thought it'd be dumb not to take advantage of the expertise out there. My question is:

"What alloy do you think would make for the best valve seat plate?"

Regards,

Ken
Re: Valve Seating
May 17, 2017 09:15AM
Ken
Without knowing the design and just making the choice on the type of metal my first choice would be 4340. The valve and seat would have to be designed so the expansion from heat would not change the matting. This metal can be heat treated and hardened.
Rolly
Re: Valve Seating
May 17, 2017 09:42AM
Rolly,

OK, that was my first choice! It's the operating temperatures that get to you since so many hot work hardened steels still lose their hardness when you start reaching the kind of superheat you want for high efficiency.
Re: Valve Seating
May 17, 2017 09:44AM
The drawing isn't quite to scale and I'm still working out the final details, but the area in green is the part about which I am inquiring.

Ken


Re: Valve Seating
May 18, 2017 09:11AM
Ken
I think I would make the valve seat removal and interracial with the valve body, then it can be machined in a lathe and sent out to be hardened and ground or if you have a tool post grinder it could be ground at home in the lathe.
Rolly
Re: Valve Seating
May 19, 2017 09:33AM
Ken
There is no reason to go over 750-800 F with superheat. Whatever pressure your running 200F over saturation temp is all you need. Follow the steam tables for expansion.
All the power plants I’ve work at the most they run is 1200F at the boiler with turbines, and that’s only because of the long runs and temperature losses in delivery.
Rolly
Re: Valve Seating
May 20, 2017 08:33AM
Hi Rolly,

Sorry for the delay in responding, my work schedule has been wildly unpredictable since the first of February.

First of all, thanks very much for taking the time to reply.

I'm looking at a high speed, single cylinder engine with a new admission valve (needed to run at high speed with short cutoff) to put out (hopefully) about 35 HP before auxilliary loads are subtracted. For the sized vehicle it is meant to fit, a very small, forced circulation boiler of high output will be needed. I plan to run about 850 F but, after years in product development, I've found that plans and reality often go their own ways until you have time to iron out the bugs. We've all seen "can't fail" projects that collapse utterly because they assumed no margin for error. An engine that can run at 1,000 F provides a safety margin if the boiler superheat control goes off the reservation. I figure overbuilding is better than rebuilding after an "oopsie".

I can try redrawing the head to re-examine the use of separate seats. The problem is the admission valve which must, necessarily, lift rather than drop. An admission valve seat would have to fit into a pocket in the head and this pocket requires material behind it to provide solid support. That leaves a significant gap between the bottom of the head and the bottom of the valve, all of which contributes to clearance volume. Minimizing clearance volume both improves thermal efficiency and contributes to better operation of this admission valve, which is how the seating plate came about. I do have a tool post grinder but the only lathe available on a regular basis, at the moment, is a Hardinge; which is just too small to accept the grinder without extensive modifications.

Regards,

Ken



Edited 1 time(s). Last edit at 05/20/2017 08:50AM by frustrated.
Re: Valve Seating
May 20, 2017 12:14PM
Hi guys, if I may interject an additional question or two: Does 4340 corrode unless you'd inject oil in the steam? As I recall the only metals mentioned in the oil-less Cyclone posts were 304 and 316 as well as an aluminium alloy. Those are very corrosion resistant, I thought 4340 is more of a crmo tough steel that rusts?
Also, may I ask how you would heat treat it in this application?

How
Re: Valve Seating
May 20, 2017 01:55PM
If you can't use a seat:

Stellite Hard Facing Alloys - Kennametal


Several items of interest in the following article... (stem clearance, and interference fit of seat)
Valves and Seats - Engine Builder Magazine
Re: Valve Seating
May 20, 2017 04:23PM
Ken my thoughts, no change in clearance.
Rolly
Yes 4340 will rust but you can harden it and its 85,000 tensile. 303 SS is soft and 304 can work harden when machined, but not hard enough for valve seats. 440C SS can be hardened as hard as 4340 but it also rusts.


Re: Valve Seating
May 24, 2017 01:35PM
How hard would be proper hardness for the 4340?

Had a knife from CPM S90V, it never stained. 'Aim for hardness 56-59'. It is a powder steel with 9% vanadium so it wears slowly. Could that be something, maybe?

[www.crucible.com]
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All files from this thread

File Name File Size   Posted by Date  
plate.png 36.4 KB open | download frustrated 05/17/2017 Read message
Valve Dr-2.jpg 23.7 KB open | download Rolly 05/20/2017 Read message
Valve Dr.jpg 18.8 KB open | download Rolly 05/20/2017 Read message