powdered metal valve seats. 6

[Youpullitcamaro]

I recently purchased a set of Edelbrock performer rpm aluminum cylinder heads for my race car.
After 1 1/2 hours testing at Daytona a rocker stud unscrewed and the cam became damaged.
To make sure that the valves in that cylinder had not been damaged I decided to do a leak down test, 3 of the cylinders leaked.
So I removed the heads and inspected the valves parts of the offending seats had began to break away (photos attached).
These seats are POWDERED METAL as confirmed by the builder and air pockets are detectable when magnified.
Fortunatley the heads were taken back and repaired with DUCTILE IRON seats new valves ETC.
I am still out a great deal of money and question the reliability of such valve seats or is it just a poor batch?

 

[dicer]

PM does not belong in an engine anyplace. Its cheap stuff, benifits only the manufacture, well that is if they don't have a warranty call on it.

 

[patprimmer]

The GM J car engine of the late 80s had sintered metal rods. They performed their job in an acceptable manner at a lower cost. Suitable for purpose at lower cost does benefit owners.

Regards
Pat
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[thruthefence]

Lycoming had a sintered oil pump drive gear for awhile on the aircraft engines. The were supposed to be removed from service have been removed by an AD note

 

[swall]

Dicer--most conrod designs for new production engines are pm, albeit re-heated and forged. The structure of the pm is conducive to the fracture split end cap procedure.

 

[btrueblood]

There are some tribology alloys (very high hardness/low wear) that can't be made reliably by any other method (short of casting in zero gee).

 

[dicer]

Someone mentioned what PM is good for. FRACTURING
And it does that very well. If you are making a nonstressed part, like a statue, PM is great stuff.

 

[patprimmer]

It really comes down to how well a part is designed and made and the manner in which it will be used. PM has proven satisfactory in many applications, but fails in others. Same can be said of any material. I like beryllium copper valve seats. Will you pay for a set of them for me for my next engine. Oh, they are quite expensive.

Regards
Pat
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[GregLocock]

dicer, both Caterpillar and BMW use PM conrods. This is an engineer's forum, not wannabehotrodders.






Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Peter7307]

"I like beryllium copper valve seats. Will you pay for a set of them for me for my next engine. Oh, they are quite expensive."

Is this material still used?
I thought it was taken out of use due to health concerns.

Pete.

 

[patprimmer]

It is still used on real serious race cars with titanium valves. It is very expensive I think partly because of the health issues and required safegards

Regards
Pat
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[dicer]

I know many of the OEM's are using PM. I also know of many of the failures of the parts as well.
The failure is most always a fracture. Someone has mentioned how big ends on Con rods made of PM are fractured at the split as part of the manufacturing process. Castiron also works for such things as crankshafts, and connecting rods, I think the keyword though is Highstress, you won't find a castiron connecting rod in an F1 engine nor in an aircraft engine. Anymore than you will find a PM rod in either of those engines.
Yes I agree they will work, but for how long? They are a failure waiting to happen.

 

[tripleZ]

First off, to answer the OP's question, that failure mode that you're seeing on that part would be a common failure for an axial pressed part (which a valve seat insert would be). For radii on the ends of parts, you're most common cracks at molding will be "lift off" cracks or shear cracks. The one in your pictures is likely a lift off crack. This will occur if the punch or die forming that feature is not properly polished to allow release of the green compact. It can also occur if the molding press timings are off, or if the lubricant in the powder is not the right type or not at the correct %. All in all, I'd say you got a part from a bad batch of parts. Hopefully this is going back to the manufacturer? Parts with this geometry can easily be pressed at 840 pieces per hour and up, so you won't be the only person seeing issues.

Any P/M house worth it's salt would have their operators checking that feature using a "peel test" or a similar means of measuring the localized density in that area.

 

[Youpullitcamaro]

It is good to see the feedback I have seen the PM rods before and have heard of their strength.
But with these valve seat failures the whole process leaves me sceptical.
If i had not stripped the engine when I did I fear that a a whole engine loss was enevitable.
I am all for advancement but most wheels are still round!

 

[tripleZ]

Now to go on to Dicer's comments...being a P/M engineer I feel the need to defend my industry. Yes, P/M parts are being used more in more in OEM applications due to the cost savings they provide (less scrap due to sintering to near net shape, less machining stock, etc.). That being said, not all P/M parts are the same. First, there are about 3 basic styles of P/M: conventional sintered, high temp sintered, and liquid-phase sintered. Let's focus on the first.

Conventional sintered parts are most commonly used in OEM applications as structural components such as bushings. They are generally about 85% or more of the theoretical density, meaning there is porosity in the parts (iron, iron-carbon, and various other derivatives might be sintered to 6.4-6.6 g/cc). The sintering temp of basic irons is usually around 2100°F, and the particle bonding is what you'd call sufficient for applications that don't involve high stresses.

High temp sintered parts can be used in a variety of structural applications. The tone wheels for your ABS systems (this is the toothed wheel pressed on to the shaft) is often 400 series SS or an Fe-P blend. The exhaust flanges holding your pipes are 409 or 410 SS and are usually P/M. These densities will be 92-95% of the theoretical density. Your iron grades will be sintered at 2300°F and your SS grades upwards of 2500°F. These types of components can be used for higher stress applications due to the lower porosity and greater particle bonding in the microstructure. But obviously they cost more. And your dimensional consistency is less than that of a conventional sintered part, so secondary operations may be required.

Liquid phase sintered components are special. We control the chemistry of the powder, the sintering atmosphere, and the sintering temperature to get portions of the compact to go liquid during sintering. The end result is a part that is 98-99.5% porosity free. And many times we can control the sinter temperature and time to achieve an optimal microstructure. These parts will be used in high wear, high stress, and high pressure applications. Currently I make M2 cam rings that run in high pressure diesel pumps, T15 spade drill bits, and M2 valve seats that ride in a variety of compressors. I also have 300 series SS parts in high wear food mixers and in high pressure valve assemblies. The major drawback? The cost is much higher and the dimensional consistency is much worse than the other two methods, meaning most often I'm providing a blank with .002-.015" of machining stock for qualification.

So what's my point? All P/M is not the same. It comes down to an agreement between P/M house and the purchaser. There have been times (it still occurs) where we'll quote a part for a major OEM where we indicate the method of pressing/sintering that should be used for the application. If they don't like the cost, then they end up playing around with lower cost options that might not have the best factors of safety considered. It all comes down to cost.

As to failures in P/M, they do occur no doubt. Sometimes it's the fault of the OEM for a poor design. Other times the P/M house fails to have the process controls in place to prevent failures such as that seen by the OP. But that can happen with any material and any processing method. It all comes down to who you're buying your parts from I guess and how much people are willing to pay for the product.

I apologize to the OP for the hijack. I just tend to find that there is a lot of mis-information out there about P/M.

 

[dicer]

Thank you for the good info tripleZ.
Bushings are a good place for PM and thats been happening for many years, as I remember Oilite bushings as a kid.
I did not know about liquid phase sintering. I can agree sintering has its place. Carbide inserts for machining is one of them, at least I think that is how they are made.
I do have a question, can grain structure be controlled in a high pressure liquid phase process? And what would be the difference between that and a normal casting of the same materials? Will it ever meet the same strength and structure as say a forged part of the same materials?
Personally I would not call a liquid phase a PM or sintering process. It would be more of a casting.
To me PM has always been a highly heated compressed powdered metal, that is like you say porus, and with the densitys involved the actual atomic adhesion would be very low as compaired to what a liquid formed chuck of metal would be, and that is why it is so prone to failure. There just isn't enough glue holding it all together.

 

[Youpullitcamaro]

TripleZ
Thanks for the input.

 

[tripleZ]

To answer your question regarding grain structure, yes we can control that in certain materials. For example, we manufacture M2 cam rings that go into diesel fuel pumps that have a certain microstructural requirement...basically there are grain size requirements, an allowable amount of carbide free areas, etc. The M2 that we manufacture has a very tight carbon spec in the incoming powder, plus some special additives to enable us to get a slightly wider sintering window. To sinter these parts, we must run them in a vacuum furnace pumped down to about 300 microns during the sintering phase. There are strict pre-heat, pre-sinter, and sintering times and temperatures required to hit the microstructure properly. After sintering, each load has small lobes pulled from throughout the build that are mounted, polished, and viewed to ensure that the load is conforming. There are other controls, but that's the basic idea.

How is the P/M process different than casting? Well, because we can use very fine graphite throughout our powder blends, and because we control the temperatures (heat & cool) and partial pressures in the sintering process very tightly, we can achieve a more uniform microstructure with a finer dispersion of smaller carbides. T15 tool steel is a perfect example. Wrought T15 has large carbides spaced throughout the matrix. It's great for wear, but grinding wheels like to pull those carbides out (as do other applications) which can drive the formation of surface cracks. For PMT15, we can get a finer carbide dispersion than wrought. Because the sinter window there is less than say M2, we will undersinter the product, leave minimal porosity, then HIP the product to get the optimum microstructure and ensure full densification. This gives you a lot of little wear resistant carbides that are less likely to induce surface cracks during machining operations. Crucible and Bohler-Uddeholm have some good information on their respective websites reqarding P/M steels vs. wrought alloys.

There are some cases where we can get better properties than wrought. Take 410L. Wrought 410L is soft and is not very wear resistant. If I high temp sinter 410L in a mixture of hydrogen and nitrogen, I can basically put nitrides all along the grain boundaries. Now, my part isn't as corrosion resistant as the 410L, but I'll beat it for wear resistance everyday. Wrought 410 will be in the HRB range if measuring surface hardness (particle and macro). My PM grade 410 might have a macro hardness of 45 HRB, but the micro hardness of the individual particles will be in the HRC scale. The difference is comparing impact to wear resistance. You're basically finding the right material for the application.

In most cases a part forged from wrought is going to have better fracture properties under extreme circumstances (probably like you see running a race car). The average vehicle, with a properly designed and manufactured P/M part that doesn't see the extremes, may well suffice for everyday applications. It just depends on your usage I guess. For example, a forged SS 434 exhaust flange is going to have better corrosion resistance than a high temp sintered exhaust flange. It has to do with how the P/M flange is manufactured. However, that corrosion resistance in the PM flange may be fine for everyday driving...just don't put the same component in a ship going to sea!

Honestly it's all about material selection. Personally, the race application was probably not the one to put a PM part into. It's unfortunate because that's how certain industries get a bad rap.

 

[patprimmer]

tripleZ

Well said and informative. It very well backs up my much more general post 20/06/09

Regards
Pat
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[dicer]

Cam rings, do they have an integral drive gear?

You mentioned wear qualities, how about tensile strength?
How about knotch sensitivity?