The smoother the surface finish that any given engine component is, the stronger it becomes.

 

Well, like most things the answer is going to be "it depends". But in general, I'd say "true", keeping in mind that there must be a point of diminshing return, and so on.

Rich

 

I was reading an article about titanium rods. One excerpt of the article was talking about how a titanium rod can be rendered useless if you put a single scratch on it the equivalent of what could be produced by 150 grit sandpaper. It made no mention of whether this was an anomaly that was attributed to the metallurgical properties of titanium alone, but I wondered about it. Apparently what happens is that as a rod heats up and is put under stress, the push-pull motion makes the molecules in the metal act in similar fashion to that of a spring being stretched and recoiled. Since a scratch is nothing more than an extremely tiny valley on the surface of the rod, it acts in similar fashion to how a faultline in the earth's crust reacts when the adjacent plates begin to move and stress against eachother: upheaval. When the upheaval first begins, it starts sort've a domino effect where whenever the adjacent walls of the valley of the scratch bang together, when they bounce back apart they rip the floor of the valley a little bit deeper. This continues to happen until the scratch evolves into a crack.

I agree about the Law of Diminishing Returns coming into effect at some point, but I'm curious if it applies to all metals in the same manner. If we can break a titanium rod with a 150 grit scratch, can we do the same with an aluminum rod?

 

I dunno, but one thing to keep in mind is to also treat your valve springs gently. Surface defects there are very often the source of failure.

Rich

 

It may not increase strength but the part will have a longer fatigue life with a polished vs cast or machined surface in general. (steel)

 

Another Urban Legend?

150 grit makes about an Ra 80 finish on soft material like aluminum and titanium. The machining for the bolts is generally in that area or rougher.

The forged surface is rougher than that. I don't put much stock in the 150- grit (Ra80) scratch being a problem.

As for the initial question, Rich was correct. It depends on the material, the condition it is in, like heat treated or shot peened or coated and how it is being used. The inside of high-end piston pins should be fairly smooth so as not to cause a stress riser from rough machine marks. Of course, sizing the ID accurately by honing or grinding after heat treating also controls the pin weight.

Going from Ra80 to Ra8 or so doesn't do a lot for the strength of the material in use. Finish is usually specified for it's intended use with a mating part.

One area where surface finish is extremely important is when the parts are being treated with Diamond Like Carbon (DLC) coatings to resist wear. Prior to coating, the parts need to be about Ra 0.5 or smoother or you effectively make the coated part into a "diamond file". That has very little to do with strength.

If all metal parts gained strength from smoother surfaces we would only see highly polished parts everywhere on high end engines internals and driveline parts. We don't.


My $.02

 

Yet interestingly, in the past 30 or so years, the aftermarket has made consistant efforts to improve their mold making techniques, and as a result have produced parts with progressively smoother and smoother finishes. The same metal comes out of these molds as did in the 1960's, and yet some of the parts that are being produced today have strength-to-yield ratios that are double and sometimes triple that of those which came from that era. I understand that forging techniques have improved as well, but can it really be argued that the only reason a company like say, Carrillo, would spend the money it takes to make investment castings be to just make their rods look "prettier?"

 

Most failures propogate from some sort of a surface imperfection. Thats simple failure analysis.

 

A more complex failure analysis on highly stressed parts like valve springs, for example, often points to inclusions (foreign substances), or (microscopic) voids within the material rather than external surface imperfections causing the failures. Comp Cams mass media ads point this out with photomicrographs.

One of the main reasons Vacuum Arc Remelted (VAR) material is stronger and longer lasting in highly loaded parts is because the melting in a vacuum then remelting in a vacuum goes a long way toward making the material much "cleaner" and therefore measurably stronger than the original 4340 (for example). It isn't that the metal's chemistry, hardness, surface finish etc. is changed, but rather that the material is more homogeneous.

Parts like F1 cranks, (and perhaps some Cup cranks) for example are produced from VAR steel. The mast majority of the VAR steel goes to aerospace use. In some cases production thru 2007 is already sold out.

"Putting a superfinish on "dirty" steel is a lot like polishing a turd." --The OldSStroker

 

Good call, didn't even really think about inclusions because the question was regarding surface finish....

 

I'm not a metalurgist, but I believe that polishing not only removes stress risers, it also work hardens the item. Not positive, just pretty sure.

 

Only if you do it wrong.

You might be thinking about shot peening which does work harden (adding compressive forces to) the surface, but it leaves a matte finish.

 

No, I was thinking about polishing.

Oh well...
Wouldn't be the first time I was wrong about something. Won't be the last, either.