This I feel is so dumb that I can't believe I'm even asking, but I'm open to learning new things. LOL I know that we shot peen rod and cranks to make them more durable in high stress situations, I did this to my first race engine, but I'm wondering if cast iron exhaust manifolds can benefit from shot peening, can they be made to resist cracking by shot peening? Any input would be appreciated, and yes you may laugh Thanks

Not a stupid question.
It makes sense.

Cracks usually form on the surface when it become too tight or stretched too much and then the surface splits which is the start of the crack.
Shot peening causes the surface of the metal to spread out and this relieves surface tension. Reducing the possibility of a crack forming.

It works in elastic metals like steel which can bend and are more malleable but I do not know if it would on cast iron because cast iron CAN be brittle and it may cause cracks. Emphasis of CAN because some cast irons are softer depending on the materials. Exhaust manifolds are pretty brittle though.

So it's good question and I know what the books and classes taught me but not sure of the correct answer.

I found a great article on shot peening cast iron car parts.

It down loads a PDF of the article

https://www.diva-portal.org › get › FULLTEXT01

From the article:


Shot peening of cast iron.
One ought to think for a material like cast iron that is so widely used for so many
years (over 2000 years), everything has already been done. Probably this is true, but
a lot of work on cast iron has been done by companies and the results have not
become public. Thus there exists a gap in knowledge in the scientific community
today about cast irons. During the last 15 years of research on cast iron, the main
area of interest has been in the solidification process and how to affect it to get the
best mechanical properties (static). Surprisingly few articles discuss how the fatigue
properties vary under different loading for the different cast irons available today.
Even fewer articles are found in the topic of shot peening of cast iron. Most of the
articles deliberate a nodular cast iron with various microstructures under different
peening conditions [55-61]. One of the earliest publications concerning shot
peening and cast iron, showed that the benefits of shot peening on the fatigue life
of nodular casting [62] is three times better than blasting. A more academic study
[56] on the shot peening effects on fatigue strength showed increased fatigue
strength of 7 to 30 % depending on microstructure. Another article also dealing
with shot peening of nodular cast iron with different microstructures resulted in 30
to 115 % increase in fatigue strength [59]. Additional work on different nodular cast
iron subjected to shot peening showed an increase in fatigue strength between 20
to 50 % once again depending on the microstructure [55]. A 60 % improvement on
the fatigue strength was shown on a Cu-Ni austempered nodular cast iron [63].
As a closing comment I would like to state that even though the existent of articles
on grey cast iron, or compacted graphite iron, is absent in the context of shot
peening and fatigue strength, it can be done and result in increased fatigue strength
with the right parameters.

And, they only dumb questions are those that go unasked!

Heck, I'll ask this: is warping the same as fatigue ?
I think of fatigue as a problem from cycling (tension/compression or stretching).
I'm not sure warpage - and subsequent cracking from fasteners remaining tight - is the same thing.
I've usually found the warpage, if caught before the cast iron cracks, fixable by planing the manifold sealing surface.

I would do a little research into this. We used to send out our slitter and chopper knives in the steel mill with great success


https://trc.nist.gov/cryogenics/aboutCryogenics.html

I am not saying I am right so feel free to say I am wrong.

I always felt that the casting process left tension in the material as it cooled. To reduce that tension it would have to be cooled down slowly over a long time. Refineries don't have multiple days to wait for most products so tension is left in the metal parts.

The machining process removes metal and creates a flat shape but it does not remove the tension. Exhaust manifolds get hot enough to let the metal move and the tension comes out as warpage. Things can be done to reduce it like stiffening ribs to control metal curl or twist but often engineers don't see the defects in field tests.

That is how I always figured it happened for exhaust manifolds.

Many other parts it would probably be fatigue deformation like you said.

Fatigue can cause warpage, but warpage and fatique are two different things, like you already said.

Those are the words of motivational speakers.
Lies.
There are plenty of stupid questions. To deny that is stupid unto itself.
The OPs question is NOT stupid. It raised an issue that I've never heard of and it has merit.

Thanks alot for the input, I just couldn't wrap my head around this given an exhaust manifold rarely has any moving parts, and everything I've ever seen that was shot peened either had moving parts or was a moving part, i e crankshafts and connecting rods. Might be worth a try. This is very interesting, thanks again!

You need to know and or remember their are different alloys of cast iron, Compact Graphite Iron is way stronger than cast grey iron, cast ductile iron is different than the first two also..
I know very little about those so before deciding to do anything to help any of them I would do a lot of research to make sure you, I, us, get it right to start with on all aspects of the alloy chosen to use

Chrysler used to let the flat head block castings sit for a year or so to season before they machined them. Wonder why?

Rods and cranks are not hollow, an exhaust manifold is. I'd be nervous that the shotblasting could break the manifold, maybe if it was filled with something that would not allow it to flex much it may be doable?

With that in mind , how are cranks and rods shot peened, is it also referred to shotblasting ? I've had a block shotblasted .

I worked in a foundry that cast iron gear boxes. After the actual casting process, each casting was tumbled through a screen and was sandblasted at the next screen to remove some of the flashing and to make the outer case uniform in color. Everything there was sand blasted.

At a different factory, we made plow shares (like used on farm field plows) Those were shot peened, we had a 12" round table blaster with a 1/2" thick rubber on top of the table to protect it You put the par on the table, as it rotated and the part went through a rubber curtain through the blaster, and came out through a rubber curtain blasted. Shot blast is normally done with sand as a blasted element, shot peened would use steel or hard brass bee bees as a blast element, depending on the material being blasted. The peening provides a deeper hard surface then a blast (by a couple thousandths). Both processes throw the element at the item through a high speed belt powered blower wheel. Those blast machines self destruct, they need to have the special alloy material armor replaced about every year or two. I worked in maintenance there, replacing the armor sucked, especially when the company thought they could stretch out the replacement time. Our machine used steel bee bees. It would blow through a piece of 1/2" thick steel plate in a day if the steel was in line with the blast wheel.

Back in those years I ran many exhaust manifolds through the shot peen table. it cleaned up those rusty exhaust manifolds really nice. Did a pretty good job on car wheels too. I had no other purpose other then cleaning those things up, so I can't tell you if it made things stronger. I can tell you we used those manifolds on a dirt track and never broke an exhaust manifold like some of the Chevy and Ford guys experienced. I assumed it was because our manifolds were Mopar stuff. I can tell you our manifolds were cherry red after each feature race. The only 2bbl carbs we were allowed to run just couldn't feed the motors enough gas. If we would have been able to use the same carbs as the other guy, they probably wouldn't had been able to keep up with us, as it was, we gave them a hard run.