Why stainless instead of carbon steel for manifolds?
Why stainless instead of carbon steel for manifolds?
Just wondering whats everyones thoughts on using stainless instead of carbon steel. Lets here the sig worthy material thanks.
Material properties aside, stainless 304 weld-els are actually cheaper then mild steel and 316 is the same price. Cost alone on a weld-el manifold puts stainless in the win column.
With regards to material properties:
Stainless Steel has alloy elements that help prevent carbon precipitation hardening of the base metal. Mild steel will pick up carbon from the carbon dioxide gas flowing in it. This carbon bonds to the iron causing cementite, or iron carbide. This is a ceramic material which is extremely hard and brittle. Hard materials, while generally stronger, are very susceptible to cracking under tensile loads. While carbon precipitation hardening occurs at low temperatures, high temperatures greatly increase the diffusion rate at which this happens. This is why products used at high temperature rarely use mild steel.
The alloy constituents used determine the grade of stainless material. Chromium and nickel are common, but titanium and molybdenum are also used in some grades. The purpose of these constituents are primarily to break up the grain structure to increase strength, but also, they often have an affinity to carbon so carbon that is diffusing into the base metal will want to bond to the alloy elements and not the base iron.
Basically, there is a lot of science behind materials and there is a reason stainless steel is unanimously determined as a superior material over mild steel in exhaust manifolds. The important question to ask is what are the different types of stainless to use and not why nobody uses mild steel.
With regards to material properties:
Stainless Steel has alloy elements that help prevent carbon precipitation hardening of the base metal. Mild steel will pick up carbon from the carbon dioxide gas flowing in it. This carbon bonds to the iron causing cementite, or iron carbide. This is a ceramic material which is extremely hard and brittle. Hard materials, while generally stronger, are very susceptible to cracking under tensile loads. While carbon precipitation hardening occurs at low temperatures, high temperatures greatly increase the diffusion rate at which this happens. This is why products used at high temperature rarely use mild steel.
The alloy constituents used determine the grade of stainless material. Chromium and nickel are common, but titanium and molybdenum are also used in some grades. The purpose of these constituents are primarily to break up the grain structure to increase strength, but also, they often have an affinity to carbon so carbon that is diffusing into the base metal will want to bond to the alloy elements and not the base iron.
Basically, there is a lot of science behind materials and there is a reason stainless steel is unanimously determined as a superior material over mild steel in exhaust manifolds. The important question to ask is what are the different types of stainless to use and not why nobody uses mild steel.
Well being that I work in the pipe fitting and welding world. I have barely seen stainless get used unless its food grade and on a few jobs that are for refining air. Most of the jobs Ive been on, which are almost all chemical refining jobs are mild steel and very high corrosive products use incoloy. Some carrying product at a higher temperature than our little 4 bangers will produce even under pressure and heat.
I can find weld els in mild way cheaper than stainless?!?!?
Another thing to add, once stainless is heated to a certain temp, welding, and boost temperatures it cooks out the chromium and begins to have "mild" steel properties. Take a piece of stainless thats not been welded and you cant pick it up with a magnet, then weld it, all the sudden it has a magnetic property. Same way with manifolds, if people would actually drive their cars and drive them hard instead of being a nut swinging contest they would see that after a good few heat cycles a form of rust will start appearing on the manifold particularly around the welds where the chromium has already been cooked out.
I can find weld els in mild way cheaper than stainless?!?!?
Another thing to add, once stainless is heated to a certain temp, welding, and boost temperatures it cooks out the chromium and begins to have "mild" steel properties. Take a piece of stainless thats not been welded and you cant pick it up with a magnet, then weld it, all the sudden it has a magnetic property. Same way with manifolds, if people would actually drive their cars and drive them hard instead of being a nut swinging contest they would see that after a good few heat cycles a form of rust will start appearing on the manifold particularly around the welds where the chromium has already been cooked out.
Last edited by canitbethislong; Nov 25, 2012 at 12:15 PM.
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how does cast iron compare to stainless manifolds? (heat, life span etc) I own a JDM Evo 7, I was wondering if the factory manifold is cast iron?
Last edited by EvoTurboTurk; Nov 27, 2012 at 05:41 AM.
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The chromium doesn't "cook out" it simply bonds to carbon, again, exactly what I said it does above. What you are suggesting is that there is mass transfer out of the metal and that is incorrect.
As far as being magnetic, many 300 series stainless materials go magnetic from work hardening too. Are you telling me simply because it has magnetic properties it is no longer stainless steel? Because if you are telling me that, you are wrong.
A oxide passivation layer will form on the surface (I suppose you could call this "rust") but once that passivation layer forms, very little further oxidization takes place. Sure, you might get superficial oxidization that looks like corrosion, but 99.5% of the rest of the material in that cross section is oxide free. Mild steel on the other hand will rust beyond a superficial layer on the surface. Also, Mild Steel exhaust components rust from the inside out usually. This is due largely to the water vapor that exists in the exhaust of a gasoline engine. The water vapor combines with other by-products of combustion and forms acids and electrolytes that promote corrosion.
As for cost, I'm just going off McMaster Carr pricing. I'm sure there are cheaper places, care to share where you are getting your prices from where the mild steel elbows in this size are cheaper? I agree, mild steel SHOULD be cheaper, but it must be that McMaster moves more stainless in this case and it makes them cheaper overall for them to sell?
Schedule 10
1-1/4" 304 stainless - long radius elbow 90* - Butt weld = $9.36 EA
1-1/4" 316 stainless - long radius elbow 90* - Butt weld = $13.28 EA
Schedule 40
1-1/4" 304 stainless - long radius elbow 90* - Butt weld = $10.26 EA
1-1/4" Mild Steel - long radius elbow 90* - Butt weld = $12.46 EA
1-1/4" 316 stainless - long radius elbow 90* - Butt weld = $12.59 EA
With regards to applications you know of seeing more corrosive materials being transferred in mild steel. This might actually be for reasons you are unaware of. Some chemicals will attack stainless steel but not mild steel due to the alloy elements, often related to hydrogen embrittlement. Often times, the alternative material in these situations is titanium or inconel, at that point it is just more economical to double or triple the wall thickness to deal with the strength lost at high temperatures and use mild steel.
I don't work in the pipe fitting field, but I do understand the fundamental processes that are going on at the material level. I also understand corrosion and high temperature creep.
If weight isn't a concern, I'd say go ahead and jump to schedule 40 mild steel...if it was cheaper. Primarily because it's easier to work with and then just ceramic coat it to protect it from corrosion on the external surfaces so it looks pretty. But if the stainless weld-els are cheaper, I see no point to go mild steel and instead I'd go to a thinner wall section to make the manifold lighter.
As far as being magnetic, many 300 series stainless materials go magnetic from work hardening too. Are you telling me simply because it has magnetic properties it is no longer stainless steel? Because if you are telling me that, you are wrong.
A oxide passivation layer will form on the surface (I suppose you could call this "rust") but once that passivation layer forms, very little further oxidization takes place. Sure, you might get superficial oxidization that looks like corrosion, but 99.5% of the rest of the material in that cross section is oxide free. Mild steel on the other hand will rust beyond a superficial layer on the surface. Also, Mild Steel exhaust components rust from the inside out usually. This is due largely to the water vapor that exists in the exhaust of a gasoline engine. The water vapor combines with other by-products of combustion and forms acids and electrolytes that promote corrosion.
As for cost, I'm just going off McMaster Carr pricing. I'm sure there are cheaper places, care to share where you are getting your prices from where the mild steel elbows in this size are cheaper? I agree, mild steel SHOULD be cheaper, but it must be that McMaster moves more stainless in this case and it makes them cheaper overall for them to sell?
Schedule 10
1-1/4" 304 stainless - long radius elbow 90* - Butt weld = $9.36 EA
1-1/4" 316 stainless - long radius elbow 90* - Butt weld = $13.28 EA
Schedule 40
1-1/4" 304 stainless - long radius elbow 90* - Butt weld = $10.26 EA
1-1/4" Mild Steel - long radius elbow 90* - Butt weld = $12.46 EA
1-1/4" 316 stainless - long radius elbow 90* - Butt weld = $12.59 EA
With regards to applications you know of seeing more corrosive materials being transferred in mild steel. This might actually be for reasons you are unaware of. Some chemicals will attack stainless steel but not mild steel due to the alloy elements, often related to hydrogen embrittlement. Often times, the alternative material in these situations is titanium or inconel, at that point it is just more economical to double or triple the wall thickness to deal with the strength lost at high temperatures and use mild steel.
I don't work in the pipe fitting field, but I do understand the fundamental processes that are going on at the material level. I also understand corrosion and high temperature creep.
If weight isn't a concern, I'd say go ahead and jump to schedule 40 mild steel...if it was cheaper. Primarily because it's easier to work with and then just ceramic coat it to protect it from corrosion on the external surfaces so it looks pretty. But if the stainless weld-els are cheaper, I see no point to go mild steel and instead I'd go to a thinner wall section to make the manifold lighter.
Last edited by 03whitegsr; Nov 27, 2012 at 08:26 AM.
No did not say it's losing it's stainless properties it takes on more of a mild property.
I have my reasoning for why most builders use stainless instead of mild. With a ceramics coating inside and out, without the cost of purge a lot of things start to equal out.
Cooking out is a term we use when in the welding field.
I have my reasoning for why most builders use stainless instead of mild. With a ceramics coating inside and out, without the cost of purge a lot of things start to equal out.
Cooking out is a term we use when in the welding field.
You speak as though your a metallurgic engineer, you know lots of the properties. Its like someone that has gone to school to learn about n.a.s.a. and fly shuttles according to paper and someone that has built them and know that paper is not the only thing that works good.
I shoot competitions with long range high powered rifles. On paper with a 115gr dtac my .243 looked like it was unbeatable. Thats compared to the 7mm bullets, and the 6.5mm bullets when it comes to drift and drop. That isnt the case, at 450yds I couldnt knock down a blackhawk 50%ipsc target. With my .260 I plowed it over, how so my 243 is faster and on paper hits harder.
When we build systems that will see 1500+* heat and **** so toxic it will turn your bone marrow into puddy and eat your body from the inside out, and stuff that is man made and is 99.99% pure. Our systems are all mild. We occasionally use stainless, like I said on food grade, and anything that has to do with purifying air or seperation of air. Much like a hydrogen plant, that was mainly stainless. When we have **** thats super toxic and corrosive and alot of heat we usually use incoloy.
I understand your theory of chromium not being theoretically cooked out of a metal. But just like aluminum which is another metal not used in turbo manifolds. Why, aluminum pipe can with stand the heat our cars push out of the exhaust and well hell it would be super light too. Ceramic coat the inside of the manifold so the heat doesnt "sink" in and destroy the metals properties.
What happens with these cars we have is we introduce them into the "unnormal" extremes, fuels, carbon, heat, pressure and weight. The metals we use stainless, give it long enough and being heated enough then toss it out in the rain and watch it rust away.
I doubt that a mild manifold is going to close up with exhaust suit. And all the other "normal" issues related to with mild steel in a controlled environment.
My theory is that most of the manifold builders are building their manifolds off of the "nutswinging" crowd and looks instead of functionability. I would much rather have a mild manifold welded up properly, ceramic coated inside the runners and outside. Then have a stainless polished manifold that costs 12-1500 and in 2-3 weeks of "good" hard usage it starts to look like the beautiful finish of rust. And take the chance of a improper weld up of a stainless piece.
Im not really trying to turn this into a metallurgy debate, Im curious to see what other people think the reasons are and maybe inform the public of a few different options.
I shoot competitions with long range high powered rifles. On paper with a 115gr dtac my .243 looked like it was unbeatable. Thats compared to the 7mm bullets, and the 6.5mm bullets when it comes to drift and drop. That isnt the case, at 450yds I couldnt knock down a blackhawk 50%ipsc target. With my .260 I plowed it over, how so my 243 is faster and on paper hits harder.
When we build systems that will see 1500+* heat and **** so toxic it will turn your bone marrow into puddy and eat your body from the inside out, and stuff that is man made and is 99.99% pure. Our systems are all mild. We occasionally use stainless, like I said on food grade, and anything that has to do with purifying air or seperation of air. Much like a hydrogen plant, that was mainly stainless. When we have **** thats super toxic and corrosive and alot of heat we usually use incoloy.
I understand your theory of chromium not being theoretically cooked out of a metal. But just like aluminum which is another metal not used in turbo manifolds. Why, aluminum pipe can with stand the heat our cars push out of the exhaust and well hell it would be super light too. Ceramic coat the inside of the manifold so the heat doesnt "sink" in and destroy the metals properties.
What happens with these cars we have is we introduce them into the "unnormal" extremes, fuels, carbon, heat, pressure and weight. The metals we use stainless, give it long enough and being heated enough then toss it out in the rain and watch it rust away.
I doubt that a mild manifold is going to close up with exhaust suit. And all the other "normal" issues related to with mild steel in a controlled environment.
My theory is that most of the manifold builders are building their manifolds off of the "nutswinging" crowd and looks instead of functionability. I would much rather have a mild manifold welded up properly, ceramic coated inside the runners and outside. Then have a stainless polished manifold that costs 12-1500 and in 2-3 weeks of "good" hard usage it starts to look like the beautiful finish of rust. And take the chance of a improper weld up of a stainless piece.
Im not really trying to turn this into a metallurgy debate, Im curious to see what other people think the reasons are and maybe inform the public of a few different options.
Yes the factory setup is cast iron, in theory you have one of the strongest exhaust manifolds available. It may not be the best bang for your power potentials if say your trying to run 7.5's in a drag car setup but for 99% of the general evo enthusiasts it would be more than suitable.
The factory manifold is not cast iron. It is cast A5N Ni-Resist alloy if I recall correctly. There is a difference between cast iron, cast steel, and cast steel alloys.
Again, where are these sources that make the mild steel cheaper? Further, who coats the inside of manifolds? Last I checked, the stuff that can actually hold up to 2000*F has to be flame sprayed on and you can't really do that inside of an intricate tube. I’m not saying it can’t be done, just I wasn’t aware it could be and if you can provide a source, I would be interested. Also, most of those ceramics are actually engineered to work with stainless materials with their lower expansion rates. Using them on mild steel could lead to the ceramic material cracking off the base metal.
I'm not a materials engineer. I do have a master's degree in mechanical engineering with an emphasis on materials and corrosion though. That does not make me an expert by any means in my own mind nor should it in anybody else’s. I'm simply asking for your data to prove your ideas, or at the very least a sound reason to your hypothesis.
Again though, this all seems academic. Stainless steel is cheaper as far as I can see. Backpurging it isn’t that big of a deal and it ensures a smooth internal flow surface anyway. As far as poor welds, well, if you backpurge and get full penetration, that's going to be enough for a 100k mile header when using pipe.
Now if you are talking 0.049" wall stainless, you could have a good point with the quality of weld argument.
Again, where are these sources that make the mild steel cheaper? Further, who coats the inside of manifolds? Last I checked, the stuff that can actually hold up to 2000*F has to be flame sprayed on and you can't really do that inside of an intricate tube. I’m not saying it can’t be done, just I wasn’t aware it could be and if you can provide a source, I would be interested. Also, most of those ceramics are actually engineered to work with stainless materials with their lower expansion rates. Using them on mild steel could lead to the ceramic material cracking off the base metal.
I'm not a materials engineer. I do have a master's degree in mechanical engineering with an emphasis on materials and corrosion though. That does not make me an expert by any means in my own mind nor should it in anybody else’s. I'm simply asking for your data to prove your ideas, or at the very least a sound reason to your hypothesis.
Again though, this all seems academic. Stainless steel is cheaper as far as I can see. Backpurging it isn’t that big of a deal and it ensures a smooth internal flow surface anyway. As far as poor welds, well, if you backpurge and get full penetration, that's going to be enough for a 100k mile header when using pipe.
Now if you are talking 0.049" wall stainless, you could have a good point with the quality of weld argument.
6.75 for ss 90
7.75 for cs 90
10.63/ft ss
6.50/ft cs
Liner expansion for cs and ss per 10^-6 in/in F°
Cs 7.3
304L 9.6
310L 9.0
316L 8.9
So if ceramic coatings are made for ss then they will be fine for cs is what that shows me. Also I've thermally ceramic coated one if my manifolds made of ss internally and it appears to have been fine without losing any coating from flaking.I guess I could cut the manifold apart for you and send it off to get tested and see how much actually stayed.
Either way I'm still looking for a reason that people don't use mild.
7.75 for cs 90
10.63/ft ss
6.50/ft cs
Liner expansion for cs and ss per 10^-6 in/in F°
Cs 7.3
304L 9.6
310L 9.0
316L 8.9
So if ceramic coatings are made for ss then they will be fine for cs is what that shows me. Also I've thermally ceramic coated one if my manifolds made of ss internally and it appears to have been fine without losing any coating from flaking.I guess I could cut the manifold apart for you and send it off to get tested and see how much actually stayed.
Either way I'm still looking for a reason that people don't use mild.
A key component of exhaust material selection is how well it retains heat. This may not be something you're concerned about in your industry, but keeping heat in the tube makes more power regardless of N/A or forced induction. I'm not talking about lower underhood temps either, although that is condusive to power production. Keeping the heat in, increases exh. velocity which helps scavenging if you talking N/A or spooling a turbo with forced induction. One of the desirable properties of stainless over mild steel is for a header is exactly this. Less heat gets out of the pipe. If you think about the amperage needed to properly weld aluminum, mild steel or stainless, you'll get what I'm saying. The material that requires the least amount of heat to weld, retains the most heat. Given equall thickness of material to be welded, stainless requires alot less amperage. If I weld 16ga 304 with 62amps, mild steel will require around 80amps and aluminum will require around 130amps for similar penetration.