Ceramic Coatings - Dumps and Front Pipes.

[Kinks]

Do I pass teacher?

you get 100 internet points

[GTSBoy]

you get 100 internet points

<<Put's them in pocket>>

They'll be useful later.

[Roy]

Or we could talk about stress corrosion cracking, pinhole corrosion, sulphidation, carburisation, hot corrosion, fluoride and chloride attack, and all the other attack vectors that I have to worry about when designing pressure vessels out of 304L, burner components out of 253MA and inconel (and other 600 series alloys), why we tried to use Haynes HR-120 despite the fact that it is miserable to weld and likes to crack from the root on short and shallow welds, etc.

Carbide precipitation, pinhole corrosion and stress corrosion cracking actually share some similarities in what happens. Any time you get ceramic forming from the alloying elements from the 300 series steels migrating out of the grains and into the grain boundary regions is not a good time. I had to investigate a terrible pressure vessel explosion where a nearly brand new vessel went pop. Turned out, putting it next to the seaside (industrial area near the Brisbane ports), operating it at the temperature that they did and with cyclical pressure loading inside was just the right recipe for chloride mediated pinhole corrosion. Left the grains almost completely depleted of chrome and a huge amount of chromium chloride ceramic material in between. The then pretty mild steel that was left in the grains didn't last more than a few hours. Quite exciting.

I guess your point about using argon is more about not using CO2. The presence of carbon is the obvious problem there.

Do I pass teacher?

LOL, so now try again and take all the noise outr of your post and stick to stuff that is pertinent and not about impressing us all

The biggest ticket item IMO the carbide precipitation/stress corrossion cracking during fabrication and operation of a tubular/stainless manifold

Re argon....argon is common for shielding gas, but often people dont bother purging for things like exhausts. In part because they are not worried about penetration, internal dicolouration etc but importantly its what using an argon purge does to stop atmosphere/oxygen from affecting the crystallline structure during welding.

[GTSBoy]

I thought you were going for the trick question where tight arses use CO2/argon mix instead of pure argon shielding gas for the back purge, thus inviting an increase in precipitation rather than suppression of it.

[Roy]

Not trying to trick anyone. You only have to look at the inside of manifolds to see that they are not even purged. Hell even on weld externals people play with current etc to try and get the blue tint etc on the weld for aesthetic reasons ! People need to change their thinking to colour = damage to material!

Use something like Varigon as the shielding gas and argon as the purge and provided you get the O2 down you barely even get a straw colour heat affected zone (HAZ). So knowing how the absence of colour in the HAZ is beneficial to the crystalline structure of the weld and its performance then you can start to defend stainless as a material, even when used as tubing vs pipe in exhaust manifolds.

But no need to use such expensive gases, straight argon is fine for what you are trying to achieve with a manifold

Then understanding what is going on in the HAZ during fabrication I think you can start to consider what is going on during normal manifold operation. Just like the argon shielding the weld pool when at high temperature and stopping it reacting with the atmosphere...ceramic coating performs a similar function in addition to reducing skin temperature.

It also provide more uniform temperature gradient across the material. So ceramic coating is good for thermal performance but also material performance. But it wont fudge poor fabrication or material selection.

So I touch on this stuff as I think you need to be on board with this sort of thinking before going on about stainless is crap and will crack. I personally think coating and wrapping manifolds are fine. I coated and wrapped my Trust stainless tubular exhaust manifold. I stopped wrapping as it simply wasnt durable enough for me,

[GTSBoy]

That's all fair. It does point out that we can't make blanket statements. It also points out that very careful (good quality welding technique in particular) fabrication work is required in order to be able to get away with wrapping.

Also, I wasn't personally saying that stainless is crap. I merely said that more often than not wrapping exhaust pipework leads to failure. I actually like coatings, although I have said many times in the past that I think it really needs to be on the inside of hot stuff, not the outside (or more to the point, not the outside only!).

[djr81]

Im not sure I subscribe to the wrapping causes cracking assertion. I would reckon much of the stainless manifolding will crack anyway.

Troy you mentioned stress corrosion cracking. We get that at work but mostly it is instigated by chloride or caustic attack at grain boundaries. Not sure I can figure the mechanism in an exhaust?

[GTSBoy]

Carbide precipitation caused by the welding. Depletes chrome from the grains in (effectively) the same way as chloride mediated SCC. Austenitic stainless steels, like most materials, rely on not just the chemistry of the alloy when it is made, but the time-temperature history (ie heat treatment, quenching, abuse by welding etc) to provide their basic properties.

[Roy]

Yeh, Stress Corrosion Cracking (SCC) is very commonly seen with chloride attack. But its not the only cause

The thing to consider is stainless becomes highly sensitised at high temperatures. So the cracks along grain boundaries will likely still be an issue at 700-800deg C exhaust gases in stainless in the absence of chlorides simply due to the sensitised nature of stainless at that temp.

[djr81]

Is the chrome depletion inherent in the welding process and is it effectively supplanted by the carbide deposition/precipitation? Or is it a poor process causing the problem?

Also is a different alloy a solution. Obviously Inconel works. Would a 321 be a better option?

Sorry for the numpty questions.

[GTSBoy]

What happens is that the chrome that is supposed to be evenly distributed throughout the whole metal (ie, evenly throughout each grain in the metal) is stripped away from the bulk of the grain and migrated toward the grain boundary where it forms various non-useful compounds (effectively ceramic materials, being chrome carbides or chrome chlorides, depending on which problem is going on) and leaving behind metal that is unprotected by sufficient chrome to prevent corrosion. The corrosion then causes metal to go away, leaving holes/weaknesses that lead to cracks and sometimes even more catastrophic failures. A requirement of SCC is that there must be stress (ie alternating loads and so on) but realistically, it's not good to have it happen even without that, and there is no shortage of stress in the applications we're talking about anyway.

I'm not sure if 321 specifically is much better off. It is a higher grade so starts out with more alloying component content, so has a head start there - meaning what might be enough damage to kill some 316 or 304 might not cause a similarly treated bit of 321 to fail. But it is prone to exactly the same problems as any other 300 series steel.

Inconel is a whole 'nother ball game. Inconel is not even considered to be a stainless steel. It is a high nickel alloy - being 60% nickel makes it hard to think of as a steel - there's not enough iron in it!! Inconel has different failure modes that come out to play. If you hold it at say 800-1000°C in the presence of a sulphur containing atmosphere, you can get completely unexpected failures where the metal just appears to evaporate. A 25mm thick piece can go away in just a few months under those conditions. The S attacks the chrome (again - it's almost always the chrome!!), forming a scale layer full of chrome that isn't stable and comes off. The next layer of parent metal can then corrode from simple oxidation (that inconel would normally be immune to given that its native scaling temperature is >1150°C) because of insufficient chrome. The material just loses scale until it is all gone.

[Roy]

Or is it a poor process causing the problem?

Imo it's a process control (lack of) causing the problem. Thre are various grades of 300 series stainless steel that lend themselves to better high temp operation but in instances their weldability, machinability etc vary and work against you. Plain old 316L is fine IMO

The trick is to control the oxygen. If you can keep the O2 content of the purge gas below 1000ppm and you get good penetration without missed egdes/undercut etc then you are going to get a good weld. Its not hard to do right but its easy to get wrong if you dont know what you are really trying to achieve for a good robust fabrication

If you get the O2 down around 150ppm then you will get a weld completely devoid of any discolouration.

I have posted this before... http://www.assda.asn.au/blog/wp-content/uploads/2010/05/Figure-2-AWS-image-2.jpg

Anything under 1000ppm you will easily get a 3 or better. In some semiconductor applications we need to get it down to a level 1 so have to go through with the timely purges to get down to 150ppm.

To get a shiny stainless finish most people pickle or polish the weld and for most applications that will be fine as it restores the chromium oxide layer and corrossion resistance of the materials surface.

But in applications where you care about the underlying material properties/metallurgy due to its intended duty ....you control the weld pool. Then the weld and its immediate area exhibit largely the properties of the annealed parent metal. How often have you seen tube flat out fail in any other area outside of the weld?

[Missileman]

I started out playing wit mitsubishi and Toyota which have the exhaust on the drivers side close to the brake booster, clutch master, steering rack / box etc. Coatings made a big improvement in temp control. ....... wish I never bought a nissan.

What nissan do you own, and when did you purchase it?

Edited January 8, 2015 by Missileman

[GTR_JOEY]

Sooooo a back to back comparison of ceramic vs non ceramic would be sweet

[niZmO_Man]

This reminds me of Manufacturing Engineering and Chem Material Science classes I did at uni.

[Missileman]

...at least i have learned a little about welding.

but how much should I spend on imperfect welding my manifold, per hill climb? Should I build with 303 0r 316 SS?

Does the cost of wrapping and proper ceramic coating, give me any realistic chance of improving my placing than some other competitor, who has thrown out their back seat..?

[Missileman]

Do I increase power, response and efficiency, or should I reduce mass, and increase reliability, and traction?

My money is on the later.

Guess I'm a flat white type of guy

[GTSBoy]

I would use 304L over the other 300 series choices. L for Low carbon. It has better creep resistance. You can also get 316L I think, which would probably be OK (if you can find it). The reality these days is that all 304 and 316 are probably the L grade anyway. Unless China spec. I would also do it from pipe and bends (stainless equivalent of steam pipe) rather than tube. But that's just the over-engineer in me.

Actually, the over-engineer in me would do it in 253MA, if I could find bends available in it, and if I could get by without a kidney. It's half the price of inconel, but that's not saying much by comparison with normal stainlesses. 253 is a lovely material for high temp applications.

[Ben C34]

This is the lamest wrap battle with ever.

[Missileman]

just disregard my posts and its not too bad lol