Significance of Carbon in Stainless Steel

try this link: www.wisegeek.com/what-is-high-carbon-stainless-steel.htm

I seem to recall that the call out of 316 ELC was used for stress corrosion resistance for certain applications with pump casings, seals, seal sleeves and impellers. The Extra Low Carbon determination was evidently beneficial based on certain services.

I once helped design a piping system for concentrated H₂SO₄. We specified 304 ELC pipe for improved corrosion resistance.

304 is stronger than 316 - in the marine field 316 is specified unless more strength is required - however 304 rusts and requires more polishing!

304 rusts - usually only in the presence of salt and heat. We use 304 on all of our on-road exhaust systems - with a lifetime corrosion guarantee.

We use 316L for marine and spark arrestor (ATEX: 94/9/EC) applications.

Stainless Steel will rust when other substances are introduced. Like using a grinder that was previously used on mild steel then using it on stainless steel or storing mild steel along with the stainless steel and they are in contact with one another.

The price of nickle is the cost driver in the price of stainless steel.

Another factor is price. 304 SS is cheaper than 316 Stainless steel. Also 304 is easier to form and weld than 316. However, 316 is more resistant to corrosion. Strength camparison between the two alloys isn't really the factor in choosing between the two, it's the corrosion resistance factor. 304 is the most commonly used. However if the environment has a high corrosive factor then choose the 316.

It's 201 or 208 Stainless Steel( I don't remember which) is slightly stronger than 304 and handles much the same as 304 and is food grade certified. It is also cheaper than 304, however it is harder to come by.

The different grades of Stainless Steel are designed for different applications.

There is also 416T Stainless Steel, which is a mirror surfaced Stainless Steel comprised of two Stainless Steel laminates with a layer of mild steel in between. If you should weld or grind the surface, you can expose the inside and get rust. It is can be formed just fine. 416T is primarily used for cosmetic applicatoins rather than structural.

You should get a Metals Handbook. It goes over the purpose of most of the major alloying elements (and carbon too I think).

If I remember right though, carbon is used for hardness. This isn't homework is it?

Carbon content effects / determines the type of steel you have. Also, the amount of hardening required. . .

. . . why are we answering this? Does Google only work for Engineers? Never mind. I suppose I get it. I can imagine the scenario. Some guy sitting in a hole some were in, I don't know. . . Louisiana, wants to find out about something that he knows nothing about. He types the question into Google and gets a million results, but he has no filter. He can't tell what's relevant or inter related. He can't pick the diamond out of the sand. Even on a beach COVERED in diamonds. So he comes here. To a place populated with boffins. Here he believes he will get handed the information he needed without ever developing a filter of his own (through hard work and study). So, what do we do? Well, most of the time we remember it, but we might Google it just to confirm some of the numbers, and lo-and-behold, there it is. Fully explained in graphic detail, on the very first link. There is more information, in greater detail, than I am prepaired to research and write out for this Yahoo, why the heck didn't he just Google it?!? Heck, I'll just post a link. The link I will post will contain all the informatin he needs. Maybe he'll read it. Probobly not. It's really not his fault though. He dosen't understand that half-way down the page is the answer he needs, but that he would be better off to read the whole thing. All he can see, is sand.

http://en.wikipedia.org/wiki/Stainless_steel

-A-

Well if more replies came with the image of the chair on the beach (with its occupant) as shown in your reply #8 then we all would ask questions.

Lower level of carbon results in a better weld.

hi all

lowering the carbon content in stainless steels is to improve the sensitization( intergranular corrosion), but decreases the ultimate tensile stength and the yield stress.

QA/QC ENG

Amr Soliman

Petrojet-Egypt

Insertion of any alloy material (including carbon) changes the mechanical, physical, and corrosion properties of the alloy. Properties of stainless steel change with carbon percentage as well as any of the other components used. The choice of stainless steel must be dictated by the requirements of the application (remember that if carbon goes down, something else must go up to keep 100% mixture) and not just by the carbon content. These are questions that can best be answered in any metalurgical text, on-line research, or by your steel supplier (assuming you have one). Purchasing a stainless steel based only on its carbon content can lead to disastrous results.

To echo -A- and others who posted here, do a little research on your own before tossing out a general question that may not have relevance to your specific requirements.

Obviously, changing the carbon content of any steel effects the properties such as strength, ductility and malleability. In addition, inlcusion of a variety of other compounds can effect reactivity and corrosion as well as strength, ductility and malleability also. The following links might help address much of the discussion above on this blog and help answer the initial question generally.

http://en.wikipedia.org/wiki/Stainless_steel

http://en.wikipedia.org/wiki/SAE_steel_grades

for more detail try this:

http://www.astm.org/BOOKSTORE/COMPS/101.htm

To add up further to what others have said, I would like to point out that,

Carbon promotes the formation of austenite for all steels including SS. It also forms a carbide with Chromium that can lead to intergranular corrosion.

When austenite SS are heated or cooled slowly through the temperature range of 425-870⁰ C (also known as sensitising temperature), the Carbon precipitates from the solid solution mainly at the grain boundaries, where it combines intergranularly with Chromium to form chromium rich carbide.

As a result, the Chromium in the adjacent matrix gets depleted and steel suffers localised intergranular attack (also known as inter crystalline corrosion), when it is exposed to certain corrosive service conditions.

The severity of such attack depends on the time and temperature of exposure, as also on the composition and heat-treated condition of the steel.

Carbide precipitation, in austenite stainless steel weld-ments could be got rid of, if they are meant to to be used in corrosive media, by using a extra low carbon grade Stainless Steel material (carbon up to 0.03%), or by using a stabilized SS with Columbium plus Tantalum or with Titanium. Alternatively the job should be Solution heat treated, but seldom done due to other complications like thickness factor and size of the job..

Hence we can cay that, there will not be any significant changes in mechanical properties due to little reduction in Carbon percentage.

But by using low carbon version, Steels are rendered sufficiently immune to carbide precipitation in the sensitising temperature of 425-870⁰ C.

The steels can then be safely welded without loss of corrosion resistance. Examples are AISI 304L and 316L types and usually employed for service below 425⁰ C. The filler metal used for welding them should also be ELC types.

Hope above clarification will help you.

Sridhar.

Good point, should have mentioned heat treatments also, especially welding. Weld SS is a critical weak point in construction of a variety of systems. If not careful the welding processes can substantially change the corrosion resistance of the SS at the welded joint and you have a serious problem in a very short time frame, relative to service life.

Thank You RCE for your comments. I will keep in mind about HT also. regards

Sridhar.

To avoid chloride or caustic induced SCC (Stress Corrosion Cracking) in SS, we can have stress relief heat treatment. As already mentioned earlier, this is not very easy.The minimum effective temperature is 900⁰C and this induces complications like distortion and re introduces significant residual stresses on cooling. Even if perfect stress relief is achieved, the service stresses themselves may be adequate to promote SCC.

Alternatively Solution Annealing could be done. But austenite SS fabrications rarely require post weld heat treatment. Critical fabrications are sometimes solution annealed to redissolve the precipitated carbides in the HAZ, which may cause inter crystalline corrosion in service, or to relieve the stresses that may lead to stress corrosion in service.

The recommended solution annealing temperatures are:

For type 302,304, 304L, 308 etc. the temp. is 1,010-1,120⁰C.

for type 309, 316 etc. the temp. is 1,038-1,120⁰C.

for type 316L the temp. is 1,038-1,110⁰C.

for type 321 the temp. is 954-1,066⁰C and like wise.

The soaking temperatures should be approximately 3 minutes for every 2.50 mm thickness. But care must be taken to cool the assembly rapidly and uniformly through 900-425⁰C range to avoid carbide precipitation.

Thick jobs should be water sprayed or quenched, though air cooling is suitable to thin sections.

The furnace should have adequate dimensions,efficient handling equipment and right cooling arrangements. If the cooling is non-uniform, it can give rise to increased residual stresses and distortion.

Thin sections must be well supported with proper fixtures during high temperature exposure to prevent sagging. If the surface develops any oxidation, then that area must be thoroughly removed by short-blasting, acid pickling or by machining.

Equally important is to ensure that the surfaces prior to Welding / Fabrication are free from carbon and hydrocarbon contamination caused by grease, oil, waxes, cutting fluids, primers etc. and this could be achieved by solvent cleaning or pickling with nitric acid solution of 10-20% strength.

Similarly, weld reinforcement on stainless steel welds must be ground out if required, as they act as stress risers and lead to stress corrosion when exposed to certain corrosive atmospheres and chemicals.

Especially the surfaces directly in contact must be buffed by Calico mops, bobs or continuous emery bands. to a degree required by service conditions to give a long life to the weld joint.

Sridhar.