Definition of stress corrosion 6

[trainguy]

I'm having a brain freeze. I'm getting confused between carbide precipitation and stress corrosion. Can someone give me a basic definition of these?

Structural engineer in distress...

tg

 

[Metalguy]

Carbide precip. is merely the formation of carbides. It can be harmful in SS because it robs the grain boundary areas of Cr, which gives corr. resistance.

SCC is a crack from combined stress and corrosion-neither of which alone may cause cracking.

Time to eat-more later.

 

[unclesyd]

This is by no means a complete discussion of either topic and feel free to post any additional question you might have.

The basic difference is that Carbide Precipitation is chemical/metallurgical change in a metal, mainly SS, where chromium carbides form close to the grain boundries and being insoluble in the metal matrix precipitate at the grain boundries. The formation of these chromium carbides causes a depletion of chromium near the grain boundries. If you have less than 10%-11% chromium in an iron alloy the material losses it's corrosion resistance. A corrodent can attack the chromium depleted area in the form of corrosion called Intergranular Corrosion. IGC is quite prevalent in the HAZ of Austenitic SS, not of the stabilized or "L" grades.

Stress Corrosion is where a metal is usually under very high tensile stresses in an enviroment that is somewhat corrosive and with the increase in stress accelerates the corrosion rate several fold. The off shoot of this that stress corrosion usually precedes Stress Corrosion Cracking or Corrosion Fatigue. Again in SS it is often seen as very shallow pits and a general rusty appearance.

 

[metengr]

To possibly add to unclesyd's description of SCC, the issue is crack propagation in a material caused by the interaction of tensile stress, susceptible material and environment. You need to have all three of these conditions for the SCC mechanism to ocurr. If you remove any of the three conditions, SCC will cease.

The definition of carbide precip provided by unclesyd is complete. I would add that carbide precip is one of the three conditions mentioned above (susceptible material) that could result in SCC.

 

[Metalguy]

SCC is either TGSCC (trans-granular) or IGSCC (intergranular). Both types can sometimes be found along the same crack path. It can get very interesting, but I suspect you just want a brief idea of what happens.

There are rare cases where carbide precip. along GB's is actually beneficial.

 

[trainguy]

Very interesting, informative, and quick! Stars for everyone, on me.

I'm guessing then that cracks adjacent to a welded repair patch on an austenitic SS railcar underframe component are probably due to carbide precipitation and subsequent intergranular corrosion. Tensile stresses at this particular location are low, but the environment can be corrosive.

Does that sound sensible?

Please note that we are getting a sample analyzed and we will hopefully get some input from the testing company's materials engineer.

Thank you all very much.

tg

 

[Metalguy]

Yes, esp. if Cl is involved. Seems it can cause SCC just "by looking" at a piece of 304! You get IGSCC near the weld, and TGSCC away from it.

The applied stresses may be low, but the RESIDUAL welding stresses are most likely right at the base metal yield strength level!

 

[metman]

trainguy,
you are quick to understand. As metengr noted you could have sucseptible material if it is not a stabilized S.S. grade. Carbide precip occurs when the unstabilized grades are heated (by welding can do it)in the 800-1500F range.

Jesus is THE life,
Leonard

 

[trainguy]

Looks like we have an easily defendable argument after looking at an old spectrometer test result of a similar undamaged component. The analysis points to 304 SS, with 0.058 Carbon, so definitely not 304L.

They simply should not have welded the repair patch.

I 'm still waiting on a chemical test of a sample drilled right at the weld's HAZ, and my guess is that the Cr count will be low. That would seal the case shut, if it isn't already.

I'll keep you posted.

Thanks again.

tg

 

[Metalguy]

Trainguy,

The Cr will probably be normal (~18%) in the HAZ if you analyze a drilled sample. The Cr doesn't disappear-it just gets tied up with C in the form of a carbide. One atom of C usually ties up 4 of Cr. While the carbides themselves resist corrosion, the GB area which supplied the Cr behaves more like carbon steel! The carbides are microscopic--but you drill probably isn't.

 

[trainguy]

Thanks, Metalguy.

Maybe I should just stick to structural eng., and stop pretending to be a metals eng!

I wish there were a way to unsend the Email to my client with my erroneus hypotheses!

I appreciate all the input.

tg

 

[Metalguy]

I don't think you need to prove anything more, given the high C content. But there are some relatively inexpensive tests for sensitization that a met. lab. can do. They start with ASTM A262 part A, and it sill probably fail that for sure.

Just send your client a "corrected" e-mail.

And don't feel bad-I hate math!

 

[metman]

Mtalguy,
I gave you a star for your elegant explanation on March 5 & 6 and especially appreciated "The carbides are microscopic--but you drill probably isn't." I love it! That comment makes up for a whole googleplex of math and BTW you faired more than tolerable with your C vs Cr ratio so you do know a little math.

Jesus is THE life,
Leonard

 

[Metalguy]

Metman,
Thanks. I'm just glad Trainguy didn't want/need a long dissertation on carbide nucleation or M/C ratios!

So, here's how YOU can earn a star: Under what circumstances (metal and usage) would GB carbides actually be beneficial? I work with something which requires a 5 hr. HT at 700+ deg. C to force carbide precip. along the GBs.

 

[trainguy]

Hi guys, I'm back on this topic.

Is there a weld procedure out there that will permit fillet welding (overhead and vertical) onto 304 stainless steel, given a corrosive environment (train track - salt etc.)?

I ask this because I'm worried that someone will say: The original car builders DID weld this 304 together, and it's NOT cracked everywhere...

The arrangement (simplified for discussion) is a rectangular patch with welds on the short sides, and on one long side supplemented with some plug welds. The patch is applied to a railcar underframe longitudinal member in a region usually in compression.

Thanks in advance,
tg

 

[metengr]

trainguy;
If you are asking could you weld a 304 ss patch plate on top of 304 ss base metal that has been in service, the answer is yes. Prior to welding, I would clean the surface of the 304 ss base metal to remove any residue/contaminants and inspect the surface using Liquid Penetrant (PT) to assure you have sound material.

If the 304 ss shows no indications from PT, I would fillet weld using the GTAW process with no preheat, and use a maximum interpass temperature of 400 deg F. The filler metal should be ER308.

After the completion of welding, I would perform a PT to assure your repair is OK.

 

[NickE]

The reply to metalguy:

Well I think that Cr{sub-n)C(sub-m) precipitation could be beneficial in two ways.

1. Mostly homogeneous neuclation-> The CrC precipitates are coherent to the matrix of the metal and very very small (almost a G-P Zone, maybe a little bigger) this causes the lattice to be under significant strain thus causing moving dislocations to leave a dislocation loop around them, the resulting stress field causes resistance to the movement of future dislocations. IE a form of Precipitation Hardening, also may increase fatigue life by preventing crack propagation.

2. Mostly Heterogeneous neucleation of M23C6, M6C, and MC -> These large precipitates (carbides of wich Cr can be used, although Ti, Mo, or W also may play a role) of wich M23C6 is the one stable to 875*C, provide an increse in creap strength for some alloys. This occurs due to the formation of large carbides (M23C6) on the grain boundry in wrought superalloys. While also surrounded by and aided by the (gamma prime) precipitate, the carbides provide some stability to the grain boundry and "pin" them into place.


(the above is condensed and drawn from "Physical Metallurgy Principals" 3rd ed. R.E. Reed-Hill and R. Abbaschian)

well do i get a star?

(I may be completly wrong about the above. If I am then please feel free to let me know and I will have this post deleted)


nick

 

[stanweld]

trainguy,
Fillet weld patches are notoriously weak in fatigue service. I suspect that corrosion fatigue played the predominant role in cracking. You must design them carefully and weld in such a way as to minimize stress at the corners. Flush patches made with full penetration welds are preferrable.

 

[metengr]

Nicke;
I will give you a star for response #2,and for your effort and interest in this forum. My source book "The Superalloys" by SIMS/HAGEL refers to the beneficial affect of grain boundary precipitates in certain superalloys that do not generate gamma prime phase. Namely, boron and zirconium in Udimet 500 increases creep life by 13 times and stress rupture strength by 1.9X. According to the book the theory is, odd-size atoms (like boron,zirconium and magnesium that are 21-29% over or undersize) segregate to grain boundaries filling vacancies and reducing grain boundary diffusion. High temperature creep and rupture failure normally initiate at grain boundaries lined with certain embrittling carbides in Udimet 500.

For nickel-base superalloys that generate gamma prime phase from the principal reaction of an aging heat treatment

MC + gamma = M23 C6 + gamma prime,

the concentration of gamma prime phase at grain boundaries is the major factor in reducing creep cracking. For Rene 80, it is described that the gamma prime phase inhibits grain boundary sliding because it engloves certain detrimental carbides.

The last example given is for a nickel-iron base superalloy like A 286 where the dominant beneficial grain-boundary carbides that are formed during a 16 hour, 1325 deg F aging treatment are M23 C6 and MC because they retard grain boundary sliding.

This is by no means a complete description because there are many superalloys; nickel- base, cobalt- base and iron-nickel base that have their own set of carbide and secondary phase reactions at elevated temperature.

 

[unclesyd]

Make every corner round if possible.
Weld a continous bead if possible.
Don't make your plug welds too small where you get a bad weld in the middle.
I would start each bead on the patch if possible.

Make sure the PT is done by an very experienced person as cracks in SS can be very tight. I would clean by grinding and immediately check afterwards while the metal is still warm.