WELDING ISSUES WITH 316/CF8M PICKLING AND PASSIVATING 2

[confused21]

HI GUYS

WE ARE HAVING SOME ISSUES WITH OUR 316/CF8M CASTINGS.

AFTER THE PICKLING AND PASSIVATING PROCESS HAS TAKEN PLACE,
THE AREAS WHERE THEY HAVE BEEN COSMETICALLY WELDED, THE WELD MARKS SHOW UP AS BLACK PATCHES AND ARE MORE PROMINENT THAN BEFORE THEY WERE
TREATED.

THIS PROBLEM AT THE MOMENT SEEMS TO BE ONLY WITH THE TIG WELDED PARTS.

ANY IDEAS GUYS

 

[jwhit]

I've seen this condition as well, never had an answer but suspect that it relates to the difference in composition and finer grain of the weld reacting differently than the original base metal while forming the passive film. I'm sure others have more knowledge on the why it occurs. BTW, is there really such a thing as a cosmetic weld? All welds, no matter how insignificant they appear must meet the applicable standards. The term cosmetic implies otherwise.

 

[confused21]

we have a standard - MSS SP-55 where the casting has to pass a visual inspection before it is passed onto the customer.

so we weld up any defects to smooth up the surface area , which will be machined out by the customer.

but this issue is has only started a few months ago , no process changes have taken place during this period.

the welding rods have been analysed and are a very similar spec to the parts they are welded onto.

just having a real game to work out why this is happening

 

[metengr]

See page 10, below

http://www.picklingandpassivation.c...mau/Avesta_Pickling_Handbook_2009_Edition.pdf

 

[jwhit]

My basis for the comment about cosmetic welding is that any weld you make has to be prepared per the base metal specification and in the case of ASTM or ASME the General Requirements, for instance, ASTM A703 for an ASTM A351 casting. If the surface anomaly is rejectable and needs repair it is a defect,regardless of how minor it is, the resultant cavity from removal of the defect is to be repaired. Generally speaking, the defect needs to be removed down to at least the minimum acceptable size before welding. In more than a few cases, depending on material and what lies below, welding over can cause more problems than letting it go. This is especially true with materials of limited ductility and poor weldability. I know this interpretation flies in the face of common foundry practice, but the practice is one of the reasons that the people in the wrought end of the business sort of look down on those of us in the foundry.

 

[EdStainless]

slight differences in chemistry and grain texture are the reason for the apparent color diff.
In many cases it is because the pickle is actually etching the residual delta ferrite out of the weld filler, leaving this material slightly porous on the surface. The pitted texture looks dark, or traps oxide leaving it with a color.
If you were to lightly blast the area before pickle it would be much less noticeable.


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[arunmrao]

lakk, As I understand, your definition of cosmetic welding is welding the surfaces to improve the aesthetics of the castings. The castings are having surface defects on a regular basis and you are attempting to salvage them.

My advice will be to minimize the occurrence of surface defect, rather than keep salvaging them which is time consuming and expensive.

You have received excellent advice about discoloration , some of which was a learning experience to me.

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[confused21]

you have given me alot to think about.

i will follow up with the way the castings are weld repaired, see how they look after pickling which is carried out offsite.

As for minimizing the occurrence of surface defects, that's another issue altogether, most castings need touching up a little.

the slag-off (koag)always makes its way into the moulds when casting.

we are currently looking at other types of ladles to help eliminate this problem and keep welding to a minimum.

Thankyou Guys

 

[jha1982]

Hello Expert's
Is it mandatory to do the Passivation or pickling of SS welding ?
if yes is any codes & standard supports documents ?

 

[EdStainless]

Only if you care about corrosion resistance.
Even welds that look bright will have improved corrosion resistance after a pickle.

Of course if you really cared you would either be using over-alloyed filler or doing post-weld annealing.

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