316 SST becoming magnetic thru welding??? 6

[drax]

We have a 316 SST plate and weld 2 SST studs on it, is it possible for this to become magnetic? Our customers concern is corrosion and he is alarmed that this is magnetic. I'm not sure what kind of weld filler was used, I was told that First of all, the studs were rusting and they told me that if you weld a non-magnetic stainless steel bolt to a non-magnetic stainless steel plate and use the proper stainless welding rod than nothing is going to be magnetic period. Is this true?

thanks in advance

 

[unclesyd]

No it is not true.
We you weld a non-magnetic SS there is a high probability that the welding will produce Ferrite, a highly magnetic phase, in the weld metal.

This is controlled by the use of a welding electrode and procedure that produces low ferrite in the weld, FN (Ferrite Number) is as the term used. Usually the FN allowed is called out in the welding specification if it is going to be problem.

Depending on the end use of the component determines whether the Ferrite is detrimental.

Come back with a few more details on the end use

 

[mcguire]

316 stainless steel, as solidified in welds, generally has from 5 to 10% delta ferrite which is a ferro-magnetic phase, so you must expect some magnetic response. If none (or very, very little) of the base metal was melted, the magnetic response will originate in the filler metal, and the amount of magnetic response will depend on its composition.
However, that's not what's important for corrosion resistance. Welds always have worse corrosion resistance than the wrought base metal in grades like 316L. Figure on losing 10C in critcal pitting temperature.
This is irrespective of any magnetic response. If you want the welds to be as good as the base metal, alloy up the filler metal in chromium and/or molybdenum, and pickle or electropolish the welds. Try to avoid abrading the welds,as this, too, screws up pitting resistance.

 

[drax]

Thanks for your help. What is a 316 rod is threaded, would it become magnetic and rust?

thanks for all of your help.

 

[mcguire]

It will become superficially magnetic. Because of the surface deformation, its corrosion resistance will be diminished. This is not primarily related to the presence of the magnetic phase, but to the exposure of sulfide inclusions and residual stress from the threading.

 

[drax]

Dumb question, but I'll ask anyway. If the studs are rusting and they are 316, what can be done to prevent it. And if I understand correctly, does the surface deformation a result of ferro-magnetic reaction.

thanks!

 

[mcguire]

To improve stud corrosion resistance at this point you can passivate or electropolish.

The second question I don't quite understand. The ferromagnetic response in deformed stainless steel is due to the tranformation of austenitic 316 to martensitic of the same composition and roughly the same corrosion resistance.
The magnetic martensitic structure doesn't cause the poorer corrosion resistance. The deformed surface does, because it exposes inclusions. Remove them chemically and the problem is largely alleviated.

 

[drax]

Thank you very much. What if the studs were made from 304 by mistake and not 316, would they exhibit more surface rust?

 

[mcguire]

yes, a significant amount more.

 

[unclesyd]

What size are the studs as you could have magnetic studs from the
thread rolling, if rolled threads or made from cold rolled bar stock.. The size affected would be anything under 1.25 " dia.

Check some of the studs that haven’t been welded on for magnetism.

 

[rmw]

mcguire,

In the third post of this thread, you said a 10C loss in critical pitting temperature.

The thread addressed welding ss to ss.

Could you restate what you said with respect to the heat affected zone of a ss, say 316 to carbon steel in a pressure vessel.

I have a customer who has welded SS bottoms and tubesheets in a sugar evaporator (copper tubes) and persists in descaling with muratic acid. He is having beau coup problems in the heat affected zone, and is trying to solve it with ceramic coating.

Do you have any feel for the cirtical pitting temp loss in a dis-similar weld as described above?

rmw

 

[metengr]

rmw;
Is the corrosion problem related to the termination point of the stainless steel overlay in the vessel, similar to a knife edge corrosion problem OR is the carbon steel substrate corroding under the SS overlay?

 

[unclesyd]

I wouldn't worry about the critical pitting temperature your problem is with the HCl used to descale the Evaporator.

You should convince them to change cleaning materials to some other acid, like sulfamic, phosphoric, or citric.

 

[mcguire]

Unclesyd is right about the muriatic. It is strictly not acceptable to use HCl with stainless. It will find the weakest link and make it weaker.
Any time you make wrought stainless molten, as in welding and even with filler metal, the portion that becomes molten refreezes with inclusions which are not at thermodynamic equilibrium with the matrix. These inclusion grow by oxygen and sulfur diffusion to them, causing chromium oxides and sulfides to form which depletes the matrix around them of chromium. These tiny regions of low chromium cause pits to nucleate more easily, hence the lower critical pitting temperature.
These can be removed by electropolishing or careful strong passivation, but muriatic only jump starts the pitting process.

 

[rmw]

UncleSyd, and Mcguire

There was never any doubt about the problem being the use of muratic acid in this situation. I thought my use of the word "persists" would communicate that. Their thinking is that since the vessel wall and tubesheet sections are thick and "massive" as in comparison to thin wall ss tubing, the corrosion rate won't matter. It is the weld joing where the SS vessel bottom skirt joins the original CS vessel side wall that is problematic.

Now, as to convincing them, and when I say this, I guess I can't e mail them this thread, as if they ever looked at their e mail anyway, these are the people who if you discovered that they had never found out about sex, would be terribly hard to convince to try it, and if they ever finally did try it, and even had the slightest malfunction, would forever say, "nope, we tried that once, and it did not work."

So, I was trying to get valid data, such as a value for critical pitting temperature reduction to use along with other printed SS data in trying to convince them to abandon their use of the muratic.

I have the potential of putting in another process that is all SS, and I don't want muratic anywhere near my HX, and have to insist on the use of sulfamic or phosphoric. But there is reluctance to change on their part.

Sulfamic and or Phosphoric (I had not thought about citric) are obviously the answer, but they are more expensive. Ironically, I wonder if the added cost of using sulfamic et al, is compared to the cost of coating the vessel walls with ceramic coatings? Strange how much money some will spend to keep their costs from going up.

Plus, I am not sure if sulfamic and/or phosphoric have a cleaning rate with their particular type of scale as does muratic.

mcguire, thanks for your explaination, as that is the type of ammunition I am looking for to use on them. Isn't the process you describe exaserbated by one side of the weld being a dis-similar metal, CS in this case, and the mixing into the weld puddle of components of the carbon steel??

I am familiar with the forged CS tubesheet overlay process, where a 'buttering' layer of 308 is put down first against the original CS and then layers of 309 (I am doing this from memory, away from a reference I could use to verify the exact metallurgies) are added to a thickness of 3/8" or so, to which SS tubes are seal welded when the Hx is built.

metengr, it is not an overlay, but the butt weld of a SS section of vessel wall some distance above the tube sheet to the CS of the original vessel, and the weld zone described by mcguire. Although, I did hear it described that the muratic acid ate away the base metal and left the actual weld bead behind untouched, so that might be part of what you were asking about, so as to give you a clue of what is going on.

This client has invested a lot of money in making the bottoms of their evaporators now to be SS, and I fear that they are rapidly destroying their gain.

To iterate yet again, I am not trying to justify or find ways to help them continue the use of muratic, but to gain ammunition to help convince them to discontinue its use.

Thanks for all the help.

rmw

 

[metengr]

rmw;
This handy corrosion chart might help to you to convince your customers on the use of muriatic acid with 3XX ss components.

http://www.nde.co.za/pdf/resistance.pdf

You also mentioned copper tubes. I would be very concerned about the condition of the copper tubing, as well, from exposure to muriatic acid.

 

[rmw]

metengr,

Thanks for the valuable post.

According to this chart, even sulfamic would be suspect, as this industry, in order to speed up the descaling in their Roberts type evaporators, puts steam on the evaporator body with the cleaning acid solucion flooding the tubes to above the top tubesheet to get some boiling action to induce some agitation.

I had not even thought about the problem with copper. The traditional set up in the industry in my area is CS vessels, and tubesheets, and copper tubes. Cleaning with muratic is the holy grail.

My early back ground was with salt water evaporators, basically made of cu/ni materials, and sulfamic was the only cleaning acid we recommended.

Thanks again, I can use the chart to good advantage.

rmw

 

[unclesyd]

To drax and all.
The thing to remember is when you use any chemical cleaner, acid/base, it is better to use a proprietary composition or formula. You will get the needed ingredients to a safe and complete job. As with the case with Sulfamic you get an inhibitor for use up to a certain temperature. Along with this there are wetting, rinsing, and other agents to due a job without harm to the equipment.
Most proprietary cleaners from reputable suppliers will do the job they are designed for without any problems if used as directed.
If you are still forced to use HCl based material here is one of the best. This is a well inhibited HCl based material that works extremely well on multi-metal systems, as with all cleaning materials of this type it can cause Chloride Induced SCC on Austenitic Stainless Steel.

http://www.rydlyme.com/home.html