This is probably a question in advance for Jody before the zoom meeting tomorrow.

I'm not fully understanding the reason and meanings behind the colour changes mainly in stainless and titanium after welding.

I understand that for stainless, colours indicate the chromium separating from the alloy. That's of course bad because it's then effects corrosion resistance etc.... But, we're melting it. What did we think would happen????

I have some example questions as topic raisers. I don't expect anyone to answer these specifically:

Is the ultimate maximum temperature the cause of the separation? (chromium from the alloy)
Does the time for which the temperature is applied impact the separation?
Is the problem only relevant to the surface of the stainless? Does it run deeper into the material?
We can of brush it back and remove the colour quite easily. Is the now uncoloured surface still likely to corrode?
Do the colours occur from lack of gas coverage around the weld?
With stainless, is there a guide to colouring. eg Straw is good, green is bad? How bad is bad?
When welding, the centre of the weld is obviously hotter for longer (being molten). The temperature naturally decreases outwards. Sometimes the weld can be silver but the surround be blue. Is that just gas coverage?

As you can see, I don't fully understand what's going on here.

For titanium, is the reason and meaning the same? Are there aluminiums or vanadiums escaping from the mix?
Do colours occur on pure titanium?
With titanium, is it all about the gas coverage? I understand the re-activeness of titanium with oxygen above 500F.
The pass/fail criteria for titanium colouring seems quite specific to the actual colour. If we brush the colour back, do we always pass? ;-)
What is the oxygen embrittlement that we hear of? Is that only about the surface or does it penetrate more deeply?

Once again, these questions are just conversation raiser. I'd simply like to gain a better understanding of what's occurring with the colours.

Regards,

Michael Kearsey

Hi Michael,
from this article on passivating stainless steel:
www.worldstainless.org/Files/issf/non-image-files/PDF/Euro_Inox/Passivating_Pickling_EN.pdf

"As heat tints are formed on the surface of
stainless steel chromium is drawn to the
surface of the steel, as chromium oxidises
more readily than the iron in the steel. This
leaves a layer at and just below the surface
with a lower chromium level than in the bulk
of the steel, and so a surface with reduced
corrosion resistance.
Visible welding heat tint on stainless steel
surfaces reduces the available corrosion
resistance of the surface."

It's important to know here that the discolouration of stainless (and titanium) only happens in the presence of atmosphere (oxygen). So, though it's in some sense true that discolouration indicates chromium separating out from the alloy, in practical terms for the welder it directly indicates contact with oxygen whilst hot. The darker the colour, the hotter it was at the moment oxygen was introduced to the material.
This isn't the same as contact with oxygen whilst molten - when that happens the metal with actually burn in contact with the oxygen, resulting in a grey/black, porous brittle weld material. Steels have a lower ignition temperature than melting temperature. This is one feature of steel which allows it to be cut with an oxyfuel flame. It doesn't melt away before you can burn a cut line through it.
So colours don't exactly occur due to lack of gas coverage around the weld... well that is kind of true... but really they occur because of the high workpiece temperature at the time at which the gas coverage is removed, for example by the welder moving forward to continue to weld, or the post flow shutting off, or a breeze coming through etc. Basically you can't keep the entire length of the weld shielded all the time unless you are using a purge chamber, but you can attempt to have the metal remain as cool as possible so that it doesn't react excessively when the shielding gas is gone. This is why chill blocks, increased welding speed, pulse, increased amounts of filler - all of which keep the workpiece cooler - help to reduce discolouration.
Mechanically brushing stainless will improve its corrosion resistance but electropolishing or pickling (passivating) is supposedly more effective. Unfortunately I can't remember where I read this.
Guide to stainless discolouration: less is better. Gray is not acceptable because the material really started to burn. That's the beginning of sugaring and that surface has completely compromised corrosion resistance. All the way from straw to dark blue is kind of fine in the sense that it's all recoverable. It all goes away with a brushing or pickling. However if you are concerned about heat for other reasons, for example distortion or loss of tensile strength, you will also be looking for minimal discolouration as an indicator of not having excessively heated the part. So less colour is good because less heat is good in basically all cases.
When the weld is silver but the surrounding is discoloured, your heat affected zone has extended beyond the width of your gas coverage. So the weld itself was sufficiently shielded but enough heat was conducted into the neighbouring surface where there was oxygen present such that discolouration occurred.
One fun thing you can do is get a thick scrap piece steel in a vise and TIG weld a load of beads on top of each other in a big mound. Let it all get way too hot so that even with good post flow it goes really dark and greys out once you're done. Then quickly grab a file and file the top of the weld material. The freshly exposed material will discolour right before your eyes. Do it again two seconds later and it will discolour again but less dark. As you repeatedly file the colours will go down through the rainbow to straw and silver. This really shows how the colour change is all about the temperature at time of exposure to oxygen.

I don't have any experience with titanium but the principle is the same. If you don't want it to react with it can't come out of the shielded zone whilst hot. I believe the discolouration is titanium oxides, not other alloying elements. AFAIK titanium is corrosion resistant because of its passive titanium oxide layer. So I'm rather certain that pure titanium will also discolour when heated in the presence of oxygen.
As far as I know, brushing back discoloured titanium isn't OK because of oxygen embrittlement. Most people say that straw is the upper limit for discolouration because anything beyond it indicates possible embrittlement. Of course it depends on the application and what the welding engineer said is allowed. I do have a friend of a friend who discoloured his titanium bicycle fork for aesthetics and it cracked whilst riding... so don't do that!!
Cheers

Hi, Michael. I'll link the Lincoln Electric Welding Guide for Stainless Steels. I'm confident you'll find it interesting if nothing else. The α-numerical ID is C6.4000 (Lincoln uses this α-# ID system for doc's.) This one and many others are free & can be downloaded as .pdf files from LE's site. This one is ~40pg.'s including an index, graphs, charts, & figures.

Direct Link - C6.4000 PDF below:
https://ch-delivery.lincolnelectric.com/api/public/content/468b884c48d34770b842ed4d20610306?v=edd177d4

5-Step map to get there:
1.) http://www.lincolnelectric.com
2.) click "Support" toward the bottom of the page to see the submenu.
3.) "Resource Center" is in that sub-menu.
4.) "Welding Guides"
5.) "Stainless Steel Welding Guide"

I listed the path so the full resource center can be easily located.

Unfortunately, I can't think of a comparable resource to point toward in regard to titanium. I'll try & track one down & post a link if I succeed. My maxim here: a document like this will be a better resource for you/others vs. my crude and partly-subjective ramblings lol 😉 Anyway, I hope someone finds this as interesting as I do!
I love seeing videos exploring these sort of topics as well. Jody's "Down & Dirty Metallurgy" video was awesome. So are those from his much older series on welding metallurgy.

Hi Alex,

Thanks for link. I've just downloaded stainless guide. Will read it tonight. Also found a guide for shelf shielded flux core which I use around the farm here. Thanks for that too.

I have also just ordered the "Metals and how to weld them" book. We're really becoming clued up here.

Thanks for your searchings.

Thanks Alex for the PDF from Lincoln, looking forward to reading that