If you are welding a reactive metal, light straw is tolerated. Some welding standards even allow dark straw, etc., but the bottom line is that the oxygen from the atmosphere has interacted with the base metal. The interaction between oxygen (nitrogen and hydrogen as well) permeates reactive base metals such as titanium and zirconium.
That isn't the case with metals such as ferrous, nonferrous, and refractory metals (some refractory metals also are reactive, example: tungsten) such as austenitic stainless steel, nickel, carbon steel, etc., where the reaction with oxygen is primarily a surface effect. That is, if you were to remove the iridescence, no damage is suffered by the substrate. That is very different than the permanent damage experienced by reactive metals where the damage typically extends through the full thickness of the base metal.
In the case of a reactive metal, the iridescence can be removed with a wire brush, but the damage is permanent. The affected weld and base metal must be removed and replaced. Welding over the affected area may look fine, but there is no recovery, the damage is done.
One of my clients forges titanium for aerospace applications. After forging at high temperatures, even with the protective measures they take, they chemically mill at least 2-inches of base metal from all exposed surfaces to get down to the unaffected base metal.
Best regards – Al
Thanks for the post Al. Now my next question. If someone wants to cheat and buff the weld to try to hide there screw up will an Xray or UT pick up the damaged weld..? And if so what does it show up as. I have heard that they can pick up sugar but if the weld is sound for the most part how does it show up. Im sure there are lots of guys that try to cheat and there has to be a way to catch them...!
The sensitivity of RT is not as impressive as some people are lead to believe. In general, a number of welding standards use 2% as the difference in thickness or density that has to be observable in an RT film. In other words a porosity void must represent at least a 2% change in thickness to be seen on the film. Because porosity can be spherical, the outer diameter of the pore hole is not seen. That is why RT is not a good method of detecting laminations in plate. The lamination does not represent 2% of the thickness of the adjacent base metal, so it isn't detected as a variation in film density (darkness).
Excessive root oxidation can be detected if it represents at least 2% of the material thickness. In most cases the excessive root oxidation is associated with excessive melt through and if the base metal is thin, the roughness of the oxide surface may be detected.
Wire brushing isn't aggressive enough to remove excessive root oxidation from many base metals. The iridescence film that forms is acceptable by many welding standards provided the base metal isn't a reactive base metal. The tint can be removed by wire brushing in many cases. When optimum weld quality is demanded and a reactive base metal is being used, each weld bead is required to be visually inspected before the welder is permitted to deposit additional weld. While the wire brushing removes the visible evidence the hot base metal and weld were exposed to atmospheric gases, the damage has been done and no amount of wire brushing will correct the situation.
Best regards - Al
Thanks for questions!
And special thanks for answers!
That was more than informative!
X-ray or UT won't catch it, discoloration is on the order of a few atomic layers thick. What you are seeing with the different colors is a layer of oxide that bends the light, thicker layers (blue-red-purple) correspond to thicker oxide films. But it's still only several atomic layers thick. That means the color is easy rubbed off with sand paper, chemically etched or ground down.
You can catch it using destructive methods, small samples can be fed to scanning electron microscopes, LEECO combustion, Auger, and other testing methods, these will tell you exactly how much oxygen nitrogen and carbon you have picked up.
For non destructive tests: eddy current testing and micro hardness can usually identify gross contamination but can't quantify the percentage of oxygen or nitrogen and may not catch small but dangerous amounts.
As a reminder "sugaring" is very severe gross contamination when welding in open air, first of all it burns out alloying elements like carbon, second it has absolutely no strength and for stainless steel no corrosion resistance. If it's left on the inside of pipes it will quickly corrode in service contaminating what ever you're sending through the pipe and possibly leading to a mechanical failure.
for reactive metals like titanium or zirconium. They will instantly accumulate large amounts of oxygen or nitrogen through the whole thickness. THis can ruin corrosion and strength, hence they generally have very tight standards on discoloration. For Zirconium it's usually 100% silver, Or they machine down the top weld layer to get to clean material.
Final note there's two types of contamination pickup: Liquid and solid. In the liquid state metals will pick up severe amounts of discoloration in miliseconds. In the solid state they can still build up enough contamination to discolor but generally it only effects that top surface. This is why some types of discoloration is lived with. unfortunately the colors are exactly the same so it's near impossible to look at a finalized weld and tell the two apart.
I would just like to reiterate, for zirconium welding it's generally standard practice to always machine or pickle away a certain thickness due to contamination concerns. To emphasize HOW reactive it is. Generally it is welded in a vacuum or a box that is vacuum evacuated and back filled with an 99.995% pure inert gas. Usually the gas quality is monitored too as it's iffy to believe vendor certs.
Long story short it's extremely reactive and even low levels of oxygen and nitrogen will destroy corrosion properties (what it's famous for)
