June 11, 2014
Q: We manufacture hydraulic cylinders. Ever since we changed our shielding gas blend, we’ve noticed a higher level of distortion. Before the change we used a 95/5 blend, but now we use 92/8, which we have documented using a 0.045-in.-dia. filler metal. Would the 92/8 blend run hotter and cause additional distortion issues?
A: That's a great question; however, there is some information missing. Welding always causes some amount of distortion. You stated that you used to use 95/5 mixed shielding gas. Was that an argon/oxygen or an argon/carbon dioxide blend? Both are typical shielding gas blends for GMAW of carbon steels as well as the 92/8 blend you are now using, which, I’m assuming, is argon/CO2.
With your previous shielding gas, were there properly developed procedure qualification records (PQR) and welding procedure specifications (WPS) and, if so, how well were those documents implemented for use in production? When you changed shielding gas, were all other variables such as arc equipment, shielding gas delivery, filler metals, base materials, fixtures, or process of manual fitting and inspection methods the same?
It is possible that when you changed your shielding gas, changes to the weld parameters were made to produce certain desirable results. One of those parameters may have been in the form of higher heat input.>
If your shielding gas has always been the Ar/CO2 mixture and you simply changed to a higher concentration of CO2, then the additional 3 percent would have little impact on distortion levels. However, changing from a 5 percent oxygen to an 8 percent CO2 blend could potentially cause some additional distortion, but again, this level would be expected to be relatively small.
Oxygen is added to argon—typically in 1 to 5 percent concentrations—to improve weld bead wetting and arc stability. You can add CO2 in a wide range of concentrations—depending upon the application, it can be as low as 2 percent all the way up to 100 percent. Carbon dioxide typically helps in the oxidation of contamination or impurities and enhances weld penetration, which increases with higher concentrations.
Due to the physical properties of the CO2 molecule, the weld process develops a hotter arc than does the oxygen molecule. This hotter arc may be the reason for an increase in distortion.
Many times when manufacturers implement production improvements, they change more than one variable, which can sometimes lead to unforeseen results. Additionally, you tend to inspect the process more carefully after you’ve made a change. It’s possible that the distortion was always there, but it was never really addressed.
As stated earlier, the gas change will result in a hotter arc temperature. To truly answer the question, you need to take a closer look at your previous process and compare all changes, no matter how insignificant they may seem. This will identify the cause for the increase in distortion.
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