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For years the pharmaceutical industry has relied heavily upon the American. Welding Society’s AWS D weld dis- coloration chart1 to gage the acceptability. AWS D () SS Discoloration – Free download as PDF File .pdf), Text File .txt) or read online for free. AWS D – Download as PDF File .pdf), Text File .txt) or view presentation slides online.

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Many methods have been devised to deliver the purge gas to the weld site, but few methods exist to control the internal gas pressure, thus the internal weld profile. Experimentation is required to establish the ideal pressure for a given tube size, wall thickness and material.

The awx weld pressure is input to the device, and it correctly offsets the pressure for the tubing configuration as tested. However, this value will ass be correct for the specific assembly tested.

Purging for Improved Weld Profile | Arc Machines, Inc.

Purging for suitable weld color is relatively simple. Once that is determined, the next problem is to correlate the required pressure at the weld with the measured pressure at the delivery inlet.

Orbital Welding Internal Pressure Control. In order for all color to be eliminated, oxygen levels must depending on the material fall below approximately 20 ppm.

Since the internal pressure at the weld-site e18.2 dependent upon the purge gas delivery pressure, the distance from the entry measurement point and the weld, and the gas venting rate, a closed loop pressure controller is required, such that a specific, low value pressure can be maintained.

The only remaining hurdle is to obtain the d18.22 inlet pressure for the known weld pressure, and fortunately, the EXEL SG-1 also provides the capability of measuring the inlet and weld pressures simultaneously.


The initial purpose of internal tube purging with inert gas usually argon was the prevention of surface oxidation on the internal diameter of the tubing surrounding the weld Heat Affected Zone. Most users are satisfied with the results obtained with pure argon as a purging medium; although a few organizations like to add a small percentage of hydrogen to produce a reducing environment.

If another configuration is to be welded, the entire experiment must be repeated. Welding Journal, July This variable orifice is especially important because as the joint is welded shut, the effective purge gas exit area is reduced, so as the joint is sealed the SG-1 is making real-time adjustments to accurately maintain the required pressure.

However, given the dangers associated with the use of hydrogen, it is hard to recommend it unless a critical need exists. The internal pressure at the weld location, typically arrived at empirically, is influenced by the gas flow rate and the size of the exit orifice. The prepurge time is an empirically derived value dependent on many factors including tube size, gas flow rate, exit orifice size, number, sizes and locations of dead-legs or other components that can trap atmosphere, and thus take more time to purge.

Since the pressures are quite d8.2, on the order of 0.

Purging for Improved Weld Profile

The combination of flow rate and prepurge time determines how much oxygen is still present, and thus, the amount of weld color caused by oxidation. The input gas is controlled both for pressure and flow, using a variable orifice iris controlled by a microprocessor in a closed loop system.

One way to establish the inlet pressure is asw repeat the empirical testing with the inlet pressure as the controlled variable. SEMI Standard F78 includes a chart with minimum and maximum purge pressures and flow rates for various tube sizes and wall thicknesses. This experimentation requires several trial pressures, with a weld coupon created for each test condition, until acceptable results are obtained; a time consuming and costly activity.


However, gauging the internal pressure such that the weld ID profile is acceptably flat is a more difficult task. However, in recent years, this focus on smooth, clean, particle-free internal weld requirement has migrated from the semiconductor industry to the biopharmaceutical industry, and on to the food equipment industry.

In addition, most standard gas pressure regulators are not nearly accurate enough to deliver the exacting performance required.

The unit then utilizes this offset for all welds performed with this same configuration. In addition, measuring the pressure some distance from the weld location due to pressured drop will not provide an accurate method of determining the pressure at the weld zone.

Figure 1 – AWS D Purging for Improved Weld Profile. Since the pressures are given as ads range, they serve as a useful starting point for the empirical determination of the best pressure for the ideal weld profile.

He is a registered professional California Electrical Engineer. In the interest of exercising control over the weld profile, the ID purge is introduced with a certain sws of pressure, and in order to maintain consistency and repeatability that pressure must be consistent throughout the weld, and from weld to weld.

It is conventional wisdom that the semiconductor industry was, ads remains, the primary driver for the continuous improvement of orbital autogenous weld profiles and internal cleanliness for process gas delivery tubing. This level of measurement and accuracy permits the EXEL SG-1 to repeatedly maintain the exact internal pressure required for a perfect weld.

Dennis Cobb has spent more than 25 years both designing and managing the design of semiconductor capital equipment.