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High Frenquecy Welding of Stainless Steel Tubing(Two)

Date: 2014-07-08

INTERNAL BEAD 
The internal bead formed during the HF welding operation can be removed by scarfing with a carbide tool or by rolling into the I.D.. Many manufacturers leave the bead in place when the tube is to be used for decorative applications. If a high quality weld is required, scarfing is highly recommended as rolling the bead into the I.D. can create cracks and stress risers on the I.D. where the hard oxidized bead material is rolled into the softer weld area.

When using internal bead removal tooling, it is essential that the tool be kept cool with sufficient clearance around the tool for coolant to freely circulate. If there is insufficient clearance, the coolant will back up into the weld and may create defects. It is also important to realize that when scarfing stainless steel, the speed at which the bead moves relative to the tooling is important. Generally, the faster the speed of the tube, the better the quality of the scarf and the longer the life of the tool. It has been recommended that a minimum welding speed of 125 FPM be maintained when I.D. scarfing stainless steel.

WELD POWER, RIPPLE, AND FREQUENCY
 The optimum weld power is the minimum power at which a satisfactory weld is being obtained. Adjustment to the weld power is generally done by observing the spume which occurs at the apex of the weld. The spume, which appears yellow or orange, usually spurts downstream of the weld on both the inside and outside of the tube. A moderate spume is the best indication of adequate weld power and steady mill conditions. Uneven spume is an indication of uneven welding conditions and usually of poor weld quality. It can be caused by mechanical irregularities in the mill, such as eccentric rolls, or by excessive weld power or both.

Some manufactures of ferritic stainless tube for automotive applications have applied an automated weld power control system based on a two-color pyrometer that is integrated into the welders speed/power control system. The pyrometer can be set to maintain a consistent weld temperature once the proper temperature has been determined. This diminishes the variability introduced when using several operators on the weld mill, all of whom see the color of the spume slightly differently.

One of the advantages of the new solid state welders is the availability of very low ripple output power as a standard feature. Welders with a .2% ripple are routinely delivered and even lower numbers are possible with new IGBT (Isolated Gate Bipolar Transistor) technology. The low ripple is mainly of concern if you are leaving the weld bead in the tube. Low ripple results in a smoother bead but it does not affect the amount of bead formed. The amount of bead formed is a function of squeeze out that is influenced by strip width, weld power, and frequency.

When using a vacuum tube welder at low power, the SCRs are phased back and this will maximize the ripple effect. To minimize the ripple, run the welder at higher power settings, at least 80% or better. Solid State welders operate at lower voltages and are equipped with better filtering to ensure low ripple over the entire power output range. It has been noted that some stainless steel tube producers prefer 200 kHz for a welding frequency stating that it provides a more ductile weld. Others insist on 400 kHz because of the narrower heat affected zone produced. At least one HF welder manufacturer can provide frequencies of 100 kHz to 800 kHz depending on customer needs and product demands.

Very little research has been done on which frequency really works best because product application, strip width and width control, welding speed, squeeze out, vee length, frequency, and welding temperature all affect weld quality. It is not likely that we will ever be able to accurately evaluate all of the possible interactions of the various parameters and establish the optimum welding frequency. One experiment was performed on 304 stainless where all parameters were held constant and only the frequency was changed. One run was made at 355 kHz and another was made at 155kHz. The microstructural analysis showed very little difference except for the amount of upset and flow angles.

As would be expected the lower frequency heated more metal, resulting in more squeezeout. Because more metal was heated, more of the edge was softened and the flow angles were steeper with the lower frequency. The differences were definable but it was not possible to categorically state that in all cases one weld would perform better than the other

It has been shown that for different sizes of pipe and different welding speeds, there is a critical welding frequency, above which the welder will operate more efficiently with less sensitivity to variations in weld parameters. This is true for carbon steel and for stainless steel. It remains true that while it is possible to achieve a satisfactory weld at virtually any frequency, welds made at higher frequencies generally are easier to control. Since control of the welding parameters is more critical with stainless, it would suggest that higher frequencies would give better long term results because of the reduced incidence of weld defects and scrap tube.

INERT GAS 
It has been often said that if the HF weld is properly executed, no cover gas is necessary. In many cases, this may be true. However, recent developments indicate that the use of a cover gas may have significant advantages when the product is intended for critical applications, such as automotive exhausts. Typically, the cover gas of choice is argon or an argon mixture.

Years ago, many experiments were conducted to develop a delivery system that would blanket the weld area without chilling the weld pool. The problem was that the argon, in its liquid state for storage, was very cold. If applied to the weld area directly, it could freeze the weld pool. If it was applied at a high pressure or velocity, it would entrain air and the oxides would still form in the weld. Additionally, the nozzle had to be nonmetallic so that it would not be subject to inductive heating. Getting the argon to the I.D. of the weld was not very successful so that even under the best conditions, the results were not significantly better than not using a cover gas.

Recent trends are directed towards enclosing the coil and the weld rolls in an airtight box and gently purging the box with Argon. A gas dam must be incorporated onto the I.D. trim bar to prevent excessive loss of gas down the I.D. of the tube. The current philosophy is simple: if you prevent any oxides from forming during the welding process, the chances of oxides being trapped on the bond plane are nil.

Having gone through the trouble of gas shielding to prevent weld defects, it should be clear that the mill environment must be kept meticulously clean also. Of critical importance is the rule of thumb that carbon steel must not be run on a stainless mill. The iron oxide (scale) that will contaminate the mill coolant is a great source of two problems. First, iron oxide scale can be rolled into the surface of the stainless and cause a rusty discoloration and second, particles of scale can be entrapped in the weld vee to create weld defects. It is just as important that any part of your mill made of iron or steel be kept clean and its contact with the strip edges be minimized.

WELD TESTING 
The design of weld test procedures is extremely important, since a poorly designed test can lead to either rejection of material which is actually within product specifications, or to approval of rejectable material.

The most common tests are flare tests and expansion tests. On heavier wall material, flattening tests are also often used. As a general rule the test should be designed to subject the tubing to elongations similar to those required for its production processing. Thus, if the final product involves flaring, a flare test is clearly appropriate. If it involves expansion, the expansion test is appropriate.

It is essential that the test tooling be properly designed and well maintained so that consistent results will be achieved. The test procedures must be carefully defined and followed so that variations introduced by human influence are minimized. For example, when flaring tube from which the I.D. bead has not been removed, small grooves must be cut in the flare mandrel to provide clearance for the bead. If this is not done, the bead will be forced into the I.D. of the tube causing stress risers which can cause premature failure.

The speed and possibly even the temperature at which the test is performed must be defined and controlled. Metallographic sectioning of the weld will reveal the degree of upset that has been achieved as well as the width and uniformity of the bond region. In austenitic stainless steel, the metal on the bond line acquires a dendritic structure. This structure will recrystalize during annealing. Before annealing, however, examination of the dendritic structure is useful in evaluating the uniformity and squeeze out of the weld. A narrow uniform layer is desirable. “Hairy” margins of the dendritic region represent grain boundary liquation and should not be confused with carbide grain boundary precipitation which generally cannot be resolved with a light microscope.

The degree of upset can sometimes be observed by examining the flow lines visible from the slight banding that occurs in most stainless steels. While it is difficult to measure quantitatively, a moderate amount of upset is essential to achieving a good weld. Generally, micros are taken prior to any subsequent annealing so that the microstructure is well defined and flow angles can be observed and measured. Non-destructive testing has evolved much like high frequency solid state welding has, gaining wider usage and acceptance. However, despite gratifying technological advances, no NDT manufacturer has been able to create a system that will absolutely guarantee weld quality. NDT systems work very well when they are a part of a well-executed quality program that includes metallographic, mechanical, and destructive testing of the product.

PROCEDURES 
Because the consequences of making a poor weld are very expensive, it is economically imperative to exercise special care at the mill when welding stainless steel tube. A standard mill set up sheet is absolutely necessary to ensure consistent and repeatable setups. The setup sheet should include all standard roll, spacer and shaft information, fin roll fin widths, girth after each pass, girth prior to weld and circumference after weld, coil and impeder dimensions, vee length, speed, power, weld temperature, gas type (if used) gas flow and pressure, sizing dimensions, etc. Each of the defined parameters must be listed with an aim and a tolerance. Each parameter should be measured and recorded at the start of each shift and after lunch break.

Purchasing should be ordering the material to written specifications which include melting, casting and rolling methods, chemistry limits, gage and gage tolerances, coil size, slit width and width tolerances, camber limits, shipping, loading and storage instructions, Etc.

Test and inspections should be detailed on a list that defines which test procedure is used, by whom, where, how often, and provide accept/reject tolerances. Each test procedure should be written to include methodology, equipment used, calibration procedures, and what to do if the test fails or passes. Each operator or inspector should have a job description that defines the tasks to be performed, personnel qualifications, and special training or education required.

CONCLUSION 
High frequency welding of stainless steel is more difficult than welding carbon steel. Mill cleanliness, exclusion of oxygen from the weld area, adherence to the best procedures, a highly disciplined and well trained crew, and a fully functioning quality program will help to ensure the maximum yield of high quality tubing.

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