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High Frenquecy Welding of Stainless Steel Tubing(One)

Date: 2014-07-08

The welding of stainless steel tubes differs substantially from the welding of high quality carbon steel tubes in a number of ways. The most significant difference is in the melting
point of the oxides that are formed on the faying edges during the welding process. The oxides formed on carbon steel generally melt at a temperature lower than the melting point of the steel. Since the oxides melt first and are fairly liquid even before the steel melts, it is easier to squeeze them out in the weld rolls. Actually, it is possible to make a fairly clean weld on carbon steel without actually melting the steel, though this is rarely done.

Stainless steel, either austenitic or ferritic, has substantial quantities of chromium as an alloying element that can oxidize during the welding process. These chromium oxides are refractory”, that is, they have very high melting points. In fact the melting point is higher than the stainless steel itself. Because of this, the stainless steel becomes soft and
even liquid before the oxides melt, making the oxides difficult to squeeze out. If left on the bond plane, these oxides constitute discontinuities that can cause brittle fracture of the weld, low formability, and possibly weld line corrosion.

It is, therefore, essential that enough metal be melted at the edges to flush the oxides out during the weld squeeze. If properly done, all of the oxides will be extruded during the squeeze out and will be contained in the ID and OD flash (Fig 1). Conversely, if too much metal is melted, the HF vee may become unstable leading to possible weld defects. It is a well established fact that the “window of success” for stainless steel is smaller than for carbon steel and greater attention must be paid to the forming and welding parameters.

MATERIAL COMPOSITION
Because carbon can combine with the chromium to form very hard chromium carbides in the weld area, keeping the carbon content low will reduce the incidence of hard welds and cracks. Additionally, when the chromium forms carbides, it is no longer in a form that will help protect the surface of the metal and weld area corrosion is possible. By specifying the L ersions such as 304L, the tube producer will reduce the potential for problems without compromising performance of the tube material.It is possible that excessive unreacted aluminum and titanium can form aluminum and titanium oxides that are also refractory and just as difficult to remove as chromium oxides. The maximum limits for soluble aluminum and titanium may have to be lower than commercial limits. This is an issue to take up with your strip supplier. In addition,large non-metallic inclusions in stainless steel can result in defects known as hook cracks when they are upset in the weld area. Low levels of sulfur and phosphorus should be specified when purchasing strip.

COIL CAMBER AND TEMPER
Flat and straight coil will run better in any mill and this is especially true of stainless. In a properly adjusted mill, soft strip will usually run more smoothly than hard material
because of the ability of the mill to correct for slight strip camber. Care should be taken to avoid introducing camber when recoiling strip after slitting.

STRIP EDGE CONDITION
Good strip edges are absolutely essential to achieving a good weld with stainless. The edges should be smooth with a minimum of burr (Fig 2). This requires careful attention to both edge condition and the set up of the slitting knives. Great care must be given to handling and transporting of the slit strip to avoid scraping or bumping of the edges as edge damage is a likely cause for weld defects. Good practice may also include the use of edge trimming on the mill prior to the first breakdown roll. This generates a small amount of trim scrap but this may be paid for by consistently higher quality welds and reduced tubular scrap.

COIL WIDTH
The designer of the tube mill’s roll tooling should provide a list of coil widths for the tube sizes to be produced. These may be only a theoretical dimension as the mill operators control the amount of squeeze out and a large squeeze out may require a wider strip width. It is absolutely essential to good weld quality that adequate squeeze out is achieved.

WELDING VEE GEOMETRY
The recommended vee angle for stainless steel is between 5 and 7 degrees. . To determine the vee angle, measure the vee width at a point 2 inches up stream from the weld roll enterline. The gap between the edges should be between .175” and .250”.The vee angle is established by the width of the fin on the last fin pass, the amount of upset in the weld rolls, the distance from the last fin pass to the weld rolls, and the springback of the edges between the last fin pass and the weld rolls.

WELDING VEE GEOMETRY
The recommended vee angle for stainless steel is between 5 and 7 degrees (Fig 3). To determine the vee angle, measure the vee width at a point 2 inches up stream from the weld roll centerline. The gap between the edges should be between .175” and .250”.The vee angle is established by the width of the fin on the last fin pass, the amount of upset in the weld olls, the distance from the last fin pass to the weld rolls, and the springback of the edges between the last fin pass and the weld rolls.

In some mills, an insulated seam guide is used between the last fin pass and the weld pressure rolls. It should not be used as a “seam spreader” to open the seam wider than its natural width, which is the width of the last fin plus the natural springback between the edges, since this can lead to production of slivers.

SLIVERS
Slivers are typically small, elongated pieces of metal which can be produced by the rubbing of the strip edges against metal mill parts. There are many places that this can take place, but two of the most likely are the I.D. mandrel support strut and the edges of the seam guide. Another source of slivers is handling damage. The coils are often stacked upright, leaning against one another or against a steel rack. Any rubbing between coils or against the rack can create slivers and edge damage. Use great care when handling the coils with a fork lift. Inserting the forks between the coils can result in slivers and edge damage also.

If slivers are allowed to form and get into the weld area, they are likely to cause welding problems. In some cases they will build up on the induction coil, finally causing an arc either from the coil to the workpiece or from one turn to the next of the coil. This arc will momentarily divert current from the welding vee causing a transient drop in the weld point temperature and possibly a weld defect.

The sliver can also be carried down into the welding vee, causing a short circuit across the vee prior to its closure point. The sliver will rapidly vaporize. However, during the time it takes to do this, the current, which normally flows from this point to the apex of the vee, is momentarily interrupted, and, again we have a potential cause for a weld defect.One of the potential benefits of using a solid state welder for stainless steel is the lower coil voltages used. The lower coil voltage is less likely to create the arc that can result
in a defect.

COINING OF THE STRIP EDGES IN THE FINPASS ROLLS
If the slitting is good, or if edge trimming is used, no significant working of the eedges should be required in the fin passes. If slitting is rough or if there is edge damage from shipping or handling, it may be possible to flatten and square them in the fin passes and still make a clean weld. However, it is also possible that this will lead to production of slivers or laps. A lap is created when a burr is rolled back into the edge but is not fused with the edge metal.

Parallelism of the edges coming together in the vee is more important in stainless steel than in carbon steel. Because stainless steel generally exhibits more spring back than carbon steel, there is a tendency for the edges to close at the inside of the tube first and then at the outside (peaked forming). The overheating at the I.D. can lead to a buildup
of melted material on the impeder and/or I.D. mandrel and the lack of heat on the O.D. can cause cold or pasty welds.

Care should be taken to keep the fin rolls clean to prevent pressing dirt, oil, etc. into the edge of the strip. Any foreign material on the strip edge, including paint, will oxidize during heating of the vee and contribute to potential weld defects. Felt wipers may be used on top of the fin rolls to scavenge the corners between the fin blade and the roll radius.

If stiff strip is being used, it is recommended that roll tooling incorporating proper edge forming be used. Without edge forming, efforts to get stiff strip to form properly in the fins may result in edge damage and a less than round shape going to the weld rolls. Because stainless steels work hardens so rapidly, it is important to use well designed tooling to minimize the springback prior to the weld rolls.

INDUCTION COIL
Induction coils can be of the same general design as for carbon steel. If a solid state welder is used, the design may differ from the vacuum tube coil design for the same size. owever, the length of the coil should be approximately equal to its inside diameter. The distance from the center line of the weld pressure rolls to the front of the induction coil should be approximately the same as the coil I.D.).

If the weld pressure rolls are too large, it may not be physically possible to get the coil this close. The longer vee will require more power to heat and slightly higher temperatures
to offset the heat lost due to the longer time it takes the current to travel from the end
of the coil to weld rolls.

The longer welding vee can result in a wider heat affected zone since the heat will have a longer time to diffuse into the edges of the strip. In extreme cases the temperature at the weld point may drop below the critical temperature needed to achieve a good weld. The merits of a wider heat affected zone are widely debated in the industry. Some feel that a wider HAZ will improve ductility needed for forming and bending. Others counter that the HAZ is a weak link in the tube, representing a potential point of corrosion which should always be kept at an absolute minimum.

WATER IN THE VEE
One of the most important requirements of making high quality welds in stainless steel is the elimination of oxygen from the weld area. Water dissociates at the welding temperature into hydrogen and oxygen that provides plenty of oxygen for the formation of metallic oxides. Therefore it is essential that the weld area be kept as dry as possible.This means that water used to cool the weld rolls must not get into the weld vee and that coolant used to cool the impeder must be routed backwards by using a return flow impeder. Coolant from the upstream forming rolls must not be allowed to flow into the weld area and fill the tube to overflowing.

SPUME
Spume is the name generally given to the small particles of metal that are ejected from the welding vee during the high frequency welding operation as a result of the high electromagnetic forces produced by the current flowing in the work coil (Fig 8). This spume tends to be in the form of very small, spherical particles of metal and metal oxides. They can be very hard and abrasive and if they are allowed to accumulate on the weld rolls, marking of the tube is likely.

The metal particles can also blow backwards and accumulate on the coils, and even on the edges of the strip, both of which can cause weld defects. The spume can be removed with felt wipers on the weld rolls, and in some cases, literally vacuumed up by using a fume collector placed over the weld area.

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