A short dissertation on TIG welding of stainless steel in welded tube manufacture
This dissertation on protection for welded Stainless Tube, was written by Lance Stagbauer. We (Alan Wrigley & Lance Stagbauer) are very old friends and workmates.
- Welding is achieved by passing the strip edges of roll-formed parent material under an arc generated from a tungsten electrode.
- The function of the non-consumable tungsten electrode is to establish an arc between it and the parent material to be welded and thus generate a pool (puddle) of molten parent material.
- The puddle will form from the melting of the two edges of the parent material with the puddle’s cross-sectional area slightly increased as a consequence of radial pressure applied by side rolls near the weld zone.
- The arc is conical and transfers from the electrode to the parent material.
- The electrode, molten puddle and adjacent parent material are partially protected from oxidation by an inert shielding gas (argon) which is introduced to the welding area via a heat resistant (usually ceramic) nozzle surrounding the electrode.
- In cross-section, the molten puddle is inversely conical (widest at the top) with the non-molten edges of the parent material supporting the puddle aided by surface tension.
- The lower portion of the conical molten puddle (inside the tube) protrudes to form the internal weld bead and evidences full weld penetration. It retains its form by the surface tension.
- Due to the parent material traversing under the electrode, the molten puddle assumes a “teardrop” shape, being rounded at the entry point into the arc influence and tapering to a point after exiting the weld zone where “freezing” of the molten material is complete.
- During this period of liquidity several forces/activities/ resultants are in effect.
- The molten material is agitated as a consequence of the magnetic influence of the arc.
- The molten material reacts to traces of oxygen from air which the shielding gas cannot exclude, particularly when external turbulence is not minimised by the use of a weld box.
- The reaction of the molten material surface to oxygen contamination results in the formation of an oxide film on the molten surface which can be likened to slag.
- The agitation mentioned in 1. can disperse some of this into the molten puddle.
- Depending on the severity of the oxygen contamination of the molten puddle, the consequences impacting on the welding integrity can be catastrophic. “Severe” contamination can comprise extremely minute oxide components.
- This situation arises from the “freezing” mechanics if the molten puddle. As mentioned above, the molten puddle is supported by the angled non-molten edges of the parent material. The molten material “freezes” from this angled face at right angles in crystalline structures called “dendrites” (dendrites = tree-like). The tops of these “trees” act as a face moving toward the centre-line of the weld from both sides thus meeting in the middle. This action tends to gather up any random oxygen contaminants mentioned in 4. into the moving “freeze” face and concentrate those at the weld centre line. Because both faces are developing from an angle they meet first at the inner section of the weld-line and continue to the surface of the weld.
With an appreciation of First Principles:
- It can be appreciated that all or any contaminants must be excluded (or at least minimised) from becoming inclusions in the molten puddle of the weld process which will then concentrated to the weld centre-line.
- By using a weld box which encloses the weld area, several contributing factors are engaged to minimise oxidation during welding,
- Random draughts are excluded.
- By-passed argon from the weld process is retained as a secondary atmosphere in the weld box.
- Less air is available to dilute the weld gas.
- Much lower argon flows are needed from the torch to minimise oxygen inclusion.
- Lower torch argon flow results in less turbulence around the arc.
- An improved atmosphere opportunity at the “tail” of the weld “teardrop” when still molten weld material can be exiting the torch gas during high speed welding.
- With correctly engineered weld boxes, less cumbersome electrode shielding nozzles can be employed thus permitting the use of improved optics for electrode/weld-seam relationship control.
- Weld boxes are only part of a successful and consistent quality assured welded tube entity. Attention must also be paid to shielding gas supply, management and reticulation. For example moisture in shielding gas will fractionate into hydrogen and oxygen in the plasma generating temperature of an arc with the oxygen forming oxides in the weld.
- “Dry” Argon on bottled form contains some moisture although very little.
- Cryogenic Argon also contains some moisture but due to its temperature, any moisture is confined to floating ice in the containing vessel. When reticulated as a liquid and gassified for use it is moisture free but must be reticulated to the welding torch using gas lines which are not susceptible to osmosis with argon. E.g. using an osmosis susceptible line can mean the argon can be under high pressure but as it is a super dry gas it can absorb moisture through the walls of the line from outside air without losing internal pressure.