What is Stainless Steel?

Stainless steel is normally joined by welding. Welding provides high strength joints with minimum flow restrictions and prevents crevice corrosion, the major concern with screw thread joints. Threaded connectors form tight crevices that often corrode. However, elimination of crevices does not guarantee trouble free operation. Extreme care must be taken during welding, as many installation problems occur because the basic rules of stainless steel welding are violated. These rules include:

• Always use oxide-grinding wheels, not silicon carbide for any dressing of weld surfaces. The carbide may react with the chromium, which decreases the corrosion resistance of the weld metal.
  
  
• Because stainless steel has lower heat conductivity than carbon steel, 30% less heat input generally is required. Also, the welds take longer to cool. Maintain short arc length and use staggered beads for very long welds to reduce heat input.
  
  
• The coefficient of thermal expansion for austenitic stainless steel is higher than for carbon steels and ferritic or martensitic stainless steels. Therefore, keep the base metal restraint to a minimum to prevent distortion of the system.
  
  
• If multiple weld passes are required, maintain the interpass temperatures at less than 200° F (100° C) to prevent cracking and distortion of the system. Avoid crater cracks by controlling the size of the termination weld pool. If crater cracks occur, remove by grinding with an aluminum oxide wheel before proceeding.

WELDING AL-6XN

Use a weld filler alloy on all field welds-for orbital welds use weld rings, for other welds, wire or weld rings may be used. The filler alloy must have higher molybdenum content than the AL6XN to compensate for alloy dilution on cooling. Typically a 9% Molybdenum alloy (Alloy 625) is used. If Alloy 625 is not available Alloy C 276 (15% Mo) may be substituted.

• Use an inert gas for both the weld and shield gas. Either helium or argon may be used, although argon is normally used. It is acceptable to use 3–5% nitrogen additions to both the torch and shielding gas to compensate for the nitrogen lost from the alloy during welding.
  
  
• Make sure the heat tint on the tubing is a light straw yellow at the darkest. A silver weld and heat-affected zone are the best. Any darker weld heat tints must be removed before placing in service. Dark blue heat tints are the most susceptible to corrosion. Remove by grinding followed by acid cleaning/passivation. A poorly cleaned surface may be just as susceptible to attack as the original heat tint.
  
  
• Do not preheat the weld unless the material is below 50º F. When the material is below the dew point, allow it to warm up to above the condensation temperature to prevent moisture condensing on the surface. Remember: moisture causes heat tints.
  
  
• Ignite the weld within the area to be welded. If that is impossible, grind the ignition point to remove it completely.

OVERALLOYING AL-6XN

Why “over alloy” AL-6XN‚ weld areas? Because of two words- Intergranular Corrosion. Although AL- 6XN is classified as a single phase alloy, when it is melted as in welding, it will solidify as a two phase alloy with:

1. Being austenite
2. Being chi phase

Chi phase, a chromium-iron-molybdenum compound depletes the grain boundary of molybdenum and chromium reducing corrosion resistance.

By over alloying as with alloy 625 weld insert rings, the alloy balance and therefore corrosion resistance is restored to the base alloy. All metals are composed of small grains that normally are oriented in a random fashion. These grains each are composed of orderly arrays of atoms, with the same spacing between the atoms in every grain. Because of the random orientation of the grains, there is a mismatch between the atomic layers where the grains meet.

Now there is a mismatch in galvanic potential between the base metal and the grain boundary, so galvanic corrosion begins. The grain boundaries corrode, allowing the central grain and the chromium carbides to drop out as so many particles of rusty sand.

The surface of the metal develops a “sugary” appearance. Several compounds may cause intergranular corrosion in addition to chi phase and chromium carbide. Another compound is sigma phase, a chromium-iron compound. Note, these are compounds, not a random mixture or alloy.

These compounds usually are formed when some type of heating occurs, such as:

• Welding
• Heat treatment
• Metal fabrication

Understanding how they form makes it relatively easy to control their formation. For example, always use a low carbon grade of stainless steel when welding is to be done. Today these grades are very common ever since the invention of argon - oxygen - decarburization (AOD) refining about 30 years ago.

Almost all stainless steel is made by this method since it allows very precise control of the alloying elements, and it is possible to obtain routinely carbon levels in the range of 0.025 percent, a level at which no chromium carbide particles form in the HAZ during welding. These grades normally are designated as “L” grades, like Types 304L, 316L or 317L. Always use the “L” grades if there is any chance that the system will be welded.

Another way of controlling the formation of chromium carbide is to use a stabilizing element addition to the stainless steel. These are titanium and niobium (columbium). The Type 304 equivalent with titanium is Type 321, and the Type 304 equivalent with niobium is Type 347. Stabilized grades should be used whenever the steel is held for long periods in the temperature range of 800 to 1500°F (425 to 800°C). Sigma or chi phase may be minimized by avoiding the temperature range where they form, or by using alloys high in nickel and nitrogen.

This is followed by a downslope which gradually terminates the current, and a postpurge to prevent oxidation of the heated material. These weld parameters are dialed into the power supply from a weld schedule sheet after determination of the proper parameters from test welds done on tubing samples.

Fusion welds with automatic orbital TIG welding equipment is practical on tubing or small diameter pipe in sizes from 1/8 inch OD tubing to 6” schedule 10 pipe, and on wall thicknesses up to 0.154 wall.

AL-6XN is easily weldable with weld parameters, including travel speed (RPM) and weld currents, comparable to 316L stainless steel. Weld appearance is excellent with a smooth, shiny, flat weld bead on both the OD and ID.

For welds with weld insert rings, the inserts are simply placed between the two sections to be welded and fusion welded as usual, except for a slight increase in welding current to compensate for the increased thickness of material contributed by the insert ring. These welds also have a pleasing appearance, with a slight crown on the OD and some inner-bead reinforcement.

AUTOGENOUS WELDING FOR AL-6XN®

Autogenous welding can be used with the following precautions:

• Use of 3 to 5 volume percent nitrogen in the shielding gas and a post-weld anneal above 2150° F (1180° C) followed by rapid cooling and pickling if a protective atmosphere was not used during annealing.
• The duration of the anneal must be sufficient to re-homogenize the weld segregation.
• The G48-B crevice test can be used to assess the quality of autogenously welded and annealed AL- 6XN alloy.

In many applications, a post-weld anneal and pickle may not be possible, as in large vessel fabrication or field welding of piping systems. In these cases, the exposure conditions must be carefully reviewed to determine if autogenous welds are satisfactory.

Autogenous AL-6XN welds are more resistant to corrosion than similar welds of types 316L, 317L and 904L. Their corrosion resistance is approximately that of alloy 904L base metal and superior to that of types 316L and 317L base metal.