Weld Defects and Practical Advice for Stainless Steel Welding

Some of the most common types of defects are defined below.

Hot cracking

This is the most common type of weld defect, and is caused by, among other things, excessively large weld pools, high impurity levels, high weldment restraint, and too thin welds. Weld-crater cracks are a type of hot cracking and occur if the arc is extinguished too quickly. Ferrite in the weld metal counteracts hot cracking. Hot cracks must be ground away.

Strike scars

Strike scars occur if the arc strays outside the joint briefly while the electrode is being struck. This type of defect has high inherent stress, often in combination with a sharp crack. It can cause stress corrosion cracking and crevice corrosion. Strike scars in duplex steels can give rise to 90-100% ferrite, resulting in embrittlement and reduced corrosion resistance. Strike scars must be ground away.

Porosity

Porosity is caused by moisture on the work metal, moisture in the electrodes, moisture in the gas (TIG, MIG), contamination of the joint (oil, paint etc).

Slag inclusions

These may result from the use of an electrode with too large a diameter in a narrow joint, or by careless welding.

Incomplete penetration

This results from using the wrong type of joint or incorrect welding parameters.

Root defect

Incomplete penetration can cause crevice corrosion and stress corrosion cracking.

Incomplete fusion

This is caused by an incorrect travel speed in MIG welding, an excessively narrow joint, excessively low welding current, or the wrong electrode angle.

Hydrogen cracking

In 13 Cr weld metal Preheat temperature too low, moisture content in covering too high.

Excessive local penetration (pipe welding)

Gap too large, heat input too high

Sink or concavity (pipe welding)

Incorrect joint design

Oxidized root side

Poor shielding can cause corrosion attacks. Remove the oxide.

Spatter

Grinding spatter can cause pitting and must therefore be removed. Weld spatter can also cause pitting.

Grinding scratches

Coarse grinding of the welded joint must be followed by fine grinding and possibly polishing.

Practical advice

Use standardized joint types. A single-U butt joint is recommended for pipe welding with TIG. The single-U butt joint is particularly advantageous in the overhead position. 

A tip is to machine single-V butt joints but grind up the single-V butt joint to a single-U butt joint in the overhead position. Tack with a gap of about 1.0-2.5 mm.

Never leave grinding burr.

Clean the joint before welding.

When tacking with TIG, use shielding gas and grind off or thin out the tacks.

When welding pipe with TIG, use pure argon and gas hoses of good quality.

Spread out the gas on the root side. Gas flow (2)-20 I/min.

Purge the pipe with 7-10 x the enclosed volume.

Keep the shielding gas on until the weld has cooled to below about 200°C.

Using a gas lens is recommended–it provides a better gas shield. Good in deep joint types, for example weldolets.

MIG welding can be carried out with pure argon or a gas mix of argon + 30% helium + 1% oxygen.

Heat input 0.5-1.5 kJ/mm (normal).

If welding with covered electrodes, do not exceed the maximum recommended current.

Extinguish the arc carefully at the end of the weld.

Do not exceed the recommended welding current.

Inter pass temperature <100°C (150°C).

The joint must be completely free of low-melting phases such as metallic copper, zinc or lead. Such phases can otherwise cause metal penetration during welding.

Submerged-arc welding and resistance welding can be used, but require special welding parameters. Information can be obtained from our technical customer service.

  

Post Weld Heat Treatment for Stainless Steel

To ensure satisfactory corrosion resistance for the welded joint, slag, spatter and oxides must be removed. Welding oxide is rich in chromium, which means that the material underneath the oxide has been depleted of chromium, thereby reducing its resistance to pitting corrosion. Post-weld treatment is therefore very important if the weld is to be exposed to acidic or neutral; chloride containing solutions such as seawater and pulp bleach plant liquids.

In these cases, pickling should be carried out to remove this oxide and enable the formation of a new protective and passivating oxide layer.

 Note that failure to use sufficient shielding gas during pipe welding may result in oxidation of the root side. In such cases the root side has to be cleaned by mechanical or chemical means.

Annealing

Stress-relief annealing of a non-stabilized stainless steel at temperatures within the range 550-650°C involves a risk of chromium carbide precipitation and might reduce the resistance to wet corrosion. Stabilized material however can undergo stress-relief annealing within the temperature range 550-650°C without any problems.

The safest method is to carry out stress-relief annealing at temperatures in excess of 1,000°C. The temperature levels can be provided by the manufacturer.

Brushing/grinding

Spatter and strike scars should be ground off, while oxide and other discoloration should be removed by brushing.

Grinding should be carried out in several stages and finished using an emery cloth with a 120 mesh or finer. If steel brushing is preferred, stainless steel brushes must be used.

Surfaces which have undergone a process of grinding should preferably be pickled or washed with dilute nitric acid to ensure full protection against corrosion.

Blasting

If blasting is used, the blasting medium must be clean and free of iron particles, iron oxides, zinc, or other similar materials.

Pickling or washing with dilute nitric acid is recommended after blasting.

Pickling

From a corrosion point of view, pickling is considered to be the best method for cleaning a welded joint. In addition to the actual cleaning process which occurs during pickling, the welded area also undergoes a new process of passivation.

This method restores the welded joint’s resistance to corrosion, partly by removing the chromium depleted layer and partly by forming a new layer of the protective oxide film.

Pickling can be performed at the location of the joint using either pickling paste or pickling fluid. All residue caused by the pickling process should be thoroughly rinsed away using clean water and dealt with in accordance with the recommendations provided by the relevant authorities.