Procedure for Welding Clad Steels

The use of a clad material, consisting of a mild or low alloy steel backing faced with stainless steel, usually from 10 - 20% of the total thickness, combines the mechanical properties of an economic backing material with the corrosion resistance of the more expensive stainless steel facing. This facing usually consists of austenitic stainless steel of the 18% chromium, 8% nickel and 18% chromium, 10% nickel type, with or without additions of molybdenum, titanium and niobium, or a martensitic stainless steel of the 13% chromium type.

The backing should be welded first, at the same time making sure that the root run of the mild steel electrode does not come into contact with the alloyed cladding. This can be achieved in two ways, either by cutting the cladding away from both sides of the root, or welding with a close butt preparation and a sufficiently large root face.

After welding the mild steel side, the root run should be back grooved and the stainless clad side welded with a stainless electrode of matching composition. The use of a more highly alloyed electrode (e.g. Afrox 309L) for the initial root run on the clad side is advisable. This applies particularly to preparations in which the backcutting of the cladding makes pick-up from the mild steel difficult to avoid. For the best resistance to corrosion, at least two layers of stainless weld metal on the clad side are recommended.

The welding of material which is clad or lined with 13% Cr (martensitic) steels usually requires a preheat of 250°C and the use of austenitic electrodes of appropriate type. Welding should be followed by a post weld heat treatment, though satisfactory results can be obtained without these precautions if, during welding, heat dissipation is kept to a minimum. This will help to temper the heat affected zone by utilising the heat build-up from adjacent weld runs.

Procedure for Welding Stainless Steels to Mild or Low Alloy Steels

Situations frequently arise when it becomes necessary to weld an austenitic stainless steel to a CMn or low alloy ferritic steel. In selecting a suitable electrode, the effect of dilution of the weld metal by the base material must be considered.

The weld metal may be diluted from 20 - 50% depending on the welding technique used, root runs in butt joints being the most greatly affected since all subsequent runs are only in partial contact with the base material and share dilution with neighbouring runs. If a CMn or low alloy steel electrode is used to weld stainless to CMn steel, the pick-up of chromium and nickel from the stainless steel side of the joint could enrich the weld metal by up to 5% chromium and 4% nickel.

This would result in a hardenable crack-sensitive weld. Austenitic stainless steel electrodes are therefore used for joining dissimilar metal combinations of stainless materials to CMn and low alloy ferritic steels. However, the correct type, which has sufficient alloying to overcome the effects of dilution from the mild or low alloy steel side of the joint, must be selected since if the weld metal does not start with an adequate alloy content the final weld may contain less than 17% chromium and 7% nickel.

Weld metal with lower chromium and nickel content is crack sensitive. Also, if as a result of dilution the weld metal is incorrectly balanced with nickel and chromium, there may not be sufficient ferrite present in the weld metal to prevent fissuring and subsequent cracking.

For these reasons, the austenitic stainless steel electrodes such as Afrox 312 or 309Mo, etc. should be used, as their composition has been specially balanced to ensure that the total alloy content is adequate to accommodate dilution effects and their ferrite content is sufficient to provide high resistance to hot cracking.

Post Weld Cleaning of Stainless Steels

Following welding, the weld and surrounding heat affected zone (HAZ) should be properly cleaned, to ensure that the entire weldment has full corrosion resistance. Depending upon the application, one or more of the following may be necessary:

• Chip or grind to remove all slag, scale and heavy oxide

• Remove all spatter

• Grind any arc strikes

• Wire brush to remove all traces of slag

• Wire brush to remove discolouration

• Grind and/or repair any crevices and pits

• Ensure all wire brushes are stainless steel, and are segregated for use only on stainless steels

• Segregate all tools for use on stainless steel, and do not allow them to become contaminated with carbon steel.

If the weldment is not properly cleaned, slag, entrapped foreign particles, and even discoloured oxides (light blue or strawcoloured or darker) may cause corrosion, depending on the environment.

Further treatments could include:

• Chemical cleaning

• Pickling – use of an acid to attack and remove contamination, oxidised areas, etc.

• Passivation – chemical treatment to form chromium rich passive oxide layer on the surface

• Mechanical polishing – to remove crevices and produce a smooth surface

• Electro-polishing (following mechanical polishing) –produces the smoothest surface finish to avoid crevices and pits. This also renders the surface less reactive than chemical passivation.