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