Page 106 - CITS - Welder - Trade Theory

WELDER - CITS

Stainless steel properties types weld decay and weldability

Objectives : At the end of this lesson you shall be able to
• explain the classification of stainless steel
• state the physical properties of stainless steel
• explain the welding procedure
• describe the weldability test of stainless steel
• state the effect of weld decay.

Classification of stainless steel:
Stainless steel is an alloy of iron, chromium, and nickel. There are many different classification of stainless steel
according to the percentage of its alloying elements. Accordingly there are three main classifications for stainless
steel.
One group is FERRITIC, which is non-hardenable and magnetic. The other group is MARTENSITE, which is hard-
enable by heat treatment and is also magnetic. The third group is ‘AUSTENITIC’ which is extremely tough and
has ductility. This is the most ideal for welding and requires no annealing after welding. But it is mildly subjected
to corrosive actions. The other groups ferrite and martensite are non-weldable. Usually the austenitic type of
stainless steel is called 18/8 stainless steel which contain 18 percent chromium 8% nickel apart from the iron
percentage. To eliminate corrosive action in this type of stainless steel stabilizing elements such as columbium,
titanium, molybdenum, zirconium etc. are added in a small percentage. So, this weldable type of stainless steel is
called a ‘stabilized type’ stainless steel. These elements also can be added to filler rods.
Physical properties of stainless steel: The coefficient of expansion of stainless steel of ferrite and martensite
are approximately the same as carbon steel whereas the austenitic type of stainless steel has about 50 to 60%
greater coefficient of expansion than carbon steel. So, while welding this type of stainless steel, distortion will be
more. The heat conductivity is approximately 40 to 50% less than that of carbon steel for austenitic type.
All these types have a brighter colour without having any stain in appearance.
Types of stainless steel filler rods: Specially treated stainless steel filler rods, which contain stabilizing elements
such as molybdenum, columbium, zirconium, titanium etc., are available.
The chromium percentage is also sometimes 1 to 1 1/2 percent more than in the base metal, so as to compensate
the losses that may occur during the welding operation from the base metal. The melting point of the filler rod also
will be 10° to 20°C less than the base metal. Filler rods of different sizes are available in the market.
Flux: A special type powdered flux which contains zinc chloride and potassium dichromate is available. During
welding powered flux is to be made into a paste form by adding water and applied on the underside of the joint.

Method of controlling distortion: Since stainless steel has a much higher coefficient of expansion with lower
thermal conductivity than mild steel, there are greater possibilities of distortion and warping.
Whenever possible clamps and jigs should be used to keep the pieces in line until they have cooled. And also a
thick metal plate of copper should be used as a backing bar during welding so as to reduce distortion in the parent
metal. Tacks at frequent intervals (i.e. pitch of tack is 20 - 25 mm) will also reduce distortion.

Welding procedure:
The type of edge preparation, nozzle size, filler rod size, pitch of tack for different thickness of sheets to be welded
are given in Table 1.

Start welding from the right edge of the joint and proceed in the leftward direction.
Keep the tip of the inner cone of the flame within 1 to 1.5 mm of the molten puddle, and hold the blowpipe at an
angle of 80-90° to the work. (Fig 1)

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CITS : C G & M - Welder - Lesson 14-26 CITS : C G & M - Welder - Lesson 14-26