Some materials are easily weldable while certain others require special procedures to weld them. These materials are called difficult to weld materials. The welding of the following such materials will be discussed in this chapter.

1. Welding of cast irons

2. Welding of aluminium and its alloys

3. Welding of low carbon HY pipe steels

4. Welding of stainless steels

In addition to the above, the welding of dissimilar metals and the hardfacing and clad­ding will also be discussed.

7.1 WELDING OF CAST IRONS

7.1.1 Composition of Cast Irons

7.1.2 Oxy-Acetylene Welding of Gray and Nodular Cast Irons

• Grey cast iron contains much of carbon in flake form. This flake carbon distribution causes it to be brittle and, therefore, the standard set for its welding is not very high.

• Nodular Iron is cast with magnesium, nickel or rare earth addition, the graphite is in the form of spheroids with ferrite or pearlite matrix. This iron has ductility in as cast state upto 4% and on annealing-upto 15—25%. Its weldability is better than that of Grey cast iron as S and P are at low level. Thus the risk of hot tearing in weld metal is reduced. Welding steps are given below.

• A 60 - 90 Vee grove is prepared.

• When repairing a crack a hole should be drilled at each end of the crack to arrest it.

• The job before welding is preheated to 300-650 C in a furnace then covered with asbestos cloth, exposing only the cavity to be welded.

• If furnace is not available the casting is covered with asbestos cloth and locally heated by gas flame. Thick sections should be preheated in a furnace.

• Filler material should have the same composition as the base metal with minimum S and P. Special rods containing Ti and high Si content are also sometimes used.

• Welding rods are square or round cast bars.

• Fluxes for grey iron filler rods are composed of borates, soda ash, and small amounts of ammonium sulphate, iron oxide, etc.

• Torch tip is one size larger than that required for steel of the same thickness.

• Adjust the torch to a neutral flame.

• Move the flame along the groove untill the entire joint is preheated to dull red.

• Concentrate the flame at the bottom of the vee with tip of inner cone about 3.0 to 6.0 mm from the metal surface. As the bottom fuses thoroughly move the flame from side to side to let the liquid metal run down to the pool and rotate the torch to mix the molten metal from side walls to mix with the metal in the pool.

• If metal gets too fluid and runs down raise the flame.

• After the weld pool is formed, heat the filler rod end by outer envelop of the flame, dip the rod into the flux.

• Introduce the Flux coated (dipped) filler rod into the molten pool and apply flame to the tip of the filler rod and the welding is carried out.

• As the weld completes, cover it with asbestos and allow it to cool slowly.

• Post welding stress relieving be carried out for complex shapes. For this purpose keep casting in a furnace at 650°C for one hour per 25 mm thickness and cooled to 260°C or below at a rate not faster than 28°C per hour.