ROAD AND WORKSHOP by Handyman
Page 63
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Arc Welding Hints—Heat Stress and Distortion
k T some time in his training, a welder -1• will discover that although his weldig is taking shape satisfactorily, other kings are happening that are not so )od—although correctly welded, the aished work is considerably misshapen. he reason for this is expansion and conaction because of localized heat. Not nly will the work alter shape when cold, tit stresses will be set up. It is imporint to learn how to avoid this distortion y certain pre-weld preparations.
First of all, it is necessary to appreciate ad remember the movements of steel nder local heat conditions. Once this is nderstood, certain standard precautions xn be taken that will soon become part I the welding operation.
Take a strip of steel, 6 in. long, 2 in.
ide and in. thick. This plate is to heated to near welding temperature own one side only. marked A on Fig. I. s the heat build up takes place, so do se stresses on that side of the plate. Because of this expansion of side A, tensile stress is produced on side B and distortion as shown in Fig. 2 is caused— in the form of a left-hand curve away from the heat.
This, however, is not the end of the movement by any means, and the next movement is most important. As further heat is poured into side A of the plate, expansion of A becoihes so great that stresses reach a point, after which, plastic shrinkage takes place. This means that side A will become shorter after cooling, whereas side B has not undergone any distortion—and will therefore be at its original length. As a result, on cooling down, the plate will now distort in right-hand curve as shown in Fig. 3.
Now check the preparations necessary
when welding two plates of 36 in. length. Look at Fig. 4 and you will See the weld proceeding along the plate A to C. As the are reaches point B, the free ends of the plates will deviate from each other. If for any reason the weld is stopped at that point, the two edges will return and close up (even overlapping each other) making it impossible to proceed further with the work.
Therefore, the welder has two alternatives: (a) to tack strOngly at 12 in. intervals with runs 2 in_ long, each strong enough to withstand stress, or (b) for this 36 in. length, increase the gap at the free end by -!7 in, before making the run, By this means free eXpansion will take place. and contraction will bring the plates back true in line when cold.