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Show Welding Guidelines Are: 1. Correct Joint preparation - use beveled jOints. This is particularly important in heavy plate. 2. Correct power setting - in general, use as low heat input as possible. 3. Use stringer beads rather than weaving. 4. Keep interpass temperatures low. 5. Maintain a reinforced, or convex bead contour. 6. Do not preheat, or postheat. 7. Bevel all butt welded plate. Implied in the suggested guidelines is an attempt to get all the weld metal to solidify at the same Instant. However, Because of the low thermal conductivity of alloys, the base metal does not dissipate the heat as rapidly and reduces the COOling rate of the weld deposit. Because of the sharp thermal gradients that might exist in such a condition, there is a tendency for bead cracks. If bead cracks do occur, it Is good practice to reduce the dwell time, or use multipass welding, laying down less metal per pass. Perhaps the most common cause of the weld failures in service is lack of adequate penetration. If penetration is not complete, a cavity remains within the joint and the repeated expansion and contraction of the metal around the cavity initiates fracture from within. This is aggravated by the fact that the absence of metal in the cavity prevents normal distribution of stresses, creating stress concentration. These sharp and irregular contours frequently existing within the void further contribute a notch or stress riser effect (Figures 7a, and 7b). To achieve complete penetration, chamfering or gapping of the joint is necessary in plate as thin as 3/16". Smoother weld contours with freedom from craters are most deSirable, especially for service in carburizlng atmospheres. Butt welding is preferable to lap type of welding and will be discussed later. Heat resisting alloys are not free machining. They require sharp and heavy tools. Machinability rating is roughly 40% that of mild steel machine stock. Relatively low speeds with cuts deep enough to get under the skin are recommended. The rake of the tool should be such that it does not rub on the material which would work harden it, causing havoc with the tool life. 6 |