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Show 3. Shapes - Certain shapes can be produced as a casting are not made as a mill form, or that cannot be fabricated economically from the wrought forms available. 4. Compositions - Some compositions are available in castings that lack sufficient ductility to be worked into wrought forms. The Advantages of Wrought Are: 1. Section Size - There is practically no limit to the section sizes available in wrought iron. Thinner sections often permit a weight reduction of 50% or more, making the initial cost of fabrication no more than, and perhaps less than, a casting; decreased tare weight results in substantial reduction in operati ng costs in many instances. 2. Thermal Fatigue - Thinner sections that reduce thermal stresses, and the inherently greater ductility of wrought materials, usually promote better resistance to thermal fatigue, especially under cyclic conditions. 3. Soundness - Wrought materials are normal ly free of internal and external defects such as shrinks, porosity, cold shuts, etc. 4. Surface Finish - The smooth surface of wrought materials is often beneficial in avoiding focal points of concentrated or accelerated corrosion or carbon attack. 5. Availability - Wrought heat resisting alloys are available from stock in numerous forms, permitting immediate procurement, and mini mizing the need for excessive inventory or maintenance supplies. 6. No pattern cost. I n fabrication, the heat resi sting alloys work differently than mild steel. Whether bending, forming shearing, machining, or wel ding, there are some differences that should be considered based on the realization of the alloy's properties. The most important thing to keep in mind when shearing an alloy for high temperature application is to avoid cracks and rough edges. Torn metal is the focal point for stresses which can lead to failure and a shorter service life. The yield strengths are a little higher in alloy in the hot rolled, annealed condition and the tensile strengths are a lot higher compared to mild steel. Shearing capacity has to be apprOXimately 50% greater. Good shearing practice is to cut 20% of the metal and the remaining 80% is fractured. A larger percent of the fracture ratio can result in tearing and develop small edge fissures which become notches when used at high temperatures due to expansion of the metal. Punching is a more critical operation and more susceptible to tearing. If economically feasible, drill rather than punch holes and on thick plates, saw cut rather than shear. 281 These fissures can also affect the ability to take forming. Use as large a bending radius as possible . The A I S I 's recommendation for austenitic steels is forming to an inside radius twice the thickness of the material, and on more highly alloyed metal it could be four times the thickness. In the real world, this is usually much too generous for most fabrications and we often see metal bent to much smaller radi i. If the inside radius has to be sharp, it is good practice to dress the edges to reduce the amount of fissures that can be generated through shearing or punching . The method for best edge conditioning is saw cutting and grinding. Welding heat resisting alloys requires specific techniques easily acqui red through experience, but does not requi re sophi sticated techniques. Welding Guidelines Are : 1. Correct joint preparation - use beveled joints. This is particularly important in heavy plate. 2. Correct power setti ng - in general, use as low heat input as possible. 3. Use stringer beads rather than weavi ng. 4. Keep interpass temperatures low. 5. Maintain a reinforced, or convex, bead contour. 6. Do not preheat, or postheat. 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 occu r, 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. I f penetration is not complete so that a cavity remains within the joint, 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 fur ther contribute a notch or stress riser effect. (Figures 5, 6a and 6b). To achieve complete penetration, chamfering or gapping of the joint is necessary. Smoother weld contours with freedom from craters are roost desirable, especially for service |
