A weld repair is made in a power transmission shaft that contained a small fatigue crack. The shaft is made from a low a

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A Weld Repair Is Made In A Power Transmission Shaft That Contained A Small Fatigue Crack The Shaft Is Made From A Low A 1 (86.66 KiB) Viewed 16 times

A weld repair is made in a power transmission shaft that contained a small fatigue crack. The shaft is made from a low alloy steel. The shaft was post-weld heat treated (tempered) after welding. The shaft is put back into use but soon fails due to the propagation of a crack and subsequent fast fracture. Which of the following answers most accurately describes what may have happened? O During welding the heat affected zone transformed to austenite. The steel has a low carbon equivalent meaning the critical cooling rate is high. This made it easy to form martensite which could crack during cooling under the thermal stresses. The postweld heat treatment would reduce the brittleness of the martensite but the crack was already present and could grow when the usage stresses were applied. O The shaft was probably initially in a quenched and tempered form. During welding the heat affected zone transformed to austenite and then cooled faster than the critical cooling rate,forming martensite. This would be quite easy due to the high carbon equivalency of the alloy. The postweld heat treatment would reduce the brittleness of the martensite but it is likely that a crack already formed during cooling in response to thermal stresses. This crack could then grow when the usage stresses were applied. O The steel used for the shaft would have a high carbon equivalency, indicating that even slow cooling could form martensite in the heat affected zone. This region would reach temperatures sufficient to form austenite. The postweld heat treatment should have increased the strength of the martensite so the failure was probably because this treatment was too short or at too low a temperature. O The temperature in the weld zone would have been very high during welding, sufficient to transform the initial microstructure to austenite. The low carbon equivalency of the alloy would result in a high critical cooling rate. Combined with the post weld heating this should have resulted in a predominantly ferrite/pearlite microstructure, which would have a low yield strength. This would encourage failure in use.