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A shear pin made from a 4340 steel (0.4% C) fails by ductile fracture. It was supposed to be initially normalised, austenitised, quenched and tempered to 100% martensite to give a hardness of 5.5 GPa. The hardness was measured to be 4.5 GPa. Which of the following summaries most accurately accounts for what may have gone wrong? O Insufficiently heating the steel would result in partial austenitisation, leading to only partial martensite after quenching and a reduction in final hardness. The temper could still occur at the correct conditions and still lead to an insufficient strength and encouraging ductile failure. O Performing a full austenitisation but then cooling slower than the critical cooling rate for the alloy would allow some ferrite/pearlite to form after the martensite reaction has finished. This would be softer than the tempered martensite and so reduce the final hardness. O Assuming a full austenitisation and then cooling faster than the critical cooling rate, the hardness would exceed the required hardness. Tempering at an excessive temperature would reduce the hardness too much and also reduce the ductility, which is consistent with the mode of failure. O If the carbon content of the alloy was lower than expected then we might get incomplete austenitisation at a temperature sufficient to fully transform correct alloy, leading to retention of softer austenite at room temperature. The resulting hardness of the tempered martensite would also be lower than desired due to reduced solution strengthening by the carbon.

A shear pin made from a 4340 steel (0.4% C) fails by ductile fracture. It was supposed to be initially normalised, austenitised, quenched and tempered to 100% martensite to give a hardness of 5.5 GPa. The hardness was measured to be 4.5 GPa. Which of the following summaries most accurately accounts for what may have gone wrong? O Insufficiently heating the steel would result in partial austenitisation, leading to only partial martensite after quenching and a reduction in final hardness. The temper could still occur at the correct conditions and still lead to an insufficient strength and encouraging ductile failure. O Performing a full austenitisation but then cooling slower than the critical cooling rate for the alloy would allow some ferrite/pearlite to form after the martensite reaction has finished. This would be softer than the tempered martensite and so reduce the final hardness. O Assuming a full austenitisation and then cooling faster than the critical cooling rate, the hardness would exceed the required hardness. Tempering at an excessive temperature would reduce the hardness too much and also reduce the ductility, which is consistent with the mode of failure. O If the carbon content of the alloy was lower than expected then we might get incomplete austenitisation at a temperature sufficient to fully transform correct alloy, leading to retention of softer austenite at room temperature. The resulting hardness of the tempered martensite would also be lower than desired due to reduced solution strengthening by the carbon.

Elements Of Electromagnetics
Elements Of Electromagnetics
7th Edition
ISBN:9780190698614
Author:Sadiku, Matthew N. O.
Publisher:Sadiku, Matthew N. O.
ChapterMA: Math Assessment
Section: Chapter Questions
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A shear pin made from a 4340 steel (0.4% C) fails by ductile fracture. It was supposed to be initially normalised, austenitised, quenched and tempered to 100% martensite to give a hardness of 5.5 GPa. The hardness was measured to be 4.5 GPa. Which of the following summaries most accurately accounts for what may have gone wrong? O Insufficiently heating the steel would result in partial austenitisation, leading to only partial martensite after quenching and a reduction in final hardness. The temper could still occur at the correct conditions and still lead to an insufficient strength and encouraging ductile failure. O Performing a full austenitisation but then cooling slower than the critical cooling rate for the alloy would allow some ferrite/pearlite to form after the martensite reaction has finished. This would be softer than the tempered martensite and so reduce the final hardness. O Assuming a full austenitisation and then cooling faster than the critical cooling rate, the hardness would exceed the required hardness. Tempering at an excessive temperature would reduce the hardness too much and also reduce the ductility, which i consistent with the mode of failure. O If the carbon content of the alloy was lower than expected then we might get incomplete austenitisation at a temperature sufficient to fully transform correct alloy, leading to retention of softer austenite at room temperature. The resulting hardness of the tempered martensite would also be lower than desired due to reduced solution strengthening by the carbon.
Transcribed Image Text:A shear pin made from a 4340 steel (0.4% C) fails by ductile fracture. It was supposed to be initially normalised, austenitised, quenched and tempered to 100% martensite to give a hardness of 5.5 GPa. The hardness was measured to be 4.5 GPa. Which of the following summaries most accurately accounts for what may have gone wrong? O Insufficiently heating the steel would result in partial austenitisation, leading to only partial martensite after quenching and a reduction in final hardness. The temper could still occur at the correct conditions and still lead to an insufficient strength and encouraging ductile failure. O Performing a full austenitisation but then cooling slower than the critical cooling rate for the alloy would allow some ferrite/pearlite to form after the martensite reaction has finished. This would be softer than the tempered martensite and so reduce the final hardness. O Assuming a full austenitisation and then cooling faster than the critical cooling rate, the hardness would exceed the required hardness. Tempering at an excessive temperature would reduce the hardness too much and also reduce the ductility, which i consistent with the mode of failure. O If the carbon content of the alloy was lower than expected then we might get incomplete austenitisation at a temperature sufficient to fully transform correct alloy, leading to retention of softer austenite at room temperature. The resulting hardness of the tempered martensite would also be lower than desired due to reduced solution strengthening by the carbon.
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