AME 2 Q1

Answer:

B. Increased hardness, decreased ductility, and formation of martensite.

Explanation:

Hardening involves heating steel to a specific temperature (austenitizing) and then rapidly cooling it (quenching). This process transforms the steel's microstructure into a hard, brittle phase called martensite. As a result, the steel becomes significantly harder but less ductile.

Why the other answers are incorrect:

• A. Increased ductility, decreased hardness, and a finer grain structure. This describes the opposite process, annealing, which softens the steel.
• C. Decreased strength, increased toughness, and a coarser grain structure. Hardening generally increases strength, not decreases it. Toughness can vary depending on the specific steel and heat treatment process. Grain size typically refines during hardening, not coarsens.
• D. No significant change in mechanical properties or internal structure. This is incorrect. Hardening drastically alters the steel's microstructure and mechanical properties.

Hardening significantly increases hardness and reduces ductility while forming a new microstructure called martensite.

Answer:

B. Low-carbon steels

Explanation:

Annealing is a heat treatment process used to soften and improve the ductility of steel. It is particularly effective on low-carbon steels as they are more prone to hardening and becoming brittle during processing.  

1. Annealing is a heat treatment process which alters the microstructure... - Bodycote

www.bodycote.com

By annealing low-carbon steels, you can restore their original softness and make them easier to form or machine.

Why the other answers are incorrect:

• A. High-carbon steels: High-carbon steels are typically hardened and tempered to achieve desired properties. Annealing would reverse this process, making them softer and less suitable for applications requiring high strength and hardness.
• C. Stainless steels: Stainless steels often undergo annealing to improve machinability or ductility before further heat treatments, but they are not primarily annealed for softening purposes.
• D. Tool steels: Tool steels are typically hardened and tempered to achieve high hardness and wear resistance. Annealing would soften them, making them unsuitable for their intended purpose.

Low-carbon steels are the primary candidates for annealing as they are more prone to hardening during processing and benefit greatly from the softening and ductility improvement provided by this heat treatment.

Answer:

C. Increased toughness, decreased brittleness, and refinement of microstructure.

Explanation:

Tempering is a heat treatment process that follows hardening. It involves reheating the hardened steel to a lower temperature, which reduces hardness but significantly increases toughness and ductility. The internal structure is also refined during this process.

Tempering is essential for achieving the desired balance of hardness and toughness in steel components.

Why the other answers are incorrect:

• A. Increased hardness, decreased ductility, and formation of martensite. This describes the hardening process, not tempering. Tempering actually reduces hardness.
• B. Decreased hardness, increased ductility, and formation of pearlite. While tempering does decrease hardness and increase ductility, the formation of pearlite is more associated with the annealing process. Tempering involves the decomposition of martensite into finer structures.
• D. No significant change in mechanical properties or internal structure. Tempering significantly alters the mechanical properties and microstructure of steel, making it tougher and less brittle.

Tempering is a crucial process for achieving the desired balance of hardness and toughness in steel components.