Question

74) Epoxy is a(n) ?

a) elastomer

b) thermoset

c) thermoplastic

d) composite

.

75) In tempering of steel, temperature treatments around 400 °C are used to?

a) reduce brittleness and increase ductility (toughen) by transforming martensite morphology

b) increase strength and hardness by locking in martensite structure

c) increase strength and hardness by causing lower density glass layer to form on surface

d) reduce brittleness and increase ductility (toughen) by causing martensite morphology to form

.

76) Surface carburizing of steel involves

a) burning excess carbon out of the iron carbon alloy to form steel.

b) introducing extra carbon into surface of austenite to harden and toughen surface with quenching and formation of martensite.

c) forming martensite on the surface through rapid cooling to harden and toughen surface with quenching.

d) introducing extra carbon into surface of cementite to harden and toughen surface with quenching.

Please answer all of them without guessing!!! Thanks

Answer 1

74)

c) thermoplastic

75)

a) reduce brittleness and increase ductility (toughen) by transforming martensite morphology

76)

b) introducing extra carbon into surface of austenite to harden and toughen surface with quenching and formation of martensite.

Explanation :

carburization is a heat treatment process in which iron or steel absorbs carbon liberated when the metal is heated in the presence of a carbon bearing material, such as charcoal or carbon monoxide, with the intent of making the metal harder. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard via the transformation from austenite to martensite,

Tempering reduces the hardness in the steel by gradually changing the martensite into a microstructure of various carbides, such as cementite, and softer ferrite (iron), forming a microstructure called "tempered martensite"