Engineering Materials
A material is known as allotropic or polymorphic if it

Has a fixed structure under all conditions
Exists in several crystal forms at different temperatures
Responds to heat treatment
Has its atoms distributed in a random pattern

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Engineering Materials
In a hardening process, the hypo-eutectoid steel is

Heated from 30°C to 50°C above the upper critical temperature and then cooled in still air
Heated from 30°C to 50°C above the upper critical temperature and then cooled slowly in the furnace
Heated below or closes to the lower critical temperature and then cooled slowly
Heated from 30°C to 50°C above the upper critical temperature and then cooled suddenly in a suitable cooling medium

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