Engineering Materials
Tensile strength of steel can be safely increased by

Adding carbon up to 6.3%
Adding carbon up to 0.83%
Adding small quantities of copper
Adding carbon up to 2.8%

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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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