Mechanical Engineering - Engineering Materials - Discussion

The correct answer is the grain size increases very rapidly.

When low-carbon steel is heated to a lower critical temperature (A1 or Ac1), the microstructure transforms, and the grain size increases rapidly. This is because:

- The steel transforms from a mixture of ferrite and pearlite to a single phase of austenite.
- The austenite grains grow rapidly as the temperature increases.
- The grain boundaries become more mobile, allowing for rapid grain growth.

This rapid grain growth can lead to a coarser microstructure, which can affect the steel's mechanical properties.

The other options are not correct because:

- There is a significant change in grain size when heating to the lower critical temperature
- The average grain size does not reach a minimum at this temperature
- The grain size does not first increase and then decrease rapidly; instead, it increases rapidly and then may stabilize or continue to grow depending on the temperature and time.

Low carbon steel is heated up to UPPER critical temperature -the average grain size is a minimum.

Because the grain size changes only when it is heated above LCT.