Tool life can be almost infinite at low cutting speeds

Question

explain me about recommended that all machining be done at low speed

Answer 1

Basically all materials have elastic and plastic deformation thresholds.

Elastic deformation is not permanent, the object springs back to its original shape once the loading is taken off.  Objects like springs are designed such that from normal compression (no load) to full compression is completely within the elastic deformation range of the materials.

Plastic deformation is permanent, the object does not spring back to its original shape once the loading is taken off, though it might spring back a small amount, this is the elastic portion of the deformation.

You can readily see both if you take a ruler, bend it a little and it springs back totally straight and is still usable.  Now, bend it around on itself really sharply and you will find that it only partially returns to its original shape, the elastic portion of the bending returns, but the total bend has been too great across the length of the bending range of the ruler and it has undergone permanent (plastic) deformation.