Question

Rank the following microstructures of steel in terms of their relative ductility, assuming they each feature the same overall composition and, as applicable, the same fractions of the phases present.

1 = highest ductility

6 = lowest ductility

Fine Pearlite, bainite, tempered martensite, spheroidite, martensite, coarse pearlite   

Rank the following microstructures of steel in terms of their relative hardness, assuming they each feature the same overall composition.

1 = highest hardness

6 = lowest hardness

Fine Pearlite, bainite, tempered martensite, spheroidite, martensite, coarse pearlite   

Answer 1

Spheroidite is a softest form of steel so it has highest ductility.

Between coarse pearlite and fine pearlite , as we know, ductility decreases as grain becomes finer and finer.

Bainite is formed by cooling austenite at cooling rate higher than that required for pearlite and lesser than that for martensite formation. So bainite has more ductility than martensite but lesser ductility than pearlite.

Even after tempering ( i.e. heat treatment to induce ductility and toughness into martensite), generally martensite has lesser ductility and higher hardness than bainite.

Martensite is hardest of all the microstructure considered here.

So the order for ductility goes like

1.Spheroidite 2.coarse pearlite 3.fine pearlite 4.Bainite 5.Tempered martensite 6.Martensite

Generally ductility and hardness don't exist together in a metal. Therefore order for hardness will be

1. Martensite 2. Tempered martensite 3. Bainite 4. Fine pearlite 5. Coarse pearlite 6. Spheroidite.

Hope the answer is satisfactory. Feel free to ask in comments.