Question

Excellent combinations of hardness, strength, and toughness are obtained from
tempered martensite. An employee in a heat treatment facility austenitized an eutectoid
steel at 9500C for 45 min and finally allowed the steel to slowly cooled in furnace. Do
you think the required tempered martensite structure was produced? If not, support
your answer with a detail heat treatment that can be employed for the steel.

Answer 1

Have explained properly. I hope, will be helpful to you.

Thank you.

Tempered martensite is formed by the heat treatment of martensite. To obtain martensite we need to provide heat treatment to austenitizing eutectoid steel at a temperature of more than 725 degree centigrade (atq 950-degree centigrade.) Then followed by sudden quenching having colling rate of more than critical cooling rate (see the TTT curve below for production of 100%martensite).

Bt clearly in question it is saying to have a slow cooling in the furnace so we will not get martensite formed and hence we cannot temper this to obtain tempered martensite. In case of slow cooling, we will end up getting Pearlite structure which is not desired.

So to obtain martensite we need a very high cooling rate that of exceeding the critical cooling rate. Which will result in very hard and strong martensite but will lack a little toughness. So we go for tempering of martensite to reduce the brittleness and introduce some toughness.

Tempering of martensite:-

Tempering is a heat treatment process which follows 3 types:-

1. High-temperature tempering -> To induce high toughness into material.

->Martensite is heated between 550-650 degree centigrate and then slowly cooled in the furnace. The obtained structure is also called Sorbite (a highly tough, hard, and strong).

1. Medium temperature tempering -> To induce medium level toughness and maintain high brittleness

-> Martensite is heated between 350 - 500 degree centigrade and then slowly cooled in the furnace. Obtained structure is also called as Toosite.

1. Low-temperature tempering. -> To gain very low toughness.

Martensite is heated ~ 250 degrees centigrade and then slowly cooled in the furnace.

From above 3 processes, anyone can be selected to temper martensite. Generally High or medium temperature tempering favors for high toughness and hardness with strength combination.