Question

(a) What is the distinction between hypoeutectoid and hypereutectoid steels?

(b) In a hypoeutectoid steel, both eutectoid and proeutectoid ferrite exist. Explain the difference between them. What will be the carbon concentration in each?

(c) In bullet format compare and contrast the expected mechanical behavior of hypoeutectoid and hypereutectoid steels in terms of: (i) Yield strength (ii) Ductility (iii) Hardness (iv) Tensile strength

Answer 1

a.)

• Steels having carbon within 0.02% – 0.8% consist of ferrite and paerlite. They are known as hypoeutectoid steel.
• Steels having greater than 0.8% carbon but less than 2.0% are known as hyper‐eutectoid stee

b.)

• The proeutectoid ferrite formed at a range of temperatures from austenite in the austenite plus ferrite region above 727C. The euctecoid ferrite formed during the eutectoid transformation as we cool below 727C. It is a part of the pearlite microconstiutents
• Another point of difference is that both hypereutectoid and hypoeutectoid steels have proeutecoid phases, while in eutecoid steel, no proeutectoid phase is present.
• Proeutectoid signifies is a phase that forms (on cooling) before the eutectoid austenite decomposes. It has a parallel with primary solids in that it is the first phase to solidify out of the austenite phase. Thus, if the steel is hypoeutectoid it will produce proeutectoid ferrite and if it is hypereutectoid it will produce proeutectoid cementite.
• The carbon concentration for both ferrites is 0.022 wt% C.

c.)

• (i) Yield strength : The hypoeutectoid steel have yield strength aroud 370 Mpa and hypereutectoid steels have little higher yield strengh.
• (ii) Ductility : The hypoeutectoid steel is the most ductile and the ductility has decreased by a factor of three for the eutectoid alloy. In hyper-eutectoid alloys the additional, brittle cementite on the pearlite grain boundaries further decreases the ductility of the alloy. The proeutectoid cementite restricts plastic deformation to the ferrite lamellae in the pearlite and fracture occurs in the brittle grain boundary zone.
• (iii) Hardness :  hypoeutectoid steels are less brittle and hypereutectoid steel contains low strength cementite at grain boundary region hence its more brittle.
• (iv) Tensile strength :- Grain boundary regions are high energy region, prone to cracking. Besides having cememtite in the grain boundaries, its strength decreases drastically. It fails at the tensile strength of Cementite, even though pearlite is present. (5000 psi). Hypoeutectoid steel contains ferrite at grain boundaries and pearlite inside grains, so grain boundaries being the high energy state region, it will fail from there.(Ferrite -40000 psi, Pearlite- 120000 psi)