Heat treatment of plain carbon steels

Question 1

Cooling medium for Annealing, Normalising, Hardening, Tempering

Answer 1

Annealing: furnance cooling
Normalising: air cooling
Hardening: Rapid quenching (e.g. water)
Tempering: air cooling (after hardening)

Question 2

Purpose of process annealing

Answer 2

Relief cold working stresses,

Soften steel for further cold working

Question 3

Process annealing temperature range and carbon content range

Answer 3

Temp. range: 550 - 650 degree C

Carbon content range: less than 0.4 %C

Question 4

Which heat treatment induces recrystallisation?

Answer 4

Process annealing/Full annealing

Question 5

Grain structure and properties after process annealing

Answer 5

Strain free, equiaxed grains

restored ductility, relieved internal stresses

Question 6

Purpose of spheroidising annealing

Answer 6

improve machinability and ductility

converting hard cementite layers into small bits to let tool bit cut through easily

Question 7

Spheroidising annealing temperature range and carbon content

Answer 7

Temp. range: 650 - 700 degree C

carbon content range: 0.5 - 1.4 %C

Question 8

Purpose of full annealing

Answer 8

completely soften steel

turn as-cast structure into uniform grain structure

Question 9

full annealing temperature range and carbon content range

Answer 9

temp. range: 30 - 50 degree C above UCT for hypoeutectoid steels
30 - 50 degree C above LCT for hypereutectoid steels

carbon content range: 0 - 1.4 %C (everything)

Question 10

Purpose of normalising process

Answer 10

obtain uniform fine grain structure in forgings

refine grain structure

Question 11

normalising process temperature range and carbon content rage
describe process

Answer 11

temp. range: 30 - 50 degree C above UCT
carbon content range: 0 - 1.4 %C (everything)

heat metal to 30 - 50 degree C above UCT and cool in still air

Question 12

difference between full annealing and normalising process

Answer 12

Cooling rate for normalising is faster than full annealing (air cool vs furnace cool)
Normalising produces finer grain size
Normalising produces higher strength

Question 13

purpose of hardening process

Answer 13

increase hardness and wear resistance of steels (cutting tools, dies)
increase strength of steels

Question 14

hardening process temperature range and carbon content range

Answer 14

temp range: 30 - 50 degree C above UCT for hypoeutectoid steel
30 - 50 degree C above LCT for hypereutectoid steel

carbon content range: 0.3 - 1.4 %C

Question 15

how is martensite formed?

Answer 15

rapid quenching in water or oil after hardening process

Question 16

properties of martensite and crystal structure

Answer 16

very hard and brittle

body-centred tetragonal (BCT)

Question 17

Speed of quenching medium from fastest to slowest

Answer 17

caustic solution, brine, water, oil, air

Question 18

Which one out of the 2 (water or oil) should be used if both is available and why?

Answer 18

Oil (less drastic and cracking or distortion in workpiece can be minimised)

Question 19

conditions to harden steel

Answer 19

quenching rate exceeds CCR (cooling rate higher than CCR)

carbon content should be more than 0.3%C

during heating/soaking austenite is formed (austenite changes to martensite)

Question 20

purpose of tempering process

Answer 20

improve toughness and ductility

remove internal stresses (induced by quenching)

Question 21

tempering temperature range

Answer 21

100 - 600 degree C

Question 22

At what temperature does tempered martensite start forming?

Answer 22

200 degree C

Question 23

effect on properties the higher the tempering temperature

Answer 23

the higher the temp, increase in ductility and toughness

decrease in strength and hardness