Steel

Question 1

Strength and hardness of steel _____ with increase of CC, but ductility and toughness _____

Answer 1

increases; decrease

Question 2

What happens if Mn is not added to steel?

Answer 2

S combines with Fe to form FeS at GB, which has low Tm. Steel containing FeS can’t be hot worked, as it will melt

Question 3

What are the effects of adding Mn to steel?

Answer 3

> increases yield strength due to substitutional solid solution hardening
decreases diffusion coefficient of C
increases hardenability
decreases ductile to brittle transition temperature

Question 4

What are the effects of adding Ca to steel?

Answer 4

> it forms calcium sulphide/silicate that remain equiaxed after hot rolling
the change from elongated to equiaxed grains improves ductility and toughness

Question 5

What are some impurities in steel?

Answer 5

a. S must be <= 0.05wt%
b. P forms very brittle Fe3P on GB that affects ductility and toughness
c. Si leads to inclusions that adversely affect fracture-sensitive properties

Question 6

Low carbon steels (CC<0.25wt%)

Answer 6

> 20-20-20 steel
low strength
high ductility, formability, weldability

Question 7

Medium carbon steel 0.3-0.6wt% C

Answer 7

> higher strength than low carbon, but lower ductility and weldability
can be heat treated to increase strength and hardness
used for hammers, axles, shafts, etc.

Question 8

High carbon steel 0.6-1wt% C

Answer 8

> high hardness, low ductility, toughness, and weldability

> used for springs, cables, cutlery, files, saw blades, etc.

Question 9

Full annealing

Answer 9

hypo and eutectoid steels are are heated and furnace cooled slowly to get coarse pearlite

Question 10

Isothermal annealing

Answer 10

> plain carbon and alloy steels to produce soft ferrite-pearlite microstructure
steel is austenized, held until transformation occurs, air cools and minimizes residual stresses

Question 11

Patented heat treatment

Answer 11

> used for steel ropes and springs

> microstructure of fine pearlite with high hardness

Question 12

Normalizing of steels

Answer 12

> steels heated and held to complete dissolution of C and cooled in air
normalizing refines grain structure and homogenizes

Question 13

Quench hardening

Answer 13

> for max hardening, steel is quenched at rates faster than critical CR to ensure austentite –> martensite
martensite forms because kinetics are faster than pearilte, even though it is metastable
martensite may transform into ferrite and carbide

Question 14

Hardenability:

Answer 14

ability of steel to form martensite on quenching from

austenitic state

Question 15

Depth “d” to which at least 50% martensite is present is a measure of:

Answer 15

the depth up to which the steel has hardened

Question 16

Depth of hardening:

Answer 16

the depth up to which the microstructure contains at least 50% martensite

Question 17

Effects on depth

Answer 17

> thickness of the component ^ d v
rate of quenching v d v
austenite grain size v d v
component size ^ d v

Question 18

Depth of hardening is ____ for AISI 1045 steel specimens thicker than about 3”

Answer 18

zero

Question 19

Critical diameter

Answer 19

the largest (thickest) bar that has 50% martensite at its center

Question 20

All the bars of diameter larger than dc have ___ ___ 50% martensite at the center, and all therefore, not
effectively hardened throughout.

Answer 20

less than

Question 21

The critical diameter dc is ________ of specimen

diameter (thickness).

Answer 21

independent

Question 22

A component of thickness ____ dC can be

effectively hardened

Answer 22

less than or equal to

Question 23

Critical diameter depends on

Answer 23

> rate of cooling v dc v

> grain size v dc v

Question 24

Ideal critical diameter

Answer 24

the diameter of the thickest
cylindrical bar whose quenched microstructure contains 50%
martensite at the center, when the rate of cooling at the
component surface is infinite

Question 25

Ideal critical diameter is a measure of

Answer 25

the intrinsic hardenability of steel

(material property), a quantative measure of it that doesn’t depend on dimensions of sample or cooling rate

Question 26

Ideal critical diameter depends on

Answer 26

> coarse austenite grains = easier to form martensite and increases Di
Di increases with increasing CC

Question 27

Hardenability depends on

Answer 27

> coarse austenite grains = increase in hardenability (not recommended as toughness and strength decrease)
increase in CC increases hardenability
cobalt decreases hardenability
Mn, Cr, Mo, and V substantially increase hardenability of steel