Steel Flashcards
Strength and hardness of steel _____ with increase of CC, but ductility and toughness _____
increases; decrease
What happens if Mn is not added to steel?
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
What are the effects of adding Mn to steel?
> increases yield strength due to substitutional solid solution hardening
decreases diffusion coefficient of C
increases hardenability
decreases ductile to brittle transition temperature
What are the effects of adding Ca to steel?
> it forms calcium sulphide/silicate that remain equiaxed after hot rolling
the change from elongated to equiaxed grains improves ductility and toughness
What are some impurities in steel?
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
Low carbon steels (CC<0.25wt%)
> 20-20-20 steel
low strength
high ductility, formability, weldability
Medium carbon steel 0.3-0.6wt% C
> 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.
High carbon steel 0.6-1wt% C
> high hardness, low ductility, toughness, and weldability
> used for springs, cables, cutlery, files, saw blades, etc.
Full annealing
hypo and eutectoid steels are are heated and furnace cooled slowly to get coarse pearlite
Isothermal annealing
> plain carbon and alloy steels to produce soft ferrite-pearlite microstructure
steel is austenized, held until transformation occurs, air cools and minimizes residual stresses
Patented heat treatment
> used for steel ropes and springs
> microstructure of fine pearlite with high hardness
Normalizing of steels
> steels heated and held to complete dissolution of C and cooled in air
normalizing refines grain structure and homogenizes
Quench hardening
> 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
Hardenability:
ability of steel to form martensite on quenching from
austenitic state
Depth “d” to which at least 50% martensite is present is a measure of:
the depth up to which the steel has hardened