MME:293 Lecture 11
Alloy Steel
Department of MME
BUET, Dhaka
Classification of steel
Two main groups :
1. Plain carbon Steel
2. Alloy steel
Plain carbon Steel :
1 .Low 2 .Medium & 3 High carbon steel
Alloy steel
Ni,V ,W etc dissolve in ferrite and Cr, Mo etc dissolves
in carbides
Reason for Alloying
Chromium steels (5xxx series)
Forms either simple or complex chromium carbide (carbide stabilizer)
Increased hardness and wear resistance of the steel is resulted
Cr increases grain growth during HT
During HT, brittleness may result from the coarse grains produced
Nickel-chromium steels (3xxx series)
Nickel – grain refiner, graphite stabilizer
Chromium – grain coarsener, carbide stabilizer
The beneficial effects of one element are stronger than the adverse effects
of the other
Steels containing 4.25% Ni and 1.25% Cr are called air-hardening steels,
Benefit ?
Manganese steels (13xx series )
Regarded as alloying element when Mn exceeds more than 1.6% in
steels
Mn-steels are used as substitutes for other more expensive low-alloy
steels
Hadfield Mn steel – 1% C, 12% Mn
After damage, it immediately becomes extremely hard .
Silicon steels (92xx series)
Regarded as alloying element when Si exceeds more than 0.6%.
Hadfield Si steels containing 3% Si (with 0.01% C) has excellent
magnetic properties
Mn (~ 1%) and Si (~ 2%) combination in steels produces unusually
high strength with good ductility and toughness
Stainless steel
steels containing at least 12% Cr are SS
Resistance to corrosion, under oxidizing conditions
Corrosion resisting property is imparted by:
Insoluble adhesive film of chromium oxide (Cr2O3)
The film is too thin to be visible but impervious to water and air
Quickly reforms (self-healing) when scratched or otherwise damaged
Strong reducing conditions cause a susceptibility to attack
Cl ion is destructive to Cr steels
Other properties of stainless steels:
strong, tough, high operating temperatures
low thermal conductivity (1/3 that of carbon steels),
difficult to machine, more expensive than carbon steel
Classifications
Classified into four types based on crystal structure and strengthening mechanisms:
Austenitic stainless steels
austenite is retained in the room temperature by Ni; best corrosion resistance
Ferritic stainless steels
contains less Ni than austenitic SS; medium corrosion resistance; less expensive
Martensitic stainless steels
least corrosion resistance; excellent for applications for springs, and cutlery
Precipitation-hardened stainless steels
Austenitic stainless steels (200 and 300 Series)
Most common stainless steel (roughly 70% of total SS production);
Cr-Ni (300 series) and Cr-Ni-Mn (200 series) SS .
The structure is austenitic; nickel is a strong austenite stabilizer
Contains 0.15% C (max), 16% Cr (min) and Ni or Mn
Addition of 2-3% Mo enhances corrosion protection in neutral salt
solutions
Addition of Ti or Nb for welded products by forming TiC or NbC
instead of (FeCr)4 C
SS containing 18% Cr plus 8% Ni
Not heat treatable, hardenable by solid solution strengthening and
cold working
Non-magnetic (although cold-worked steels show some degrees of
magnetism)
Most expensive due to high Cr and Ni content
Used for flatware, cookware, architecture, automotive, etc.
The optimum temperature for precipitation of carbide is ~ 650 C
The precipitation of (CrFe)4C at grain boundaries causes the
concentration of Cr in the adjacent austenite to fall
known as intergranular corrosion and also “weld decay”
It can be eliminated by:
lowering carbon to 0.03%,
use Ti or Nb to remove the carbon as
TiC or NbC, without lowering the Cr
content of the austenite
Ferritic stainless steels (400 Series)
SS containing 18% Cr
Martensitic stainless steels (400 and 500 Series)
SS containing 12% Cr
