Metal Alloy Classification :
Metal alloys, by virtue of composition, are often grouped
into two classes—ferrous and nonferrous.
Steels are alloys of iron and carbon plus other alloying
elements. In steels, carbon present in atomic form, and
occupies interstitial sites of Fe microstructure. Alloying
additions are necessary for many reasons including:
improving properties, improving corrosion resistance,
etc. Arguably steels are well known and most used materials
than any other materials.
Classification Of Steels:
1) Mechanical properties of steels are very
sensitive to carbon content. Hence, it is practical
to classify steels based on their carbon content.
Thus steels are basically three kinds:
- Low carbon steels (% wt of C < 0.3),
- Medium carbon steels (0.3 <% wt of C < 0.6) and
- High carbon steels (% wt of C > 0.6).
2) The other parameter available for classification of steels
is amount of alloying additions, and based on this steels are
two kinds: (plain) carbon steels and alloy-steels.
Low carbon steels:
These are arguably
in the greatest quantities than other alloys. Carbon present in
these alloys is limited, and is not enough to strengthen these
materials by heat treatment; hence these alloys are
strengthened by cold work. Their microstructure consists of
ferrite and pearlite, and these alloys are thus relatively
soft, ductile combined with high toughness. Hence these
materials are easily machinable and weldable.
Typical applications of these alloys include:
structural shapes, tin cans, automobile body components,
A special group of ferrous alloys with noticeable amount of
alloying additions are known as HSLA
(high-strength low-alloy) steels. Common alloying elements are:
Cu, V, Ni, W, Cr, Mo, etc. These alloys can be strengthened by
heat treatment, and yet the same time they are ductile,
formable. Typical applications of these HSLA steels
include: support columns, bridges, pressure
|Table: Designation of low alloy steel|
Medium carbon steels:
These are stronger than low carbon steels. However these are of
less ductile than low carbon steels. These alloys can be heat
treated to improve their strength. Usual heat treatment cycle
consists of austenitizing, quenching, and tempering at suitable
conditions to acquire required hardness. They are often used in
tempered condition. As hardenability of these alloys is low,
only thin sections can be heat treated using very high quench
rates. Ni, Cr and Mo alloying additions improve their
Typical applications include: railway tracks &
wheels, gears, other machine parts which may require good
combination of strength and toughness.
High carbon steels:
These are strongest and hardest of carbon steels, and of course
their ductility is very limited. These are heat treatable, and
mostly used in hardened and tempered conditions. They possess
very high wear resistance, and capable of holding sharp
Thus these are used for tool application such as
knives, razors, hacksaw blades, etc.
With addition of alloying
element like Cr, V, Mo, W which forms hard
carbides by reacting with carbon present, wear resistance of
high carbon steels can be improved considerably.
The name comes from their high resistance to corrosion i.e.
they are rustless (stainless).Steels are made highly
corrosion resistant by addition of special alloying
elements, especially a minimum of 12% Cr along with Ni and Mo.
Stainless steels are mainly three kinds: ferritic
& hardenable Cr steels, austenitic and precipitation
hardenable (martensitic, semi-austenitic) steels. This
classification is based on prominent constituent of the
Typical applications include cutlery, razor blades,
surgical knives, etc.
Ferritic stainless steels are principally Fe-Cr-C
alloys with 12-14% Cr. They also contain small additions of Mo,
V, Nb, and Ni.
Austenitic stainless steels usually contain 18%
Cr and 8% Ni in addition to other minor alloying elements. Ni
stabilizes the austenitic phase assisted by C and N. Other
alloying additions include Ti, Nb, Mo (prevent weld decay), Mn
and Cu (helps in stabilizing austenite). By alloying additions,
for martensitic steels Ms is made to be above the room
temperature. These alloys are heat treatable. Major alloying
elements are: Cr, Mn and Mo.
Ferritic and austenitic steels are hardened and strengthened by
cold work because they are not heat treatable. On the other
hand martensitic steels are heat treatable.
Austenitic steels are most corrosion resistant, and they are
produced in large quantities. Austenitic steels are
non-magnetic as against ferritic and martensitic steels, which
Tool and Die
Tool and die steels are
specially alloyed steels designed for high strength,
impact toughness, and Wear resistance at room
temperature.Tool and die steels are among the most
important materials and
are used Widely in casting, forming, and machining operations
for both metallic and nonmetallic materials. They generally
consist of high-speed steels (molybdenum and tungsten types),
hot and cold-Work steels, and shock-resisting steels.
|Table : Application Of Tool Steel.|
(HSS) are the most highly alloyed tool and die steels. First
devel-oped in the early 19005, they maintain their hardness
and strength at elevated operating temperatures. There are
two basic types of high-speed steels: the molybdenum type
(M-series) and the tungsten type (T-series). The M-series
steels contain up to about 10% molybdenum with chromium,
vanadium, tungsten, and cobalt as other alloying elements.
The T-series steels contain 12 to 18% tungsten with chromium,
vanadium, and cobalt as other alloying elements. The M-series
steels, undergo less steels generally have higher abrasion
resistance than T-series distortion in heat treatment, and
are less expensive. The M-series constitutes about 95% of all
the high-speed steels produced in the United States.
High-speed steel tools can be coated with titanium nitride
and titanium carbide for improved Wear resistance