IV.
Types and Uses of Alloy Steels
High- Alloy Steel
- Defined by a high percentage of alloying elements
- Most common high alloy steel – Stainless Steel (contains at least 12%
chromium)
- High alloy steels: in which sum of total Alloying elements is greater than 5%
Low-Alloy Steel
- Have much lower percentage of alloying elements, usually 1-5%
- These steels have very different strength and uses depending on the chosen
alloy
Uses of Alloy Steel
Use of Alloy Steel is mainly in automobile industry and machinery parts. Alloy steel can be
used in process area where carbon steel has limitation, below are few application of alloy steel:
Low alloy steels are widely used for the production of pipes, automotive and aerospace
bodies, railway lines and offshore and onshore structural engineering plates. High-
strength low alloy steel is used in these areas due to its corrosion resistance properties.
High-alloy steels are defined by a high percentage of alloying elements. The most common
high-alloy steel is stainless steel, which contains at least 12 percent chromium. Stainless
steel is generally split into three basic types: martensitic, ferritic, and austenitic. Martensitic
steels contain the least amount of chromium, have a high hardenability, and are typically
used for cutlery. Ferritic steels contains 12 to 27 percent chromium and are often used in
automobiles and industrial equipment. Austenitic steels contain high levels of nickel, carbon,
manganese, or nitrogen and are often used to store corrosive liquids and mining, chemical,
or pharmacy equipment.
Commonly used alloying elements are:
Chromium – Increases Resistance to corrosion and oxidation. Increases harden ability and wear
resistance. Increases high temperature strength.
Nickel – Increases harden ability. Improves toughness. Increases impact strength at low
temperature.
Molybdenum – Increases harden ability, high temperature hardness and wear resistance.
Enhances the effects of other alloying elements. Eliminate temper brittleness in steels. Increases
high temperature strength.
Manganese – Increases harden ability. Combines with sulphur to reduce its adverse effects.
Vanadium – Increases harden ability, high temperature hardness and wear resistance. Improves
fatigue resistance.
Titanium – Strongest carbide former. Added to stainless steels to prevent precipitation of
chromium carbide.
Silicon – Removes oxygen in steel making. Improves toughness. Increases hardness ability
Boron – Increases harden ability. Produces fine grain size.
Aluminium – Forms nitride in nitriding steels. Produces fine grain size in casting. Removes
oxygen in steel melting.
Cobalt – Increases heat and wear-resistance.
Tungsten – Increases hardness at elevated temperatures. Refines grain size.
The most commonly used grades of Alloy Bar:
Grade 4140 - Chromium Molybdenum Steel
Form: Round Bar, Rectangular Bar, Square Bar, Round Tube, Bright Bar, Hex Bar, Flat
Bar, Black Bar, Forged Bar and Plate
Typical Application: Drill collars, high temperature bolts, sprockets, kelly bars, reamer
bodies, rotary table shafting, oil well tool joints, spindles, stay bolts, tractor axles, tractor
arms, axle shafts, valves, bolts, subs, couplings, trailer axles, winch shafts, piston rods,
rams, hydraulic machinery shafts, precision lead screws, chain links, zinc die-casting
dies.
Grade 4340 - Nickel-Chromium-Molybdenum Steel
Form: Round Bar, Rectangular Bar, Square Bar, Round Tube, Bright Bar, Hex Bar, Flat
Bar, Black Bar, Forged Bar and Plate
Typical Application : Crank shafts, axle shafts, dies, subs, gear shafts, gears, mine
drilling parts, oil well fishing tools and parts, etc.
Grade 6150 – Chromium Vanadium Steel
Form: Round Bar, Rectangular Bar, Square Bar, Round Tube, Bright Bar, Hex Bar, Flat
Bar, Black Bar, Forged Bar and Plate
Typical Applications: Gears, pinions, shafting, axles, machinery parts, springs, heavy
duty pins, bolting, hand tools.
Grade 8620 – HSLA -Nickel-Chromium-Molybdenum Steel
Form: Round Bar, Rectangular Bar, Square Bar, Round Tube, Bright Bar, Hex Bar, Flat
Bar, Black Bar, Forged Bar and Plate
Typical Applications: Because of the fine combination of properties that may be
developed, as well as the ease with which it may be processed, this material is ideal for a
wide variety of applications. Gears, pinions, oil pump piston rods and liners, spline
shafts, piston pins, cams, jigs, plastic molds, oil tool slips, jaws, gauges,etc.
V. Manufacture of Steel
Basic Materials
Iron Ore – are rocks and minerals from which metallic iron can be economically
extracted.
Coal – Metallurgical coal or Coking coal is a vital ingredient in the steel making process
and the primary source of carbon.
Limestone – is used as a flux in the blast furnace. The limestone is very important in the
blast furnace iron making process.
1. The first step in the steel manufacture begins at the blast furnace. To separate iron
from iron ore, coke (substance when gas is taken out of coal), limestone and dolomite are
charged into the blast furnace.
2. Temperature raised to 1600oF. This high temp causes the coke to burn and melt the
iron. This red hot iron drained at an opening at the base of the furnace. Natural gas is
often injected to reduce the amount of coke consumed. The dolomite and limestone
combine with the non-ferrous elements of the ore to form a slag, which floats on the top
of the molten iron and is removed separately. The product of the blast furnace is known
as “Pig Iron” the basic ingredient of steel. It takes 2 tons of iron ore, 2/3 ton of coke, ½
ton of limestone, 4 tons of air to make 1 ton of Pig iron. Some of the pig iron goes to
the foundries to make iron castings, but the vast majority is re melted and used in the
production of steel in steel furnace.
Several types of furnaces are used for the production of steel including:
Open Hearth Furnace
The open-heart furnace (OHF) uses the heat of combustion of
gaseous or liquid fuels to convert a charge of scrap and liquid
blast-furnace iron to liquid steel.
Bessemer Furnace
Bessemer Furnace is the first method discovered for mass-
producing steel from molten pig iron before the development
of OHF. The key principle is the removal of impurities from
the iron by oxidation by blowing air through the molten iron.
Electric Furnace
Electric Furnace is a heating chamber with electricity as the
heat source for achieving very high temperatures to melt and
alloy metals refractories.
New Oxygen Furnace
New Oxygen Furnace or Basic Oxygen Furnace uses pure
oxygen to convert a charge of liquid blast-furnace iron and
scrap into steel.
3. Continuous casting. A dynamic casting method in which molten metal is poured
continuously from a tundish (molten metal reservoir) into a water-cooled mold (e.g.,
copper) and solidified metal is continuously withdrawn in the form of a plate or round
billet.
