STAINLESS STEEL

• Stainless steel is an iron based alloy made of some of the basic elements
such as: Iron, Chromium, Silicon, Nickel, Carbon, Copper, etc.
• It has excellent resistance to corrosion due to its chromium content,
which normally range from 10.5 to 20 percent of the alloy.
• There are five families of stainless steel these are:
Ferritic Austenitic
Martensitic Duplex
Precipitation Hardening
What is Stainless Steel?

FERRITIC
• These steels are based on Chromium with small amounts of
Carbon usually less than 0.10%.
• They have a similar microstructure to carbon and low alloy steels.
• They are usually limited in use. Ferritic steels are also chosen for
their resistance to stress corrosion cracking and are magnetic.

AUSTENITI
C
• These steels are the most common. Their microstructure is derived from the
addition of Nickel, Manganese and Nitrogen.
• This structure gives these steels their characteristic combination of weld
ability and formability.
• They cannot be hardened by heat treatment but have the useful property of
being able to be work hardened to high strength levels whilst retaining a
useful level of ductility and toughness.
• They are nominally non-magnetic but usually exhibit some magnetic response
depending on the composition and the work hardening of the steel.

MARTENSITIC
• These steels are similar to ferritic steels in being based on Chromium but
have higher Carbon levels up to as high as 1%.
• This allows them to be hardened and tempered much like carbon and low-
alloy steels.
• They are used where high strength and moderate corrosion resistance is
required.
• They are more common in long products than in sheet and plate form.
They have generally low weldability and formability and are magnetic.

DUPLEX
• These steels have a microstructure which is approximately 50% ferritic and
50% austenitic. This gives them a higher strength than either ferritic or
austenitic steels.
• They are resistant to stress corrosion cracking.
• They are weldable but need care in selection of welding consumables and
heat input.
• They have moderate formability. They are magnetic but not so much as the
ferritic, martensitic.

PRECIPITATION HARDENING (PH)
• These steels can develop very high strength by adding elements such as
Copper, Niobium and Aluminium to the steel.
• With a suitable “aging” heat treatment, very fine particles form in the
matrix of the steel which imparts strength.
• These steels can be machined to quite intricate shapes requiring good
tolerances.
• Corrosion resistance is comparable to standard austenitic steels

THE MANUFACTURING
PROCESS
The manufacture of stainless steel involves
a series of processes. First step in making
stainless steel:
• The raw materials are melted together in
an electric furnace. This step usually is 8
to 12 hours of intense heat.
• Next, the mixture is cast into one of
several shapes, these include blooms
(rectangular shapes), billets (round or
square shapes) and slabs.

• After steel goes through forming operations,
beginning with hot rolling, in which the steel is
heated and passed through huge rolls.
• Blooms and billets are formed into bar and wire,
while slabs are formed into plate, strip, and sheet.
Bars are available in all grades and come in rounds,
squares, octagons, or hexagons.
• After the stainless steel is formed, most types must go
through an annealing step. Annealing is a heat
treatment in which the steel is heated and cooled
under controlled conditions to relieve internal
stresses and soften the metal.

• Annealing causes a scale or build-up to form on the
steel. The scale can be removed using several processes.
One of the most common methods:
Pickling, uses a nitric-hydrofluoric acid bath to descale
the steel.
Electro cleaning, an electric current is applied to the
surface using a cathode and phosphoric acid, and the
scale is removed.
• The annealing and descaling steps occur at different
stages depending on the type of steel being worked.

• Bar and wire go through further forming steps (more hot rolling,
forging, or extruding) after the initial hot rolling before being
annealed and descaled.
• Sheet and strip, on the other hand, go through an initial annealing and
descaling step immediately after hot rolling.
• After cold rolling (passing through rolls at a relatively low
temperature), which produces a further reduction in thickness, sheet
and strip are annealed and descaled again. A final cold rolling step
then prepares the steel for final processing.

• Cutting operations are usually necessary to obtain the desired blank
shape or size to trim the part to final size. Mechanical cutting is
accomplished by a variety of methods:
 straight shearing using guillotine knives
 circle shearing using circular knives horizontally and vertically
positioned
 sawing using high speed steel blades
 blanking using metal punches and dies to punch out the shape by
shearing
 nibbling, cutting by blanking out a series of overlapping holes and
is ideally suited for irregular shapes.
 flame cutting, which involves a flame-fired torch using oxygen
and propane in conjunction with iron powder.

• Surface finish is an important specification for stainless steel products. A smooth surface as
obtained by polishing also provides better corrosion resistance. There are a variety of
methods used for finishing:
 A dull finish is produced by hot rolling, annealing, and descaling.
 A bright finish is obtained by first hot rolling and then cold rolling on polished rolls.
 A highly reflective finish is produced by cold rolling in combination with annealing in a
controlled atmosphere furnace, by grinding with abrasives, or by buffing a finely ground
surface.
• After the stainless steel in its various forms are packed and shipped to the fabricator or end
user, a variety of other processes are needed.
• Further shaping is accomplished using a variety of methods, such as roll forming, press
forming, forging, press drawing, and extrusion.
• Additional heat treating (annealing), machining, and cleaning processes are also often
required.

CHARACTERISTICS OF STAINLESS STEELS
• The characteristics of the broad group of stainless steels can be viewed as
compared to the more familiar plain carbon "mild" steels. As a generalisation the
stainless steels have:
Higher work hardening rate Higher ductility
Higher strength and hardness Higher hot strength
Higher corrosion resistance Higher cryogenic toughness
Lower magnetic response (austenitic only)
Must retain corrosion resistant surface in the finished product.

Alloy Group Magnetic Response1 Work Hardening Rate Corrosion Resistance2 Hard enable
Austenitic Generally No Very High High By Cold Work
Duplex Yes Medium Very High No
Ferritic Yes Medium Medium No
Martensitic Yes Medium Medium Quench & Temper
Precipitation Hardening
Yes Medium Medium Age Harden

Alloy Group Ductility High Temperature
Resistance
Low Temperature
Resistance3
Weldability
Austenitic Very High Very High Very High Very High
Duplex Medium Low Medium High
Ferritic Medium High Low Low
Martensitic Low Low Low Low
Precipitation
Hardening
Medium Low Low High

APPLICATIONS OF STAINLESS STEEL
bulk materials handling equipment
 building exteriors and roofing
 automobile components (exhaust,
trim/decorative, engine, fasteners, tubing)
chemical processing plants (scrubbers
and heat exchangers)
pulp and paper manufacturing
STAINLESS STEELS ARE USED IN AN ALMOST ENDLESS NUMBER OF APPLICATIONS AND INDUSTRIES:
petroleum refining,
water supply piping
consumer products
marine and shipbuilding
 pollution control
 sporting goods (snow skis)
 transportation (rail cars)