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 STAINLESS STEEL

Composition :

When an alloy of steel contains more than approximately 10 ½% Chromium it can beclassified as a stainless steel. This is because Chromium has a high affinity for Oxygen

and forms a stable Oxide film on the surface of the steel. This film is resistant to further 

chemical or physical change.tainless steels can be di!ided into four ma"or groups# namely $artensitic# erritic#

&ustenitic and 'uplex.

The Martensitic Group

This group contains a minimum of 1(% Chrome and usually a maximum of 1)% *ith

Carbon in the range of 0.0+ to (.0%. 'ue to the high Carbon content of the steel it

responds *ell to heat treatment to gi!e !arious mechanical strengths# such as hardness.

When heat treated this group of steels sho* a useful combination of corrosion resistance

and mechanical properties that ,ualify them for a *ide range of applications. These steelsare all magnetic.

The Ferritic Group

This group contains a minimum of 1-% Chrome and Carbon in the range 0.0+ 0.(%.

The increase in Chromium gi!es increased corrosion resistance at high temperatures.

/o*e!er it can not be heat treated therefore its applications are limited. These steels aremagnetic.

 

The Austenitic Group

This group contains Chromium in the range 1- (% and ic2el in the range + (0%#

*ith !arious elements added in an effort to achie!e desired properties. When fullyannealed this group exhibits a useful range of mechanical and physical properties. These

steel are normally non magnetic.

 

Duplex Stainless Steels

This relati!ely ne* group has a balance of Chromium# ic2el# $olybdenum and itrogen to gi!e a near e,ual mix of austenite and ferrite. The result is a high strength#

highly corrosion resistant material. 3ecommended extended use *ithin temperature limitsof 0 to 4500 6C due to embrittlement. They are referred to by 7 numbers# or 

manufacturer8s trade names.

 

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9roperties:

/igh oxidationresistance in air   at ambient  temperature  is normally achie!ed *ith

additions of a minimum of 15% by *eight; chromium# and up to (<% is used for harshen!ironments.=1(> The chromium forms a passi!ation layer of chromium???; oxide Cr (O5;

*hen exposed to oxygen. The layer is too thin to be !isible# and the metal remainslustrous. The layer is imper!ious to *ater  and air# protecting the metal beneath. &lso# this

layer ,uic2ly reforms *hen the surface is scratched. This phenomenon is called passi!ation and is seen in other metals# such as aluminium  and titanium. Corrosion

resistance can be ad!ersely affected if the component is used in a nonoxygenated

en!ironment# a typical example being under*ater 2eel bolts buried in timber .

When stainless steel parts such as nuts and  bolts are forced together# the oxide layer can

 be scraped off# causing the parts to *eld  together. When disassembled# the *elded

material may be torn and pitted# an effect 2no*n as galling. This destructi!e galling can

 be best a!oided by the use of dissimilar materials for the parts forced together# for 

example bron@e and stainless steel# or e!en different types of stainless steels martensiticagainst austenitic;# *hen metaltometal *ear is a concern. itronic alloys reduce the

tendency to gall through selecti!e alloying *ith manganese and nitrogen. ?n addition#threaded "oints may be lubricated to pre!ent galling.

&pplications :

tainless steelAs resistance to corrosion and staining# lo* maintenance and familiar lustre

ma2e it an ideal material for many applications. There are o!er 10 grades of stainlesssteel# of *hich fifteen are most commonly used. The alloy is milled into coils# sheets#

 plates# bars# *ire# and tubing to be used in coo2*are#   cutlery# hard*are# surgical

instruments# ma"or appliances# industrial e,uipment for example# in sugar refineries; andas an automoti!e and aerospace structural alloy and construction material in large buildings. torage tan2s and tan2ers used to transport orange "uice and other food are

often made of stainless steel# because of its corrosion resistance and antibacterial

 properties. This also influences its use in commercial 2itchens and food processing plants# as it can be steamcleaned and sterili@ed and does not need paint or other surface

finishes.

tainless steel is used for "e*ellery and *atches *ith 51<B being the type commonly

used for such applications. ?t can be refinished by any "e*eler and *ill not oxidi@e or turn blac2.

ome firearms incorporate stainless steel components as an alternati!e to  blued  or 

 par2eri@ed steel. ome handgun models# such as the mith Wesson $odel <0 and the

Colt  $1D11 pistol# can be made entirely from stainless steel. This gi!es a highluster finish similar in appearance to nic2el plating. 7nli2e plating# the finish is not sub"ect to

fla2ing# peeling# *earoff from rubbing as *hen repeatedly remo!ed from a holster;# or 

rust *hen scratched.

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ome automoti!e manufacturers  use stainless steel as decorati!e highlights in their 

!ehicles.

 PHOSPHER BRONZE 

Composition

Phosphor bronze  is an alloy  of copper   *ith 5. to 10% of tin  and a significant

 phosphorus content of up to 1%. The phosphorus is added as deoxidi@ing agent during

melting

Properties :

These alloys are notable for their toughness# strength# lo* coefficient of friction# and fine

grain. The phosphorus also impro!es the fluidity  of the molten metal and thereby

impro!es the castability# and impro!es mechanical properties by cleaning up the grain boundaries.

Uses :

9hosphor bron@e is used for springs# bolts and !arious other items used in situations

*here resistance to fatigue# *ear and chemical corrosion are re,uired e.g.# a ship8s propellers in a marine en!ironment;. The alloy is also used in some dental bridges.

Erades &# C and F G C1000# (100# 0-00 are commonly used nonferrous spring alloys.

The combination of good physical properties# fair electrical conducti!ity and moderate

cost ma2e 9hosphor Hron@e round# s,uare# flat and special shaped *ire desirable formany springs and electrical contacts and a *ide !ariety of *ire forms *here cost of

 properties does not prescribe Heryllium copper . =