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MINI - ENCYCLOPEDIA OF STAINLESS STEEL



AUSTENITIC
R
EFERS TO 300 SERIES STAINLESS , THE MOST POPULAR OF STAINLESS ALLOYS ACCOUNTING FOR 85 - 90% OF STAINLESS FASTENERS SOLD, NAMED AFTER SIR ROBERT WILLAMS AUSTEN , AN ENGLISH METALLURGIST , AUSTENITIC STEEL IS A CRYSTAL STRUCTURE FORMED BY HEATING STEEL , CHROMIUM AND NICKEL TO A HIGH TEMPERATURE WHERE IT FORMS THE CHARACTERISTICS OF 300 SERIES STAINLESS STEEL . AN AUSTENITE IS A MOLECULAR STRUCTURE WHERE 8 ATOMS OF IRON SURROUND ONE ATOM OF CARBON , THUS LIMITING THE CORROSIVE EFFECTS OF CARBON .

AUSTENITIC FASTENERS GAVE HIGH LEVEL OF CORRSION RESISTANCE IN THE STAINLESS STEEL FAMILY , CANNOT BE HARDENED BY HEAT TREATMENT AND ARE ALMOST ALWAYS NON - MAGNETIC . SOMETIMES HEAT AND FRICTION IN COLD FORMING CAN CAUSE AUSTENITIC STAINLESS TO TAKE SLIGHT MAGNETISM , BUT HE CORROSIONRESISTANT PROPERTIES REMAIN THE SAME . MOST COMMONLY USED GRADE IS TERMED AS 304 Q GRADE .

TYPICAL INDUSTRIES USING 304 GRADE FASTENERS INCLUDE :

FOOD INDUSTRY
DAIRY INDUSTRY
WINE BREWERIES
CHEMICAL INDY.
PULP & PAPER
PHARMACEUTICAL
SHIPS & BOATS
SWIMMING POOL
POLLUTION CONTROL
ELECTRONICS
COMPUTERS
MEDICAL & HOSPITAL
TEXTILES
FURNACES
ELECTRICALS

TYPE 316 STAINLESS HAS ADDED NICKEL AND ADDED MOLYBDENUM . THE MOLYBDENUM ( CALLED MOLY ) INCREASES CORROSION RESISTANCE TO CHLORIDES AND SULFATES, INCLDG.SULFUROUS ACIDS IN PULP INDUSTRIES . IT HAS SUPERIOR TENSILE STRENGTH AT HIGHER TEMPERATURES COMPARED TO 304 , BESIDES PULP AND PAPER , TYPICAL INDUSTRIES USING 316 ARE LISTED BELOW :

INDUSTRIES USING 316 GRADED FASTENERS ARE :
CHEMICAL PLANTS
TEXTILE INDUSTRY
INK & BLEACH
SHIPS & RELATED
ACID PLANTS
PHOTOGRAPHIC INDYS
RUBBER INDYS


CARBIDE PRECIPITATION

CARBON THAT BREAKS LOOSE FROM ITS BOND WITHIN THE STAINLESS SOLUTION WHEN MATERIAL IS HEATED BETWEEN 800 - 1400 DEGREES. UNDER SEVERE CONDITIONS , IT CAN RESULT IN EXTRA OXIDATION AND SURFACE CORRSION .

 

CARBON

ADDS STRENGTH TO STAINLESS STEEL , BUT ALSO LOWERS CORROSION RESISTANCE THE MORE CARBON THERE IS , THE MORE CHROMIUM BE ADDED , BECAUSE CARBON OFFSETS 17 TIMES ITS OWN WEIGHT IN CHROMIUM TO FORM CARBIDES , THUS REDUCING CHROMIUM AVAILABLE FOR RESISTING CORROSION

 

CHROMIUM
A BLUE WHITE METAL , CHROMIUM IS THE MOST IMPORTANT ELEMENT PROVIDING CORROSION RESISTANCE IN STAINLESS STEEL . BY ADDING 12% CHROMIUM TO ORDINARY STEEL , STAINLESS STEEL IS FORMED . CHROMIUM OFFSETS THE CORROSIVE EFFECTS OF CARBON FOUND IN STEEL AND IS THE PRIMARY FACTOR IN THE ABILITY OF STAINLESS TO FORM PASSIVE FILM ON IT'S SURFACE PROVIDING CORROSION RESISTANCE .

CREEP STRENGTH :
A MEASURE OF THE RESISTANCE OF FASTENERS TO STRESS UNDER ELEVATED TEMPERATURES.AT HIGHER TEMPERATURES , A FASTENER CAN CHANGE IN DIMENSION UNDER THE SAME LOAD, AND IS CALLED CREEP . C REEP CAN CAUSE THE LOOSENING OF FASTENERS AS TEMPERATURE INCREASES .

DISCONTINUITIES :
A VARIETY OF SMALL OR LARGE DISFIGURATIONS IN A FASTENER SUCH AS PITS , TOOLMARKS , VOIDS , LAPS , FOLDS AND SEAMS AND INCLUSIONS .

M INOR DISCONTINUITIES ARE PERMISSIBLE IN BOTH COMMERCIAL AS WELL AS SPECIAL TAILORMADE FASTENERS AND THOSE MADE TO VARIOUS SPECS .

DRAWING :
WHERE RAW - MATERIAL SHAPED LIKE WIRE IS PULLED THROUGH A DIE TO REDUCE ITS DIAMETER TO THAT NEEDED FOR PARTICULAR FASTENER BEING MANUFACTURED .

DUCTILITY :
THE ABILITY OF A FASTENER TO DEFORM BEFORE BREAKING ( FOR EXAMPLE AN ELASTIC WOULD BE MORE DUCTILE THAN DIAMOND ) . D UCTILITY IS A MEASUREMENT SIMILAR TO ELONGATION .


ELONGATION :
STRETCHING A FASTENER TO THE POIN THAT IT BREAKS . THE PERCENTAGE ELONGATION AT S TRETCHING A FASTENER TO THE POIN THAT IT BREAKS . THE PERCENTAGE ELONGATION AT RUPTURE ( SAME AS MEASURE OF DUCTILITY ) IS DETERMINED BY DIVIDING THE TOTAL LENGTH AFTER STRETCHING TO THE ORIGINAL LENGTH. ELONGATION DECREASES AS THE STRENGTH AND HARDNESS INCREASES .


MAGNETISM :
AS RELATED TO STAINLESS STEEL FASTENERS , 300 SERIES STAINLESS IS NON MAGNETIC IN TIS RAW MATERIAL CONDITION . COLD WORKING CAN SOMETIMES INDUCE TRACES OF MAGNETISM IN 300 SERIES , DEPENDING ON THE SEVERITY OF COLD WORKING AND CHEMICAL COMPOSITION OF STAINLESS . A RISE IN MAGNETISM IS RELATED TO INCREASE IN TENSILE STRENGTH AND WORK HARDENING CAUSED BY THE HEAT AND FRICTION OF COLD FORMING AND DOES NOT REDUCE CORROSION RESISTANCE OR CAUSE ANY MOLECULAR CHANGE IN AUSTENITIC

RAW - MATERIAL .
A HIGHER PORTION OF NICKEL CAN INCREASE STABILITY IN STAINLESS , THUS DECREASING WORK HARDENING AND ANY POSSIBILITIES OF MAGNETISM . MANY STAINLESS SPECS ALLOW 2.0 MAGNETIC PERMEABILITY WHICH TRANSLATES TO LOW / MEDIUM MAGNETISM .

MANGANESE :
A NON - MAGNETIC METAL WHICH IMPROVES STRENGTH AND HARDNESS .

MOLYBDENUM
NICKNAMED MOLY , MOLYBDENUM IS A METAL ADDED TO 316 STAINLESS STEEL , SHARPLY INCREASING CORROSION RESISTANCE TO CHLORIDES AND SULFATES , ESPECIALLY VARIOUS SULFUROUS ACIDS IN THE PULP INDUSTRY . MOLYBDENUM HELPS REDUCE HARDNESS AND INCREASES TENSILE STRENGTH AT HIGHER TEMPERATURES .

NICKEL
A METAL ADDED TO 300 SERIES STAINLESS TO PROVIDE CORROSION RESISTANCE , INCREASED STRENGTH IN BOTH HIGH AND LOW TEMPERATURES AND INCREASED TOUGHNESS IN LOW TEMPERATURES . NICKEL LOWER THE EFFECTS OF WORK HARDENING , THUS REDUCING TRACES OF MAGNETISM CAUSED BY COLD FORMING AND MAKING MATERIAL FLOW MORE FREELY IN MANUFACTURING .

 

PASSIVATING
TECHNICALLY , PASSIVATING IS THE PROCESS OF DIPPING FASTENERS INTO NITRIC-ACID SOLUTION TO RAPIDLY FORM CHROMIUM OXIDE ON THE SURFACE OF THE MATERIAL , CREATING A PASSIVE FILM THAT PROTECTS STAINLESS FROM FURTHE OXIDATION . THE PUPOSE OF PASSIVATION IS TO REMOVE BOTH GREASE LEFT FROM MANUFACTURING AND TRACES OF STEEL PARTICLES WHICH MAY HAVE RUBBED OFF MANUFACTURING TOOLS ON TO THE FASTENERS .

PASSIVE FILM
THE MAJOR CHARATERISTIC OF STAINLESS IS ITS ABILITY TO FORM A THIN LAYER OF PROTECTION CALLED PASSIVE FILM ON ITS OUTSIDE SURFACE . THIS FILM RESULTS FROM A CONTINUAL PROCESS OF LOW OXIDATION , SO OXYGEN FROM THE ATMOSPHERE IS NEEDED FOR THE PASSIVE FILM TO EXIST . ONCE FORMED IT PREVENTS FURTHER OXIDATION OR CORRSION FROM OCCURING . EVEN IF CHIPPED OR SCRATCHED , A NEW PASSIVE FILM ON STAINLESS WILL FORM.

PROOF LOAD :
A TEST LOAD THAT A FASTENER MUST UNDERGO WITHOUT SHOWING SIGNIFICANT DEFORMATION . IT IS USUALLY 90% OF THE YIELD STRENGTH.

 

TENSILE STRENGTH :
A COMMON MEASURE TO COMPARE THE STRENGTH OF A FASTENER . IT IS THE LOAD NEEDED TO PULL THE FASTENER APART .

YIELD STRENGTH
THE AMOUNT OF PRESSURE REQUIRED TO CAUSE PERMANENT DEFORMITY .

TORQUE OR TORSION STRENGTH
TORQUE IS THE FORCE USED IN TWISTING, SUCH AS TIGHTENING A FASTENER.TORSION STRENGTH IS THE AMOUNT OF FORCE NEEDED TO TWIST A FASTENER APART . BOTH MEASURES CONSIDER THE AMOUNT OF PRESSURE APPLIED TO THE FASTENER AND THE LENGTH OF THE WRENCH USED IN THE APPLICATION .

STAINLESS STEEL
W ITH THE ADDITION OF 12% CHROMIUM TO IRON , STAINLESS STEEL IS FORMED . THE CHROMIUM PROTECTS THE IRON AGAINST MOST CORROSION OR RED COLOUR RUST , THUS THE TERM STAINLESS STEEL . THE ABILITY OF STAINLESS TO FORM A THIN LAYER OF PROTECTION ON ITS OUTSIDE SURFACE , CALLED A " PASSIVE FILM " , IS ITS MOST IMPORTANT CHARACTERISTICS IN PREVENTING CORROSION .
T HE OVERRIDING PURPOSE OF STAINLESS STEEL IS TO PROVIDE CORROSION RESISTANCE AGAINST
( a ) ATMOSPHERIC CONDITIONS SUCH AS CARBON DIOXIDE , MOISTURE , ELECTRICALS FIELDS , SULFUR , SALT AND CHLORIDES COMPOUNDS
( b ) NATURAL AND ARTIFICIALLY PRODUCED CHEMICALS
( c ) EXTREME WHETHER WHERE COLD TEMPERATURES CAUSE BRITTLENESS AND HOT TEMPERATURES REDUCE STRENGTH AND INCREASE CORROSION

 

CHEMICAL COMPOSITION OF S.S FASTENERS
( %VALUES IN MAX.UNLESS OTHERWISE STATED )

Elements
C
Si
Mn
P
S
Cr
Mo
Ni
Steel Group
A2 ( 304 )
0.08
1
2
0.05
0.03
15 - 20
8 - 11 '
A4 ( 316 )
0.08
1
2
0.045
0.03
16 - 18.5
2 - 3 '
8 - 11 '

ALL DIMENSIONAL , MECHANICAL AND CHEMICAL PROPERTIES ARE AS PER SPECIFIED STANDARDS . THE DESIGNATION SYSTEM COMPLY WITH ' I S O ' 3506 STANDARD . A270 DONATES A = AUSTENITIC CHROMIUM - NICKEL STEEL , 2 = COLD HEADING STEEL ALLOYED WITH CHROMIUM AND NICKEL NORMALLY CALLED AS 304 GRADE . 4 = COLD HEADING STEEL ALLOYED WITH CHROMIUM , NICKEL AND MOLYBDENUM . 70 = PROPERTY CLASS , ie : TENSILE STRENGHT WHICH IS 10 TIMES OF THE DENOTED FIGURE IN N / mm 2 min .