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Stainless Steel Basic Properties

What Is A2-70 Stainless Steel ?

 

DIN/ISO A2 Stainless Steel Is A Corrosion Resistant Steel; It Is Also Known As ASTM-304.

A2 / ASTM-304 Is An 18/8 Stainless Steel : This Designates A Metallurgical Content Of 18% Chromium & 8% Nickel.

A Bolt Marked A2-70 Is A 304 Stainless Steel Bolt With A 700 N/mm2 Tensile Strength (See Below Table).
The -70 Representing The Tensile Strength Divided By 10.
-50 & -80 Grades Are Also Available But Are Not All That Common.

A2 / 304 Stainless Steel Is What Is Known As An Austenitic Stainless Steel : It Is (Mostly) Non-Magnetic.

Though Some Residual Magnetism Can Be Introduced When The Material Is Cold Worked.

A2 / 304 Stainless Steel Is One Of The Most Highly Corrosion-Resistant Materials Available To The Designer And Engineer.

How Do A2-70's Material Properties Compare To OEM Mild Steel Bolts ?

 
stainless steel bolt and nut strength ratings versus mild steel bolts and nuts

Tensile Strength Denotes The Load At Which The Material Breaks.

0.2% Yield Strength Denotes The Load That Will PERMANENTLY Deform / Stretch The Material By 0.2% Of Its Original Size.
This Is Sometimes Referred To As The Elastic Limit.

As Can Be Seen : A2-70 Stainless Exceeds Ordinary Low Grade Steels For Strength.  Though It Is Weaker Than 8:8 Heat Treated Steels, Which Are Quite Commonplace.

It Does Mean That You Have To Think A Little About Mechanical Loadings Before You Replace An 8:8 Steel Bolt With An A2-70 Stainless Steel Bolt.! If You Are Replacing A 4:6 Or A 5:8 Bolt Then There Are No Concerns.

Where Can I Use A2-70 Stainless Steel Bolts ?

 

A2-70 Can Be Safely Used To Hold Car Body Panels And Bumpers; Wing (Fender) Bolts, Hinge Bolts, Handles, Locks, Light Fittings Etc.

A2-70 Can Be Safely Used On Non-Mechanically Loaded Engine Components Where A Simple Clamping Action Is Required (Subject To Torque Limitations).
i.e: Inlet & Exhaust Manifold Attachment, Ancilliary Attachment, Pipe Clamps, Flange Joining etc.

A2-70 Can Also Be Used For Securing Access Panels, Cover Plates And Accessory Items To The Vehicle.

Where Should I NOT Use A2-70 Stainless Steel Bolts ?

 

A2-70 Should Not Be Used In Heavily Loaded Mechanical Or Structural Areas.

  • Areas Of Direct Loading On The Suspension.
  • Areas Of Direct Loading On The Steering Column And Linkages.
  • Active Engine Componets (Con-Rods, Crank Shaft, Clutch, Transmission & Power Off-Takes)
  • Active Gearbox And Power-Train Components.

Any Nut Or Bolt Stamped With A 10:9 Or 12:9 Should Be Replaced With An Identically Rated (Or Higher) Component AND NOT An A2-70 Component.

The Below Table Gives A Rough Guide To The Maximum Torque Wrench Settings To Be Used On The Most Popular Metric Sizes.

stainless nut, bolt and screw torque wrench settings

What Is The Difference Between A2 & A4 Stainless Steel ?

 

A4 Grades Of Stainless Steel Are Usually Reserved For Use In Highly Caustic Environments Where Improved Resistance To Pitting Corrosion Is Required, i.e Marine & Exhausts.
Pitting Corrosion Occures Once The Chromium Becomes Exhausted Through Self-Passivation After Caustic Attack - The Effects Are Usually Local To The Exposed Areas. The Pitting Forms As The Iron Atoms Become Exposed To Oxygen And Oxidise To Form 'Rust'.

The Improved Resistance Comes From The Addition Of A Higher Proportion Of Chromium, Which Helps To Form A Protective Oxide Layer On The Iron - The More Chromium You Have The Longer The Protection It Offers Lasts.

However, Whilst Some Properties Are Improved, Others Deteriorate. The Extra Chromium Makes The Metal More Succeptable To Fatigue And Cracking : OK If You Are Rolling Serenly Around On The High Seas : Not So OK If You Are Driving An Old Car Like A VW.!

This Extra Protection Is Not Necessary For Automotive Use As The Component Life Of A2-70 Grades Is Generally Massive (Lifetime Guarantees Are Often Given On Items Made From These Materials).
A2-70 components Will Outlive Your Vehicle.

Professional Engineers Agree That A2 Stainless Is The Optimum Selection For Automotive Use.

How Much Of An Issue Is Galvanic Corrosion ?

 

Galvanic Corrosion Takes Place When Two Dissimilar Metals Are Electrically Coupled In The Prescence Of An Electrolyt : The Electrolyt Assisting In The Transfer Of The Charged Atoms / Molecules Necessary For The Corrosion To Take Place.
The More Reactive Of The Two Metals (The Anode) Will Corrode In Preferance To The Less Reactive Metal (The Cathode).

Steel Structures In The Maritime Environment Are Often Protected By Zinc Annodes. The (More Reactive) Zinc Corrodes And Dissolves In The Prescence Of The Salt Water (The Electrolyt), Thereby Offering Some Anti-Corrosion Protection To The Steel (Cathode). The Servicable Life Of The Steel Is Thus Extended.

The Rate At Which Galvanic Corrosion Can Proceed Is Goverened By Two Key Factors.

  • Availabilty Of Electrolytic Liquid (Such As Salt Water) To Facilitate Ion Transfer
  • The Relative Exposed Area Of The Two Dissimilar Metals That Are Exposed And Electrically Coupled In The Electrolyt

Stainless Steel Is Less Reactive Than Ordinary Mild Steel Or Cast Irons ; Therefore The Iron/Steel Will Corrode Galvanically (If The Above Conditions Are Met) Protecting The Stainless Steel.

In Terms Of A Cast Iron Or Aluminium Engine Block With Stainless Exhaust Studs The Exposed Area Of The Engine Block Is Massive Compared To The Small Exposed Area Of The Studs. This Minimises The Galvanic Effects Should An Electrolyt Be Introduced ; This Is Unlikey On A Hot Engine Unless You Wash It With A Water Based Cleansing Agent.

Fuel/Oil Misting & Residues Around Engines Will Offer Protection As It Forms A Dielectric Insulation Barrier To Electron Flow. This Barrier Stops Electrically Charged Ions Flowing And Thereby Stops The Formation Of A Galvanic Corrosion Cell.

Thread Locking Compounds And Lubricants Also Provide A Dielectric Barrier, As Does Plating In Inert Metals (Gold, Chrome Etc) And Painting.

The Stainless Steel Exhaust Aftermarket Thrives Without Any Reported Galvanic Corrosion Issues Despite The Relative Exposed Areas Being Closer Than With A Few Threaded Studs. The Hot And Dry External Environment Of The Exhaust Helps To Drive Water Based Electrolytic Liquids Away.

The DeLorean Motor Company Also Opted To Make A Whole Car Bodyshell Out Of Stainless Steel : Coupling It To An Iron Engine.




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