Monday, August 20, 2012

Thread Galling of Stainless steel fasteners Prevention from another author

Written by:J oe Greenslade

A few times each year we receive calls from fastener suppliers who are in conflict with their customer over the quality of stainless steel bolts and nuts. The customer's complaint is that during installation the bolts are twisting off and/or the bolt's threads are seizing to the nut's thread. The frustration of the supplier is that all required inspections of the fasteners indicate they are acceptable, but the fact remains that they are not working. 

This problem is called "thread galling." According to the Industrial Fastener Institute's 6th Edition Standards Book (page B-28), 

Thread galling seems to be the most prevalent with fasteners made of stainless steel, aluminum, titanium, and other alloys which self-generate an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing - the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out. 

Carpenter Technologies, the fastener industry's largest supplier of stainless steel raw material, refers to this type of galling in their technical guide as "cold welding." Anyone who has seen a bolt and nut with this problem understands the graphic nature of this description. 

The IFI and Carpenter Technologies give three suggestions for dealing with the problem of thread galling in the use of stainless steel fasteners: 

1. Slowing down the installation RPM speed will frequently reduce, or sometimes solve completely, the problem. As the installation RPM increases, the heat generated during tightening increases. As the heat increases, so does the tendency for the occurrence of thread galling. 
2. Lubricating the internal and/or external threads frequently eliminates thread galling. The suggested lubricants should contain substantial amounts of molybdenum disulfide (moly), graphite, mica, or talc. Some proprietary, extreme pressure waxes may also be effective. You must be aware of the end use of the fasteners before settling on a lubricant. Stainless steel is frequently used in food related applications, which may make some lubricants unacceptable. Lubricants can be applied at the point of assembly or pre-applied as a batch process similar to plating. Several chemical companies offer anti-galling lubricants. One such source, EM Corporation, suggests their Permaslik¨ RAC product for use at the point of assembly. They suggest Everlube¨ 620C for batch, pre-applying to stainless steel fasteners. 
3. Using different stainless alloy grades for the bolt and the nut reduces galling. The key here is the mating of materials having different hardnesses. If one of the components is 316 and the other is 304 they're less likely to gall than if they're both of the same alloy grade. This is because different alloys work-harden at different rates.
Another factor affecting thread galling in stainless steel fastener applications is thread roughness. The rougher the thread flanks, the greater the likelihood galling will occur. In an application where the bolt is galling with the internal thread, the bolt is usually presumed to be at fault, because it is the breaking component. Generally, it is the internal thread that is causing the problem instead of the bolt. This is because most bolt threads are smoother than most nut threads. Bolt threads are generally rolled, therefore, their thread flanks are relatively smooth. Internal threads are always cut, producing rougher thread flanks than those of the bolts they are mating with. The reason galling problems are inconsistent is probably due largely to the inconsistencies in the tapping operation. Rougher than normal internal threads may be the result of the use of dull taps or the tapping may have been done at an inappropriately high RPM. 

Fortunately, stainless steel bolt and nut galling problems do not occur everyday, but when they do it usually creates a customer crisis. Knowledge of why this occurs and how to remedy it can save the supplier much grief and many headaches. 

Below are the questions that should be asked and the suggestions that should be made immediately when you are confronted with a customer's complaint about thread galling: 


1. Are you using the same driver RPM you have used in the past to install these stainless fasteners? 
If they say they are driving them faster than in the past or if they say this is a new application, suggest they immediately try slowing the driver RPM and see if the problem goes away. In general, a stainless bolt of a given size should be driven slower than a steel bolt of the same size. 

2. Are the bolts and/or internal threads lubricated? 
If they say, "no", suggest they try lubricating the bolts and/or internal threads with one of the lubricants listed above. If this eliminates the galling, you might want to batch lubricate the remainder of the order to eliminate the extra work of applying lubricant at the point of assembly. 

In applications where galling is a repetitive problem, it is advisable to supply the fasteners with pre-applied lubrication to eliminate future problems 

3. Are you using the same grade of stainless steel for the bolts and nuts? 
If the answer is, "yes", you can suggest changing one or the other to a different grade. 

Be sure the suggested grade meets their corrosion needs and changing the material does not cause a procurement problem. 

When thread galling occurs in stainless steel bolt and nut applications, don't panic. Try the suggestions listed above. One, or a combination of these, will probably resolve the problem immediately.

Tuesday, August 14, 2012

Galvanic Corrosion & BUMAX fasterners

One of the biggest questions I get today is about Galvanic corrosion and can BUMAX help.  The answer is yes, but it is not the end all product.  While the inherent properties of BUMAX reduces galvanic corrosion, it doesn’t fully prevent it.  BUMAX brand can be coated to provide maximum protection while retaining all the properties of 316L but with the increased strength.  BUMAX Fasteners for Galvanic Corrosion due exist.  If you would like more information email  or look at .  Below is additional information on what is Galvanic Corrosion.

 Galvanic corrosion, also called Bimetallic Corrosion, takes place when two dissimilar metals are electrically connected in an electrically conducting fluid.

The likelihood and severity of galvanic corrosion depends on several factors like:

  1. Corrosion potential of metals in the environment considered
  2. Relative area of the two metals
  3. Type of electrolyte

The corrosion potentials of the stainless steelsare noble than the corrosion potentials of aluminum, as can be seen in the galvanic series of metals in seawater below.

This mean that there will be no galvaniccorrosion on stainless steel when placed in contact with aluminum while aluminum will corrodes.

Permissible couples to avoid galvanic corrosion of aluminium when placed in contact with more noble materials are limited to a potential difference of 0.10 V on the galvanic series.

Anodic current density and hence corrosion rates can be reduced by increasing the anode-to-cathode surface area ratio; if a small area of stainless steel is placed in contact with a big area of aluminum the rate of aluminum galvanic corrosion is low due to the effect of the relative areas.

It is possible to avoid stainless steel aluminium galvanic corrosion isolating the two material by means of an electrical insulating material, like rubber.

In the presence of crevices stainless steels may exhibit less noble potentials due to oxygen depletion within the crevice.

Therefore, coupling a relatively large area with the small-area characteristic of a crevice may result in rapid attack of the material within the crevice leading to stainless steel corrosion.

Under some circumstances, coupling stainless steel to a more active metal can shift the stainless steel corrosion potential from a passive range to an active range. This shift results in an increase in the corrosion rate of the stainless steel due to stainless steel galvanic corrosion.

Stainless steel fasteners in contact with more active metals can become embrittled from the hydrogen
generated by the stainless steel aluminum galvanic corrosion couple.

The table below reports the Corrosion potentials in flowing sea water at ambient temperature.

The unshaded symbols show ranges exhibited by stainless steels in acidic water such as may exist in crevices or in stagnant or low velocity or poorly aerated water where Stainless Steel become active, while the shaded areas show the potentials of Stainless Steel when is in passive state.

This table is taken from: Atlas Steel Technical Note No. 7 "Galvanic Corrosion"

Tuesday, August 7, 2012

What is A2-70 Stainless Steel Fasteners and Material

What Is A2-70 Stainless Steel ?

DIN/ISO A2 Stainless Steel Is 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 ?

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.

How Much Of An Issue Is Galvanic Corrosion ?
Galvanic Corrosion Takes Place When Two Dissimilar Metals Are Electrically Coupled In The Presence 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 Anodes. The (More Reactive) Zinc Corrodes And Dissolves In The Presence 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 Governed By Two Key Factors.
  • Availability 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 Aluminum 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 Minimizes The Galvanic Effects Should An Electrolyt Be Introduced ; This Is Unlikely 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.

Friday, August 3, 2012

So one of the most overlooked asspect of fasteners, Nuts and Washers.  I always have to try to remember what size relates to the size of the bolt.  See below


Hex NutMachine Screw NutHex NutJam NutNylock NutMachine Screw Nut
* This is the diameter across the flats. It is also the size of wrench to use.

Wednesday, August 1, 2012

What size wrench to use with your Bolts

This ia always a fun question.  You know the bolt size, but forgot what size socket or wrench you need.  Here is your answer.

Bolt Diameter
Head/Wrench Size
Heavy Hex
ANSI - American National Standards Institute
ISO - International Organization for Standardisation
DIN - Deutsches Institut fur Normung
JIS - Japanese Industrial Standard
* Indicates ISO standard.