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HYDROGEN EMBRITTLEMENT OF HIGH

STRENGTH BOLTS

American Galvanizers Association

Tom Langill

October 3rd, 2012

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Hydrogen Embrittlement• Chemical Processes generate hydrogen• Hydrogen goes into the atmosphere or diffuses into the

steel• Issue is trapped hydrogen – Hydrogen does not release

at high temperatures• Trapped hydrogen starts moving at later date when part

is under external stress• Movement of hydrogen causes micro-cracking which

expands under stress – failure• Dissimilar metal interactions can generate hydrogen and

cause embrittlement

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Hydrogen in the Hot-Dip Galvanizing Process

• Main source of hydrogen is the pickling operation

• Fluxing can also generate some hydrogen• Temperature of the galvanizing bath drives

out most hydrogen• Previous work showed only steels with over

150 KSI strength trapped hydrogen• Hydrogen embrittlement does not occur

immediately after galvanizing

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ASTM A490 Bolts

• High Strength Bolts – UTS 150 KSI to 173 KSI

• Rockwell “C” Hardness - 33 to 39• Practical limit is determined by Wedge Tensile

Test of Bolts• Failure mode in Wedge Tensile Test is

separation of bolt head from shaft• Qualification of corrosion protection systems is

per Industrial Fastener Institute (IFI) Specification 144

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IFI 144 REQUIREMENTS

• Rotational Capacity• Salt Spray• Cyclic Testing• Hydrogen Embrittlement• Adhesion Test• Paintability• Coating Thickness

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ROTATIONAL CAPACITY

• Full size bolts tested with lubricant• Tighten greater than 110% of Proof Load• Skidmore-Wilhelm Tension Calibrator• ¾” and 1” bolts tested• Apply 35 Kips then add 5 degrees rotation• Measured Torque on ¾” – 417 Ft-lbs (547 Ft-

lbs maximum)• Measured Tension on ¾” – 44 Kips (40 Kips

minimum)• ¾” bolts passed the test

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ROTATIONAL CAPACITY

• Apply 64 Kips then add 5 degrees rotation• Measured Torque for 1” – 966 Ft-lbs (1333

Ft-lbs maximum)• Measured Tension for 1” – 79 Kips ( 74 Kips

minimum)• 1” bolts passed test

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SALT SPRAY TESTING

• ASTM B117 Test Procedure• 1000 Hours test duration• ASTM D1654 Inspection after Testing• Measure % of red rust on bolts• % converted to rating number• Ratings from 0 to 10• Higher number is better performer

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SALT SPRAY TESTING• 10 bolts of each size – ¾” and 1” tested per B117

•Head and Body inspected separately per D1654

•For 1” Bolts - Head Rating Average = 6.3; Body Rating = 5.9

•For ¾” Bolts – Head Rating Average = 7.1; Body Rating Average = 7.3

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CYCLIC TESTING• GM 9540P Test Procedure

•80 cycles duration – each cycle 24 hours

•Parts inspected every 8 cycles for red rust

•5 bolts of each size tested under load

•Then 5 bolts of each size tested under no load

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CYCLIC TESTING• Head and Body

inspected separately per D1654

• For 1” Bolts - Head Rating Average = 9.0; Body Rating = 9.0

• For ¾” Bolts – Head Rating Average = 9.0; Body Rating Average = 9.0

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HYDROGEN EMBRITTLEMENT

• Bolts harden through heat treatment to maximum allowed by A490 specification

• Two tests were performed to evaluate embrittlement potential – ASTM F606 and ASTM F1624

• Tests were performed on actual galvanized A490 bolts

• Bolts were mechanically cleaned and flash pickled before galvanizing

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HYDROGEN EMBRITTLEMENT

• ASTM F606 Test Conditions– 4 degree wedge under bolt head– Tightening torque giving tensile load of 37,575

lbs for ¾” bolts = 75% UTS minimum– Tightening torque giving tensile load of 68,175

lbs for 1” bolts = 75% UTS minimum– Load applied for 48 hours– 20x scope used to examine bolt after testing

• Examination found no signs of cracking• Samples met the specification requirements

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• ASTM F1624 Test Conditions– Rising Step Stress– Bolts machined to hemisphere cross section– Coated to electrically isolate galvanized steel– Internal Hydrogen Embrittlement (IHE) – air– Environmental Hydrogen Embrittlement (EHE) – Immersed

in salt water with imposed potential of -1.15 V

• Perform Fast Fracture Test – No Step Stress• Determine Step Rate Through Series of

Tests under Rising Steps

HYDROGEN EMBRITTLEMENT

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HYDROGEN EMBRITTLEMENT

• Rising Step Stress Tests used to determine optimum test conditions– Length of Step– Time of Step– Overall Test Time– Step Stress Increase

• Optimum Condition Reached When No Further Change to Stress Value for Initiation of Cracking

HYDROGEN EMBRITTLEMENT

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HYDROGEN EMBRITTLEMENT

• Evaluation of Hydrogen Embrittlement Potential is Stress at Cracking Divided by Fast Fracture Load

• For Hot-Dip Galvanized 1” Bolts the Average Stress at Cracking was 449 lbs.

• The Fast Fracture Load of 1” Bolts was 603 lbs.

• The Ratio of Cracking Stress to Fast Fracture Strength is 65%

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HYDROGEN EMBRITTLEMENT

• Acceptance Criteria in Accordance with ASTM F519 – Minimum 60% FFS

• Hot-Dip Galvanized Bolts Tested at 65% of FFS in Step Stress Testing

• Bolts of 1” Diameter or Less are Not Susceptible to Hydrogen Embrittlement

• Test Performed with Scribe Mark in Coating to Simulate Coating Defect

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ADHESION TESTS

• Test Conducted in Accordance with ASTM B571

• ASTM A153 is also Referenced in Adhesion Test

• Tests using the Stout Knife Test in Accordance with A153 were Performed by RMMP Labs

• Bolts of 1” and ¾” were Tested and Passed All Requirements

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PAINTABILITY

• Test were Conducted in Accordance with IFI 144

• 10 Bolts of Each Size (1” and ¾”) were Painted

• Spray Paint was Approximately 12” from the Bolts

• Three Layers of Paint were Applied• Paint Layers Dried in 1 Hour Exceeding

the 48 Hour Minimum

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COATING THICKNESS

• Test were Conducted in Accordance with ASTM E376

• Ten Bolts of Each Size (1” and ¾”) were measured with Oxford Instruments CMI200

• 21 Measurements were Taken on Each Bolt to Insure Reproducible Results

• All Measurements Exceeded the Specification Minimum of 2.1 mils.

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ACTION PLAN

• ASTM Just Approved a Version of IFI 144• New Specification is ASTM F2660• Compare Results to Requirements of

F2660• Publish Paper of Results in ASTM

Materials and Characterization Journal• Request Change to ASTM A490

Specification – Allow Hot-Dip Galvanizing of A490 Bolts

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