Metal finishing Q&As since 1989

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Above What Tensile Strength do Screws Require De-Embrittlement?

Dear Sir,
We are heavily involved in fasteners of various types, with our own zinc plating plant in-house.
We are discussing at what tensile strength de-emspecbrittlement is necessitated.
I know spring washers/HSFG- ASTM A325 hex bolts need de-embrittlement due to their pliability/elongation factors, and I reckon for HT screws we should de-enbrit at gr 10.9 and higher - any thoughts ?

Robert Morrison

2006

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First of two simultaneous responses -- 2006

Several issues.
Zinc electroplating is prohibited for ASTM A325 bolts.
Zinc coating by hot dip galvanizing per ASTM ASTM A153 or mechanically plating per ASTM B695 is allowed. However, due to thread tolerance, strength and quality issues, coating of A325 fasteners (bolt and nut sets) may only be performed by the bolt manufacturer. For usage information, download the free 'June 30, 2004 RCSC Specification for Structural Joints Using ASTM A325 or ASTM A490M Bolts' at www.boltcouncil.org.

I believe electroplating is prohibited for the highest strength fasteners, SAE Grade 8 (UTS 150 ksi) or Metric Grade 10.9 (1040 MPa), or above. SAE SAE J 1648 states "It may be necessary to provide coatings other than electroplating for fasteners with hardness above 40 HRC (390 HV)." Note that improper pickling can induce hydrogen embrittlement in galvanized steel.

When electroplating high strength steel, pre-plating stress relief and post-plating hydrogen embrittlement bake-out are prescribed in plating specifications such as ASTM B633. E.g., cold-worked steel of UTS 1000 MPa or above shall be stress relieved at 190 °C for 4 hours, and electroplated parts of UTS 1200 MPa shall be baked at 190 °C or above for 3+ hours within 4 hours of plating.

ASTM B839-04 [withdrawn] (ISO/DIS) ISO 10587 & ASTM F519 are suitable tests for hydrogen embrittlement of plated bolts.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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s

Second of two simultaneous responses --

Most consensus standards use the threshold of > 320 HV (32 HRC) or > 1000 MPa tensile strength for requiring embrittlement relief. SAE USCAR 5, SAE AMS2759/9, DCX PS-9500, Ford WSS-M99A3-A, and ASTM B850-98 are the most common North American standards for embrittlement relief. ISO 9588 is probably more common in the rest of the world.

Toby Padfield

2006