Letter 40031

Above what tensile strength do screws require de-embrittlement? [UK] 

March 24, 2006

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-enbrittlement is necessitated.
I know spring washers/HSFG- A325 [link is to spec at TechStreet] 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
metal supplies - Aberdeen, UK

Several issues.
Zinc electroplating is prohibited for ASTM A325 bolts.
Zinc coating by hot dip galvanizing per ASTM A153 [link is to spec at TechStreet] or mechanically plating per B695 [link is to spec at TechStreet] 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 A490 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 J1648 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 B-633 [link is to spec at TechStreet]. 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 (ISO/DIS 10587) & F519 [link is to spec at TechStreet] are suitable tests for hydrogen embrittlement of plated bolts.

Ken Vlach - Goleta, California

Most consensus standards use the threshold of > 320 HV (32 HRC) or > 1000 MPa tensile strength for requiring embrittlement relief. SAE/USCAR-5, SAE AMS 2759/9B, DCX PS-9500, Ford WSS-M99A3-A, and ASTM B 850 are the most common North American standards for embrittlement relief. ISO 9588 is probably more common in the rest of the world.

Toby Padfield
Automotive module supplier - MI, USA

I want to answer or follow-up on this subject publicly (in non-commercial fashion).

 

I want to post a new question or inquiry of my own on a different subject.