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Above What Tensile Strength do Screws Require De-Embrittlement?
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 [affil link] 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 ?
First of two simultaneous responses -- 2006
Zinc electroplating is prohibited for ASTM A325 [affil link] bolts.
Zinc coating by hot dip galvanizing per ASTM ASTM A153 [affil link] or mechanically plating per ASTM B695 [affil link] 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 [affil link] or ASTM A490M [affil link] 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 [affil link] 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 [affil link] . 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ASTM B839-04 [affil link / withdrawn] (ISO/DIS) ISO 10587 [affil link] & ASTM F519 [affil link] are suitable tests for hydrogen embrittlement of plated bolts.
- Goleta, California
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Second of two simultaneous responses -- 2006
Most consensus standards use the threshold of > 320 HV (32 HRC) or > 1000 MPa tensile strength for requiring embrittlement relief. SAE USCAR 5 [affil link], SAE AMS2759/9 [affil link], DCX PS-9500, Ford WSS-M99A3-A, and ASTM B850-98 [affil link] are the most common North American standards for embrittlement relief. ISO 9588 [affil link] is probably more common in the rest of the world.Toby Padfield
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