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Baking of Fasteners to Relieve Hydrogen Embrittlement

Q. Hi,

When the fasteners or screws parts (e.g., 200 kg) are treated in the oven for Hydrogen embrittlement relief, should the duration of baking (e.g.. 190 °C at 4 hours - ASTM B633 be timed from the temperature of the air in the oven, or from the time the fasteners & screws reach the desired temperature? (difference in timing is due to the temperature lag between the oven and screws).

What I mean is: Does the baking temperature stated by any specification refer to the temperature of the oven or is it the temperature of the actual baked parts?

Hope that as many in this business of baking & heat treatment can advise what is the industry practice and standard. Can anyone advise on the procedure to baking in the oven?

Thanks in advance.

Ivy Goh
Plating shop - Singapore

A. It is absolutely, positively when the parts reach temperature; since the amount of heater input, velocity of air movement and the mass of the load all contribute to the length of time that it takes for the center of a mass of bolts to reach desired temperature.
Relief does not really start till you reach the specified temperature.
A critical factor that you left out is using the absolute minimum amount of time from plating to the oven. Extremely important!

James Watts
- Navarre, Florida

A. Agree with James Watts. You need to embed some thermocouples within the loads in order to certify the baking process.

You haven't mentioned the grade of steel or fastener specification. Higher strength steels are more susceptible to hydrogen embrittlement and any delay in bake-out. Abnormally low tensile and Charpy impact test results could be due to hydrogen embrittlement. Some fastener standards generally prohibit the electroplating of higher strength steels; e.g., ASTM A325 only allows hot-dipped or mechanically deposited zinc coatings.

ASTM F1941/F1941M-16, 'Standard Specification for Electrodeposited Coatings on Threaded Fasteners,' describes process qualification via hydrogen embrittlement testing per ASTM F606, ASTM F1624, ASTM F1940 or NASM 1312-5. Lab-supplied test specimens are processed alongside the production parts through the entire cleaning/plating/baking process. The processed specimens are returned to the lab for the hydrogen embrittlement testing.

IFI-142, Hydrogen Embrittlement Risk Management (available from Industrial Fasteners Institute, gives detailed information on minimizing hydrogen pick-up during processing.

Ken Vlach [deceased]
- Goleta, California

contributor of the year 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, continue to benefit from.


Q. I would like to know when would you NOT BAKE out a fastener? If that fastener had a 31C rockwell hardness (B633) and had been electroplated, phosphate, or had been cleaned with an alkaline? I would think that you would want to mitigate the risk as much as possible? But, does baking out the metal reduce the hydrogen, or eliminate the hydrogen and cause a void of hydrogen?

Ian MacMoy
Product Development for Fasteners in oil and gas industry - Houston, Texas, USA
October 21, 2014

A. Articles with a Rockwell "C" hardness of less than 32 are not susceptible to hydrogen embrittlement and therefore do not need baking, regardless of the surface finishing process.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
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November 1, 2014

Rockwell hardness vs. stress factor

Q. ASTM is having a meeting in Anaheim, CA the week of May 17th 2015, to discuss B633 and F1941 and their classification of which hardness to refer to, 31C or 39C, for bake out. My question is, do you really get a stress factor from measuring the Rockwell hardness? Aren't those two different things? What formula do you run to calculate stress from hardness?

Ian MacMoy
- Houston, Texas
February 3, 2015


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