Heat Treat Industry News
July 09, 2010
Stress Corrosion Cracking, Hydrogen Embrittlement of Nickel Alloys
Question
What can you tell me about stress corrosion cracking (SCC) and hydrogen embrittlement of nickel alloys?
Answer
One of the most important criteria (for material selection) is resistance to stress corrosion cracking and hydrogen embrittlement. The risk of these occurring increases at higher temperatures and pressures and is mitigated by low stress levels. Therefore, material selection, fabrication processes and material thickness should be chosen with a view toward achieving low residual stresses. For example, in piping, typical methods of lowering stresses once the operating temperature and pressure have been decided are closer pipe support spacing, thicker pipe walls and thermal relieving of residual welding stresses.
Test methods to differentiate between hydrogen embrittlement and stress corrosion cracking include:
1. Tensile and notched tensile property evaluation (per ASTM G 142-98)
2. K1H fracture test (to evaluate the threshold stress intensity factor for hydrogen stress cracking)
3. Slow strain rate (SSR) test
4. Disc pressure test (to measure the susceptibility to hydrogen embrittlement of metallic material under high-pressure hydrogen (ASTM F1459)
Hydrogen Bake-Out
With respect to plating, specifications now state that a plated carbon-steel fastener "shall be baked for not less than 23 hours at 375+25°F within 2 hours after plating to provide hydrogen embrittlement relief" (per MIL-N-25027D). In the past, the plating specifications required baking at 375+25°F for only 3 hours within 4 hours after plating. This treatment was found to be inadequate, and most plating specifications were revised in 1981-82 to reflect the longer baking time. Hydrogen embrittlement problems also increase as the fastener strength increases.
by Daniel H. Herring - July 7, 2010
Daniel H. Herring
dherring@heat-treat-doctor.com
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.
http://www.industrialheating.com/
For more information on our services, please contact us via e-mail
Robert Gutierrez - RGutierrez@HoustonHeatTreat.com
Mark Tate - Sales or Technical assistance - MTate@HoustonHeattreat.com
or call us at 281-590-9600.
