Hydrogen Embrittlement or Carburization
Hydrogen Embrittlement or Carburization
(OP)
How can I tell?
We have a stamped "spring" that get post machined. Material is cold rolled carbon strip, untempered, per ASTM A682 that is .059-.065 thick. It gets heat treated to Rc38-43 per AMS-H-6875. It then gets plated per MIL-STD-171 #1.1.2.2 with a post bake for 3 hours minimum immediately following at 350-400F. Alternate finish is 1.9.2.2 of MIL-STD-171.
(Note:
1.1.2.2 = Cadmium Plate per QQ-P-416, Type II, Class 2, .0003 inch thick, normal color
1.9.2.2 = Electrodeposited zinc per ASTM B633, Type II with supplementary chromate treatment, Class 2 .0005 inches thick, normal color)
Our vendor certs say heat treat was hardened at 1500F for 30 minutes, then oil quenched. Tempered at 875F for 30 minutes and air cooled. Hardness measured Rc38.0-40.0.
I don't have a copy of the plating cert at this time.
The parts are breaking when bent with an axisymmetric load by brittle fracture. It's either hydrogen embrittlement from the plating, or carborization from the heat treat. How can tell the difference with but a few pieces to sample? We now have a new lot of parts that do not exhibit the failure, how can we verify the vendor certs?
We actually did have ONE sample sent to an independent metallurgical lab for analysis. The material was too thin for a Rc measurement, so they got a Ra=72 and converted it to Rc of 43. They then took a cross section and made Tukon Vickers impressions transverse from surface to core every .002 inches. A carborized surface was found at .016" from each side. Surface hardness was Rc51.8 and core hardness Rc26.0.
We need to determine, if possible, root cause and a corrective action. If not possible, at least determine a verification process because the default sample plan didn't work. (Note that the next lot of parts were fine because we told the vendors to baby-sit their processes.) I have several ASTM specs currently at my desk waiting for my tired eyes. Pointing me to any references or historical accounts of this failure mode is welcome.
We have a stamped "spring" that get post machined. Material is cold rolled carbon strip, untempered, per ASTM A682 that is .059-.065 thick. It gets heat treated to Rc38-43 per AMS-H-6875. It then gets plated per MIL-STD-171 #1.1.2.2 with a post bake for 3 hours minimum immediately following at 350-400F. Alternate finish is 1.9.2.2 of MIL-STD-171.
(Note:
1.1.2.2 = Cadmium Plate per QQ-P-416, Type II, Class 2, .0003 inch thick, normal color
1.9.2.2 = Electrodeposited zinc per ASTM B633, Type II with supplementary chromate treatment, Class 2 .0005 inches thick, normal color)
Our vendor certs say heat treat was hardened at 1500F for 30 minutes, then oil quenched. Tempered at 875F for 30 minutes and air cooled. Hardness measured Rc38.0-40.0.
I don't have a copy of the plating cert at this time.
The parts are breaking when bent with an axisymmetric load by brittle fracture. It's either hydrogen embrittlement from the plating, or carborization from the heat treat. How can tell the difference with but a few pieces to sample? We now have a new lot of parts that do not exhibit the failure, how can we verify the vendor certs?
We actually did have ONE sample sent to an independent metallurgical lab for analysis. The material was too thin for a Rc measurement, so they got a Ra=72 and converted it to Rc of 43. They then took a cross section and made Tukon Vickers impressions transverse from surface to core every .002 inches. A carborized surface was found at .016" from each side. Surface hardness was Rc51.8 and core hardness Rc26.0.
We need to determine, if possible, root cause and a corrective action. If not possible, at least determine a verification process because the default sample plan didn't work. (Note that the next lot of parts were fine because we told the vendors to baby-sit their processes.) I have several ASTM specs currently at my desk waiting for my tired eyes. Pointing me to any references or historical accounts of this failure mode is welcome.
--Scott
http://wertel.eng.pro





RE: Hydrogen Embrittlement or Carburization
http://www.read-eurowire.com/ist.cfm
RE: Hydrogen Embrittlement or Carburization
Either way, go info though. Thanks.
--Scott
http://wertel.eng.pro
RE: Hydrogen Embrittlement or Carburization
RE: Hydrogen Embrittlement or Carburization
Somebody help me out here as I was told the H2 bakeout time had been increased.
RE: Hydrogen Embrittlement or Carburization
I am leaning to say that it is H2 embrittlement and longer baking is the way to go, even though I am not an expert. To confirm it, you may send a broken sample with brittle fracture to a metallurgical lab and let them take some microstructure pictures for you. If it is because of carburization, you should see brittle at surface and ductile in the core. Microstructure can tell people a lot more than people want to say...
But I am puzzled to ask where carburization happened? During quenching and tempering?
RE: Hydrogen Embrittlement or Carburization
Actual Material:
Cold Rolled Annealed 1050 #2F SHRD
Reflattened ASTM A682 (.062” x 4.250” x 96”)
GA tolerance P/M .002
Width tolerance P/M .015
Chemical composition:
C=0.500
Mn=0.740
P=0.010
S=0.001
Si=0.180
Ni=0.020
Cr=0.050
Mo=0.010
Al=0.013
Cu=0.030
Ti=0.003
Actual Finish:
1.1.2.2 of MIL-STD-171E with Molycote 3400A.
Parts were baked for 23 hours at 375F +/-25F within 4 hours of plating to relieve hydrogen embrittlement.
Parts were baked for 1 hour at 248-266F after plating to improve adhesion of dry film.
Some parts were baked for 5 hours at 375F +/-25F prior to cleaning and plating to relieve damaging residual stress and then baked for 23 hours at 375F +/-25F within 4 hours of plating to relieve hydrogen embrittlement.
I am trying to follow up with the vendors to see what they used for cleaning (acidic cleaner) and/or corrosion resistance during shipping (oil coat).
--Scott
http://wertel.eng.pro