swertel
Mechanical
- Dec 21, 2000
- 2,067
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.
--Scott
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
