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Defective Manufacture of Oil Field Stripper Bowl
A 12 3/4 inch diameter forging of AISI 4130 was
purchased by a machine shop in order to fabricate an
oil field stripper bowl adapter ring. The forging
ingot had been heat treated to a surface hardness
consistent with the stripper bowl engineering
requirements. Subsequently, the ingot was machined
to accommodate the 11 3/4" O. D. of the subject stripper bowl
test plug.
Photograph A:
Section of failed oil field stripper bowl
During testing, the stripper bowl failed, resulting
in a fatality. Testing of the stripper bowl for
hardness, tensile and yield strength resulted in
values of hardness, yield strength and tensile
strength being below the engineering drawing requirements.
The machine shop was unaware that AISI4130 was
shallow hardening. That is, the surface hardness of
the raw heat treated ingot did
indeed meet the required hardness, however, after
machining off 1/2" from the AISI4130 ingot O.D., the
hardness was now less than required by the
engineering drawings. Subsequent testing of the
stripper bowl resulted in failure. A more
appropriate material choice would have been AISI4140
which is through hardening (i.e. the hardness is
consistent throughout the thickness of the heat
treated ingot).
The results of the laboratory testing and accident
investigation indicated that the surface of the raw
ingot met the hardness required by the engineering
drawing. However, subsequent to that hardness
testing, some of the surface of the ingot was
machined away (in order to accommodate the stripper
bowl test plug). The resulting hardness of the
stripper bowl adapter ring was less than specified
on the engineering drawing.
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Dr.
R. Craig Jerner, Ph.D., PE specializes in accident
investigation and metallurgical failure analysis,
with over 30 years experience as a metallurgical
consultant and accident investigator. He has
testified as a metallurgical expert in over 250
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