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 drawing requirements. Subsequently, the ingot was machined to accommodate the 11 3/4" O. D. of the subject stripper bowl test plug. A section of the subject stripper bowl is shown in Photograph A.
Photograph A Section of failed oil field stripper bowl
During pressure testing, the stripper bowl failed, and the stripper bowl plug was ejected. The ejected plug became airborne and crashed through a machine shop roof fatally injuring a worker. Testing of the stripper bowl for hardness, tensile and yield strength resulted in values of hardness, yield strength and tensile strength which were below the engineering drawing requirements.
The machine shop was unaware that AISI 4130 is a shallow hardening steel. 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 AISI 4130 ingot O.D., the hardness was, at that new surface location, less than required by the engineering drawings. Subsequent pressure testing of the stripper bowl resulted in failure. A more appropriate material choice would have been AISI 4140 which is through hardening. The hardness of AISI 4140 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, 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 theads were less than specified on the engineering drawing.