Examination of BSU-86/B Setscrews

Authors: Marc S. Pepi; ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD

Abstract:

A total of four used and four new setscrews from a BSU-86/B bomb fin assembly were examined to determine the probable cause of internal socket head stripping. The intent was to compare the properties of the used setscrews to those of the new. The U.S. Army Research Laboratory (ARL), Weapons and Materials Research Directorate (WMRD), performed a dimensional verification to ensure that the parts undergoing investigation met the governing requirements. The wall thickness at the internal socket points was found to be less than nominal in most setscrews. The chemical composition of two used and two new bolts compared favorably to American Institute for Steel and Iron (AISI) 4037 steel, which was in accordance with the governing requirement. The core hardness of the parts fell within the required range. Vickers microhardness profiles from the surface of the parts showed that a gradient existed in one of the used and in one of the new setscrews. Metallography was subsequently used to determine the cause of the hardness gradients. A layer of total decarburization was revealed on most of the setscrews upon etching with 2% nital. Although the thickness of the layer met the requirements of FF-S-200, the presence of 'total' decarburization was not in accordance with MIL-H-6875 for Class A material. The micro structure of each setscrew examined was finely tempered martensite, which conformed to an austenitized, quenched, and tempered part. It was concluded that no trends existed between the used and new setscrews examined. Recommendations were offered concerning the required torque, as well as the use of automatic tools.

Limitations:

APPROVED FOR PUBLIC RELEASE