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Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.machtools.c9001519
EISBN: 978-1-62708-223-5
Abstract
An AISI 4340 Ni-Cr-Mo alloy steel draw-in bolt and the collet from a vertical-spindle milling machine broke during routine cutting of blind recesses after relatively long service life. Based on fracture surface features, it was suspected that the draw-in bolt was the first to fracture, followed by failure of the collet, which shattered one of its arms when it struck the work table. Scanning electron microscopy showed the presence of hairline crack indications along grain facets on the fracture surface of the bolt. This, coupled with stepwise cracking in the material, generally raised suspicion of hydrogen embrittlement. It appeared that fracture in service progressed transgranularly to produce delayed failure under dynamic loading. The pickling process used to remove heat scale was suspected to be the source of hydrogen on the surface of the bolt. The manufacturer was requested to change its cleaning practice from pickling to grit blasting.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.machtools.c9001155
EISBN: 978-1-62708-223-5
Abstract
Milling machine arbors were inserted with satellite spindles having a maximum speed of 1500 rpm, and broke out between the groove and the flange. The appearance of the fracture surface was the same on both arbors. The pronounced scan lines characterized the fractures as fatigue fractures. The appearance of the fracture in the arbors indicated ductile fatigue fracture which had its origin in the radii between groove and flange. These radii of 0.15 and 0.2 mm were too small for the load on the milling machine. In addition there were grooves at the base of the radii which had an unfavorable effect on the life of the component by acting as notches with their resulting stress concentration. Considering the great hardness of the case, the small radii would have been critical even without grooves. Measures were taken so that the critical radius of the milling machine was increased and the surface roughness measured more precisely.