Title page for ETD etd-07282008-135105
|Type of Document
||Halley, William G.
||Fracture path transitions in peels tests of medium carbon steel spot welds
||Materials Engineering Science
|Gordon, Ronald S.
|Lytton, Jack L.
|Dowling, Norman E.
|Eiss, Norman S. Jr.
|Reynolds, William T. Jr.
|Date of Defense
Fracture path transition, from interfacial fracture to a pulled button, in peel tests
of spot welds in SAE 1039 steel was evaluated to determine the controlling material
properties. Welds were tested in the as welded condition and after tempering at various
temperatures to develop a range of hardness and strength in the weld metal.
Two transitions were found, from complete interfacial fracture to partial
interfacial and from partial interfacial fracture to a pulled button. Samples tempered at
less than 350 C exhibited complete interfacial fracture while those tempered at 500 C or
higher pulled full buttons. Each transition was accompanied by a large increase in the
energy absorbed during fracture.
Both partial and complete interfacial fracture occurred by intergranular fracture
along prior austenite grain boundaries. Optical microscopy utilizing a tint etch indicated
that austenite existed as films on prior austenite grain boundaries of samples tempered at
less than 500 C and TEM confirmed that these films were austenite.
Weld metal toughness was found to control the fracture path. If fracture initiation
was delayed until the applied load caused plastic deformation of the coupons pulled
button fracture occurred. Fracture initiation prior to plastic deformation of the coupons
resulted in interfacial or partial interfacial fractures.
A small secondary hardening peak was observed in samples tempered at 450 C.
Secondary hardening, which normally results from alloy carbide precipitation, was due to
AlN precipitation in this aluminum killed plain carbon steel.
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