The effect of galvanizing on the mechanical resistance and fatigue toughness of a spot welded assembly made of AISI410 martensite

Welding of high strength steels containing martensite is difficult due to the appearance of brittle grains after fast cooling. Galvanizing spot-welded specimens made of AISI410 solves this problem, and strongly improves the performances of spot welds. The mechanisms responsible for these achievements are presently analyzed. The changes of the mechanical strength and fatigue toughness of spot-welded samples made of AISI410 after galvanizing have been studied. AISI410 is a chromium-rich martensite. Tensile tests on normalized specimens indicate that the yield stress is close to 1010 MPa and the UTS at similar to 1270 MPa. The fatigue limit of raw AISI410 is at 690 MPa (Delta sigma(L) at R=0.1). Specimens were cut following the rolling direction and spot-welded. Metallography reveals that spot welding produces: - hard martensite at the center of the nugget; - a heat affected zone, which has been forged by the welding electrodes and recrystallized at high temperature; - and finally, a ring of grains tempered in a subcritical temperature range. Spot welds were tested in tensile-shear mode. Both overload failure and fatigue toughness were evaluated. Annealing and galvanizing post-treatment have been used to improve the mechanical properties of the spot-welded specimens. The improvement of the mechanical performances of spot-welded specimens after post-treatments are reported as follows. First, we observed significant changes in the modes of rupture both in overload failure and fatigue tests. The mechanical strength of untreated spot welds is controlled by the brittleness of the nugget and the forged heat affected zone. After post-treatment, the ultimate tensile strength is increased, and rupture results from fissures propagating exclusively into the zone recrystallized in the subcritical range of temperature. This mode is genuine to spot welds in martensite because of the high strength of the base metal. Second, three modes of rupture depending on the force range control the fatigue toughness. Annealing and galvanizing modify the ranges. Galvanizing increases the endurance, and the fatigue limit by a factor similar to 3.5. This effect is attributed to zinc brazing that strongly changes the distribution of stresses under tensile-shear solicitation. Hardness measurements and tensile test on specific samples clarify the mechanical behavior of the layers around the spot, and shed some light on the mechanisms of rupture. Lifetimes are controlled by the initiation of fatigue cracks. (C) 2013 Elsevier B.V. All rights reserved.