Fatigue and Corrosion Fatigue Fracture of a Plasma Nitrided, Laser Hardened, and Galvanized B-Mn Steel

Sammanfattning: Fundamental and technological aspects of fatigue and fracture of conventional and advanced material are topics of primary importance to the development of materials science and engineering. Surface treatments have been established to improve the surface properties of material against surface phenomenon such as wear, corrosion, fatigue and corrosion fatigue. This thesis deals with the influence of two modem (plasma nitriding and laser hardening), and one conventional (hot dip galvanizing), surface treatments on the fatigue, corrosion fatigue (in seawater) and fracture of a B-Mn steel. The main approach adopted here focuses the attention on fatigue crack initiation mechanisms.Constant stress amplitude plane reversed bending fatigue tests (R = -1) at room temperature and at a frequency of 47 Hz in air and sea water were conducted using cylindrical untreated and surface treated quenched and tempered (Q&T) steel specimens made of a B-Mn SS2131(∼AISI 15B21H) steel with stress concentration factors of Kt =1.05 and 1.7.S-N curves show that surface treatments improved the fatigue limit by 53 % and 115 % (plasma nitriding), and 18% and 56 % (laser hardening) of smooth and notched Q&T specimens; whereas hot dip galvanizing slightly decrease the air fatigue limit of smooth (-9 %) and notched (-10 %) Q&T specimens. The small cracks formed in the coating during galvanizing could be the cause of this reduction. Compressive stresses and improved strength (hardness) introduced by plasma nitriding and laser hardening play the principal role for the improvement of fatigue strength, subsurface crack nucleation, fish-eye asymmetry, and reduction of stress intensity factors. Plasma nitrided (PN) and laser hardened (LH) materials behaves as composites. Laser process parameters have to be optimised in order to avoid the formation of a damaged area (characterised by a low hardness and low compressive residual stresses) around the overlap zone which promotes the formation of long narrow crack along the edge of the overlap zone causing premature failure.Sea water suppress the fatigue limit and reduces fatigue strength (especially at long lives) of smooth and notched Q&T, PN and LH specimens, but smooth and notched hot dip galvanized (HDG) specimens still exhibit a corrosion fatigue limit (at 2x107 cycles) with values a little lower than the corresponding air fatigue limits. Physical and electrochemical corrosion protection by hot dip galvanizing are the causes for the good corrosion fatigue properties obtained. Plasma nitriding improves the corrosion fatigue resistance of Q&T specimens; it is associated to the good corrosion resistance of ε and -phases, the enhancement of corrosion and fatigue by compressive residual stresses and the consumption of H+ ions during reduction of nitrogen. Mechanisms for nucleation of pits, and corrosion fatigue cracks have been proposed to explain corrosion fatigue strength reduction due to sea water. A derived equation predict well the corrosion fatigue strength of notched specimens from the observed air fatigue strength of smooth specimens.