Influence of microstructural morphology and prestraining on short fatigue crack propagation in dual-phase steels

The ferrite-martensite dual phase (DP) steels offer a better combination of strength and ductility than other conventional steels with equivalent stat ic strength which are generally used in the automobile industry. The contin uous martensite phase in DP steels seems to play an important role in deter mining fatigue properties and be a key factor to gain a higher resistance t o fatigue. Additionally it is very important from a practical point of view to improve the fatigue properties after prestraining or cold working. The effects of microstructural morphology and prestraining on the propagation b ehavior of short fatigue cracks in DP steels were investigated by the in-si tu observation in a scanning electron microscope. The material with martens ites continuously surrounding individual ferrites exhibited a higher fatigu e strength and longer fatigue life than those of the material with martensi te phase dispersed in the ferrite matrix. Cold rolling prior to fatigue tes ting resulted in a decrease of fatigue life for both materials, especially an extreme decrease for the martensite-dispersed material. It was found tha t the fatigue crack propagation was sensitive to microstructural morphology and prestraining only in the small crack within a range of approximately 2 50 mum and the propagation life of this region governed the total fatigue l ife.