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 static strength which are generally used in the automobile industry. The continuous martensite phase in DP steels seems to play an important role in determining fatigue properties and be a key factor to gain a higher resistance to 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 behavior of short fatigue cracks in DP steels were investigated by the in-situ observation in a scanning electron microscope. The material with martensites continuously surrounding individual ferrites exhibited a higher fatigue strength and longer fatigue life than those of the material with martensite phase dispersed in the ferrite matrix. Cold rolling prior to fatigue testing resulted in a decrease of fatigue life for both materials, especially an extreme decrease for the martensite-dispersed material. It was found that the fatigue crack propagation was sensitive to microstructural morphology and prestraining only in the small crack within a range of approximately 250μm and the propagation life of this region governed the total fatigue life.