Effect of trace ferrite on mechanical properties of a tempered HSLA steel

The influence of trace ferrite on tensile and impact properties of a HSLA steel quenched from 850 degrees C and tempered at different temperatures has been examined. SEM and TEM analyses show that the quenched microstructure is martensite with similar to 3 vol% ferrite, and the evolution of martensite at different tempering conditions is same to that of specimens with single martensite. The tensile strength is determined by the strength of ferrite and martensite according to the law of mixtures. The yield strength is a result of precipitation strengthening and tempering softening, and the yielding point appears when the mobile dislocations are insufficient. Both yield and tensile strength are reduced by the trace ferrite when tempering at temperatures 180-350 degrees C due to the strength difference between ferrite and martensite. Moreover, the ferrite improves the elongation but deteriorates the impact property for its low strength, and the tempered martensite embrittlement is advanced to lower tempering temperature 250 degrees C due to the carbide precipitation facilitated by the trace ferrite. However, the influence of trace ferrite is reduced after tempering at higher temperatures 450 degrees C and 550 degrees C. High temperature tempering is essential for specimens with trace ferrite to achieve optimal comprehensive mechanical properties.