Changes in Mechanical Properties of Dual-Phase Steel Due to Post-Welded Microstructure and Loading Geometry

Since the early 2000’s adoption of dual-phase steels have increased for automotive construction.  Dual-phase steels have a multiphase microstructure containing ferrite martensitic and sometimes bainite.  This structure gives the material strength and high formability allowing automotive designers to down-gauge parts for weight reduction.  Dual-phase steels are often highly alloyed.  When a steel is welded, areas of the HAZ that are heated above the austenitic temperature can cool to martensite and bainite.  However, in the sub-critical HAZ, martensite in the microstructure tempers, forming a soft zone.  This creates a large heterogeneity in the mechanical properties across the weldment.  The degree of heterogeneity depends on: base material structure, material chemistry, and heat input.  Failure will not necessarily occur in the softened HAZ.  The properties of a laser or gas metal arc weld depends on joint geometry as well as local weld properties.  Welds may fail in the sub-critical HAZ when they are welded in the butt-joint configuration and pulled in uniaxial tension.  However, when welds are lap fillet welded or undergo Olsen testing, the weak point can be made to shift from the softened area of the HAZ to other areas of the weldment.  Varying welding heat can also  change local weld properties either by changing the cooling rate of the of the super-critical HAZ or changing the length of time the sub-critical area of the HAZ is at tempering temperature.  Depending on where the failure occurs, this will change the strength of the weldment.  Ultimately, the strength of dual-phase steel weldments depends on three major factors:  the joint geometry, steel microstructure and chemistry, and heat input.  Design of safe joints is balancing these factors.  This is harder than when welding HSLA or low carbon steels where weldments are more homogenous, however, it is very achievable.