Abstract
This work gives an insight into the transient softening at the fusion boundary of resistance spot welds on hot stamped steel. Metallographic investigations and hardness mapping were combined with finite phase–field modeling of phase evolution at the fusion boundary. Saturation of weld nugget growth in the welding process was observed. For industrially relevant, long welding times, the fusion boundary of a spot weld is therefore isothermally soaked between the peritectic and solidus temperatures. This leads to a carbon segregation to the liquid phase due to higher carbon solubility and possibly to δ-Fe formation at the fusion boundary. Both results in a local carbon depletion at the fusion boundary. This finding is in good agreement with carbon content measurements at the fusion boundary and the results of hardness measurements.
Highlights
Advanced and ultra-high strength steels (AHSS/UHSS) are used in the production of crash-relevant parts in the automotive industry
This study examined the softening, observed the fusion boundary of resistance spot welds on a press hardening steel 22MnB5, using phase field modeling and experimental observations
Phase field modeling conducted to examine the material behavior during this isothermal holding has shown that carbon content in austenite reduces from 0.22 wt % to 0.151 wt %, confirming the hypothesis suggesting that carbon redistribution at the fusion boundary is caused by the combination of segregation due to the solubility differences between solid and liquid and the delta-ferrite formation in the peritectic region
Summary
Advanced and ultra-high strength steels (AHSS/UHSS) are used in the production of crash-relevant parts in the automotive industry. The most common production relevant strategies to increase the process window are welding with longer welding times and welding current modulation [2,3,4], resulting in welding times of up to several seconds, e.g., in the case of welding with production-related disturbances like gaps [3]. Opting for this solution causes a broader heat-affected zone (HAZ), and can lead to softening at the fusion boundary, described for 22MnB5 with Al–Si [5,6,7,8] and Zn coatings [9]. The presence of the fusion boundary softening for 22MnB5 is argued by Lu et al [11], their experimental approach was not optimal for detecting the softened region, as will be discussed further
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