Subcritical heat affected zone softening in hot-stamped boron steel during resistance spot welding

Fracture modes of resistance spot welded ultra-high strength hot-stamped boron steel via lap-shear test are different from that of the traditional advanced high strength steel due to the difference in geometrical size and material property of the spot welds. In this paper, lap-shear fracture modes of resistance spot welding joints were analyzed and joint characteristics that affecting the fracture behavior were discussed. Three fracture modes were found to change from interfacial fracture (IF) to pull-out fracture (PF) with the increase of nugget diameter. For PF I mode, the fracture initiated at the transition zone between the fusion zone and upper-critical heat affected zone (HAZ) and propagated along the thickness of the nugget. For PF II mode, during which the failure initiated at the sub-critical HAZ where the softest zone occurred, and it propagated to the base material. Obvious hardness decrease was observed in the transition zone with the formation of the delta ferrite at the fusion boundary due to the relatively high amount of alloying element in the hot-stamped boron steel, which could provide the reason for route of PF I extending along this zone. Fluctuation in the hardness in the transition zone led to the existence of both PF I and PF II at the same welding current.

Section: Microhardness Distributionsupporting

confidence: 80%

Analysis of Fracture Modes of Resistance Spot Welded Hot-Stamped Boron Steel

Zhao

Zhang

Lai

2018

“…This effect has also been reported for dual phase steels [10]. The presence of the fusion boundary softening for 22MnB5 is argued by Lu et al [11], however their experimental approach was not optimal for detecting the softened region, as will be discussed further.…”

Section: Introductionsupporting

confidence: 54%

“…Hardness mapping (Figure 8) reveals a softened region at the fusion boundary, where the position of the softened region corresponds to the position of the blue-etched halo, as can be seen in Figure 9a,b. According to the results of the present study, the softened region at the fusion boundary is not detected by a commonly used hardness traverse measurement (e.g., [11]) ( Figure 7) due to the small size of the softened region compared to the step of the hardness measurement and under circumstances the size of the indentation. To detect the softened region at the fusion boundary, measurement with a low loading force and small distance between the indentations are required [6,7,10].…”

Section: Metallographic Investigations and Hardness Measurementscontrasting

confidence: 46%

Transient Softening at the Fusion Boundary of Resistance Spot Welds: A Phase Field Simulation and Experimental Investigations for Al–Si-coated 22MnB5

Sherepenko

Kazemi

Rosemann

et al. 2019

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.

“…In contrast, a pre-formed part is cold-stamped firstly in case of the indirect hot stamping process, and only quenching and calibration are performed after austenitizing in the press [9]. On the other hand, boron steel sheets have been widely used for the production of high strength panels of car bodies and vehicle chassis structures due to its excellent formability [10,11]. The ultimate tensile strength of hot-stamping boron steel sheets would reach up to 1500 MPa with a full martensite microstructure [12,13].…”

Section: Introductionmentioning

confidence: 99%

Enhancement of Fatigue Endurance by Al-Si Coating in Hot-Stamping Boron Steel Sheet

Tan

et al. 2019

Most structural components undertake cyclic loads in engineering and failures always cause catastrophic economic losses and casualties. In the present work, the phase evolution of Al-Si coating of high-strength boron steel during hot stamping was investigated. Two types of 1500 MPa grade boron steel sheets, one with Al-Si coating and the other without, were studied to reveal the effect on the high-cycle fatigue behavior. The as-received continuously hot-dip Al-Si coating was composed of α(Al), eutectic Al-Si and τ5. After hot stamping at 1193 K, three phases formed in this coating: β2, Fe(Al,Si)2 and α(Fe). The experimental results showed that the endurance limit of the coated steel sheet was 370 MPa under 107 fully reversed tension-compression loading cycles as opposed to 305 MPa in the uncoated sheet. Both the coated and the uncoated specimens showed surface-induced transgranular fatigue fractures. In the uncoated sheet, the fatigue cracks were generated from the decarburization surface, but the Al-Si coating effectively prevented the occurrence of near-surface decarburization during high-temperature hot stamping, and the only cracks in the coated steel sheet were initiated at wire-cutting surfaces.