The Role of HAZ Softening on Cross-Tension Mechanical Performance of Martensitic Advanced High Strength Steel Resistance Spot Welds

Abstract

Giga-grade martensitic advanced high-strength steels are prone to sub-critical heat-affected zone (SCHAZ) softening during resistance spot welding. The article aims at understanding the role of HAZ softening on the fracture mode, load-bearing capacity, and energy absorption capability of MS1400 resistance spot welds during the cross-tension test. The highest load-bearing capacity was obtained when pullout failure was initiated from the martensitic coarse-grained HAZ. However, more severe HAZ softening and formation of a wider softened zone, promoted at high heat input conditions, encourages strain localization in SCHAZ, promoting transition in failure location to sub-critical HAZ. This change in pullout failure location is responsible for the observed reduction in the weld peak load at high welding currents. Therefore, control of martensite tempering in the HAZ is critical to obtain strong and reliable resistance spot welds in martensitic advanced high-strength steel sheets. To preclude the detrimental effect of the martensite tempering on the weld strength, the minimum welding current, which enables pullout failure mode, should be used for resistance spot welding of MS1400 advanced martensitic steel.

Validated Multi-Physical Finite Element Modelling of the Spot Welding Process of the Advanced High Strength Steel DP1200HD

“…Figure 7 shows the difference between the base material, with hardness values of slightly below HV 400, and the HAZ and the FZ with hardness values well above HV 500. Dual phase steels such as the investigated DP1200 here are relatively resistant against HAZ softening [ 47 ]. The present results support these findings.…”

Investigation of Liquid Metal Embrittlement during Resistance Spot Welding of Martensitic Steel with Zn Jet Vapor-Deposited Coating

“…The IF is characteristic of a smaller nugget diameter with a lower load-bearing capacity beginning at the weld notch and propagating through the FZ. The IF exhibits brittle behavior, and thus, is usually undesirable [31,33,34,39]. The transition from IF to PF occurred at 6.5 kA for the coated sample when a critical nugget size of 6.62 mm was reached, similar to that of the uncoated sample (6.7 mm) showing PF.…”

“…The microstructure in the heat-affected zone (HAZ) followed the temperature gradient from the near vicinity of the FZ with the highest temperature to the cool base metal. Therefore, three HAZs can be clearly distinguished, typically referred to as the upper-critical heat affected zone (UCHAZ) consisting of martensite, the inter-critical heat-affected zone (ICHAZ) with a dual-phase microstructure of martensite and ferrite, and the sub-critical heat-affected zone (SCHAZ) formed by the tempered martensite of the base metal [31][32][33][34].…”

Microstructure and Local Mechanical Properties of the Heat-Affected Zone of a Resistance Spot Welded Medium-Mn Steel

“…In addition to the FZ, the microstructure of the heat-affected zone (HAZ) also influences the mechanical performance of spot-welded AHSS sheets [ 12 , 13 , 20 , 21 , 22 ]. Generally, the HAZ of AHSS can be subdivided into the upper critical heat-affected zone (UCHAZ), where the temperature during welding exceeds the A 3 temperature; the intercritical heat-affected zone (ICHAZ), where the peak temperature is between A 3 and A 1 ; and the subcritical heat-affected zone (SCHAZ), where the peak temperature is below A 1 [ 23 ].…”