Published: 14 August 2026

A research team from the Polytechnic University of Bari (Dipartimento di Meccanica, Matematica e Management) and the University of Pisa has published a new open-access study in Metals that isolates the heat-affected zone (HAZ) behavior of two modern automotive advanced high-strength steels—PHS1500 (press-hardened steel) and Q&P1180 (quenching and partitioning steel)—when subjected to resistance spot welding (RSW) thermal cycles.

The work is particularly relevant for body-in-white manufacturing, where RSW remains the dominant joining process and HAZ softening can become the failure-initiation point in crash-loaded components.

Rather than cross-sectioning production spot welds—where it is difficult to attribute microstructural changes to a specific thermal history—the authors physically reproduced individual HAZ sub-regions on a Gleeble thermomechanical simulator. They calibrated finite-element thermal models for each alloy to map local peak temperatures along longitudinal microhardness tracks, then compared SEM micrographs and Vickers microhardness profiles (HV0.1) across sub-critical, inter-critical, and upper-critical HAZ conditions for both 300 ms and longer thermal cycles.

This combined experimental–numerical approach let them attribute hardness changes unambiguously to local peak temperature, rather than to the coupled thermal-electrical-mechanical field of an actual spot weld.

• PHS1500 softened most severely near the Ac₁ transformation region, where hardness fell to roughly 350 HV0.1—an approximately 42% reduction versus its fully martensitic base metal. The minimum was narrow and sharply localized.
• Q&P1180 showed its hardness minimum at a lower peak temperature, around 600 °C, and the softening profile was much smoother and more contained. The researchers attribute the difference to the alloy's higher manganese content, which stabilizes retained austenite and lowers the martensite-start temperature.
• At short sub-critical exposures, Q&P1180 did not show pronounced local softening; tempering of pre-existing martensite was partially offset by decomposition of carbon-enriched retained austenite into fine transition carbides and fresh secondary martensite.
• At the upper-critical HAZ condition near the fusion boundary (~1400 °C), both steels converged to nearly identical hardness levels because both transformed to a predominantly coarse martensitic morphology.
• At the lower upper-critical target (1000 °C), Q&P1180 was slightly softer than PHS1500, consistent with a small fraction of interlath retained austenite or other partially transformed softer constituents surviving the rapid cool.
• Increasing weld time generally lowered cooling rates and coarsened martensite blocks, with the effect being more detrimental in PHS1500 than in Q&P1180.

The authors argue that Q&P1180's smoother softening gradient makes its cross-tensile and peel response less sensitive to weld schedule variation than PHS1500, while PHS1500 requires tighter control of hold time and electrode force to keep the soft Ac₁ zone from acting as a crack path. They also suggest the Gleeble-plus-FE methodology can be used to screen RSW schedules computationally before line trials, reducing qualification cost for AHSS-intensive vehicle platforms.

The study joins a growing body of 2026 welding research focused on predicting and controlling HAZ properties in third-generation automotive steels, alongside parallel MDPI papers on laser/GMAW comparison of coated automotive steel and on post-processing of additively manufactured 316L.