# Impact of Substrate Preheating on Weld Quality, Microstructure, Corrosion Resistance, and Mechanical Properties in Gas Tungsten Arc Welding of UNS S32750 Super Duplex Stainless Steel

## TL;DR

Preheating the substrate during welding of a super duplex stainless steel improves its microstructure, mechanical properties, and corrosion resistance.

## Contribution

This study demonstrates that substrate preheating during GTAW of UNS S32750 SDSS enhances phase balance and performance without using costly filler materials.

## Key findings

- Preheating to 300°C increased the austenite volume fraction in the fusion zone from 16% to 42%.
- Microhardness decreased from 341 HV at room temperature to 314 HV at 300°C preheating.
- Corrosion resistance improved with preheating, with T300 showing the lowest corrosion current.

## Abstract

Super duplex stainless steels (SDSS) are materials known for their exceptional mechanical strength and high resistance to corrosion due to their dual- phase microstructure consisting of ferrite and austenite in roughly equal proportions. However, the Gas Tungsten Arc Welding (GTAW) process used to join SDSS often causes microstructural imbalances, mainly ferritic structures, or the formation of harmful intermetallic phases, which can weaken the material’ s desirable properties. This study examines the effect of substrate preheating on the microstructure, mechanical properties, and corrosion resistance of UNS S32750 SDSS welds produced by GTAW. Preheating the substrate was considered as a strategy to improve phase balance in the fusion zone by extending the time within the ferrite- to- austenite transformation temperature range, thus slowing the cooling rates. Four conditions were tested: welding at room temperature (RT) and preheating to 100 °C (T100), 200 °C (T200), and 300 °C (T300). Welding parameters remained constant. The fusion zone microstructure was analyzed using metallographic techniques, while mechanical properties were evaluated through microhardness tests. Corrosion resistance was assessed with potential dynamic polarization in a 3.5% NaCl solution. The results showed significant improvements in microstructural balance with higher preheating temperatures. The austenite volume fraction in the fusion zone increased from about 16% at RT to 42% at T 300. Mechanical testing indicated a decrease in microhardness from 341 HV at RT to 314 HV at T 300, reflecting the increased austenite content and its associated toughness. Corrosion tests demonstrated enhanced resistance under preheated conditions, with T 300 exhibiting the highest corrosion potential and the lowest corrosion current, nearing the performance of the base metal. These findings suggest that preheating is a practical, cost- effective method for optimizing the GTAW process for SDSS, eliminating the need for expensive filler materials and stabilizing the microstructure elements.