AISI 430 ferritic stainless steel is popular in modern industry, while conventional welding methods with filler metals produce welded joints with tensile strength (586 MPa) and elongation (7.35%), which is insufficient to meet the growing engineering requirements. In this work, the elongation of the joint is doubled (15.11%) while yield strength remains unchanged after post-weld heat treatment (PWHT). Microstructural analysis of heat affected zone (HAZ) reveals the transformation process between equiaxed ferrite, intergranular martensite, and intragranular acicular martensite in the welded joint at 750 °C and 800 °C. Additionally, molecular dynamics simulations demonstrate the impact of various types of martensite on single crystals of ferritic stainless steel under tension. The results indicate that intergranular martensite and acicular martensite demonstrate transgranular fracture, while granular martensite exhibits intragranular fracture. Intergranular martensite and granular martensite are distributed near high-strain regions within the crystal, whereas acicular martensite is concentrated at the grain boundaries, away from the high-strain regions. The comparison of hardening parameters for different types of martensite reveals that granular martensite (58.98) has higher ductility than acicular martensite (97.40) and intergranular martensite (111.54). These findings are valuable for developing advanced stainless steel welded joints that balance high ductility and strength, meeting modern engineering demands.