Microstructural Evolution of Laser-Welded Dissimilar Lap Joints of Martensitic Abrasion Resistant Steel and Cold-Worked
Laser welding of dissimilar metals such as martensitic abrasion resistant steel and cold-worked austenitic stainless steel produces welded lap joints with distinct microstructural characteristics. The martensitic abrasion resistant steel, a product of surface hardening via high-temperature exposure, provides high strength to the weldment due to its retained ferritic structure. The austenitic stainless steel, a cold-worked product, adds corrosion resistance to the weldment as its higher ductility enables the material to maintain a seamless bond.
To start, the laser welding of the two materials produces a number of heat affected zones (HAZ) that experience dramatic metallurgical changes during the welding process. In the martensitic abrasion resistant steel, the HAZ consists of a single ferrite grain interspersed with austenite particles, a phenomenon commonly known as “partial transformation”. Heat from the laser affects the face of the steel, which consists of mostly ferrite grains surrounded by austenite at the edges. This results in a decrease in strength of the HAZ due to the emergence of the austenite, which has a much lower strength than ferrite.
In the cold-worked austenitic stainless steel component, a finer grain and less drastic HAZ microstructure is observed. The cold-worked component experiences localized heating from the laser, resulting in a larger volume of austenite along the weldment interface with the relatively smaller HAZ around the weld due to its higher plasticity. Both the martensitic abrasion resistant steel and the austenitic stainless steel present HAZs that are richer in austenite than the original base material and rely on each other for strength and corrosion resistance.
The joining of martensitic abrasion resistant steel and cold-worked austenitic stainless steel is a common welding approach as it interlocks their respective properties while maintaining an aesthetically pleasing weldment. When the two materials are brought together, the austenite in the martensite’s HAZ reacts with the edges of the stainless steel component, forming a “stair-stepping” pattern of microstructural discontinuities. At the welded joint interface, grain refinement with higher hardness is observed due to the redistribution of the austenite structure.
The laser-welded joint of martensitic abrasion resistant steel and cold-worked austenitic stainless steel forms a secure, corrosion-resistant weldment with improved fatigue strength at the joint. The outcome is a finer, more uniform grain that exhibits both strength and corrosion resistance at the weldment. All in all, the microstructural evolution of this kind of dissimilar weldment is a cooperative effort to increase joint strength and corrosion resistance without negatively impacting the aesthetics of the weldment.
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