First off, unless something has changed, there are no bona-fide metallurists here. And none of us can know the original composition of your broken hammer, why it failed, or exactly how it was repaired in the past.
Welding produces a Heat Affected Zone (HAZ) around the weld that can vary according to the temperature and welding process used. Stress relieving or annealing can even out or change the microstructure, but rehardening or retempering steel of unknown composition involves a lot of guesswork that may or may not be successful.
Here is a brief summary of welding metallurgy that may give you an idea what you're up against:
"Welding metallurgy focuses on the changes that occur in steel's microstructure and properties during and after the welding process. This includes understanding the behavior of different steel types, the impact of heat on the base metal and weld pool, and the formation of the heat-affected zone (HAZ). Welding metallurgy is crucial for ensuring the strength, toughness, and corrosion resistance of welded joints.
Key aspects of welding metallurgy in steel:
Microstructure:
Welding alters the microstructure of the steel, particularly in the HAZ, where the base metal is heated but not melted.
Phase Transformations:
Heat causes phase transformations in steel, such as the formation of ferrite, austenite, and martensite, which can affect the mechanical properties.
Weldability:
Different steel types have varying weldability, depending on their composition and microstructure.
HAZ:
The HAZ is a critical zone where changes in microstructure can lead to defects like cracks if not managed properly.
Post-Weld Heat Treatment (PWHT):
PWHT can help relieve residual stresses, modify the microstructure, and improve the mechanical properties of the weld.
Weld Metal:
The composition and properties of the weld metal, which is a mixture of the filler metal and base metal, also play a role in the overall weld quality.
Defects:
Welding can introduce defects like cracks, porosity, and inclusions, which can be minimized by controlling the welding process and selecting appropriate materials.
Factors affecting the metallurgy of welded steel:
Base metal composition:
The chemical composition of the steel, including the presence of carbon, alloying elements, and impurities, influences its weldability and the resulting microstructure.
Filler metal:
The composition of the filler metal used during welding can also affect the weld's properties.
Heat input:
The amount of heat applied during welding affects the size of the HAZ, the cooling rate, and the resulting microstructure.
Welding process:
Different welding processes, such as shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and gas tungsten arc welding (GTAW), can produce different microstructures and properties.
Cooling rate:
The rate at which the weld cools after welding affects the grain size, phase transformations, and residual stresses."
If the last repair was done by "dripping molten metal" on the end of the broken part, and then reshaping it, I can understand why it failed. If your new laser welding repair is properly done and stress relieved, it may stand a better chance. But I'd be taking a wild assed guess to suggest how it should be hardened. Nothing ventured, nothing gained, so hopefully it holds up for a while. I have an 1850's vintage percussion boot pistol that has a broken hammer nose. I plan to attempt a repair by TIG welding a piece on. I will probably anneal it by burying it in the coals of a wood fire for hours, slowly cooling, and then either rehardening and drawing, or just case hardening. But I know it may fail and I'll end up making a new hammer. You might want to keep your eyes peeled for a suitable replacement.
As for your other unbroken hammer.... if it ain't broke, don't fix it.
