Looks as though you did the soldering with a torch? I can see a bluish tinge on the edges? While the soldering itself looks great. Higher carbon steels like piano wire will sometimes get brittle if they are heated red and then cooled quickly. You can correct this by warming it back up but not enough to melt the solder and let it cool slowly.

You may have already done it this way? And also they may have just supplied mild steel wire?