I venture to guess the cracking is tied to the Nickel plating. I'm no expert on plating, but there is a process called High-phosphorus Electroless Nickel which contains 9 to 11% Phosphorus. Phosphorus is known to promote hot cracking.
Again a guess, but I don't think the case hardening is related to the weld centerline cracking. Generally, centerline cracking is hot cracking.
If the cracking occured in the basemetal HAZ, that might lead you to look at HAZ hardness and Hydrogen embrittlement. The base metal carbon is low, the very thin case is adding little carbon to the melt, it really shouldn't matter?