Jurgen,

You're quite right or course; though the crownsheet rarely actually melts, the softening can be enough to 'un-zip' the stays (even in a steel firebox), resulting in the crown sheet imploding into the grate.

The US-built S160 2-8-0s seem to have had a propensity to do exactly this in the UK as the drivers were unfamiliar with the boiler water guages and the fusible plugs were close to useless.

Steph