I would start taking them outdoors for day outings and I would remove or wean the heat lamp.
https://www.backyardchickens.com/t/735392/redneck-fungshui-brooding-17-degree-temperatures/0_20
They are probably feathered enough to go in the coop now. They surely should be at 6 weeks. What breed are they? Have you been taking them outdoors for short periods to acclimate them? Do they still have a heat lamp on them?