Processes for activating chromium polymerization catalysts, which can use lower maximum activation temperatures and shorter activation times than conventional activation methods, and provide polyethylenes with high melt indices, broader molecular weight distributions, and lower long chain branching content. The activation process can comprise heating a supported chromium catalyst in an inert atmosphere to a first temperature (T1) for a first hold time (tH1), followed by allowing the chromium catalyst to attain a second temperature (T2) in the inert atmosphere, then contacting the chromium catalyst with an oxidative atmosphere for a second hold time (tH2), in which T2 can be less than or equal to T1. Additional activation treatments and conditioning steps are disclosed which can be used to enhance the melt index potential of Phillips (Cr/silica) catalysts.