72 hour transit if I travel from HK to Chengdu and then back to HK

I am a US citizen, traveling from USA to Australia, then to Hong Kong, from Hong Kong to Chengdu, I will be in Chengdu less than 2 days, flying back to Hong Kong and one day later, back to USA. I have arranged a place to stay with Airbnb with a lady in Chengdu, so I have a receipt for payment Will I qualify for 72 hour transit or need to get Chinese visa? Nancy