Restore whatsapp chat if no SD-card
You cannot recover them because you erased them when formatting, and Whatsapp can't recover them for you from their servers because they are deleted as soon as they are received by the client (you):

http://www.quora.com/How-long-does-WhatsApp-keep-the-delivered-data-like-images-or-etc-in-the-server
https://www.whatsapp.com/faq/en/android/20887921