After Enterprise portal in being installed and have configured in AX 2012, sometimes username and password keeps prompt for Home | Role Center page. However, this is irrespective to AX as any web application can be configured in IE settings for username and password.
First option Steps outlined on this MSDN page resolves this issue.
Second option
IE explorer | Internet options | Security | Intranet (or your settings), Select custom level scroll to the bottom | User Authentication and select autologon depending upon your setup
