Both metals that are being joined together need to be of equal temperature during the brazing process for a successful braze joint.
A slower heating cycle is better then a faster one. If productivity rates are critical, it is better to process multiple parts at the same time.
When using a braze preform ring for induction brazing, make sure there is good physical contact between the part and the ring. A loose ring will need much higher temperature to melt and will also cause non-uniform joints.
The parts being brazed must be clean at the joint area.