ABS melts at 110C. It gets fluid enough to extrude at about 140C. Higher temperatures are needed to make it stick to the previous layer. In order to weld to a layer that has cooled back down to room temperature the average of the two temperatures has to be above the melting point.
I.e. (t + Ta) / 2 > Tm.
t > 2 * Tm - Ta = 2 * 105 - 20 = 190C
The longer the interface temperature is higher than the melting point, the stronger the weld. I use 240C guarantee a good weld.