As you may already know, ABS is one of the trickiest materials to print with, partly because of its high thermal coefficient, which in turns leads with warping and cracks when not dealt with properly.
The keys to successfully deal with this aspect of ABS are two: cooling it slowly and cooling it uniformly. Typically you achieve this by using an enclosure.
Enclosures can range in quality and price dramatically: there are professional grade ones that are fire and sound proof, maintain a negative pressure, filter the exhaust air and cost a few thousands euros... or you could get away with something as simple as using a cardboard box. Probably, one of the most common solutions is to use an IKEA lack table and some acrylic or wooden panes as it provides a durable, effective solution at a very low cost.
Anyway: the reason why an enclosure works is because it traps the air that has been heated by your heating bed, thus "immersing" the full print (not just the bottom layer) in it. In turn, this means that:
- The difference between the extruded temperature and the final one is less.
- The gradient of temperatures across the height of the printer volume is also less.
- The print is shielded by random air drafts, like the one you would generate by approaching the printer to check it
- When you stop applying heat (print is over, heating bed off) the print will cool very gently.
Now, about that fan... Using a fan actually increases the speed at which your part cools, so - intuitively - one may believe a fan is a bad idea with ABS. However a fan is essential if your print has any type of non-trivial overhangs, and - in general - prints done using a fan (in any material) have better details. Again, with an enclosure, using a fan is seldom a problem as your fan will cool the ABS rapidly but only down to the temperature of the air in the enclosure (most commonly somewhere between 50°C and 80°C).
If your print is very small (like a benchy) and you don't want to try with an enclosure, you could still try to improve your situation by:
- simply reducing the speed of the fan (so to only "partially cool" the part)
- using a fan duct that distributes the air jet more heavenly around the extruded filament. Typically these are 3D printable parts that you find in forums and groups of a given printer model users. Here's a semi-circular one for the Anet A8, for example.