I have bent sugar maple even with more than one growth ring on the outer side. However the exposed ring ran from top to bottom and there was no grain runout. I have also bent pieces that had little leftover "bands" of outer growth ring on top, which never lifted.
It seems that that the best strategy is to soak wood for a long time and simmer it for lengthy time periods (30m - hour) rather than boil it.
The problem with bending wood is that while it takes the compression fairly well, it cannot stretch. However, you can back the pieces you want bend with something that can take a lot of tensile stress -- just like we back bows with sinew.
One option is thin steel. JWBbows demonstrates a bend of sugar maple using a steel backing in this video (not however that you can achieve this bend even without the backing):
https://www.youtube.com/watch?v=qQKNOTMAe4wThis would seem to work best for steam bends. However, I think there is an even easier way to do this -- use a stretchy cordage to back the piece "cable style", like the Inuit cable bows. You can do simple cable backings fairly quickly.
Another option is to give your piece a traditional sinew backing, and expose it to very low-temperature ambient heat for an hour or more. It's amazing how little heat you can use to achieve severe bends in wood. Time, rather than heat, seems to be the buggest factor.
For example, Adam Karpowics described bending the deflex on a Scythian bow as follows: he humidified the sinew-backed wood core for several days at +60% RH. Then he exposed the bending portions to low heat in a ln environment of near 100% humidity (perhaps augmented by a pot of boiling water, in a tent). The heat was applied for an hour, making sure the surface of the bow never exceeded 108 degrees farenheit. Then he bent scythian inner limbs in to their characteristic extreme deflex.
Moistened wood reacts to very low temperatures. It does not appear to make much of a difference if the temperature is high or low -- what matters is the duration of energy transfer.
Even 125 degree water can give you a serious burn if you leave your finger in there for too long, so don't underestimate the power of low heat and sustained energy transfer. The increased duration of exposure is what gives your wood the mobility to bend. As others have said, too much temperature destroys wood. Most people use temperatures that are way too high and exposure times that are too low, IMO. The longer you heat the wood, the longer it will retain the heat.
