Ok, let me tell you my experiences.
I usually quench only once, too, and results have been different.
I use a coal made out of oak and chestnut, in my Iron Age forge (a pit in which leather is immitted by a leather bellows), and forge only raw iron, not steel. Despite the simplicity of the design, the forge is capable of reaching high temperatures: I melted the iron twice.
After all this time, I understood that a successful forging somehow depends on the ingot's dimensions, on the time it's been heated and hammered, and in the color it has when it's quenched.
Basically, a 3 millimeters thick ingot, requires about 4 hours of forging to get enough carbon molecules. Best quench is at cherry red, or bright orange. More will cause the iron to be too brittle.
Pay particular attention when you have to weld, for you need to work at very high temp (coal white, just before melting temp), and after the welding you'll have to quench; then heat again to mantain the weld but remove the brittleness.
This weekend I'm going to study temperatures and test different queches (water, frozen water, water with oil surface, oil) for my thesis, by making 4 blades out of iron ingots; I'll then try to bend them and see the differences. Stay tuned
Greetings,
Mauro.