The impact toughness of quenched steel after tempering at 250-400�C is lower than that obtained on tempering at temperatures below 250�C. If brittle steel tempered at 250-400�C is heated above 400�C and transferred into a tough state, a second tempering at 250-400�C cannot return it to the brittle state. The rate of cooling from the tempering temperature within 250-400�C has no effect on impact toughness.
From your site you linked. That 250-400 degree range is in the straw heat range, which I was always taught was 'knife' hardness, 58-60C Rockwell. The steel will then darken through the blue/plum colors then get a vibrant bright blue then going to a lighter blue...that's where I stop. I've made new hammer springs for revolvers (flat spring) this way and they've held their power and certainly weren't brittle. You need to draw the harness back and it will give you the 'toughness' or basically impact resistance yet still have enough hardness against deformation.
