Carbides in 6150 1

[swall]

Anyone got a feel for amount of carbides that would be considered typical for Q&T SAE 6150 steel? Starting material is spheroidize annealed, austenitizing temp is 1600F, oil quench, and temper to hardness of Rc 46-51. Section size is appoximately 0.20"

 

[redpicker]

If you are seeing carbides optically, chances are they are undissolved carbides from the spheroidizing operation. What is your austenitizing time? Sure, the phase diagram indicates that 0.50% carbon steel would be fully austenitic at 1600F, but that is at equilibrium. With a spherodized structure, it can take a while to reach equilibrium.

Try 1700F austenitize. The ferrite in the spherodized structure is not going to want to become austenitic until it either goes above 1670F or absorbs enough carbon. Since the structure is spheroidized, the carbon must diffuse through pretty large ferrite grains, which is a slow process. At 1700, all of the ferrite will transform to austenite and the carbides will dissolve much more quickly. You can drop back down to 1600 prior to quenching, if you need to.

rp