Help with 1018

I don't do a lot of steel work, but I've done some. However, my experiences this afternoon confused me, so I thought I'd ask you experts.

Material: 1018 Cold rolled. Mcmaster, claims rockwell 70B.
Cutter: 1/4" carbide 2fl uncoated square center cutting EM. 1.25" stickout
Cut: 45 thou deep slot, ramped plunge. 5.8ipm 4500rpm, flood coolant. (Conservative setting in G-Wizard.)

I got no more than 1-2 inches in this cut before the EM snapped. Not knowing if the bit was dull to begin with, I put another brand new EM. It made it through the cut (4 inches long), but the cutting edges were completely gone, just fractured off the EM.

Laster investigation shows that this was probably a aluminum cutting carbide. I know for sure it came from McMaster, it was an OSG I think. (At home now)

I proceeded to mill the next path, which is a 1/32 carbide EM, ultratool. It had no problem at 1.8ipm 4500rpm 5 tho DOC. Cutting edge intact after 30 minutes of cutting.

So, was it my cut or the tool? Is an aluminum helix EM unsuitable for steel? Is it that this type of carbide is too brittle? Is carbide too brittle for 1018? If so, why does a 1/2" and a 1/32" work fine?

Thanks for the help guys!
Tom