ZOB Controller described a technical loss of separation event when step-climbing two air carrier aircraft but failing to note the absence of FL180 due to low altimeter settings.

Narrative

This is a situation type of mistake on my part; strictly my control error. I was climbing Air Carrier X underneath Air Carrier Y. The Air Carrier Y flight was not a high-performance climb; so I was step-climbing the Air Carrier X to the vacated altitudes of the Air Carrier Y while keeping in mind their very different climb rates. I was climbing the Air Carrier X to 1;000 FT below the Air Carrier Y; instead of assigning the altitude once the Air Carrier Y had left it; trying to keep a very positive separation; since the Air Carrier X could possibly catch him via a greater climb rate. I called the traffic to both aircraft; and explained the climb procedure to both aircraft. As the Air Carrier Y climbed through FL180; I assigned Air Carrier X to climb to 170. This is where my error occurred. FL180 was unusable for separation due to low altimeters. I had overlooked this critical detail since I was so focused on managing their climb rates; and by the time I realized what I had done; Air Carrier X was well through 16;500 and Air Carrier Y had barely left 18;200. I expedited Air Carrier Y through FL190; but the aircraft were within 5nm before Air Carrier Y passed FL190. I had my 'carefully' established 1000 feet the entire time; but because of the altimeter settings I may not have had proper altitude separation. The conflict alert never went off; but I alerted the watch desk after my session on the RADAR finished. Recommendation; I can only say that I always try to pay attention to the low altimeter settings. I had it in the back of my mind; but didn't think about it until it was too late to apply proper separation procedures. At the time; I felt as though I was doing everything correctly; extra carefully; even! It might have been complacency; which is a hard thing to accept when I was trying my best to keep those planes apart; but it was an oversight; and a mistake; on my part.