the issue isn't whether they could *technically* replace you (or afford to, for that matter), it's actually if they *feel* that they can or cannot replace you.
Before pursuing THAT route too strongly, I'd sniff around and see if the managers and decision-makers (who often aren't the managers ;) like you, and if not, why they don't. If your attitude at work reflects the "I can't be replaced" theory, I think you'll find the raise more of an uphill struggle than it might otherwise have been - because, regardless of actual merit, if they feel you're already well-compensated, they'll just find another fresh-out-of-school newbie to take over, and accept the short-term loss of said newbie getting up to speed.
Contrariwise, if you've gone out of your way to make life easier for the higher-ups, then you stand a pretty good chance, because they'll be willing to go to bat for you with THEIR higher-ups, and so on.
Is that right? Not really. But it's how it works. It's not about performance alone, necessarily, but about perception of performance.
--sofaspud
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