1045 vs 1018
I know what the chemical difference is, but why would one alloy be used over the other? Are the mechanical properties vastly different? These plates ( .750 by 12 by 24) have to be machined all over ( lots of holes for mounting, milling, and sufrace ground flat and paralell) to hold tolerences and they are just a support and alignment frame for a very light duty mechanism. Before I suggest a cost reduction to my customer, I want to make sure I'm not missing something.
1045 may warp less than 1018 when machining off both sides, grinding, drilling holes, etc.. 1018 is full of stress, and hard to keep flat when doing that much machining.
There is a large difference between the two steels. It take very little carbon change to effect the physical proporties of the steel. If the customer is knowlegable in what they want then I would just quote the price. It would be best to look up the aplication of each steel in the machinry's handbook and use that to decide if 1045 is too good to use. There are so many variables to consider that it takes someone with a lot of knowage to pick out the best steel for a job, most of the time you just try to pick one that will just work. If your in mass production then you should put more time into picking the correct steel. As for your application I would say that 1018 doesn't machine or grind very well. Also is it going to be hardened. 1018 would be cheaper and easyer to get. 1045 would be stronger, hardenable, easyer to finish, and more fatige resistant. Too many variables that need to be considered before a material can be chosen.
This begs another question: Hot Rolled or Cold Rolled?
Cold Rolled 1018 will be very close to the .750 dimension. But if it has to be ground for finish or size across the broad faces it cannot be used. Cold Rolled steel will have internal stresses that cause the part to spring when machined or ground.
Hot Rolled will not have the same internal stresses and it will be fairly stable after machining.
Hot Rolled, Normalized, Annealed, Quenched and Tempered, or Cold Drawn (Cold Rolled)
The brinell hardness for 1045 will be either higher or the same as 1018.
Talk to your steel supplier, if they are any good they should be able to help.
It might be worth inquiring why they are specifing 1045 if you are having trouble getting it. It could be specified for strength or hardenability, but in either case it is pretty safe to say that it could be replaced with 4140 which seems to be more common these days. I have worked for a few companies that had a lot of legacy parts that specified materials like this. Even if it is a recent print it could be that a rookie draftsman / engineer made the print and just copied it from an older similar design or he just pulled it out of thin air.
Are these burnouts to begin with? Or are they pieces cut from 3/4 x 12 flat? I ask because, as far as I know, there's no such thing as 1018 or 1045 plate.
In the hot rolled annealled state there's little difference in physical properties of 1018 and 1045. 1045 won't mill as well as 1018. For what you've described, it seems like Blanchard ground A36 plate would be a good starting point. From what I've seen over the last few years, the quality and consistency of plate is far better than that of HR low carbon flats, which are going to be stocked as A36, and not 1018, by most warehouses.
Another option, if they want a little more strength, would be burnouts from A572 Grade 50 plate. (50ksi min yield vs 36ksi for A36) Its decent to machine (as plate goes) and has always seemed somewhat more consistent than A36 when I've had occasion to make parts from it. I priced that material in 1/2" plate earlier this week from Ryerson, and it was 15 cents/lb more than A36. Given that your blanks would weigh about 60# each, the $9 or so premium over A36 would be worthwhile IMO.
If they insist on a medium carbon material, I think its Ryerson that has a material called Freemax 45 that's good to machine. Not sure if its resulfurized or some other process to help machinability, but it cuts pretty nice. Been 20+ years since I bought any of it though, so I don't remember any prices relative to A36. Doesn't stick in my mind as being particularly expensive though, and the difference between it and plain old 1040 or 1045 flat would be worth several bucks on a part with a lot of machining. The same thing is likely sold under other names, but Freemax is the one I remember.
I have to start with 7/8 or 1 inch plate, have the outside shape and some windows lazer cut, then blachard grind, finish machine, surface grind, jig bore dowels and put in tapped holes. Flatness, paralellism, size and location of holes require it. I am going to reccomend HRS plate. I think it's a draftsmen choice, not an engineering recomendation. I know the customer well enough to make the call. There is very little structural load to this. Mainly support and location of internal parts. All the moving wear parts in the assembly are hard A-2. All other support plates in the assembly are CRS. I know they would go for 4140, but I really don't think it's needed in this application.
1045 is available in plate. I buy 1045 burnouts quite often. 1018 I don't think is available, since they just use A-36, which really is equivalent to 1018/1020 for the most part.