Alloy for Smooth, Lapped Surface

You might look into some mold making steel. I think a fine grain structure is more important than paticular alloying elements when looking strictly at surface finish.

What is the temperature you are working on?

Carbon steels anyway are not much advisable for higher temperatures. Unfortunately the elements that make it costly (Ni- strength, cr- wear resistance, Mo - high temperature strength) are necessary.

What is the maximum temperature expected on the component? If it is of the order of 600-700⁰F or less, try high carbon steel, hardened and then ground.

Thanks for the input.

Temperature is in the 450 to 480 °F, there is very little mechanical load other than the gas pressure of perhaps 700 - 800 psi. The piston size is rather large and specific load is not very high. From a strength point of view a plain carbon steel would do such as 1018. Perhaps a 1050 is better, but as far as I know carbon does not do too much for surface quality.

Reading up on mold steel per the suggestion of dgibson it is mostly chromium content that provides the smooth mirror like finish.

Perhaps a 4118 or 4130 will be better than a 1050.

Forgive me for having a different opinion, but alloying elements will not help appreciably for surface finish, though adding nickel could hurt it by toughening the chip. Nitrogen can strengthen the ferrite, resulting in a crisper chip in the carbon range you suggest. If your part will not fail if anisotropic, then resulfurization will result in better surface finish, as the manganese sulfides will result in better chip separation and controlled built up edge on the tool. Rephosphorizing too. This is why 12L14 gives such great surface finishes. Best finish I ever achieved was with 12L14 plus tellurium. THESE ADDITIONS WERE NOT ALLOYS. But carbon too low for your purpose. Frankly, if not resulfurized, the surface finish is pretty much dependent on machining practice,Tooling coating, speed feed depth of cut, nose radius, honed/ not honed, lubricant, filtering etc etc^2, not the alloy. look at all the different steel grades used for bearings. Take heed of the Lincoln quote" I i had 8 hours to cut down a tree, i'd spend 6 hours sharpening my axe." or something like that. Look at the process. PS- WHo makes 13XX anymore? Thats wierd. milo

Agree with Milo completely. Listen to the man. The other opinions so far are not worth the pixels.

Titi-the-rabbit.

We use the following materials and is normal ground (in cylindrical grinding machine) and finish achieved is upto 0.2μ without too much effort (with normal stabilised process)

AISI 1035, 1040

ASTM A216WCB

Nod Iron GG25

BS970 (40CrNiMo)/ AISI 8620 (alloy steel) and other number of alloy and tool steels amongst other.

Note: these are cylidrical shaft shaped (as is obvious from cylinderial grinding)

Quite a few of these are also surface ground/ segmental ground.

Getting a finish is not a problem as you can see even from carbon steels of larger grains (Nod Irons) or smaller ones (HT Forgings).

The next question is the corrosive attack (normal ambient humidity, temperature) for which unfortunately the carbon steels are especially weak.

Certain cases of course we hard plate the surfaces , which gives us corrosion protection too.

Your temperature of application is anyway low enough for any type of steel.

Thanks dgibson, that is a great link for the polishing process itself.

Also thanks to the others and to Milo. The 12L14 may be a good choice, the lead may do the trick and C at 0.15% is ok for that service. I may also want to consider Nodular Iron as suggested by guest.

Great help. Thank you all.