Tool Steel Selection

A tool steel is any higher carbon steel that can be heat treated to resist abrasion and deformation. Typically, in our industry, we use tool steels for anything that touches the customer part or that would be in a high wear part of the equipment itself. The good news is that since material optimization isn't usually top priority in our one-off world, and most of the time, we are asking far less of our materials than they are capable of, you can pick one tool steel and be right about 80-90% of the time. The bad news is that the other 10-20% can bite you pretty badly when you get it wrong. Hopefully, this will help you identify those exceptions.

Side note: I've seen a move away from having designers specify material sizes and sometimes, even the materials themselves. I personally think that's a mistake. Material selection is fundamental to the function and cost of the machine, and should be a part of your thought process during design. So, even if your company puts that responsibility elsewhere, it's still a good ability to develop because it makes you a better designer.

Tool Steel Size Tolerance & Pricing

Bar Stock

Regardless of the type of tool steel you buy, there are few basic shapes that you will be ordering. Rectangular bar stock can be ordered from a wide variety of sizes. At 1/4" thickness and above, you can get widths up to 12" at 1/4" increments, with thickness incrementing by 1/8". Below 1/4", the width selection narrows down considerably, so you'll have to check availability a little more closely. Depending on the type of tool steel, costs for bar stock is between $4-6/lb for whole bar lengths or random drops (the leftovers they have laying around). A $3-5 cut charge will be added to each piece that a steel company cuts to length.

Round stock comes in 1/16" diameter increments from 1/4" diameter to 1". From 1" to 3-1/4" diameter, it's 1/8" increments, and above that, it's 1/4" increments. Costs for round stock range between $3-5/lb for whole bar lengths or random drops. The same $3-5 cut charge will be added per cut.

The main thing to keep in mind about bar and round stock tool steel is that the finish is scaly and sometimes has tool marks, so it is not suitable for use as-is. The good news is that it is 0.015-.020" [.5-.8mm] oversized, so if you want a 0.500" finished dimension, you can still order 1/2" stock. The bad news is that at least four if not all six sides will need to be machined. It can take 20-30 minutes to square up a block on a mill, and even longer to finish grind it, so that should be factored into your decision when choosing between bar stock and pre-ground stock.  

Pre Ground Stock

Flat Ground Stock (FGS) is, as the name implies, already ground. It comes with very good thickness and flatness tolerance, most of the time, in the +0/-.001 [.02mm] range. This saves you all of the machining noted above, but there are a few catches: you can only get it in 18 and 36" lengths, the price jumps to between $14-18/lb depending on which length and size you get, and it's only available in the more popular tool steels. But, depending on cut charges, the machining savings, and how much of the piece you will use, it's a very viable option in a lot of cases. FGS comes in many thicknesses and widths, sometimes in 1/16" increments in thickness and 1/4" width increments on smaller sizes, but you should check on a particular size before choosing it. There is also an “Oversize” (abbreviated OS in catalogs and on websites) option that has a tolerance of +.010”[.25mm]/+.015”[.38mm] which would allow post-heat treat grinding to nominal if you are worried about movement on very close tolerance areas, but if you have to do too much post processing, it’s probably more economical to just choose normal bar stock.

Drill Rod (DR) is similar to FGS (including the OS option), but for rounds. It has the same tight tolerances and can be very cost effective depending on the situation. Drill rod is only available in 36" lengths, and costs in that $14-18/lb range per piece. Diameter increments are all the way down to 1/64" on smaller diameters 1/8" increments from 1-2", 2.5", and then 1" increments at 3" and above.

Why Does This Matter?

To give a real world example of the choice, let's say you have a detail that is 1/4 x 2 x 3 and has some holes, doesn't require re-op after heat treat. You could specify A2 bar stock and get a piece that's 1/4 x 2 x 3 1/8 for around $7 ($2.12 for the per-pound charge plus $5 cut fee). It would take about a half hour to machine it to final condition, so let's say $30 of burdened labor cost plus material, for a total of $37 before you start poking the holes. Or, you could buy an 18" piece of A2FGS for $62 and 10 minutes ($10) to cut it and square it, total cost at the same state: $72. You would still have almost 15" of the FGS left, so that could enter into your decision if your company does a good job of not just reordering the next time. Or, you could say that in this case, $37 is cheaper than $72, so the right way to go is A2 bar. But, in that same scenario, if there were 5 of the same detail, the scale would tip very favorably to the FGS. The point is, such a simple decision does have impact and that's why it's good to learn about it and think about it during your designs.

The Most Used Tool Steels in Our Industry

A2 (air hardened)

This is the steel for the 80-90% of your wear details. It's extremely stable through heat treat, so the need for secondary ops are limited, works pretty well when there are varied thicknesses in a detail, shows good wear and good impact resistance. If you don't have one of the special circumstances below, choose this steel. Typical hardness is R/c 56-60 (see Heat Treating post for more info on understanding how you can use that process to your advantage).

S7

This steel is the best choice for details that have a lot of thin walls or thick to thin transitions or if the detail is going to see a lot of shock or impact load (I think the S actually stands for Shock). The reason why we don't just use it all the time is that it's considerably more difficult to machine than A2 and also moves a little more during heat treat (added post heat treat ops), plus it's slightly more expensive. The biggest thing to note with the use of this steel is that it should only be heat treated to R/c 50-55 if you want to take maximum advantage of its shock resistant properties (see Heat Treating post).

D2

D2 is basically the best material for punches and dies. It is less impact resistant, but is very wear resistant. It holds shape extremely well during heat treat. The downside is that it is a difficult material to machine and grind, so unless you are really making a punch or a die, it's not the best choice for our business. I have limited experience with this metal, but I believe it should be hardened above 60 R/c for maximum benefit.

O1

Oil hardened steel was the primary tool steel used before air hardened steel came about. Except for the situation described in the Dissimilar Metals paragraph, its use should really be avoided. It isn't a bad steel, but it moves a lot more during heat treat, so it requires a lot more secondary ops. It can be hardened across the full range just like A2, but target 56-60 normally.

M2

M2 is a high speed steel used in cutters and drill bits. It has limited use in custom automation, but I mention it because it holds shape and hardness well with high heat and/or friction. It's difficult to machine, so I wouldn't recommend unless the circumstance requires it. Very high hardness is usually specified for this steel. Application specific research should be done before using it.

4140PHT

The PHT stands for pre-heat treat. It is technically not a tool steel, but falls somewhere between a low carbon steel and a tool steel so it does have its applications. It starts off at a Rockwell C hardness of around 35-40, so it is machine-able in its natural state. Unfortunately, this is probably one of the most misused materials in our industry. Everyone sees it as a shortcut that allows for details to be machined complete without extra heat treating or processing. But, as in most shortcuts, it can get you into trouble. In reality, any consistent steel on steel contact will wear it out. If cold rolled steel (see Low Carbon Steel Selection post) would wear out in a month, PHT may take a two or three, but that's still not a production worthy solution. It also dings and dents almost as easily as low carbon steel. BUT, if you are using it against a polymer (see Plastics and Specialty Material Selection post) or a non-ferrous material like bronze, or if it has a very low duty cycle, it can be a cost effective and proper application. It typically runs around .005" [0.1mm] undersize.

Heat Treatable Stainless Steel

416 and 441 Stainless does have sufficient carbon to be heat treatable. However, unlike other types of stainless, these steels are magnetic and are not fully corrosion resistant, so take that into account in your selection. 416 machines more easily (a VERY relative term, since all stainless takes about 2x the time to machine as carbon steels), but corrodes more. 441 has better mechanical properties and resists corroding, but is more expensive and very difficult to machine.

Dissimilar Metals Rule

Ok, this is a weird one, and you will likely never see it while a machine is on your floor, but over time, if two pieces of metal that routinely rub one another under load are the same metal (A2 on A2, let's say), they will begin to "gall" each other, which means they will literally start to exchange material at the molecular level. This will cause all kinds of fit issues, and can actually seize up the tooling. So, the best choice is to use some type of bushing material (see Bushings & Bearings post), but in certain instances where that's not possible (a fine-blanking die, for instance), you should make sure that the two pieces are made of different materials. That's the main reason I even listed D2 and O1 as possible steels, because really, we don't use them very often.