Mold steels, martensite and buckets of water

How hard is hard? If you’re a toolmaker or moldmaker, that question determines everything from the machinability and wear resistance to the final price tag of molds, tools and dies. Steel has been the preferred material for a generation because of a weird property of iron that lets you alter its physical properties just by heating and cooling, no chemistry required. The property is the ability of iron to dissolve carbon in it like sugar in your coffee, up to a practical limit of about six or seven percent.

For most plain carbon steels, though, we really just look at percentages of about 1.5 and less. To get steel to really harden, you need to transform it into something called martensite. Making martensite requires carbon, roughly 0.2 percent or better, then heat, then cooling. The heat gets the carbon moving around inside the iron “lattice” and the cooling locks it in place when it’s where you want it.

Keeping the carbon atoms from going back to their original positions during cooling means you have to cool quickly, hence quenching. The faster you can get the block cooled, the more martensite you can lock in, and the harder the part. Quenching is rate-sensitive, and that’s a good thing, because an all martensite structure is brittle as glass, so brittle that dropping a water-quenched small mold part off a workbench will break it into several pieces.

The solution is to “temper” the part by reheating it gently to get the carbon to move around a little, just enough to reorganize into one or more of the other “ites”, like cementite or spheroidite. This makes the part a little more ductile, and a lot tougher. That’s great, but consider what happens when you quench a large block or mold part to get martensite. The outer surfaces of the part cool rapidly, but they insulate the inner regions, which cool much more slowly. If the core of the part cools below the “critical cooling rate”, another “ite” begins to form, called pearlite, which is softer. Even worse, the differential cooling rates can cause distortion and cracking. That’s why “through hardness” of thick section mold components is so hard to achieve, and why heat treating is the price it is. You just can’t throw steel into a bucket of water.

Heat treaters use exotic heating and cooling processes like “martempering” and “austempering” using everything from baths of molten salt to cryogenics to get ever larger mold parts hard and dimensionally stable.

How hard can steel get? For an SAE 1095 steel, the top end is 53 on the Rockwell C scale, which is damn hard for a plain carbon steel. This material also breaks at a measly 12 foot-pounds of impact force, which is damn brittle. Can you have it both ways? The simple answer is “no”, but in the real world, other ingredients like manganese, nickel and chromium make strong, ductile and expensive steels. And plating/spraying technologies allow hard surfaces on relatively soft parts. Add non-ferrous metals like copper/beryllium and aluminum and there are lots of choices, most of which are expensive. Some materials such as aluminum or copper-based alloys give excellent heat transfer, so sometimes you can win back the higher cost with faster cycle times. For plain old steel, however, a hundred years of research, and the ease with which temperature controls hardness and ductility, means that it’s not going away any time soon.