Die Ring Metallurgy

The basic physical properties of the die materials are controlled by the heat treating process and the composition of the die’s steel.

Carbon and Chromium
The two key elements in our dies are carbon and chromium. The chromium content of the steel affects the corrosion resistance and the wear resistance of the die. A free-chromium content above 12% classifies a steel as stainless. We use a steel mixture called X46Cr13 which can be described as Stainless steel dies, but we prefer to call them Chrome steel dies for the simple reason that there is no such thing as a rust free “stainless steel” die ring.
Chromium carbides are formed during the heat treat process through a combination of chromium and carbon atoms, which increases wear resistance.

Heat Treating: carburizing vs. vacuum hardening
There are two different ways in which dies are commonly hardened; carburizing or vacuum hardening and the method used depend on the type of material used to manufacture the die.

Case carburizing dies can be done in a pit furnace and vacuum hardening is done in a vacuum furnace by the addition of a carbon rich gas, such as propane. Alloy Die are processed in a pit furnace and chrome steel Dies are vacuum hardened.
When carbon is added into the atmosphere of either of these furnaces, it soaks into the steel to form a hard outer case. Case carburized dies tend to create more friction in the pelleting chamber, meaning they will usually provide better quality pellets at the expense of production capacity.

Vacuum hardening is a process that gives the same relative hardness throughout the thickness of the die, not only the outer case like you would find in carborized dies. Due to its homogeneous hardness throughout, Chrome steel offers excellent die lifespans and are much easier to break in.
There are three common types of dies being used in the industry today:
Alloy is a medium-grade carbon steel which is case carburized for a hard outer case of about 6mm thick to 57 HRC and a very soft core.
Chrome Steel (the most common of all) is vacuum hardened to 52 HRC through the thickness of the die.

Die Material Application
Each material has characteristics that may make it more desirable than another for an application.

Alloy is the most breakage and crack resistant die material currently offered, which means that it is best suited for heavy tramp metal situations. Alloy has been used in heavily abrasive situations where die life with other die materials is not significantly longer in order to keep cost/ton ratios low. Alloy is also used extensively with high mineral content applications.

Chrome Steel allows high throughput and die life. It should be used in pure feed material applications, high throughput applications and extended die life applications. Chrome Steel needs more effective thickness to achieve a pellet quality similar to carburized dies and pellet quality can be devastated by changes in thickness of as little as 2mm. These dies are the kings of industry because of their low running cost, high throughput and anti seize properties, but they will not tolerate low plasticity, frictional and high shear properties to name but a few changes in input material properties. These dies MUST be expertly matched to a certain mixture or pellet quality will suffer. When changing from a carburized die to Chrome steel always increase the blank thickness to avoid cracking.