Fig. 1. Typical 4140 gas-nitrided ring gear (Photograph courtesy of Nitrex Metal, Inc.)

I have several questions. The first is about hot-forming dies. These are not “impact” dies. They are used for a slow press at high pressure. We heat them up on two “platen” machines to 1450°F (790°C). They currently do not have any surface treatment on them. I am wondering if gas or ion nitriding would help extend their life?

My second question has to do with dies made of A2, D2 and some 4140 steels. These are hydraulic and mechanical (blanking, punching) dies. What is the likelihood of extending die life and reducing rework frequency with gas nitriding?

Fig. 2. Typical load of cast iron differential carriers ready for plasma nitriding (Photograph courtesy of Surface Combustion, Inc.)

Regarding your question about hot forming dies, nitriding (gas or plasma) normally provides additional wear benefits for hot forming dies made of H-11 and H-13 but cannot be expected to work well for this application if indeed the dies are exposed to temperatures as high as 1450°F (790°C). The concern is that the nitrided case will simply diffuse away over time. It depends to a large extent on the material being used and the severity of the application.

Regarding your second question, the life of blanking tools is generally not improved by nitriding. Life will be shorter because the edges are prone to chipping (unless the dies are used for thin or soft sheet). By contrast, wear of extrusion or embossing dies made of A2 or D2 will see prolonged life due to nitriding.

SAE 4140 is an excellent nitriding steel, and it is usually selective hardened to about 50 HRC for long blanking runs on soft material. The inside or outside corners are likely to have soft spots, however, and these areas will wear rapidly.

Beyond the answer to the question, many other materials and component parts are also capable of being gas (Fig. 1) or plasma (Fig. 2) nitrided as well.