Boriding Tool Steels for Wear Resistance
Boriding diffuses boron into the surface of a finished tool and forms an iron boride layer that resists metal to metal wear. The layer is hard, it does not depend on a coating adhering to the steel, and the published results are measured on the same grades a die shop already uses. The cost of the process is temperature. A usual boron treatment runs at 800 to 900 °C (1470 to 1650 °F), which is hot enough to austenitize the part, so boriding has to be built into the heat treatment route rather than added after it.
What the process does
Boron atoms from a solid, liquid, gas or plasma atmosphere around the finished part diffuse into the surface and form a hard, water resistant iron boride layer. Nothing is deposited on top of the steel, so there is no coating to flake off the way a plated layer can. The published gains are on tool steels rather than on plain carbon steel. Metal to metal wear testing showed a three fold improvement in wear resistance for borided O1 and O2 tool steels and over a two fold increase for borided A2 tool steel.
Measured gains on tools and dies
Two die results in the source show what the wear number means in production. A borided A2 tool steel die showed twice the life of an uncoated O2 tool steel die in a deep drawing operation that made low carbon steel cups, so the comparison is between a treated grade and an untreated grade, not between two identical tools. An H13 roller built to flange milk cans was borided and produced three times as many cans before it wore out. The pattern across the three results is that the smaller the gain on the wear bench, the smaller the production gain, and that toughness of the base grade still decides whether the tool survives the loads it sees.
Process window
Boriding can be run at temperatures as low as 600 °C (1100 °F), but usual practice is a period from 1 to 6 h at 800 to 900 °C (1470 to 1650 °F). The layer that forms is between 13 and 130 µm (0.0005 and 0.005 in.) thick. A borided surface is dull rather than bright, because the layer is microrough at the scale of the boride needles that grow into the steel.
The process temperature is the constraint that governs where boriding can be used. Because the treatment runs in the austenitizing range, either the boron treatment itself acts as the austenitizing step with the part quenched from the bath, or the part has to be reaustenitized and hardened after boriding. Either route changes the dimensions of the part, which limits the process to applications where a tolerance of about 25 µm (0.001 in.) can be accepted. A die that has to hold a finished dimension tighter than that is a candidate for a lower temperature process.
What these numbers assume
The wear and life comparisons come from separate published studies on different machines and different workpieces, so they are evidence that the treatment pays on adhesive and abrasive wear and not a set of interchangeable multipliers. Boride layer thickness, the temperature and time of the cycle, the base grade and the surface finish before treatment all move the result, and the layer is brittle in bending even though it is hard in sliding. Confirm the grade, the cycle and the tolerance stack on a first article before boriding goes into a route card.
Source: ASM Handbook, Volume 5, Surface Engineering, ASM International, 1994.