Question:
I am working on a project that involves heating a ¼-inch-thick boron steel in a continuous belt furnace up to 1825°F. The through-furnace time is approximately 5 minutes followed by forming the part to shape in a press, after which it drops into a polymer quenchant. From the quench tank it goes into a tempering furnace set at 525°F. Time through this continuous belt furnace is 45 minutes.

The part has an as-quenched hardness around 52-54 HRC, and we are trying to get the part down into the 40-46 HRC range. We have set this hardness range because we want to achieve a part that doesn't crack but will have comparable wear to our previous design (made from high-carbon steel).

My question is that some of the parts are then sent out for hardfacing of the high wear areas. This process consists of applying the slurry, going through a drying oven, fusing at around 2000°F (at heat for approximately 1.5 minutes) and cooling down to around 1600°F. The part then drops into a quenchant – all on a continuous belt. The part is then tempered at 600°F. A lot of these parts are failing in the field in an embrittlement manner.

I know the 500-700°F range should not be used, but I would appreciate any other advice you might have.

Answer:
Let me try to provide you with a simple answer to a complex question about tempered-martensite embrittlement (TME). Reheating above austenitizing temperature will change the microstructure and, in principle, negate the effects of TME. However, one must consider a number of extraneous factors such as the thickness of the material, heating effects, etc. to determine if it the part is being through-heated during hardfacing. If all areas (including the core) do not exceed the upper critical temperature (Ac3), then the part will remain prone to failure by TME.

Temper embrittlement effects can be diminished by keeping silicon and phosphorus levels as low as possible, adding up to 0.15% molybdenum and avoiding the embrittlement heat-treating conditions. Susceptibility also depends on impurity control, and here is where the steelmaking process is critical.