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Wearing

December 30, 2017 at 8:52 PM by Dr. Drang

I shoveled our driveway and sidewalks this morning, and as I was finishing up, I noticed I was leaving behind little ridges of snow. This is why.

Very worn snow shovel

Years of being pushed across asphalt and concrete had worn the leading edge of the shovel all the way back to the flutes that give the shovel its bending stiffness.12

For contrast, here’s another of our shovels that’s certainly worn, but not nearly as much:

Less worn snow shovel

The shovel would’ve lasted longer if its lip were harder than the rest of the blade. I don’t know a lot about plastics manufacturing, but I suspect that’s not possible—or economically feasible, at any rate—in a single injection molded part. (If you know for sure, please educate me.)

In steel, which I know a bit more about, hardening certain areas of a part for wear resistance is commonplace. The iron/carbon microstructure of steel can be manipulated through heat treatment processes to give a wide variety of properties, and manufacturers have long known how to harden the surfaces of gears, camshafts, bearings, and other parts that see a lot of rubbing during use.

The iron/carbon system can take many forms, which metallurgists know by names like pearlite, ferrite, and martensite. By manipulating the carbon content and putting the steel through controlled heating and cooling cycles, they can choose which form goes where to make the part perform the way it needs to.

You might wonder why parts are hardened only at the surface instead of all the way through. Two reasons:

  1. It’s more expensive to harden a part all the way through.
  2. Through-hardened parts are more susceptible to fracture under impact loading. The softer core of a surface-hardened part makes the piece as a whole more resilient.

When surface-hardened (often called case-hardened) parts do break, the heat treatment is a prime suspect. Was it hard enough? Too hard? Was the depth of hardening what it was supposed to be? To answer these questions, the part is sectioned and polished to examine its microstructure at the surface and in the core. Then a microhardness traverse is performed, in which a series of tiny controlled indentations are made into the polished cross-section to see how the hardness changes from the surface inward.

Because I’m not a metallurgist by training, a lot of what they do seems like magic to me. Carburizing, which is sometimes part of surface hardening, is one of those magical processes. If the steel in a part is low in carbon, say less than 0.3%, it can’t be hardened by heating and cooling alone—it needs more carbon. Carburization is the heating of the part in a carbon-rich environment for a extended period. The carbon slowly diffuses into the solid steel (that’s the part that seems magical), raising the carbon content of the surface and allowing it to be hardened.

If there were a analog to carburization for plastic (I think the shovel is HDPE, but I’m not sure) I might be willing to try it out on my less worn shovels. But the one I used today has seen its last snowfall.

  1. I don’t know if “flutes” is the term of art in the shovel business, but it’s the first word that came to mind. ↩

  2. Also, you might notice some areas on the shovel are lighter in color than the rest of it. That’s called “stress whitening” (which definitely is a term of art in my business), and it’s common in colored plastics that have been overstressed at some point in their life. ↩

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