# Hardening metal banding for making putty knives.



## starringemma (Aug 15, 2012)

I acquired a bunch of one and a half inch crate banding and cut them into 6, 7, 8 inch pieces. Now that I know how to peen a brass rivet in a wood handle I thought I could make some cool putty knives with black walnut grips with oak and turquoise inlays. The banding is not hardened, it will kink when you bend it unlike a putty knife. A putty knife has some flex to it but will spring back straight. I watched some videos on tempering knife blades but a knife blade has no spring in it at all… I know this by trying to use one as a screw driver.

Does anyone know about heat treating metal somewhere between hardened and not hardened?


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## Gunney (Nov 2, 2012)

I don't think you can heat treat crate banding. In order for heat treating to work, the metal has to have a certain amount of carbon content and I suspect the amount in the banding is too low. To make a putty knife, I would look for spring steel (I know that McMaster-Carr sells it), which can be hardened.

As far as the process of hardening, the metal is heated to a certain temperature that varies depending on the exact composition of the steel, usually somewhere in the 1500 to 2000 degree Fahrenheit range, and held there for a certain period of time. It is then quenched in a quenching medium that is again dependent on the specific type of steel, but could be oil, air, water, or brine. At this point, most all steels are too hard and brittle to be of much practical use and must then be tempered. Tempering involves heating the steel again, but to a lower temperature (400 to 1000 degrees depending on the steel), holding that temperature for a certain time, then allowing it to cool. After tempering, the steel will not be as hard and therefore not as brittle. The trick would be finding the proper tempering regimen that would leave it hard enough that it would spring back to its original shape, but not so hard that it would snap.

There is a process called "case hardening" which allows you to impart a thin, hardened surface layer to low-carbon steels, but it is more involved and I don't know if it would work with something as thin as the banding.


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## JAAune (Jan 22, 2012)

What Gunney said above is all useful information and I won't repeat it. There are a few extra details that I will add though.

The temperature that the steel needs to be heated to is called the transformation temperature. It varies based upon the alloy and carbon content of the steel. One thing that is constant however, is that steel becomes non-magnetic when it reaches that temperature. At this point the steel will be hot enough to glow. The color varies based upon temperature.

So if you have scrap steel of unknown quality, you just have to heat it up until it reaches the non-magnetic state. Heat the steel and pull it out of the heat source and see if the magnet sticks. Remember the color of the steel so you won't have to use the magnet while doing the actual heat treatment.

One thing that is important is to quench the steel after it's brought up to temperature. The faster you get it from the fire into the bucket the better. Do not bring it past the transformation temperature and wait for it to drop before dunking. That results in an inferior tool.

Here's a link that will get you up to speed on heat treatment: Heat treating tool steel

Note the tempering chart near the bottom. For a flexible putty knife somewhere around 45 Rockwell C should do the trick.

In that article, I didn't see any mention of W-1 steel. It's water-hardening, inexpensive and if properly hardened and tempered is excellent for woodworking applications. I recommend going with W-1 if you decide to purchase stock for cutting tools in the future. Old truck springs are decent and cheap but for putty knives, it would probably take too long for you to grind them down.

If you want something quick and easy, cheap hand saws obtained from garage sales are a good source of material. The blades should already be hardened just right for a putty knife.


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