How much does Quenching increase Cast Bullet Hardness?

How much does Quenching increase Cast Bullet Hardness?

If you’ve been following my articles for any length of time, chances are you already know I water-quench virtually all my cast bullets.  While the fact that quenching increases hardness is fairly well known and agreed upon, I’ve always wondered just how effective it really is.  Over the years I’ve seen and heard all kinds of opinions on this, but for whatever reason I’ve never come across any actual test data to back any of them up.

With that in mind, and a little help from the Lee Precision lead hardness test kit, I decided to do some of my own testing with water-quenching.

Preparation

I began this experiment by casting up some fresh test-bullets from my standard wheel-weight alloy, using an aluminum six-cavity mould by Lee Precision.  Once I’d obtained fairly consistent fill, I produced several dozen air-cooled bullets which were cast onto a sheet of aluminum foil.  These were then allowed to cool using nothing but the ambient air temperature.

With those out of the way, I moved on to casting the quenched bullets, which were immediately dropped into a bucket of water to cool.  Once again, these were produced using the exact same Lee Precision mould, with the same alloy, from the same pot, running at an identical temperature.  In order to further eliminate any atmospheric variables like air temperature, pressure, and humidity, both sets of bullets were produced in the same casting session.

With the requisite bullets cast, I proceeded to sort them into two bags, each labeled with the date they were cast, and also indicating which ones were water-quenched for easy identification.

Method

In order to get as accurate a reading as possible, I randomly selected five bullets from each bag, and proceeded to test them individually using the Lee Precision lead hardness test kit.  Details on this kit and it’s operation can be found a previous article here.  The results of each of the five tests were then totaled and divided by five to obtain an average value.

Air-Cooled

Starting with the air-cooled bullets, the measurements were as follows:

Bullet 10.066″11.8BHN
Bullet 20.068″11.0BHN
Bullet 30.068″11.0BHN
Bullet 40.068″11.0BHN
Bullet 50.068″11.0BHN

Water-Quenched

Moving on to the water-quenched bullets, the measurements were as follows:

Bullet 10.048″22.7BHN
Bullet 20.048″22.7BHN
Bullet 30.050″20.9BHN
Bullet 40.046″24.8BHN
Bullet 50.048″22.7BHN

Results

The bullets allowed to air-cool reached a final hardness averaging 11.8BHN.  When loading magnum ammunition I generally aim for 18BHN or higher, so that should give you some insight into just how soft these air-cooled bullets are.  By contrast, the water-quenched variety reached an average of 22.7BHN– a massive increase of more than 100%.

I have to admit, I was genuinely surprised by just how much of a difference water-quenching made in my bullet hardness.  Before reading the results, I’d suspected the difference might by two or three points of the Brinell hardness scale– a far cry from the reality of things.  With that said, as the tests included five different samples from each group, all of which came from the same alloy, furnace, and mould, and all of which were cast in the same session, there are virtually no other variables left to account for.  With that being the case, I’m confident these results, while surprising, are accurate.

Summary

So how much DOES quenching increase cast bullet hardness?  I’m loathe to assign a percentage to it overall since these results apply only to my own wheel-weight alloy, and it’s certainly possible that some metals are more affected by quenching than others.  With that being the case, it’s entirely possible that you could see more, or less, depending on what kind of alloy you’re using, but I think it’s safe to say quenching makes a major difference in bullet hardness, at least initially.

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shinypartsup
shinypartsup

I’m surprised the difference was that great. I’d be interested to see if other casters repeat the test and get even remotely similar results. I thought Fortune Cookie LC45 did something similar, but I may be mistaken.