Adventures in Lead Hardness Testing

There has been quite a bit of discussion on various forums about the effectiveness of inexpensive lead hardness testers and their accuracy. Lance Shay published a work at the Los Angeles Silhouette Club site which evaluated the accuracy and precision of the various testers available to hobbyists. My take away from the article was that while there are some testers that are relatively precise, none are particularly accurate.

I read with interest the method used to do the actual laboratory hardness testing which was used as the "standard" in the tests. Essentially they used a Rockwell Hardness tester to test the sample and report it's value. This article seeks to evaluate the test method using an ordinary Rockwell Hardness tester and report the findings as an accurate method to determine lead alloy hardness.

     

I won't bore you too much with the math behind the method. However, it is important to understand what is going on behind the scenes to understand why this is an accurate method.

The formula at the right shows the standard Brinell formula used to calculate hardness. A true Brinell tester uses a 10mm steel ball and 3000 KgF to conduct the test. However, in looking at the formula, it occurred to me that an ordinary Rockwell Hardness tester should yield good results. The only variables to calculate the BHN number are the KgF used, the diameter in mm of the indenter, and the diameter in mm of the test indentation. Since a Rockwell tester uses standard fixed weights to supply the force, and the ball can be measured with a micrometer, all that remains is to measure the diameter of the test indentations.

 
     
Armed with that knowledge, I dug out an ingot that was cast from range scrap and milled it flat on both sides to give an accurate reading in the tester. The Rockwell Tester relies on having a flat, parallel surface on the item being tested. Since lead is very soft, it took no time at all to prepare an ingot for evaluation. This also served to remove any surface hardness which may have resulted from aging of the alloy. The alloy in question was cast several months ago. Lab analysis revealed that the sample contained 1.8% Antimony and .12% Tin with the balance being Lead and trace elements.  
     

The Rockwell tester I used was in good order and is calibrated. It is a TT type twin tester which can be used to test hardness in thick materials as well as sheet metals.

At can be seen in the photo at the right, the milled ingot is placed on the testing table under the indenter. I happened to have a 12.70 mm steel ball indenter which I used for the test. I presume it was used on a true Brinell tester but can't be sure. In the test done by Shay in the LASC article cited above, their tester used a 1/16" steel ball and a 15 KgF force.

I reasoned that the closer you got to the true Brinell test, the more accurate the test is likely to be. On this test, I used 100 KgF of force and the 12.7 mm steel ball. I would have used the 150 KgF force which is the limit of the tester, but, it was too much force for the alloy being tested and moved the Tester's needle outside it's range of movement.

 
     

On a twin tester, as with all testers, the Force used for the test is accomplished by adding or removing stacked iron weights at the rear of the tester.

A twin tester can conduct both superficial and normal tests. Weights are graduated in 15, 30, 45, 60, 100 and 150 KgF. The test most are familiar with is the Rockwell C test which uses a 120 degree conical diamond indenter and the 150 KgF weights. The C test is used primarily to test hardened steel (and is the reason I have my tester in the first place).

 

 
     

To conduct the test, the platform is raised with the sample until the sample comes in contact with the indenter. The platform is further raised until the small arrow is vertical which indicates that the minor load of 10 KgF has been properly applied.

Next, a lever is tripped which releases the major load of 150 KgF and the indenter sinks into the sample. Since lead is very soft, I experimented with various forces to find the one which gave a good range of movement on the dial and didn't cause the dial indicator to "bottom out".

For normal testing of steel and other materials, the resulting hardness value can be read directly from the dial. However, since we are doing a Brinell test, and recalling the formula above, we can't use the dial for any of the direct readings and must measure them by hand. This is the beauty of the Brinell test and also the simple fact that allows us to use a rockwell tester to test lead.

Essentially, we are only using the tester to accurately apply the "F" to the lead. This is done very accurately since the tester has calibrated weights and uses the force of gravity to drop that weight the same way every time.

It is my opinion that this is where regular testers fall short. All rely on a spring or force be applied with a hammer which depending on the user are likely to be inaccurate. While one can argue about how accurate is accurate enough, the work cited above reveals that none are really very accurate or precise.

The major load was applied for 60 seconds which was the amount of time that was required for the needle on the dial to stop moving.

 
     

The results of the test can be seen at right. All that remained was to measure the diameter of the resulting test dimples to provide the value Di in the formula above.

I tested the sample 3 times and used a 10X loupe and a pair of electronic calipers to measure the test spots accurately directly in mm. All were essentially the same size, varying by only +/- .03 mm. The measurements were averaged and the 3.40 mm value was used to calculate the hardness number.

Using the formula above, the BHN number was calculated to be 10.81. While calculating the value was not hard, it was much easier by using the calculator found online here.

 
     

I experimented with using different forces of 60 and even 45 KgF, but all the results came out to be between 10.5 and 10.9. I also tested pure lead purchased from Rotometals and found the hardness was calculated at 4.6 BHN. Widely reported values for the hardness of pure lead is 5. The Rotometals sample was assumed to be pure, but could contain some impurities which affected the test value. Further, the test was carried out on the surface of the unmilled ingot and likely did impact the accuracy.

So, what does this all mean? I purchased a Saeco hardness tester which uses a steel needle and spring to measure hardness directly. The arbitrary reading is converted from the chart and a resulting BHN is supplied. When testing bullets cast from the same alloy that I tested, the Saeco gave readings of 14 which is quite a bit harder than the actual hardness.

The purpose of this study was to evaluate the effectiveness of using a Rockwell Type hardness tester to conduct a true Brinell measurement and compare to other test methods. It is my conclusion that based on thelimited tests conducted herethat the method works, and that I found that my Saeco tester read high which was similar to the results of the LASC experiment.