Hardness testing

Subject : Seal face hardness testing 12.8

The ideal seal face combination is a good grade of carbon/graphite running against a corrosion resistance hard face materials. In another paper I covered the different grades of carbon/graphite, but just what do we mean when we say, "hard face"? There are a lot of them available that include:

Several grades of ceramic along with different grades of ni-resist and stellite
Both tungsten and nickel base tungsten carbide
Alpha sintered and reaction bonded silicone carbide

In the following paragraphs we will be looking at the common methods seal people use to measure the hardness of a seal face and then we will be looking at a chart to learn how to convert from one method to another.

The first method we will look at is called the Brinell test method

In this test a hardened steel ball is forced into the material at a given pressure. The width of the depression then becomes the measure of the material's hardness.

Because the ball deforms on very hard surfaces, this test is somewhat limited in its use.

The Moh's scale is the next one, but I have no illustration to show you. This method compares scratch hardness with ten minerals used as standards. Unlike the other scales mentioned above; in this method the steps are not equal. The difference between #9 and #10 is about as great as the difference between #1 and #9.

The oldest method of testing hardness was to use a hard file on the test piece and see how difficult it is to remove material. One look at the following comparisons and you can learn why this method is seldom used any more, but if you do not have any test equipment it is better than nothing.

Above a number of 350 the standard machining operations of turning, boring, drilling, and tapping become uneconomical. You can compare a Brinell reading to the following file readings :

100 Metal removed easily by the file.
200 Slightly more pressure needed to remove metal.
300 Metal shows resistance to the file.
400 Takes more pressure on the file.
500 File removes almost no metal.
600 Metal cannot be filed.

The next method is called the Rockwell hardness test.

This is the most widely used test in the seal business. Hardness is read on two different scales.

The most popular is the "C" scale that uses a diamond cone. The less popular "B" scale utilizes a ball similar to that used in the Brinell test.

Mechanical seal faces should read at least 60 on the Rockwell "C" scale. You can consult the following chart to convert this reading to other scales.

Rockwell C	Brinell Scale	Scleroscope	Moh
72	772	106
70	760	102	8.5
69	755	98
67	725	94
66	712	93
65	699	92
63	672	89	8.0
62	657	86
61	645	85
60	631	84
59	617
58	603	81
57	590
56	577	78	7.5
55	562	75
53	536	73
52	523
51	510	71
49	486
48	473	66
47	462	64	7.0

The last illustration describes the Scleroscope test.

Although widely used in industry we seldom find this method used in the seal business. In this test we let a weight with a hard round end fall ten inches (255 mm) through a glass tube. We get our reading from determining how high the weight bounced off the test sample. The harder the material, the higher the bounce.

The tube on the scale is marked in 140 increments. On this scale glass would read 130 and hardened steel would record about 110.

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