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:

In the following paragraphs we'll 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 :

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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