Testing the Hardness of Metals

"Machinery", which I guess was a magazine, was published by The Industrial Press, New York; it was subtitled "The Open Window to the Machinery Industry".

The 1924 edition of Machinery's Handbook has 1592 pages. The copyright has expired.

DISCLAIMER: DON'T WRITE FOR ADVICE ON THIS STUFF. I DON'T HAVE ANY. -- I just had the book and figured that some of the material in it would be useful to a far wider audience than would see it on my bookshelf.

Turner's Sclerometer. -- In this form of test a weighted diamond point is drawn, once forward and once backward, over the smooth surface of the material to be tested. The hardness number is the weight in grams required to produce a standard scratch. The scratch selected is one which is just visible to the naked eye as a dark line on a bright reflecting surface. It is also the scratch which can just be felt with the edge of a quill when the latter is drawn over the smooth surface at right angles to a series of such scratches produced by regularly increasing weights.

Shore's Scleroscope. -- In this instrument, a small cylinder of steel, with a hardened point, is allowed to fall upon the smooth surface of the metal to be tested, and the height of the rebound of the hammer is taken as the measure of hardness. The hammer weighs about 40 grains, the height of the rebound of hardened steel is in the neighborhood of 100 on the scale, or about 6-1/4 inches, while the total fall is about 10 inches or 254 millimeters.

Brinell's Test. -- In this method, a hardened steel ball is pressed into the smooth surface of the metal so as to make an indentation of a size such as can be conveniently measured under the microscope. The spherical area of the indentation being calculated and the pressure being known, the stress per unit of area when the ball comes to rest is calculated, and the hardness number obtained. Within certain limits, the value obtained is independent of the size of the ball and of the amount of pressure.

Keep's Test. -- In this form of apparatus a standard steel drill is caused to make a definite number of revolutions while it is pressed with standard force against the specimen to be tested. The hardness is automatically recorded on a diagram on which a dead soft material gives a horizontal line, while a material as hard as the drill itself gives a vertical line, intermediate hardness being represented by the corresponding angle between 0 and 90 degrees.

Comparison between Testing Methods. -- Each form of test has its advantages and its limitations. The sclerometer is cheap, portable, and easily applied, but it is not applicable to materials which do not possess a fairly smooth reflecting surface and the standard scratch is only definitely recognized after some experience. The Short test is simple, rapid and definite for materials for which it is suited, but results obtained vary somewhat with the size and thickness of the sample. As a comparative measure of the hardness of material of the same quality and structure, however, it is quite accurate, but it is not reliable for comparing the hardness of two different metals. The Brinell test is especially useful for constructive materials. It is definite, and, with the new appliances recently brought out, easily applied. It cannot be applied, however, to very brittle materials, such as glass, nor is it satisfactory for use on hardened high-carbon steel. Keep's test is especially suited for castings of all kinds, as it records not only the surface hardness, but also the hardness of the whole thickness, and gives indications of blow-holes, hard streaks and spongy places. Obviously, it cannot be applied to materials too hard to be conveniently drilled by a hardened steel drill.

The accompanying table gives values obtained on the same materials by the scleroscope, sclerometer, and the Brinell test, the figures being reduced to a common unit, assumed as 1 as a starting point; thus the actual Brinell numerals have been divided by 6, thereby reducing the hardness values for purposes of comparison.

1st column:  Diameter of Impression, mm.
2nd column:  Hardness Numeral for 3000 kg pressure
3rd column:  Hardness Numeral for 500 kg pressure

Dia.    Pressure     Dia.    Pressure    Dia.    Pressure      Dia.    Pressure      Dia.    Pressure      
 in   3000   500      in   3000   500     in    3000   500      in    3000   500      in    3000   500
 mm     kg    kg      mm     kg   kg      mm     kg     kg      mm     kg     kg      mm     kg     kg
----   ---   ---     ----   ---   --     ----   ---   ----     ----   ---   ----     ----   ---   ----
2.00   946   158     3.00   418   70     4.00   228   38.0     5.00   143   23.8     6.00   95    15.9
2.05   898   150     3.05   402   67     4.05   223   37.0     5.05   140   23.3     6.05   94    15.6
2.10   857   143     3.10   387   65     4.10   217   36.0     5.10   137   22.8     6.10   92    15.3
2.15   817   136     3.15   375   63     4.15   212   35.0     5.15   134   22.3     6.15   90    15.1
2.20   782   130     3.20   364   61     4.20   207   34.5     5.20   131   21.8     6.20   89    14.8
2.25   744   124     3.25   351   59     4.25   202   33.6     5.25   128   21.5     6.25   87    14.5
2.30   713   119     3.30   340   57     4.30   196   32.6     5.30   126   21.0     6.30   86    14.3
2.35   683   114     3.35   332   55     4.35   192   32.0     5.35   124   20.6     6.35   84    14.0
2.40   652   109     3.40   321   54     4.40   187   31.2     5.40   121   20.1     6.40   82    13.8
2.45   627   105     3.45   311   52     4.45   183   30.4     5.45   118   19.7     6.45   81    13.5
2.50   600   100     3.50   302   50     4.50   179   29.7     5.50   116   19.3     6.50   80    13.3
2.55   578    96     3.55   293   49     4.55   174   29.1     5.55   114   19.0     6.55   79    13.1
2.60   555    93     3.60   286   48     4.60   170   28.4     5.60   112   18.3     6.60   77    12.8
2.65   532    89     3.65   277   46     4.65   166   27.8     5.65   109   18.2     6.65   76    12.6
2.70   512    86     3.70   269   45     4.70   163   27.2     5.70   107   17.8     6.70   74    12.4
2.75   495    83     3.75   262   44     4.75   159   26.5     5.75   105   17.5     6.75   73    12.2
2.80   477    80     3.80   255   43     4.80   156   25.9     5.80   103   17.2     6.80   71.5  11.9
2.85   460    77     3.85   248   41     4.85   153   25.4     5.85   101   16.9     6.85   70    11.7
2.90   444    74     3.90   241   40     4.90   149   24.9     5.90    99   16.6     6.90   69    11.5
2.95   430    73     3.95   235   39     4.95   146   24.4     5.95    97   16.2     6.95   68    11.3

Application of the Brinell Method. -- The Brinell method, as mentioned, consist in partly forcing a hardened steel ball into the sample to be tested so as to effect a slight spherical impression. The diameter of the impression is measured and the surface of the spherical cavity calculated. The pressure required in kilograms for effecting the impression is divided by the area of the impression in square millimeters; the quotient is an expression of the hardness of the material tested, and is called the hardness numeral. The standard diameter of the ball is 10 millimeters (0.3937 inch) and the pressure, 3000 kilograms (6614 pounds) in the case of iron and steel, while in the case of softer metals, a pressure of 500 kilograms (1102 pounds) is used. The diameter of the impression in the original instrument is measured by means of a microscope, after which the hardness numeral may be obtained without calculation directly from the table of "Hardness Numerals -- Brinell System". Instruments have been constructed later so as to eliminate the need of the use of a microscope for measuring the diameter of the impression.

Relation between Hardness of Materials and Ultimate Strength. -- A constant relationship exists between the hardness numeral as determined by the Brinell test and the ultimate strength of the material tested. The coefficients by which the hardness numerals must be multiplied to obtain the ultimate strength in kilograms per square millimeter may be determined by tests, and are constant for each class and kind of material, but they differ slightly for different materials and for materials treated in a different manner. The following coefficients are given for different grades of steel:

Steels, extra soft               K = 0.360
Steels, soft and semi-hard       K = 0.355
Steels, semi-hard                K = 0.353
Steels, hard                     K = 0.349

Accuracy of Brinell Hardness Test. -- When commercial apparatus, as ordinarily used for making the Brinell test, is employed, and the test is carried out with ordinary care and precaution, it is reliable within an error of five Brinell units above or below the actual hardness. In other words, if the hardness of two pieces of metal is tested, and the difference on the Brinell scale is more than ten hardness units, it is certain that there is an absolute difference in the hardness of the pieces tested. With regard to the conditions under which the tests should be made, it may be stated that the pressure should be gradually applied for two minutes or more, and the pressure should be kept on the test piece for a period of at least five minutes.

Relation between Hardness and Wear of Steel. -- There is no definite relation between hardness, as measured by the Brinell hardness testing method, and wear. While, in general, a high Brinell hardness number may be expected to indicate a metal which will give better wear, there are so many exceptions that this test for indication wearing properties would be unreliable. As an example, Hadfield's manganese steel,which has a low Brinell hardness number, is one of the best steels as far as wear is concerned. The relation of either Brinell tests of ordinary wear tests to wear in actual practice is a subject which requires further investigation. Wear tests should be made along different lines, according to the actual uses to which the metal is to be put.

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