Hardness Testing

HARDNESS TESTING AND AGE HARDENING

1.  OBJECTIVE:

Our primary aim is to measure the Rockwell Hardness values for different materials and estimate ultimate tensile strengths by the aid of conversion tables. We will also focus on age hardening, the info on which can be found in the last part of this experiment sheet.

2.  HARDNESS TESTING THEORY:

Hardness is usually defined as the resistance of a material to plastic penetration of its surface. There are three main types of tests used to determine hardness:

• Scratch tests are the simplest form of hardness tests. In this test, various materials are rated on their ability to scratch one another. Mohs hardness test is of this type. This test is used mainly in mineralogy.
• In Dynamic Hardness tests, an object of standard mass and dimensions is bounced back from a surface after falling by its own weight. The height of the rebound is indicated. Shore hardness is measured by this method.
• Static Indentation tests are based on the relation of indentation of the specimen by a penetrator under a given load. The relationship of total test force to the area or depth of indentation provides a measure of hardness. The Rockwell, Brinell, Krioop, Vickers, and ultrasonic hardness tests are of this type.

 

For engineering purposes, only the static indentation tests are used.

BRINELL HARDNESS TEST:

This test consists of applying a constant load, usually between 500 and 3000 kgf for a specified time (10 to 30 s) using a 5- or 10-mm diameter hardened steel or tungsten carbide ball on the flat surface of a workpiece.

Figure 1.  Brinell Hardness Test Schematic

Hardness is determined by taking the mean diameter of the indentation and calculating the Brinell hardness number (BHM or HB) by dividing the applied load by the surface area of the indentation according to following formula :   

where P is load in kg; D ball diameter in mm; and d is the diameter of the indentation in mm.

Calculations have already been made and are available in tabular form for various combinations of diameters of impressions and load.

The Brinell hardness number followed by the symbol HB without any suffix numbers denotes standard test conditions using a ball of 10 mm diameter and a load of 3,000 kg applied for 10 to 15 s. For other conditions, the hardness number and symbol HB are supplemented by numbers indicating the test conditions in the following order: diameter of ball, load, and duration of loading.

For example, 75 HB 10/500/30 indicates a Brinell hardness of 75 measured with a ball of 10 mm diameter and a load of 500 kg applied for 30s.

However, the BHN is not a satisfactory physical concept since the above equation does not give the mean pressure over the surface of the indentation. Meyer suggested that a more rational definition of hardness than that proposed by Brinell, would be one based on the projected area of the impression rather than the surface area. The mean pressure between surface of the indenter and the indentation is equal to the load divided by the projected area of the indentation. Meyer proposed that this mean pressure should be taken as the measure of hardness. It is referred to as the Meyer hardness.

VICKERS HARDNESS TEST:

The Vickers hardness test uses a square base diamond pyramid as the indenter. The included angle between the opposite faces of the pyramid is l36°. The Vickers hardness tester operates on the same basic principle as the Brinell tester, the numbers being expressed in the terms of load and area of the impression. As a result of the indenter’s shape, the impression on the surface of the specimen will be a square. The length of the diagonal of the square is measured through a microscope fitted with an ocular micrometer that contains movable knife-edges. The Vickers hardness values are calculated by the formula:

where P is the applied load in kg, and d is the diagonal length in mm.

Microhardness Test:

This term, unfortunately, is misleading, as it could refer to the testing of small hardness values when it actually means the use of small indentations. Test loads are between 1 and 1,000 g. Two types of indenters are used for Microhardness testing: the 136° square-base Vickers diamond pyramid described previously, and the elongated Knoop diamond indenter.

Figure 2.  Vickers Hardness Testing Schematic

ROCKWELL HARDNESS TEST:

This hardness test uses a direct reading instrument based on the principle of differential depth measurement. Rockwell testing differs from Brinell testing in that the Rockwell hardness number is based on an inverse relationship to the measurement of the additional depth to which an indenter is forced by a heavy (major) load beyond the depth resulting from a previously applied (minor) load. Initially a minor load is applied, and a zero datum position is established. The major load is then applied for a specified period and removed, leaving the minor load applied. The resulting Rockwell number represents the difference in depth from zero datum position as a result of the application of major load. The entire procedure requires only 5 to 10 s.

Use of a minor load greatly increases the accuracy of this type of test, because it eliminates the effects of backlash in the measuring system and causes the indenter to break through slight surface roughness.

The 1200 sphero-conical diamond indenter is used mainly for testing hard materials such as hardened steels and cemented carbides. Hardened steel ball indenters with diameters 1/16, 1/8, 1/4, 1/2 in. are used for testing softer materials such as fully annealed steels, softer grades of cast irons, and a wide variety of nonferrous metals.

In Rockwell testing, the minor load is 10 kgf, and the major load is 60, 100 or 150 kgf. In superficial Rockwell testing, the minor load is 3 kgf, and major loads are 15, 30 or 45 kgf. In both tests, the indenter may be either a diamond cone or steel ball, depending principally on the characteristics of the material being tested.

1. Depth of indentation under preliminary load (10 kg)
2. Increase in depth of indentation under additional load (140 kg)
3. Permanent increase of depth of indentation under preliminary load after removal of  additional load, the increase being expressed in units of 0002 mm
4. Rockwell hardness HRC = 100—e

Figure 3.  Rockwell Hardness Tesing Schematic

There are 30 different Rockwell scales, defined by the combination of the indenter and minor and major loads. The majority of applications are covered by the Rockwell C and B scales for testing steel, brass, and other materials.

 

TEST LOCATION:
           
If indentation is placed too close to the edge of specimen, the workpiece edge will bulge, and the hardness number will decrease accordingly. To ensure an accurate test, the distance from the center of the indentation to the edge of the specimen must be at least two and one-half diameters.

An indentation hardness test cold works the surrounding material. If another indentation is placed within this cold worked area, the reading usually will be higher than the real value. Generally, the softer the material, the more critical the spacing of indentations becomes. However, a distance three diameters from the center of one indentation to another is sufficient for most materials.

HARDNESS TESTING IN ESTIMATING OTHER MATERIAL PROPERTIES:

Hardness testing has always appeared attractive as a means of estimating other mechanical properties of metals. There is an empirical relation between those properties for most steels as follows:

  (in kg/mm2)

This equation is used to predict tensile strength of steels by means of hardness measurement. A reasonable prediction of ultimate tensile strength may also be obtained using the relation:

where VHN is the Vickers Hardness number and n is the Meyer’s index.

The 0.2 percent offset yield strength can be determined with good precision from Vickers hardness number according to the relation: (Hint: For steels, the yield strength can generally be taken as 80% of the UTS as an approximation)

3.  AGE HARDENING

Many Aluminum based alloys, certain steels, nickel-based superalloys and titanium alloys are strengthened by a heat treatment process known as age hardening. The first step in the precipitation hardening process is to solution treat the alloy to a temperature T1 where only the primary phase a exists and then hold the sample at that temperature until all the q (secondary) phase dissolves in the primary phase. The alloy is then rapidly cooled (quenched) to a low temperature. The precipitation of the q -phase is thus prevented because the speed of formation of q - phase particles cannot reach the speed of the rapid cooling. The aging process which follows shortly thereafter can either be applied at room temperature or at an elevated temperature T2 which is still in the two-phase region. During aging, the intermediate phase “trapped” in the primary a phase continously transforms into small precipitates of the secondary q -phase through diffusion mechanisms. The precipitate clusters formed in this way tend to strengthen the material.

Figure 4.  Age Hardening Process (image from www.farmingdale.edu)

In the early stages of aging, a large number of very small clusters of the secondary q -phase are formed. As the composition reaches its saturated normal state, the material reaches its maximum hardness. The precipitates, however, continue to grow. The fine precipitates disappear. They have grown larger, and as a result the tensile strength of the material decreases. This is called overaging.

It must be noted that the age hardening process can only be applied on materials which exhibit decreasing solubility with decreasing temperature. 

 

Figure 5.  Hardness and Tensile Strength with increasing time in age-hardened alloys (image from www.farmingdale.edu)

4. TASKS :

1.         Find Hardness Conversion Tables from the American Society for Mechanical
Engineers (ASME) Handbooks located in the References Section of the Library.

Convert following HRC's to HB and HV values using the tables : 40, 57, 62
   
2.         Using the above converted values, calculate the UTS and YS values using the empirical formulas given in the laboratory sheets. See the hint that is given prior to the formula for YS0.2 .

3.         How would you measure the hardness of an unknown alloy ?

!!! An additional task concerning age hardening will be given to you at the end of the laboratory session !!!

 

 

 

 

REFERENCES:

• ASM Metals’ Handbook
• Mechanical Testing of Materials, A. J. Fenner, Philosophical Library Inc., 1965
• Pollack, H. W., Materials Science and Metallurgy, Reston Pub. Co., Reston:1981
• Van Vlack, L. H., Materials Science for Engineers, Addison Wesly Pub. Co., Mass:1970
• Standards:

Conversion : ASTM E 140: 1956
Brinnell : DIN 50351, ASTM E 10-73, BS 240: Parti: 1961, TS 189
Vickers: DIN 50133, ASTM E 92-72, BS 427: Part 1: 1961, TS 207

Rockwell: DIN50103, ASTM E l8-74, BS 4175: Part 1: 1967, TS 14O