Hardness Testing

In this, the third and final article on mechanical testing, we will look at Hardness Testing as it applies to metals and welds.

Hardness testing is a common method of testing metals. It is quite easily and economically performed, and there is a clear affiliation between hardness and some other mechanical properties of steel. High hardness usually means high tensile strength with possibly lower ductility and conversely low hardness equates to low strength. 

When we look at how hardness pertains to welding, it is important as this property impacts the susceptibility of certain materials to problems such as hydrogen induced cold cracking (HICC) and stress corrosion cracking (SCC). In certain cases, when designing weld procedures, weld zone hardness is often specified not to exceed a certain critical value.

There are three main types of harness testing methods for metals, we will be looking at and explaining each one briefly.

1. Brinell Hardness Test

2. Rockwell Hardness Test

3. Vickers Hardness Test

In general, hardness testing is an indentation test whereby a penetrator of a given material, shape and size is forced into the surface of the material being tested by a given load. The degree of hardness is indicated by the depth penetrated or by the area of resulting impression. The Brinell, Vickers and Rockwell hardness testers all operate on this principle.

Brinell Hardness Test

The Brinell tester commonly consists of a vertically mounted hydraulic cylinder, which is used to force a ball into the surface of the metal to be tested. For steel applications, the ball is 10 mm (0.394 in) in diameter, the force applied is 3000 kg (6600 lb.) and the force is maintained for 30 seconds. The diameter of the impression is read and the Brinell number can be calculated from this. The Brinell hardness value is designated by the letters HB (Hardness Brinell) Figure 1 illustrates a typical Brinell hardness tester.

Figure1, The Brinell Hardness tester in action.

Figure 2 shows the schematic of the actual application of the ball being pressed into the material.

Figure 2, Brinell hardness test schematic.

The size of the indicator, however, limits this method when trying to test certain narrow regions in a sample such as the heat affected zone (HAZ) of the weld. The Brinell impression, being large, can only be used for obtaining hardness values over a relatively large area such as the face of a weld or on cast or wrought materials.

Rockwell Hardness Test

The Rockwell test determines the hardness of a material by measuring the depth of penetration of an indenter under a heavy load compared to the penetration made by a preload method. The Rockwell test has different scales, which are denoted by a single letter, and these use different loads or indenters. Impressions are expressed by HRx, where x represents the scale (e.g., HRC 62 is Rockwell C scale 62). 

The Rockwell hardness test uses both a diamond cone (for the hardest materials) and various sizes of steel balls for medium to hard materials. The two most commonly used in testing welds are Rockwell C and the Rockwell B. In the Rockwell C approach, a major load of 150 kg is used and the indenter is a conical shaped diamond. In the Rockwell B, the major load is 100 kg and a 1.5 mm steel ball is used for the indenter. 

A Rockwell test is made by elevating a specimen against the indenter until a minor load is applied. For B and C scales this is 10 kg. When this load is applied, the dial indicator is adjusted to a “set” position. The major load is then applied, and the loading rate and time of application are governed by the machine. On completion of the application, the major load is removed, and the hardness number is read directly from a dial indicator. 

Since there is no need to measure the indentation, this method delivers immediate results. However, the large load applied does not permit this method to be used for thin test specimens. Therefore, a typical rule is that the test piece must be approximately 10 times the depth of the indentation. Figure 3 shows a typical Rockwell Hardness testing equipment.

Figure 3, Rockwell hardness testing equipment.

Vickers Hardness Test

The Vickers hardness test is widely used for testing welds because the size of the indentation is small and it is easy to locate in specific regions of the weld such as weld metal, HAZ and base metal. 

The indenter used is a square-based diamond pyramid which is brought to within 1 mm of the indenter and the loading mechanism is tripped. The load is usually in the range 5–100 kg. 

When viewed through a microscope, the impression in the specimen appears as a dark square. The diagonals of this square are measured to give a visual reading and from this the hardness number can be calculated. Again, there are tables available that make calculation unnecessary. 

The unit of hardness given by the test is known as the Vickers Pyramid Number (HV). The appearance of the indentation in the microscope is illustrated in Figure 4, which shows how the diagonal (d1 & d2)  is measured.

Figure 4, View of indentation under the microscope and method of measuring the width.

Vickers hardness numbers are reported as xxxHVyy, the first digits “xxx” represent the hardness number, HV gives the hardness test method (Vickers) the last digits “yy” give the load used in kg. For example, 440HV30, means 440 is the hardness number, HV means the hardness test method is HV (Vickers) and the number 30 means, 30 kg load was used.

As noted, the Vickers method is widely used for testing welds because the indenter is smaller than the other methods and it uses a single applied load. When performing Vickers hardness testing, measurements are made in various regions of the weld that can provide required information. For example, the hardness of the heat affected zone (HAZ) which is the parent metal just adjacent to the weld fusion line might, indicate how rapidly the weld has cooled and whether it might be prone to cracking. Selecting a welding procedure that keeps the HAZ hardness below a certain level is a common method for reducing the probability of hydrogen induced cold cracking (HICC).

A comparison between the weld metal hardness and the parent material can give a rough guide as to whether the weld metal overmatches the parent material in strength. 

When performing a hardness test on a weld, information from several regions traversing the weld are obtained. Figure 5, shows a typical test pattern of indentations in a groove weld through the base metal, HAZ and weld bead.

Figure 5, Typical results from hardness test of a groove weld.

Figure 6 shows the detailed pattern for measuring the hardness of a fillet weld geometry.

Figure 6, Typical pattern of hardness test of fillet weld.

In conclusion Table 1 shows the difference between the hardness test methods discussed and their application, if any, to welds

Table 1, Difference between the hardness test methods.