Hardness can be described as the ability of a
material to resist permanent or plastic deformation, and is usually measured by
its resistance to indentation by an indenter of a standard shape and size.
The hardness test is by far one of the most valuable
and the most widely used mechanical test for evaluating the properties of metals
as well as certain other materials. In general, an indenter is pressed into the
surface of the metal to be tested under a specific load for a definite time
interval, and a measurement is made of the size or depth of the indentation.
The main purpose of the hardness test is to
determine the suitability of a material, or the particular treatment to which
the material has been subjected to Hardness testing may be used alone or to complement
other test methods.
This is what makes the hardness method so popular
because of the relationship that exists between hardness and other properties
of the material.
For instance, both the hardness test and the tension
test measure the resistance of a metal to plastic flow.
Such correlations are approximate and must be used
with caution when applied to welded joints or any metal with a heterogeneous
structure.
It should be noted that hardness is not a
fundamental property of a material and a hardness value is an arbitrary number.
There are no absolute standards of hardness and it has no quantitative value,
except in terms of a given load applied in a specified manner for a specified
duration and a specified penetrator shape.
Measurements of hardness can provide information about
the metallurgical changes caused by welding. For example, in alloy steels a high
hardness could indicate the presence of untempered martensite in the weld heat
affected zone, while a low hardness may indicate an over tempered condition. In
cold-worked or age-hardened metal, welding may result in significantly lower
heat-affected zone hardness due to recrystallization or over aging.
Hardness testing is divided into two categories:
Macro hardness and micro hardness
The hardness testing methods in use today for testing
metals are:
Brinell
Rockwell
Vickers
Knoop
Comments
Post a Comment