©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A1.6.8
Creep
Strength - If a load below a metal's tensile strength
is applied at room temperature
(72°F), it will cause some initial elongation, but there will be no further
measur- able elongation
if the load is kept at a constant level. If that same load were applied
to a metal heated
to a high temperature, the situation would change. Although the load is
held at a constant
level, the metal will gradually continue to elongate. This characteristic
is called creep.
Eventually, the material may rupture depending on the temperature of the
metal, the degree of load applied and
the length of time that it is applied. All three of these
factors determine a metal's ability
to resist creep, and therefore, its creep strength. 1.6.9
Oxidation
Resistance - The atoms of metal have a tendency
to unite with oxy- gen
in the air to form oxide compounds, the most visible being rust and scale. In
some metals, these
oxides will adhere very tightly to the skin of the metal and effectively seal
it from further
oxidation as is evident in stainless steel. These materials have high oxidation
resistance. In other metals,
the bond is very loose, creating a situation where the oxides
will flake off, and the metal gradually
deteriorates as the time of exposure is extended. 1.6.10
Hardness
Test - The resistance of a metal to indentation
is a measure of its hardness
and an indication of the materials's strength. To test for hardness, a fixed
load forces an
indenter into the test material (Figure 7). The depth of the penetration
or the size of
the impression is measured. The measurement is converted into a hardness
number through
the use of a variety of established tables. The most common tables are the
Brinell, Vickers,
Knoop and Rockwell. The Rockwell is further divided into different scales,
and FIGURE 7
HARDNESS TEST
SHAPE
OF INDENTER INDENTER
DESCRIPTION ROCKWELL
A Diamond
C Cone
D B
1/16 in. Diameter
F Steel
Sphere G
1/8 in. Diameter
E Steel
Sphere 10 mm Sphere
of Steel BRINNELL
or Tungsten Carbide
VICKERS Diamond
Pyramid KNOOP
Diamond Pyramid
} }
Types
of Indenters - Hardness Tests
