©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A 1.4.0.1
However, if the lattice contains two
or more types of atoms, as in any alloy-steel,
it may start to melt at one temperature
but not be completely
molten until it has been heated to
a higher temperature (See Figure 2). This creates
a situation where there is a combination
of liquids and solids within a range
of temperatures. 1.4.0.2
Each metal has a characteristic
crystal structure that forms during
solidification and often remains the
permanent form
of the material as long as it remains at room
temperature. However, some metals may
undergo an alteration in the crystalline form
as the temperature is changed. This is known as phase transformation. For
example, pure
iron solidifies at 2795°F, the delta structure transforms into a structure
referred to as gamma
iron. Gamma iron is commonly known as austenite and is a nonmagnetic
structure. At a temperature of
1670°F., the pure iron structure transforms back to the delta
iron form, but at this temperature,
the metal is known as alpha iron. These two phases are
given different names to differentiate
between the high temperature phase (delta) and the low
temperature phase (alpha). The capability of the atoms of a material to
transform into two
or more crystalline structures at different temperatures is defined as allotropic.
Steels and
iron are allotropic metals. 1.4.1
Grains
and Grain
Boundaries -
As the metal is cooled to its freezing point, a small
group of atoms begin to assemble into crystalline form (refer to Figure 3). These
small crystals scattered throughout
the body of the liquid are oriented in all directions and
as solidification continues, more crystals
are formed from the surrounding liquid. Often, they
take the form of dendrites, or a treelike structure. As crystallization
continues, the crystals
begin to touch one another, their free growth hampered, and the remaining liquid
freezes to the adjacent crystals until
solidification is complete. The solid is now composed
of individual crystals that usually
meet at different orientations. Where these crystals meet
is called a grain boundary.
1.4.1.2 A
number of conditions influence the initial grain size. It is important to
know that cooling
rate and temperature has an important influence on the newly solidified grain
structure and grain size. To
illustrate differences in grain formation, let's look at the cooling
phases in a weld.
FIGURE 2 TIME
Liquid
Liquid
and Solid
Solid
Solid-Liquid
Transformation, Alloy Metal
