©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A
are
index numbers. The 77 and 79 refer to the year that the standards for these
steels were originally
adopted or the date of their latest revision. 1.3.0.5
The ASTM designation may be further
subdivided into Grades or Classes. Since many
standards for ferrous metals are written to cover forms of steel (i.e., sheet,
bar, plate, etc.)
or particular products fabricated from steel (i.e., steel rail, pipe, chain, etc.),
the user may select
from a number of different types of steel under the same classification. The
different types are than placed under
grades or classes as a way of indicating the differences
in such things as chemistries, properties, heat treatment, etc. An example
of a full designation
is A285-78 Grade A or A485-79 Class 70. 1.3.0.6
The American Society of Mechanical
Engineers (ASME) maintains a widely used ASME
Boiler and Pressure Vessel Code. The material specification as adopted by
the ASME is identified
with a prefix letter “S”, while the remainder
is identical with ASTM with the
exception that the date of adoption or revision by ASTM is not shown. Therefore,
a common example
of an ASME classification is SA 387 Grade 11, Class 1. 1.4
CRYSTALLINE
STRUCTURE OF METALS When
a liquid metal is cooled, its atoms will assemble into a regular crystal pattern
and we say the
liquid has solidified or crystallized. All metals solidify as a crystalline
material. In a crystal
the atoms or molecules are held in a fixed position and are not free to move about
as are the molecules of a liquid or
gas. This fixed position is called a crystal lattice. As the
temperature of a crystal is raised,
more thermal energy is absorbed by the atoms or molecules
and their movement increases. As the distance between
the atoms increases, the lattice breaks down and the
crystal melts. If a lattice contains only one type of atom,
as in pure iron, the conditions are
the same at all points throughout
the lattice, and the crystal melts at a single temperature
(see Figure 1). FIGURE
1 4000
3000 2000
1000 TIME
SOLID-LIQUID TRANSFORMATION, PURE IRON
LIQUID 2795°F
SOLID
