©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A rate
of the metal. The preheat temperature may vary from 150°F to 1000°F,
but more commonly
it is held in the 300°F to 400°F range. The thicker the weld metal,
the more likely
will it be necessary to preheat, because the heat will be conducted away from
the weld zone
more rapidly as the mass increases. 1.5.2
Stress
Relieving - Metals expand when heated and contract
when cooled. The amount
of expansion is directly proportional to the amount of heat applied. In
a weldment, the
metal closest to the weld is subjected to the highest temperature, and as the
distance from
the weld zone increases, the maximum temperature reached decreases. This
nonuni- form heating
causes nonuniform expansion and contraction and can cause distortion and
internal stresses within the weldment.
Depending on its composition and usage, the metal may
not be able to resist these stresses and cracking or early failure of the part
may occur. One
way to minimize these stresses or to relieve them is by uniformly heating the
structure after
it has been welded. The metal is heated to temperatures just below the point
where a microstructure
change would occur and then it is cooled at a slow rate.
1.5.3
Hardening
- The hardness of steel may be increased by heating it
to 50°F to 100°F
above the temperature that a microstructure change occurs, and then placing the
metal in a liquid solution that rapidly
cools it. This rapid cooling, known as "quenching,"
locks in place microstructures known
as "martensite" that contribute to a metal's hardness
characteristic. The quenching
solutions used in this process are rated according to the
speed that they cool the metal, i.e.,
Oil (fast), Water (faster), Salt Brine (fastest). 1.5.4
Tempering
- After a metal is quenches, it is then usually tempered.
Tempering is a
process where the metal is reheated to somewhere below 1335°F, held at that
tempera- ture
for a length of time, and then cooled to room temperature. Tempering reduces
the brittleness
that is characteristic in hardened steels, thereby producing a good balance
between high strength and toughness.
The term toughness, as it applies to metals, usually
refers to resistance to brittle fracture
or notch toughness under certain environmental conditions.
More information on these properties will be covered later in this lesson
and in subsequent
lessons. Steels that respond to this type of treatment are known as "quenched
and tempered steels."
1.5.5
Annealing
- A metal that is annealed is heated to a temperature
50° to 100° above
where a microstructure change occurs, held at that temperature for a sufficient
time for a uniform
change to take place, and then cooled at a very slow rate, usually in a fur-
nace. The principal reason for
annealing is to soften steel and create a uniform fine grain
structure. Welded parts are seldom
annealed for the high temperatures would cause distortion.
