©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
V 5.2.0.4
The chrome-nickel stainless alloys
conduct heat only 40% to 50% as fast as carbon steel
and in the straight chrome types, heat conductivity is 50% to 65% that of carbon
steel as shown
in Figure 6. This means that the heat remains in the vicinity of the arc
for a longer period
of time instead of being dispersed throughout the weldment rapidly, as it does
when welding materials
of high thermal conductivity. This is another reason that lower amperages
are required to weld these steels.
CARBON STEEL
CHROMIUM- NICKEL
TYPES STRAIGHT
CHROMIUM TYPES
AT 20° - 100° C
.020 .040
.060 .080
.100 .120
CAL/SEC/SQ CM
THERMAL CONDUCTIVITY
FIGURE 6 5.3 STAINLESS
STEEL TYPES As
already mentioned, there are three principal categories of stainless steels: austenitic,
martensitic, and ferritic. The
names are derived from the crystalline structure of the steel
normally found at room temperature.
When low carbon steel is heated above 1550°F, the
atoms of the steel are rearranged from
the structure called ferrite at room temperatures to the
crystal structure called austenite.
On cooling, the low carbon steel atoms return to their original
structure — ferrite. The
high temperature structure, austenite, is non-magnetic, plastic and has
lower strength and greater ductility
than the room temperature form of ferrite. 5.3.0.1
When more than 17% chromium and 7%
nickel are added to the steel, the high temperature
crystalline structure of the steel — austenite, is stabilized so that it
persists at all temperatures
from the very lowest to almost melting. This alloy combination is the basis
for the austenitic
category of stainless steels . Many alloy additions are made to that base
as modifi- cations
for different service requirements.
