Chapter 8 Manual Welding Techniques
131
Joint Preparation
Joint edges prepared by thermal cutting processes
have a heavy oxide fi lm on the surface, as shown in
Figure 8-3. This oxide should be completely removed
prior to welding in order to prevent porosity in the
weld metal.
Joint edges prepared by shearing should be sharp
without tearing or ridges. Otherwise, dirt or oil can
become trapped in the joint and result in faulty welds.
Rough edges can be removed prior to welding with a
light grinding.
Weld Backing for Steel
Groove welds that are welded from one side of
the joint, where 100% penetration is required, can be
backed with argon gas or a solid bar. Solid bars can be
made of copper or stainless steel.
Excluding air from the backside or penetration
side of the joint will assist in wetting of the penetra-
tion. A wetted penetration has a smooth junction and
even fl ow of the penetration onto the base metal. The
exclusion of air also prevents the formation of scale
and oxide.
Preheating Steels
Carbon steels less than 1″ (25.4 mm) thick and
with less than .30% carbon generally do not require
preheat. Welding on highly restrained joints is
an exception. These joints should be preheated to
50°–100°F (10°–38°C) to minimize shrinkage cracks in
the base metal and the weld deposit.
Low-alloy steels, such as the chrome-moly steels,
have hard heat-affected zones after welding if the
preheat temperature is too low. The hard heat-affected
zones are caused by the rapid cooling rate of the base
material and the formation of martensitic grain struc-
tures. A 200°–400°F (93°–204°C) preheat temperature
slows down the cooling rate and prevents the forma-
tion of a martensitic structure.
Martensite is a metallurgical term that defi nes
a type of grain structure obtained by heating and
quenching. The martensitic grain structure makes the
metal hard. Parts that are welded are, in effect, heated
and quenched. If the carbon content of the material is
suffi cient, and the preheat temperature is too low, the
material in the heat affected zone will harden during
welding. This results in high tensile properties with
very low ductility. The formation of hard martensite-
type grains increases the possibility of cracking as the
weld metal cools and shrinks.
Tool steels have a very high carbon content and
are prone to cracking in the heat-affected zone without
suffi cient preheat. Figure 8-4 lists some recommended
preheat temperature ranges for welding tool steels.
Quenched and tempered steel requires preheat
and interpass temperature control to retain the orig-
inal mechanical properties of the metal. The manu-
facturer’s recommendations for these temperatures
should be strictly followed.
Torch Angles and Bead
Placement
Proper manipulation of the welding torch is
very important in making a good weld. The torch
is usually held like a pencil, as shown in Figure 8-5.
When welding is done in the fl at position, the hand
should be placed lightly on a surface, so the hand can
Figure 8-3.
The oxide scale and small gouge indentations
were formed on this piece of metal by the oxyacetylene
cutting process. (Mark Prosser)
Annealed Base Hardened Base
Material
Material Preheat Material Preheat
Type
and Postheat and Postheat
Temperature Temperature
W1, W2 250–450°F (121–232°C) 250–450°F (121–232°C)
S1 300–500°F (149–260°C) 300–500°F (149–260°C)
S5 300–500°F (149–260°C) 300–500°F (149–260°C)
S7 300–500°F (149–260°C) 300–500°F (149–260°C)
01 300–400°F (149–204°C) 300–400°F (149–204°C)
06 300–400°F (149–204°C) 300–400°F (149–204°C)
A2 300–500°F (149–260°C) 300–400°F (149–204°C)
A4 300–500°F (149–260°C) 300–400°F (149–204°C)
D2 700–900°F (371–482°C) 700–900°F (371–482°C)
H11, H12, H13 900–1200°F (482–649°C) 700–1000°F (371–538°C)
M1, M2, M10 950–1100°F (510–593°C) 950–1050°F (510–566°C)
Figure 8-4.
Thoroughly preheat the part to be welded
to the required temperature. Do not allow the part to
cool below the minimum temperature until postheat is
complete.