voestalpine bohler welding URANOS 2700 PMC Betriebsanleitung Seite 54

D.C.R.P. (Direct Current Reverse Polarity)
The reverse polarity is used for welding alloys covered with a
layer of refractory oxide with higher melting temperature com-
pared with metals.
High currents cannot be used, since they would cause excessive
wear on the electrode.
7.2.1 Steel TIG welding
The TIG procedure is very effective for welding both carbon and
alloyed steel, for first runs on pipes and for welding where good
appearance is important.
Straight polarity is required (D.C.S.P .).
Preparing the edges
Careful cleaning and preparation of the edges are required.
Choosing and preparing the electrode
You are advised to use thorium tungsten electrodes (2%
thorium-red coloured) or alternatively cerium or lanthanum
electrodes with the following diameters:
Ø electrode (mm)
1.0
1.6
2.4
The electrode must be sharpened as shown in the figure.
(°)
30
60÷90
90÷120
Filler metal
The filler rods must have mechanical characteristics comparable
to those of the parent metal.
Do not use strips obtained from the parent metal, since they
may contain working impurities that can negatively affect the
quality of the welds.
Shielding gas
Tipically, pure argon (99.99%) is used.
Welding Ø Electrode
current (A) (mm)
6-70 1.0
60-140 1.6
120-240 2.4
7.2.2 Copper TIG welding
Since TIG welding is a process characterized by high heat con-
centration, it is particularly suitable for welding materials with
high thermal conductivity, like copper.
For TIG welding of copper, follow the same directions as for TIG
welding of steel or special instructions.
Consult the instruction manual of the system.
54
current range (A)
15÷75
60÷150
130÷240
current range (A)
0÷30
30÷120
120÷250
Gas nozzle Argon flow
n° Ø (mm) (l/min)
5-6
4/5 6/8.0
6-7
4/5/6 6.5/8.0/9.5
7-8
6/7 9.5/11.0
7.3 Continuous wire welding (MIG/MAG)
Introduction
A MIG system consists of a direct current power source, wire
feeder, wire spool, torch and gas.
MIG manual welding system
The current is transferred to the arc through the fusible elec-
trode (wire connected to positive pole); in this procedure the
melted metal is transferred onto the workpiece through the arc
stream. The automatic feeding of the continuous filler material
electrode (wire) is necessary to refill the wire that has melted
during welding.
Methods
In MIG welding, two main metal transfer mechanisms are pre-
sent and they can be classified according to the means by which
metal is transferred from the electrode to the workpiece. The
first one, defined "SHORT-ARC", produces a small, fast-solidi-
fying weld pool where metal is transferred from the electrode
to the workpiece only for a short period when the electrode is
in contact with the weld pool. In this timeframe, the electrode
comes into direct contact with the weld pool generating a short
circuit that melts the wire which is therefore interrupted. The
arc then turn on again and the cycle is repeated (Fig. 1a).
Fig. 1a
Fig. 1b
SHORT cycle (a) and SPRAY ARC welding (b)
Another mechanism for metal transfer is called the "SPRAY-
ARC" method, where the metal transfer occurs in the form of
very small drops that are formed and detached from the tip of
the wire and transferred to the weld pool through the arc stream
(Fig. 1b).