Page 210 - CITS - Welder - Trade Theory

WELDER - CITS

Lb// in minimum tensile strength steel with the specific composition range. Whereas butt welds can be made with
steels having tensile strength of 45 to 70,000 lb// in2 with specific and preferred composition range. When steels
with outside the preferred range of composition it is necessary to reduce welding speed and current to avoid
cracking.

Joint design: Because of its high penetration the submerged arc process requires less deposited metal and
therefore a change in joint design. Frequently multi pass welds will be changed to a single pass automatic welds.
The Vee groove should be made within 10% of the expected bead width and it should be only as deep as required
to eliminate unnecessary built up.
The groove is primarily used to prevent built up of weld rather than secure penetration. In general, for automatic
use, Vee groove should be wider than deep otherwise the following may result:

(i) Internal bead cracking is likely to result from internal shrinkage.
(ii) If the arc voltage happens to be high with this narrow groove, there will be a possibility of slag inclusion and
incomplete fusion at the bottom of the Vee.
(iii) With a low voltage, an undercut is apt to appear at the edges of the bead.

(iv) There is a tendency of arc to wander to one side resulting in weld being off the seam.
A good rule to follow is to have weld 1.5 times as wide as it is deep. In designing joints, consideration must also
be given to the possibility of burn through caused by the penetrating arc.

Joint edges and Edge preparation: Joint edges must be clean of all foreign matters such as moisture, rust, dirt,
oil grease and paint primer if satisfactory welds are to be obtained. It is important that the abutting edges are dry
and free from foreign matter. It is very important that flux also to be free from contamination by foreign matter,
such as grease, water, mill scale and iron oxide.
Fit up: For uniform results the fit up must be accurately controlled and should be as indicated in the procedure
tables. For sheet metal, seams should be tight where no gap is specified and the minimum offset of the edges
must not be over 10% of the thickness and never be more than 2.4 mm. Drive fits are to be avoided, as they
increase tendency for cracking and porosity on heavy plates. A slight gap of 0.4 to 0.8 mm is helpful to minimise
rigidity of the joint and reduce the tendency for cracking.

Gaps over 0.8 mm should be sealed for satisfactory automatic welding. For square butt joints up through 9.6 mm
(3/8") thickness, the sealing bead should be on the side of the first automatic pass. For plates over 9.6 mm seal
on the second pass side.

On butt welds with beveled edges, make the sealing bead on the automatic first pass side. As a general rule E
6010-11 or E 6015-16 should be used for sealing square butt welds and beveled butts. Never use electrode for
sealing that will given an oxidised bead, such as, AWS E-6012 or E- 6013. Tack welds should be made with E
6010 or E 6015.

If work is held firmly against a platen, gap can be allowed since the granular flux fills up the gap and supports the
molten metal. As long as the flux is held absolutely stationary, the molten metal will be supported. If the molten
flux runs out of the joint the molten metal easily follows.

Whenever plate edges, Vee or Square butt, are open over 0.8 mm allowing flux to fall into the seam, precautions
must be observed in order to eliminate trapped slag porosity. This type of porosity consists of short large diameter
hole compared to long small holes found in the presence of moisture or rust.
The joint must be prepared that the automatic weld completely remelts and eliminates all the flux in the groove
or Vee. This may require an increased cross section manual seal bead or a change of design to a 'U' groove or
wider Vee (90°) so that the arc reaches the bottom of Vee.
Tack welds: When tack welds are used for holding the edges together, they should be as small in cross section
as practical. Avoid the use of short heavy build up tacks. Small fast bead with minimum penetration are the most
desirable. E 6010 is most satisfactory. Do not use E 6012, E 6013 or E 4510 as these may result in some porosity.
This is especially true at the higher automatic speeds.
Submerged Arc Welding Consumables: Continuous bare wires in the form of coils and dry granular fluxes
are used in combination as consumables for the saw of mild steel, low-alloy steels, stainless steels, non-ferrous
alloys and for surfacing applications. The bare wires are usually solid wires, but in recent times flux cored wires
(i.e. tubular wires carrying flux in the core) have been introduces, which also have to be used in combination with
fluxes.

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CITS : C G & M - Welder - Lesson 61-76