STR654 - Lecture 3b 2020.ppt

Sherif A. Mourad - Faculty of Engineering,

Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١

STR654:

INSPECTION, MAINTENANCE and

REPAIR of STEEL STRUCTURES

Lecture 3b

Sherif A. Mourad

Professor of Steel Structures and Bridges

Faculty of Engineering,

Cairo University

WELDING PROCESSES

� Weldability of steel.

� Welding positions.

� Arc welding techniques.

� Other welding techniques.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢

Weldability of steel

Weldability is the capacity of a metal to be

welded under the fabrication conditions

imposed into a specific, suitably designed

structure and to perform satisfactorily in the

intended service.

Weldability is improved by low-carbon, fine-

grain size and restricted thickness.

Weldability of steel


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٣

Welding Positions

� Flat position� Weld metal can be deposited faster.

� Typically used for shop welds, where manipulating devices can be used to rotate the work in a flat position.

� Over head position

� Electrode diameters below 5 mm are to be utilized to avoid metal run down.

� Horizontal-vertical position.

Welding Position


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٤

Arc Welding

� Shielded Metal Arc Welding

� Submerged Arc Welding

� Gas Metal Arc Welding

� Flux Cored Arc welding

� Gas Tungsten Arc Welding

Arc Welding

� Plasma Arc Welding

� Electrogas Welding

� Electroslag Welding


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٥

Arc Welding

Shielded Metal Arc Welding

Shielded Metal arc Welding Process


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٦


Definition:

� Heating Source is electrical arc between the

consumable electrode and the work piece

� No Pressure is applied


Equipment:

� Welding power source.

� Electrode holder.

� Ground clamp.

� Welding cables & connectors.

� Accessories (chipping hammer, wire brush ,…)

� Protective equipment (helmet, glove, …)


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٧


� Shielding is provided by the electrode covering

� Mainly gas, very little slag→ small pool, all positions

� Mainly slag, very little gases → high amperage, high deposition rate, i.e. flat position


Process Advantages:� The equipment is relatively simple, not expensive and

portable

� The filler metal and the means of its protection as well as the weld metal from harmful oxidation during welding are provided by the covered electrode

� Auxiliary gas shielding or granular flux is not required


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٨


� The process is less sensitive to wind and draft

than the other gas shielded arc welding

processes

� It can be used in areas of limited access

� The process is suitable for most commonly used

metals and alloys


Process Disadvantages:

� Low deposition rate because of the limited

electrode length

� Slag removal required

� It could not be automated


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٩

Gas Metal Arc Welding

Gas Metal Arc Welding Process


Definition:

� It uses an arc between a continuous filler metal

electrode and the weld pool

� Externally applied gas or flux cored into the wire

or both for shielding

� Without the application of pressure


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٠


Advantages:

� The only consumable electrode process that welds all commercially available alloys

� No restrictions due to limited electrode length

� All positions

� High deposition rate


� High speed of welding

� Long weld bead

� Deeper penetration

� Minimal Post weld cleaning


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١١


Thus the process is considered well suited for:

� High production and automated welding

applications

� Robotics use


Limitations:

� Complex welding equipment

� More difficult in hand - to - reach places

� Air drafted

� High level of radiated heat and arc intensity


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٢


Gas Metal arc Welding Terminology


Welding ampere varies with wire feed speed (WFS)

or melting rate (MR) as follows:

WFS = aI + bLI2

� a > Constant dependent on polarity, composition

� of the electrode (for the electrode heating)

� b > Constant for electrical resistance heating

� L > Electrode extension or stick out

� I > Welding current


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٣

Flux Cored Arc Welding

Gas Shielded Flux Cored Arc Welding

Flux Cored Arc Welding

Self-Shielded Flux Cored Arc Welding


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٤

Advantages of FCAW

� High-quality weld metal deposit.

� Excellent weld appearance-smooth,

uniform welds.

� Excellent contour of horizontal fillet welds.

Advantages of FCAW

� Many steels weldable over a wide thickness

range.

� High operating factor - easily mechanized.

� High deposition rate - high current

density.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٥

Advantages of FCAW

� Relatively high electrode deposit

efficiency.

� Economical engineering joint designs.

� Visible arc - easy to use.

Advantages of FCAW

� Less pre-cleaning required than GMAW.

� Reduced distortion over SMAW.

� Up to 4 times greater deposition rate than

SMAW.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٦

Advantages of FCAW

� Use of self-shielded electrodes eliminates

need for flux handling or gas apparatus,

and is more tolerant to windy conditions

present in outdoor construction (see

disadvantage "6" below for gas shields).

Advantages of FCAW

� Higher tolerance for contaminants that

may cause weld cracking.

� Resistant to under-bead cracking.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٧

Limitations of FCAW

� FCAW is presently limited to welding

ferrous metals and nickel base alloys.

� The process produces a slag covering

which must be removed.

Limitations of FCAW

� FCAW electrode wire is more expensive on

a weight basis than solid electrode wires,

except for some high alloy steels.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٨

Limitations of FCAW

� The equipment is more expensive and

complex than that required for SMAW

however, increased productivity usually

compensates for this.

� The wire feeder and power source must be

fairly close to the point of welding.

Limitations of FCAW

� For the gas shielded version, the external

shield may be adversely affected by

breezes and drafts. Except in very high

winds this is not a problem with self-

shielded electrodes because the shield is

generated at the end of the electrode,

which is exactly where it is required.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ١٩

Limitations of FCAW

� Equipment is more complex than that for

SMAW, so more maintenance is required.

� More smoke and fumes are generated

(compared to GMAW and SAW).

Gas Tungsten Arc welding

Gas Tungsten Arc Welding Process


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٠


Advantages

� High quality welds.

� No spatter.

� With / Without filler.

� Root pass control.


� Relatively inexpensive.

� Welding variables control.

� All Metals

� Independent control of heat source and filler

metal addition.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢١


Limitations

� Low deposition rates.

� More welder coordination.

� Less economic.


� Possible draft.

� Tungsten inclusions.

� Low tolerance for contamination.

� Coolant leakage


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٢

Submerged Arc welding

Submerged Arc Welding Process


Definition:

� Heating with an arc between a bare consumable metal electrode and the work piece.

� Arc and molten metal are submerged in a blanket of granular fusible flux on the work

� Pressure is not used

� Sometimes additional filler metal is obtained from a welding rode or metal granules


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٣


Fused fluxes:

� Raw materials are powdered, dry mixed –

melted in electric furnace - cooled - crushed

and screened for size


Advantages:

� Good chemical homogeneity

� not hygnoscopic, easily recycled without

chemical changes


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٤


Main disadvantage is when adding

deoxidizer or Ferro-alloys during

fabrication losses and segregation occurs


Bonded Fluxes:

� Raw materials are powdered, dry mixed and bonded with K or Na-Silicate.

� Then wet mix is palletized and backed at high temperature (lower than fused fluxes).

� Pellets are crushed and screened


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٥


Advantages:

� Easy additions of de oxidizers and alloying.

� Color identification.


Disadvantages:

� Tendency to absorb moisture.

� Possible gas evolution when molten.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٦


Mechanically Mixed Fluxes:

� Two or more fused or bonded fluxes are mixed.


Advantages:

� Available commercial.

� fluxes are mixed to give any precise properties.


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٧


Disadvantages:

� Segregation during shipment, storage, feeding

and recovery.

� Inconsistency from mix to mix.



Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٨


Plasma Arc Welding

Plasma Arc Welding


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٢٩

Electrogas Welding

Electrogas Welding With a Solid Wire Electrode

Electrogas Welding

Electrogas Welding with a Flux Cored Electrode


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٣٠

Electroslag Welding

Non-Consumable Guide Method of Electroslag Welding (Three Electrodes)

Electroslag Welding

Consumable Guide Method of Electroslag Welding


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٣١

Resistance Welding

� Resistance Spot Welding

� Resistance Seem Welding

� Resistance Projection Welding

Resistance Welding

Resistance Spot Welding


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٣٢

Resistance Welding

Basic Single Impulse Welding Cycle for Spot Welding

Resistance Welding

Enhanced Welding Cycle


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٣٣

Oxyfuel Gas Welding

Basic elements of an Oxyfuel Gas Welding Torch

Solid State Welding

� Diffusion Welding

� Friction Welding


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٣٤

Friction Welding

Friction Welding


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٣٥

Electron Beam Welding

A Simplified Representation of anElectron Beam Gun Column

Laser Beam Welding

Elements of a Typical Laser Welding System


Cairo University

November ٢٠٢٠

STR654 Lecture 3b ٣٦

Thermal Spraying

Cross Section of a Typical wire Flame Spray Gun

Documents

STR654 - Lecture 3b 2020.ppt