A lot more depends on selection of the electrode type for successfully carrying out weld with SMAW process. Wrong selection of electrode type may lead to more defects, more repairs. The selection of correct type of electrode coating results in weld metal with desired quality characteristics at low cost. In general, welding electrode is selected in such a way that characteristics of weld metal are similar to or better than the base material while keeping in mind the welding position and weld joints design as they significantly affect the properties of the weld.

The covering of the stick electrode consists of a multitude of components which are mainly mineral, below figure shows coating raw material and its effect on the welding characteristics.

Stick electrodes are, according to their covering compositions, categorized into four different types:

Cellulosic electrodes (Symbol C)

These electrodes are composed of large amount of hydrocarbon compounds and calcium carbonates besides other constituents and are found suitable for

1. All welding positions especially for vertical and overhead welding position and
2. Realizing high mechanical properties in a weld metal of radiographic quality.

These are preferred for vertical downward welding. However, these produce high hydrogen content in weld metal besides deep penetration.

Rutile electrode (Symbol R)

These electrodes predominantly contain rutile (TiO₂) besides other constituents and are known to offer almost 100% weld metal recovery, easy arc striking and restriking. These are found suitable for

1. Fillet welds,
2. Welding of sheet metal,
3. Good gap bridging capability,
4. Free from spatter losses and
5. All position welding.

These are recommended for welding low strength steel (<440 MPa). For welding of high strength steel (>440 MPa) generally weld metal should have low hydrogen level and therefore weld joints is developed using basic, rutile, basic-rutile and Zircon based electrode.

Basic electrode (Symbol B)

These electrodes have basic (alkali) coatings containing calcium carbonate / calcium fluoride. The basic electrodes are preferred over other electrode for developing weld joints of high strength steel (480-550 MPa) with weld metal having

1. Low hydrogen,
2. Good low temperature toughness,
3. Resistance to hot and cold cracking.

However, these electrodes suffer from comparatively poor slag detachability. The welding speed and deposition rate offered by the basic electrodes especially in vertical welding position is much higher than the rutile and acidic electrode. Basic electrodes can sustain higher welding current even in vertical welding position.

Acidic electrode (Symbol A)

Coating

1. Easier arc striking than basic electrodes but poorer arc striking than rutile electrodes,
2. Moderate welding speed,
3. Smooth weld bead
4. Good slag detachability.

However, acidic electrode has been replaced by rutile electrode and basic electrode for flat and positional welding respectively. The ductility and toughness weld metal developed by acidic electrode are better than those developed from rutile electrodes however yield and ultimate tensile strength are found inferior. This type of electrode results in minimal penetration which is good for very thin sheet but these are sensitive to moisture pick up.

Below figure compares the characteristics of the four different types

Apart from these categories other type of coatings such as Iron powder coating electrode, basic-rutile electrode, etc.  are also available, which are basically general modifications of the four above mentioned categories to fetch more advantages by combining the positives of the each type.

Reference: Technical document, MMAW, Aachen, ISF & Technical document, Nptel

Keep reading, Happy welding,

Thank you

KP Bhatt

 

Have you ever thought how human brains functions???

Training, inputs, target, feedback system, correction, learning, testing, validation, output, multivariate analysis, benchmarking, randomizing… these are few terms which when combined together and functioned together, wonders happens and that’s the secret behind functioning of human brain,

Now think… what if these all terms made to function together under one artificial system, will it simulate human brain?

Answer is YES… Researchers have successfully developed Artificial Intelligence System as popularly known to be Artificial Neural Network. These has its own axon, neurons, synapses, dendrites just as biological human brain has.

It has started creating wonders in many other fields. Here, we will see how this can be benefited in welding field. These are some of the examples, Self-Learning robots, Prediction model for the experimentation, online defect detecting technique, Precision positioning and welding.

• Self-Learning Robots: Lappeenranta University of Technology (LUT) is developing an entirely new kind of welding system, one which solves quality and productivity problems related to automated and mechanized welding. The system is self-adjusting, flexible and adaptable, such that it can be integrated as part of different robotic systems and different manufacturers’ power supplies. Its self-adjusting properties are based on a new kind of sensor system which is controlled by a neural network program. Most often in welding a monitoring sensor is used which tracks the bevel angle, an essential part of the welding process. In the system being developed by LUT, there are also monitoring sensors for the thermal profile (the weld pool’s heat values) and the weld form. The monitoring data is transferred from the sensors to the neural network, which is able to deduce and react to simultaneous changes in multiple variables. In the gas-shielded arc welding process, factors affecting outcome quality include the welding current, the arc voltage, the wire feeding and transporting speeds, and the position of the welding gun. With the help of the neural network, a regulating window can be set for these system variables, and they can then be controlled so that they remain within certain limits, which ensures that final product is as required. In practice this means that when the welding values approach the boundary values set in the parameter window, the system corrects the process so that the welding values move back towards the center of the value range and the possible defect is prevented.
• Prediction Model: With the help of supervised learning in neural network one can predict things. This can be useful while development of any new welding process. Weld bead geometry, strength, chemical composition of weld, ferrite number and many other responses can be predicted using Neural Network. This can save time and money of your development project.
• Online defect monitoring: Capturing the sound of welding and studying its waveform, one can gather many information. Defects will have certain waveform. These waveform is captured by experimentation for number of defects and processed in neural network model. This model can tell us while welding where the defect is and its type if it has occurred.

Many such technological development can be possible if we start applying Neural network for all the small work we carry out on the daily basis.

Reference: Article – The welding system of the future is self-learning.

Keep reading, Happy welding

Thank you,

KP Bhatt