What is welding?

 

Wikipedia defines welding as “a fabrication process that joins materials, usually metals.” This is done by melting the part of the materials to be joined and adding some additional molten joining material. When the molten material cools, it forms a strong bond or joint.

Welding is the most widely practiced way of joining metals together due to the efficiency and economy of the process. It has been estimated that approximately 50% of the Gross National Product of the USA arises from activities that are in some for or another related, perhaps remotely, to welding. As an example, farming may appear to have nothing to do with welding, but the equipment the framer uses to grow and harvest his crops will have used welding in their manufacturing process.

Until the advent of the 20th century, welding was confined to the blacksmith shops where two pieces of metal were heated to very high temperatures in a forge and then hammered together until the joining occurred. This is what is called forge welding.

With the introduction of electricity into industrial processes, welding became both quicker and easy, and also more economical. Today there are 4 types of welding techniques that are commonly used.

The most basic for is Arc Welding where the parts to be joined together are brought into contact with strong electrical current and heated. The molten parts are then joined together to form a weld. This is the low technology end of welding in its cheapest and least sophisticated form.

Gas Welding is generally used for repair work especially in the case hollow items like tubes and pipes. Hot gas is forced onto the surfaces to be welded. This procedure requires the parts to be subjected to less heat and is suitable for material that may be damaged by exposure to high temperature. For this reason it is used in the jewellery industry which has to work with soft metal with low melting points.

Resistance Welding requires that an additional sheet of material is used to cover the pieces that are to be welded together. This  provides great strength to a weld, but the process requires expensive equipment and also the use of additional material to encase the weld which makes it expensive and not suitable for all applications.

Laser Welding is the most modern technology available. High intensity lasers can be tightly focused and produce controllable heat on the surfaces to be welded very quickly. This is perfect for material which can be damaged by prolonged exposure to extreme heat. Laser welding is very accurate and can be used to produce even the smallest of welds. However, because of the high capital cost involved in purchasing this equipment, the cost of welding is also high.

Although the perception of welding is that of a simple process of heating and joining, it is a high technology industry with huge  amounts being spent of research and development to find stronger, more accurate and cheaper methods. The welding process plays a big role in metallurgy with a constant effort underway to find newer and more weld friendly alloys.

 

Sourced:  From www.welderworld.com

IMPORTANCE OF GOOD QUALITY WELDS IN TRAILERS

The topic of discussion today is one aspect of structural fabrication i.e welding to which the transporters do not give much importance to.  The general feeling in transporters is that welding is essential but it is  only as an ornamental requirement and does not deserve much skill and attention. This kind of mind-set is prevalent and shared by most of the transporters.  However it must be noted that the stability and the strength of the structure totally depends on the quality of the welds.

This aspect need not be discussed where both the parties i.e the manufacturer and the customer have a sound engineering background.  The problem arises where one of the parties does not have a sound engineering background and cannot appreciate the topic of discussion.

Let us understand what happens when a welding joint in a trailer or a semi trailer fails. 

1. It can lead to serious accidents.
2. It can cause distortion of structure and lead to unnecessary maintenance.
3. Tyre wear and tear.
4. Increase in down time of a productive vehicle.

The following pictures are classic example of inadequate welding and welding failure.

The  picture on the left shows a structural joint in which there is incomplete and poor quality of weld.  The picture on the right shows a failure of a weld joint due to a defect called as lack of fusion.  In this case the welder has failed to fuse the parent metals while depositing the filler material.  This type of defect shown in the picture is probably because of  low current setting coupled with inadequate skill of the welder.

The adjoining picture is a classic example of good fillet weld joint.












So in order to avoid failure and breakdown due to defective welding the following should be insisted upon.  The job of fabrication of a trailer body/frame is nothing short of structural fabrication and all the requirements relevant to fabrication and welding codes as per ASME should be insisted upon.  The following are the basic documents that the client should insist upon.

1. Material test reports for physical testing and chemical analysis  conforming to ST-42 or any other equivalent standard should be insisted upon. It is important to understand that while freezing a standard for the material to be used, the end application has to be kept in mind.
   
   
2. Once the materials to be used are tested and found satisfactory, the next step will be to establish a welding procedure.  There are standard formats in ASME sec IX which clearly define the parameters that have to be recorded. This procedure qualification has to be witnessed by an  welding inspector.  This process is called as establishing a “PQR”, the full form is “Procedure Qualification Record”.By carrying out this exercise we achieve the following. The material suitability of  the welding electrodes and the parent metal used in the structure is tested for physical and chemical properties.  The position of the weld joint and the welders skill in that position is both tested.  There are other intricate parameters which are also established like welding process, size of electrode, current, voltage, flow of gas in case of MIG/TIG, surface preparation, geometry of the weld joint, cleaning processes etc. The test sample of the welded material is sent to approved laboratories for non destructive and destructive testing as per relevant codes.
   
   
3. The next step is qualification of welders as per the established PQR and this process is called “WPS”.......

(To be continued……)

JAGDISH S KOLUR

Partner - BELCO ENGINEERS

Telefax: +91-22-27423841

Mobile:  +91-9819076201

E-Mail:  jagdish@belcoengineers.com