The WelderDestiny Compass

The Primary Essential Variable - Issue #024

Wednesday, June 14, 2017 / Perth Australia / By Niekie Jooste

In this edition of "The WelderDestiny Compass":

• Let's Talk Welding
• Welding Process as Primary Essential Variable
• The Common Problems
• Welding Process Categories
  

Let's Talk Welding

A friend who reads this newsletter asked me the other day when I am going to start writing about welding. Seeing as the idea was to first cover some broader topics to set some expectations for the future, and then start looking at more detailed welding topics to prepare ourselves for that future, I think that maybe it is time to start focusing a bit on the welding side of things!

I think that the best way to deal with technical welding details within the context of a professional welding career, is to consider the typical essential variables for welding as listed in the welding codes. In essence, if we understand everything about the essential variables, then we will be well on our way to understanding the science of welding.

The welding codes just tend to give a list of essential variables, without explaining why they are included. Also, some codes include variables that are not considered in other codes. Why is this?

As the welding processes are typically the primary essential variable in the welding codes, we will start by looking at the broad implications of the welding processes themselves. We will then in subsequent newsletters start looking at the other essential variables.

If you want me to discuss a particular welding essential variable, please feel free to send me an e-mail with your request.

If you would like to add your ideas to this week’s discussion, then please send me an e-mail with your ideas, (Send your e-mails to: compass@welderdestiny.com) or complete the comment form on the page below.

Now let's get stuck into this week’s topics...

Welding Process as Primary Essential Variable

Most welding codes use the different welding processes as the basis for listing the other essential variables. This makes sense, because many of the variables are only applicable to a specific welding process, and not applicable to others.

Codes such as ASME IX includes a different table of essential variables for each different welding process. Some codes, like AWS D1.1, use a single table, but they have separate columns for each of the welding processes, that indicate which of the essential variables are applicable to the specific welding process. Still further codes, like ISO 15614 Part 1, deals with the welding process as though it is just one of the essential variables. This code does however have a section dedicated to essential variables that are specific to particular processes.

The way I am going to tackle this issue of welding process as essential variable, is to identify the broad categories within which welding processes fit. By looking at the implications of these broad groupings, we will then be able to see how a process that includes a number of these groupings will act in a broader sense.

We will just be looking at the arc welding processes, and particularly focus on those aspects where skill play a role. We will not delve into the machine welding aspects.

The Common Problems

To start out, we need to appreciate that in arc welding we will be heating the base metal and filler metal to high temperatures (above the melting temperature of the metals involved) using the welding arc. In this process, there are common problems to solve, regardless of the welding process used. The common problems are:

• The high temperature metals will oxidise very rapidly. A means needs to be found to minimise this oxidation if we want to achieve a good weld.
• The arc needs to be stable. An unstable arc that moves unpredictably will not result in good welds. In this regard, it important to note that when an arc is struck between metals, the gas in the area is ionised and becomes a plasma. Different plasmas have different thermal properties, making some plasmas more stable than others. Plasmas containing high quantities of oxygen are quite unstable.
• Managing and removing impurities that may be present in the weld pool.

Let us now look at how these problems are solved by different welding processes.

Welding Process Categories

I will now list different welding process categories. Some welding processes will be a combination of these categories, which means that they will have the advantages and disadvantages of the different categories that are applicable to them.

• Fluxing Welding Processes: Under the high temperatures of the welding arc, some parts of the flux is turned into a gas. Typically this is carbon dioxide, carbon monoxide and sometimes hydrogen. This gas acts as a shield to displace the oxygen, thereby solving the oxide problem. In addition, the evolved gas forms a stable plasma, solving the arc stability problem. The molten slag can also help to remove impurities from the molten weld pool.
• Gas Shielded Welding Processes: The gas is provided from some kind of storage, such as a gas bottle, and flows through the welding gun or torch in such a way that it makes a covering over the welding arc and adjacent base metal, excluding the oxygen and providing a suitable gas to provide a stable arc. In the case of purely inert gasses, the shielding gas cannot provide any advantages in terms of chemical reactions to condition the weld pool in any way. Active gasses on the other hand can result in both advantageous and disadvantageous chemical reactions, which can add some elements to the weld pool, or remove some elements from the weld pool.
• Non-Consumable Electrode Welding Processes: The shielding gas used for these processes need to be inert, as any active gas will result in the tungsten electrode oxidising, degrading or melting. The main shielding gasses are argon or helium. Helium forming a "hotter" plasma, but it is also much more expensive than the argon gas. Where the higher energies are needed, it is common practice to mix some helium with the argon. A principal feature of this type of welding is that it is possible to fuse two pieces of metal together, without actually adding any filler metal. This is called autogenous welding. While autogenous welding is possible, it is only used under specific circumstances.
• Consumable Electrode Processes: Most of the arc welding processes are consumable electrode processes. Here, the arc is established between the "filler metal" electrode and the work piece. In this situation, the object is to balance the amount of energy in the electrode to that in the work piece. Too much energy in the electrode, and too little in the base metal, and the electrode will melt without the base metal melting. This typically results in lack of fusion defects. Too much energy in the work piece and not enough in the electrode leads to low deposition rates, making the welding less productive.
• Continuous Wire Feeding Processes: In a welding process such as this, the wire is fed through the welding gun in the form of a continuous wire. While the welding gun's trigger is pulled, the wire feeds through the gun and automatically feeds into the weld arc. The big issue here is that there needs to be a mechanism to ensure that more wire is burned off when the wire feeds faster, and less wire burned off when the wire feeds slower. To achieve this, these welding processes typically use "constant voltage" power sources. Here the voltage is set on the power source, and the amperage increases automatically as the wire feed speed increases.
• Manual Welding Processes: In a manual process, it depends on the welder to manually manipulate the arc and feed the filler, and manage the arc length. If the welder pushes the electrode too close to the work piece, it will short circuit and "stick". If the welder pulls the electrode too far away from the work piece, the arc will become unstable and eventually extinguish. Typically these processes are welded using "constant current" power sources. Here the amperage is set on the machine, and the voltage is controlled by the welder, based on the arc length that they use while welding.

You can now start taking each welding process, and the specific process variant that you want to use, and apply the principles stated above to give you a very good idea of what the features of the process will be, how much skill will be needed and how productive it may be.

We can also get a conceptual appreciation of why the welding process is the primary essential variable in the welding codes, and how the other essential variables depend very much on the welding process that we will be using.

If you are interested in a more in-depth discussion on this topic, click here to explore it on the WelderDestiny website...

Yours in welding

Niekie Jooste

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