Welding Tech: Stick electrodes for carbon steel - understand your options

Whether or not you specify electrodes for the welding procedure, understand how the choice is made

The electrode chosen for use in any shielded metal arc welding (SMAW, or stick welding) of carbon and low alloy steel is usually written into a welding procedure specification (WPS). The process involved in making the correct choice involve a variety of considerations that we will review here. Understanding each step can help ensure that the correct electrode is used. Should a weld fail, reviewing these considerations may help to identify the cause.

Tensile Strength

When it comes to selecting a filler metal for SMAW on carbon steel, usually the first consideration for a welding engineer (or whoever is writing the WPS) is to try to match the strength of the electrode with the base or parent metal.

The rule of thumb is to choose a weld metal with a tensile strength slightly higher than that of the parent metal. To do this, it’s necessary to know the grade of the parent metal. Once the grade is known, it’s possible to match it to the specifications supplied with the filler metal.

For example, the specified minimum tensile strength of CSA G40.21 350W steel is 450 MPa. The minimum tensile strength of weld metal made with an E49XX carbon steel electrode or an E49XX-X low alloy steel electrode is 490 MPa. So both electrodes are suitable for this type of steel.

Carbon Equivalent

Depending on the grade of steel being welded, there may be a risk of hydrogen-induced cracking in the weld metal or weld heat affected zone. The presence of hydrogen in steel causes the steel to become brittle and susceptible to cracking after welding has been completed. The higher the strength of the steel, the greater its susceptibility to cracking.

To prevent cracking, typically a welding engineer will consider what is called the carbon equivalent (CE) of the steel that will be welded. Various codes and standards provide formulae to calculate the CE of the parent metal, based on its chemical composition. The chemical composition can be found in the standard to which the steel was made. The standard will have a table that gives the amounts of the various alloying elements, such as carbon, manganese, silica, nickel, chromium, and vanadium that are in the steel. By entering the percentage of each element into the equation, you arrive at a number.

Research has shown that if the number you arrive at from your calculation is greater than 0.40, then the steel contains sufficient carbon and other alloying elements that it may be susceptible to hydrogen-induced cracking.

Typically, to avoid this defect, an electrode that has a basic coating is used. The last two digits in the electrode designation will be 16 or 18. The coating on these electrodes is designed to limit the amount of hydrogen that the liquid weld metal absorbs during welding.

It may also be necessary to preheat the parent metal before starting the weld to reduce the cooling rate of the weld zone and to allow any hydrogen that does enter the liquid weld pool to diffuse out of the steel and into the air. In some applications involving high strength steels, it may also be necessary to maintain the preheat temperature for a specified time after welding has been completed before the assembly is allowed to slowly cool to room temperature.

Welding Position

The next thing that needs to be considered is in which welding position the joints will be made. If the workpiece is small and can be easily manipulated on a work bench, then an electrode designed to make welds in the flat or horizontal position can be used. This may be advantageous for weld metal deposition rates, because the welder can work more quickly.

If the workpiece is large, or if it’s an assembly that cannot be moved and requires vertical or overhead welds, then the electrode has to be designated for use in all positions. This means that the second-to-last digit in its designation is 1. For example, an E4918 electrode will deposit weld metal with a minimum tensile strength of 490 MPa and is for use in all welding positions (the 1). The 8 means it has a basic or low-hydrogen coating.

Weathering Steels

Some grades of steel are known as weathering steels. They contain a small amount of copper and are designed to be used without any protective coating. They go into service unpainted and are designed to be used this way because the copper makes the oxide layer that forms on their surface adhere to the surface and prevent further oxidation.

The electrodes to weld weathering steel may be required to make welds that are similar to both the corrosion resistance and the in-service colour of the parent metal. Standards such as CSA W59 specify the required electrode. The standard also allows the use of different electrodes if the colour match is not important and only the corrosion resistance needs to be matched.

High-strength Steel Designations

For welding high-strength steels, the electrode designation should have more than the standard four or five digits a plain carbon steel electrode has. A plain carbon steel electrode is formatted such as in the previous 4918 example. Higher-strength steel electrodes include suffixes like -A1, -B2H,-NI3, -D1, and -D2. These suffixes indicate the specific chemical compositions and applicability of the electrodes. When welding high-strength steel greater than 550 MPa tensile strength, it’s important to pay attention to the suffix because it tells you whether the electrode can be used for the application, such as if the welded assembly is to be postweld heat treated.

For instance, many years ago I was teaching a course at a company that rebuilt large equipment parts. I mentioned that a particular electrode, E5518-B9, contains vanadium and shouldn’t be used when an assembly is going to be postweld heat-treated because vanadium in weld metal that is postweld heat-treated will cause the weld metal to become brittle and fracture easily.

When I said that, two people in the back of the room quickly left the class. They came back shortly thereafter with a box of electrodes. They noted that every once in a while when they sent repaired equipment back into service the welds would immediately fail. The electrodes they brought were, indeed, vanadium bearing.

The crux of the matter was that the purchasing department had been told to buy E5518-class electrodes, but that was all that they had been told. Purchasing’s goal was to buy the cheapest electrodes they could given the specifications provided. Therefore, when vanadium bearing electrodes would occasionally be on sale at the same time that they had a requisition for electrodes to fill, they would buy E5518-B9 electrodes. Most other types of E5518 electrodes, -B1, -B2, -B3, -B3L, -B6, -C1, -C2 and -D3 for example, do not contain vanadium and purchasing them would not be a problem. This is why the weld failures didn’t happen all the time, only occasionally. The issue was that purchasing hadn’t been given sufficient information to buy the correct electrodes.

This is a good reminder that a series of missteps always occur before someone welds with the wrong electrode and a failure occurs. It is a chain of events that leads to the failure and if one link is broken the failure is avoided. Several higher-strength electrodes may meet the same strength, weldability and hydrogen requirements, and each will have a specific chemical composition that gives it different properties designed for specific applications.

Whoever is specifying the electrodes for a job needs to consider the welding procedure that is going to be used and the end use of the final welded product. All of that needs to be well-understood before selecting the correct electrode to avoid problems when production starts and parts are put into service.

Ken Thorn is global welding engineer, Canadian Welding Bureau, ken.thorn@cwbgroup.org.

CWB Group, 800-844-6790, www.cwbgroup.org