Basics of welding carbon steels

The majority of carbon steels will come in shapes you are familiar with or will become familiar with while welding and fabricating. Plates, pipes, H-beams, and C-channel for example are common shapes you will use to fabricate. As a welder and fabricator, the WPS and blueprints will provide you with the instructions you need to complete the fabrication to the required specifications. If the WPS or blueprint has something that isn’t clear to you, stop and ask a colleague or supervisor for clarification. Spending a few minutes to clarify something is better than spending an hour or more on rework.

Steels can be hot rolled, cold rolled, quenched and tempered, or processed in other ways to achieve the desired properties. When you weld, you are destroying those treatments and mechanical workings of the base material because the metal melts within the weld pool returning the metal to its original state prior to treatment. In areas where the base material does not melt, there is a heat affected zone (HAZ) where grain growth, allotropic changes, and absorption of elements into the crystal structure can occur. The heat applied to the base metal in the HAZ will change the mechanical properties of the base material and/or result in a weld with material properties that are less than desired. The end result could be failure of the weld joint due to cracking which is why following a WPS is so important. If a structural failure occurs, as welders we want that failure to occur in the base material away from the weld and not because of the weld.

The HAZ is a complex set of sub zones shown in the V-Groove Butt Joint of Figure 20.6 below. The weld metal is at the center, followed by a penetration zone directly next to the weld metal which represents the melted base material that fuses with the weld metal. Moving further outward we next reach the coarse-grained HAZ that experiences elevated temperatures from welding but not hot enough to melt the metal. In this zone the heat causes the metal grains to grow becoming more coarse. The further away we move from the weld bead itself the less heat there is in the base metal due to dissipation of heat. The Fine-grained HAZ has grains that are affected by the heat but grow to a lesser extent. This zone may have been tempered from the heat. Outside of the fine grained HAZ is the base metal which may have been heated up, but has not undergone any mechanical property changes. It is important to note that other sources may break up the HAZ into a more detailed structure. For the purposes of this chapter, this level of detail in the HAZ is adequate.

Steels from the mill are processed through cold and hot rolling. Rolling is a process where the steel is forced through rollers to flatten it out. Finer grained steel structures have a higher toughness. Figure 20.7 shows two metals being rolled together. The process for cold and hot rolling a single piece of steel is similar.

Figure 20.8 below shows a hot ingot that is being sent to a hot rolling process. This ingot of steel will be sent back and forth through a set of rollers that will gradually get closer together so the steel flattens out.

Figure 20.9 shows the results of the hot rolling process that has occurred to the ingot pictured in Figure 20.9.

These cold and hot rolling processes break the larger grain structures into a finer grain structure. Welding on steels that have been cold or hot rolled to form finer grains will result in a coarse grain structure in the HAZ. Post Weld Heat Treatment (PWHT) will relieve some of the residual tensile stresses that exist from the weld metal shrinking after it cools. Temperatures for PWHT are carefully controlled in duration and magnitude based on the base material. Too high a temperature can transform the metal’s crystalline structure into something undesirable where there is a loss of strength, or being held for too long can cause grain growth which results in reduced toughness.

Because of these changes that occur due to the heat of welding to the surrounding base metal, it is important to follow all of the temperatures and other parameters and techniques listed on the WPS.

Whenever you are welding, you should generally know which metal or metal group you are welding on for a more successful weld. Ideally you would know the metal composition because the metal is labeled in one form or another, or can be determined by a device called a spectrometer which will analyze the metal by its light emissions. Figure 20.10 shows a similar but larger hand held spectrometer used for analyzing metal but is used to analyze pavement.

One property of metal which may be useful to the welder in identifying the type of steel they have in front of them is a file test. Table 20.1 contains information to help identify which type of steel you have based on its Brinell hardness which is one hardness scale used by metallurgists to rank the hardness of metals. The hardness of steel in Table 20.1 was able to be determined based on the hardness of a standard steel file with that of the metal being tested. Iron becomes steel with the addition of carbon.

Note. From Operator’s Circular Welding Theory And Application TC 9-237 (Table 7-6), by the U.S. Army (1993).

Table 20.2 lists the carbon contests of steel and cast iron for comparison.

Note. From Operator’s Circular Welding Theory And Application TC 9-237 (Table 7-7), by the U.S. Army (1993).

Another identification test that can be performed in the shop is a spark test that is conducted by grinding a metal sample on a grinding wheel and observing the sparks. Figure 20.11 below shows a black and white image of spark test results, color images are available online through an internet search.

Note. “Operator’s Circular Welding Theory And Application”. TC 9-237. Department of the Army, 1993. Pp. 7-13.

Lastly, the chip test is conducted by using a sharp cold chisel to remove material. Material may be small broken pieces or one long strip.

Note. From Steelworker, Volume 1, NAVEDTRA 14250 by NETPDC, (Table 1-4), 1996, U.S. Navy.

The table above describes the chip characteristics of different cast irons and steels. This is useful information if you have an unknown metal that you may think is cast iron or steel and are looking for a relatively easy way of determining its type.

Knowing the temperature of steel is important to a welder. Accurate measurements of metal surface temperatures can be taken using a hand held industrial infrared thermometer, using heat sticks to determine if the temperature has exceeded the value of the stick material, or by the least accurate method of visually observing the color of the steel. Table 20.5 below provides the colors steel may emit at different temperatures.

Note. From Steelworker, Volume 1, NAVEDTRA 14250 by NETPDC, (Table 2-1), 1996, U.S. Navy.

Uses of carbon steels in industry today

Everything from buildings, ships, bridges, railroads, cars and trucks, shelves, home appliances such as washing machines and dryers, water heaters, pressure vessels, tanks, pipes, and many more daily-used items that improve our everyday lives are made out of carbon steels or have carbon steel components.

Figure 20.12 below shows the uses of steel in a large building. Notice the size of the small trailer in the middle of the structure on the ground.

Figure 20.13 below shows a steel ship being built.

Figure 20.14 shows railcars and presumably a railroad that they are riding upon, which is also made of steel.

Other uses of steel are for cars and trucks. The sanitation truck in Figure 20.15 above is made of steel and is welded. Figure 20.17 shows two bridges, where the one on the left was built in the 1920’s while the one on the right was built in the 1990’s. Notice how the new bridge uses the same construction style to match the old bridge.

Figure 20.17 above shows a steel pipeline being constructed. Figure 20.19 below shows a steel water tank in green being used for the local towns water supply.

Welding processes used with carbon steels in industry today

The following list is not exhaustive but is a list with the welding processes you will encounter as a student in your technical program for welding carbon steels. Generally speaking all welding processes at your local community college or vocational school are applicable to welding carbon steels. Table 20.6 below offers a very general overview of how the processes are used, but remember there are always exceptions to generalizations.

Note. Heliarc is the original trade name by the Union Carbide Corporation

It is important that you learn to use as many welding processes as possible while in your welding program because many processes are used together in industry such as using GTAW for root passes followed by SMAW on pipe welding. These next few sections provide a high level overview of the welding processes. Please see the chapters on these welding processes for more information.

Shielded Metal Arc Welding (SMAW)

Shielded Metal Arc Welding is a tried and true welding process that has relatively inexpensive equipment that can get into tight spaces to weld. The process has various electrode types of different alloys, tensile strengths, and sometimes hydrogen contents. Low hydrogen electrodes are used for applications where cracking is an issue such as when welding thick base materials. If low hydrogen electrodes are not used, that hydrogen may not have enough time to diffuse out of the base material which could lead to cracks. Use of low hydrogen electrodes and even a post weld heat treatment may be used to reduce and remove hydrogen from the weldment. Chapter 8 discusses SMAW in detail.

Gas Metal Arc Welding (GMAW)

Gas Metal Arc Welding (GMAW) is a process that does not produce a slag, is a higher deposition process than SMAW meaning that it can lay down more weld. GMAW has the ability to deposit more weld metal than SMAW due to the filler wire being continuously fed into the welding gun with the pull of the trigger. The drawback of GMAW is that it suffers from being susceptible to air drafts which does not make it a good candidate for field welding. Air drafts will blow away the shielding gas that is needed to protect the weld metal while it is hot from the elements such as nitrogen and oxygen that are in the air which will result in porosity in the weld. Chapter 10 discusses GMAW in detail.

Flux Cored Arc Welding (FCAW)

Flux Cored Arc Welding is a semi automatic wire feed process that is being adopted by welders in the field. This process has a self shielded and a gas shielded variation. The self shielded FCAW, or FCAW-S, is similar to SMAW in that the electrode contains flux material that shields the weld from the air. The other variation is FCAW-G which uses a cored electrode that has a flux in it and a shielding gas which together protect the weld from the air.

One question that might be asked is “Can I use a FCAW-S wire with a shielding gas if I run out of FCAW-G wire?” You certainly could set up a machine to do this, but it is not advised because the elements that are in the FCAW-S wire will not react with the air to form a slag as they are intended and will instead end up in the weld pool which will likely negatively affect the chemistry of the weld and in turn the mechanical properties of the weldment. This is similar to hauling construction materials in a compact car vs. a truck; you can do it but unintended damage is likely to occur. If it is a code weld, you can not use FCAW-S wire because it is not listed on the FCAW-G WPS.

The benefits of using FCAW are that it has a higher deposition rate and it can be considered a low hydrogen process even though the hydrogen levels in the electrode are not as low as some low hydrogen SMAW electrodes. The downside to FCAW is that it may not have the versatility that SMAW has for tight spaces, and the welding machine needs to be close by because longer welding leads will suffer from worn out liners because the wire will rub more on the liner as it makes its way through the lead. Chapter 9 discusses FCAW in detail.

Gas Tungsten Arc Welding (GTAW)

Gas Tungsten Arc Welding (GTAW) is also used to weld carbon steels. This process produces high quality welds due to the control the welder has over the heat input and the general welding process. These welders are highly trained professionals.

The downside to GTAW is that it is not a field welding process and is typically done in a shop due to the shielding gas being susceptible to drafts blowing it away. Like FCAW, the welding leads are short because the welding machine is typically close by. Also GTAW suffers from a low deposition rate and would take a very long time to fill groove welds that are easily filled by SMAW, FCAW, and GMAW. For this reason, GTAW might be used for the root pass of a groove weld in a pipe butt joint, while the interpasses and cover passes are welding using another process. This is an important example as to why you should be learning as many processes as you can at school because it is not uncommon to find multiple processes being used together. Chapter 11 discusses GTAW in detail.

Other Processes

As mentioned in the previous section for GTAW, sometimes multiple processes are used together such as a GTAW root pass on a pipe weld, followed by SMAW. Sometimes even GMAW and FCAW can be used in succession. If you have a particular welding industry in mind to work in upon graduation, it is highly recommended that you intern at a company or at the very least go on a company tour and find out what welding processes they use. You might be surprised and find that you should learn an additional process you didn’t think was used in that industry.

While not covered in this book due to its limited instruction in community colleges, Submerged Arc Welding (SAW) is a process that is used to weld thick pieces of carbon steel together in bridge building, and construction of very large weldments making it worthwhile to introduce to you since you may find it during your welding career. The process uses a granular flux and is welded in the flat and horizontal positions only. Welding in the horizontal position requires the use of barriers that keep the flux covering the weld pool because the flux would otherwise fall out of the weld joint.

Figure 20.19 below shows a submerged arc weld being performed on a very large weldment for underwater wind turbine foundations shown in Figure 20.21. Notice the granular flux that is used which protects the weld. This flux is typically low hydrogen and must be kept in an oven at about 150 °F to up to 550 °F. As with the name, the arc is submerged and you cannot see it. SAW is a neat process to use that you should try out if given the opportunity.

The flux in Figure 20.17 above is covering the hot weld bead. Sometimes this flux will be recycled. Note that the welding does not emit fumes or smoke through the flux. SAW is used primarily on thick materials and can be used to make I-Beams and pipes.

This section discussed the uses of carbon steel and the different welding processes used to weld it. The most important thing to note is how important it is to learn more than one welding process because you may have to use multiple processes at a time during your career. Welding pipe with a GTAW root followed by SMAW interpass and cover passes is a great example.

Attributions

1. Figure 20.6: Weld Heat Affected Zone by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
2. Figure 20.7: Rolling Cladded Steel by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
3. Figure 20.8: Heat 67-V1-83 being hot rolled prior to first pass by U.S. Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
4. Figure 20.9: Heat 67-V1-83 being hot rolled after last pass by U.S. Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
5. Figure 20.10: Handheld XRF spectrometer by U.S. Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
6. Figure 20.11: Characteristics of sparks generated by the grinding of metals by Headquarters, Department of the Army in the Public Domain; United States government work
7. Figure 20.12: Field-assembled building by U.S. Department of Labor, Occupational Safety and Health Administration in the Public Domain; United States government work
8. Figure 20.13: Shipbuilding and Ship Repair by U.S. Environmental Protection Agency in the Public Domain; United States government work
9. Figure 20.14: Sealed steel containers are loaded onto railcars at the Moab Uranium Mill Tailings Remedial Action Project site in preparation for transport of uranium mill tailings to the Crescent Junction disposal cell, 30 miles north of Moab. by U.S. Department of Energy, Office of Environmental Management in the Public Domain; United States government work
10. Figure 20.15: Airflow Deflector Side Guard by U.S. Department of Transportation, Federal Highway Administration in the Public Domain; United States government work
11. Figure 20.16: View of the historic (left) and modern (right) Navajo Bridge by U.S. Department of the Interior, National Park Service in the Public Domain; United States government work
12. Figure 20.17: A pipeline being lowered into the trench by U.S. Department of Transportation, Pipeline and Hazardous Materials Safety Administration in the Public Domain; United States government work
13. Figure 20.18: Erwin Water Storage Tank Replacement by U.S. Environmental Protection Agency, Office of Ground Water and Drinking Water in the Public Domain; United States government work
14. Figure 20.19: UP schweissen WEA 00037 by Martinhannes is released under CC BY-SA 4.0