20.5 Welding Cast Iron and Cast Steel
David Colameco, M.Ed.
Basics of welding cast iron and cast steel
Cast Iron and Cast Steel are like many other casts such as Cast Aluminum and even Cast Stainless Steel in that they tend to be porous and are brittle. As a student you start by welding low carbon steel, such as A36, because it is easy to weld on and is used throughout industry. Once you have the techniques down to weld low carbon steel, it is much easier to learn how to weld on other alloys that are much more difficult to weld such as stainless steel, chrome-moly, and casts for example.
In the previous sections we discussed how cleanliness and shielding gas were important for welding stainless steel above and beyond the requirements for mild steel. For cast iron and cast steel, cleanliness is also important because many casts are repairs of equipment that were used in service and could be covered in or have soaked up oil, greases, fuels, and other hydrocarbons used to lubricate the machine parts or fuel the parts if it is an engine block for example. You may not be able to remove all of the impurities but you must do the best that you can.
If you are repairing a casting, it is very important to identify the size of the repair. If you are repairing a crack in a casting you must determine how long the crack is. First clean the surface of the casting in the area of the crack by removing any paint but also being careful not to smear metal over the crack through mechanical removal of the paint. You can use dye penetrant tests if doing a repair at home and follow the instructions for timing of the application. The crack is likely longer than you can see with the naked eye, and the dye penetrant might show you a crack that is a few inches longer than first thought.
- The next step is to end drill the cracks to prevent them from spreading any further. Drilling the ends of the cracks removes the sharp point of the crack and replaces it with a round hole. Structurally round holes do not allow a stress concentration to form and the stress flows around the hole. When you end drill the hole make sure to countersink it so you can inspect the metal below the surface to see if the crack is extending below the surface.
Preheating cast iron and cast steel parts is very important due to the brittle nature of casts. The thermal conductivity, the ability of heat to travel through a material, is lower than other materials. Due to this inability of heat to move through the material quickly, the heated metal will naturally expand while nearby metal that is not heated has not expanded as much. This creates thermal stresses and can cause casts to fail by cracking.
It is equally important to have all of your welding equipment and materials ready to go before preheating begins. That way once preheating is completed you are ready to start welding and are not off spending 30 minutes looking for something while the part cools off. Preheating of casts costs money, and any additional preheating caused by delays means more money. If you are independently doing repairs, this added cost could cause you to spend more money on a job than you bring in. With all of the equipment for welding in place, you can begin preheating.
Due to the thermal stresses discussed above, it is very important to slowly and evenly heat up castings to their required preheat temperature. Larger casting generally requires more preheat than smaller ones, and castings with different sized parts sticking out from the main fabrication require more preheat also. Check the base material’s temperature often and use additional preheat to maintain the temperature as needed.
Now that the base materials have been prepped, welding should start immediately. If the heat of welding reveals new cracks in the casting, stop welding and address the crack as if it were an original crack. This will likely mean that the weldment will have to cool back down to room temperature. This must be done slowly. Air cooling is generally too fast. If you used an oven to heat the cast, place the weldment back into the turned off oven and allow it to cool inside. The thermal insulation of the oven will slow the cooling process. Welding blankets can also be used to slow the cooling process.
If using GTAW as the welding process, you will likely see a lot of junk and dirt floating to the surface of your weld pool. This is normal and you will not be able to remove all of this junk, however you can add more weld metal in an effort to coax the dirt out and replace it with sound weld metal. There are many good videos online that show what this junk looks like and how the welders in those videos handle it.
After the welding repair is completed it is very important to slowly cool the weldment. Air cooling is too fast for almost all cast irons and steels. Place the weldment back in the heating oven with it turned off to allow the oven’s insulation to slow the cooling process, use welding blankets, or for smaller weldments bury the weldment in sand to slow the cooling process.
If you are interested in welding on cast iron, many welders will try welding on old cast iron skillets. Just make sure not to destroy a family favorite skillet that has not been used in a while but is still loved. Check yard sales or second hand stores for used skillets at a reasonable price to try welding.
Uses of cast iron and cast steel in industry today
Casts in general are an older process where liquid metal is poured into a mold. Originally this process was used before foundries were invented to produce the wrought products that we primarily use today. Therefore you will find a lot of cast iron pieces from historical fabrications or today being used in mass production of parts such as pots and pans, wheels, and other simple shapes that are mass produced. Figure 20.29 shows historic cast iron stoves while Figure 20.33 shows modern cast iron pots used today.
The railroad industry uses cast wheels in addition to wrought wheels. Figure 20.31 shows images of failed wheels, with the central photo showing a characteristic shattering of a cast on the top left most cracked surface shown.
One of the largest cast iron structures in the United States is the US Capitol Dome. This dome is more than 8.9 million pounds of cast iron and shown in Figure 20.32 to Figure 20.37.
Welding processes used with cast iron and cast steel in industry today
SMAW, OAW, GTAW and GMAW are used to weld cast irons and cast steels. If spatter is an issue when working on parts that have been machined/polished/prepared in a particular way that could be damaged by spatter, then GTAW is the process to use. For other parts SMAW and GMAW will likely be used. Which process is used will primarily come down to how critical the part is and which process is available. If a welding shop primarily repairs casting, then depending upon the size of the casts and the quality of the repair needed a process will be chosen. For casts that need to be repaired for the lowest cost, SMAW and GMAW might be chosen because using GTAW will take a lot more time and be a more expensive weld repair. If a cast engine block is being welded, then GTAW would likely be chosen because the weld is critical to the engine’s performance and a failure of the engine could result in injury or loss of life.
Oxy-acetylene welding has historically been used with casts due to its high heat input which helps maintain the preheat of the cast. However, OAW is not widely used today and some welding programs will spend two weeks out of a ten week quarter welding with OAW only to switch over to OAC (cutting) since most welders will need to use OAC while fabricating.
Attributions
- Figure 20.29: Three 10-Plate Cast Iron Stoves on display at Hopewell Furnace’s Visitor Center. by U.S. Department of the Interior, National Park Service in the Public Domain; United States government work
- Figure 20.30: Campfire Permits by U.S. Department of Agriculture, Forest Service in the Public Domain; United States government work
- Figure 20.31: Example of Vertical Split Rim, Shattered Rim, and Thermal Crack Failed Wheels by U.S. Department of Transportation, Federal Railroad Administration in the Public Domain; United States government work
- Figure 20.32: Capitol Dome by Architect of the Capitol. in the Public Domain; United States government work.
- Figure 20.33: Capitol Dome Rotunda by Architect of the Capitol. in the Public Domain; United States government work.
- Figure 20.34: Looking straight up in the U.S. Capitol Rotunda by Architect of the Capitol. in the Public Domain; United States government work.
Heat added to a weldment or base materials before welding begins. Preheat is done on thick materials and materials that are colder that the required minimum temperature specified in a WPS.