Do You Have To Preheat A 7018 Welding Rod?

The 7018 welding rod is a commonly used low-hydrogen type covered electrode, widely applied in welding fields that require high strength and toughness of welds, such as structural steel welding and pressure vessel manufacturing. For many welders, a key question is whether preheating is necessary when using 7018 welding rods. The answer is not absolute-it depends on specific welding conditions, but in most cases, preheating is recommended or even required.​
1. Why preheating is important for 7018 welding rods?​
1.1 Prevent hydrogen-induced cracks​
The biggest feature of 7018 welding rods is their low hydrogen content in the coating, which is designed to reduce the risk of hydrogen-induced cracks in welds. However, if the welding rod absorbs moisture from the environment (such as being stored in a humid place), the hydrogen content will increase significantly. Preheating the welding rod can evaporate the absorbed moisture, reducing the hydrogen entering the weld pool during welding. Hydrogen-induced cracks usually occur in the heat-affected zone (HAZ) or the weld metal after cooling, which can seriously affect the mechanical properties of the welded joint, especially for high-strength steels or thick workpieces.​
1.2 Improve weldability​
Preheating the base metal (not just the welding rod itself) can slow down the cooling rate of the weld and the heat-affected zone. For 7018 welding rods, which are often used to weld medium and high-strength steels, a too-fast cooling rate may cause the formation of hard and brittle structures (such as martensite) in the heat-affected zone, reducing the toughness of the joint and increasing the risk of cracking. Proper preheating can reduce the temperature gradient between the weld and the base metal, allowing the metal to cool more evenly and avoiding excessive brittleness.​
1.3 Ensure fusion quality​
In the welding of thick workpieces, if there is no preheating, the heat from the arc is easily absorbed by the cold base metal, making it difficult to form a sufficient welding pool. This may lead to defects such as incomplete fusion and insufficient penetration. Preheating raises the temperature of the base metal in advance, ensuring that the heat input during welding is enough to melt the base metal and the welding rod, thus obtaining a well-formed weld.​
2. When is preheating of 7018 welding rods necessary?​
2.1 Thickness of the base metal​
When the thickness of the base metal exceeds a certain range, preheating becomes necessary. Generally, if the thickness of the base metal is more than 12mm, preheating is recommended. For workpieces with a thickness of more than 25mm, preheating is almost mandatory. This is because thick workpieces have a large heat capacity and cool quickly after welding. Without preheating, the risk of hydrogen-induced cracks and cold cracks in the weld and heat-affected zone increases significantly.​
2.2 Material of the base metal​
If the base metal is high-strength low-alloy steel, carbon-manganese steel, or other materials with high hardenability, preheating must be carried out when using 7018 welding rods. These materials are more sensitive to hydrogen and cooling rate, and even for thin workpieces, preheating is required to reduce the hardening tendency. For example, when welding A514 high-strength steel with 7018 welding rods, the preheating temperature is usually required to be between 150 - 260°C, depending on the thickness of the workpiece.​
2.3 Environmental conditions​
In low-temperature environments (below 0°C), even for thin low-carbon steel workpieces, preheating of 7018 welding rods and the base metal is necessary. Low ambient temperatures will accelerate the cooling rate of the weld, increasing the risk of cracking. In addition, if the workpiece is in a humid environment, the welding rod is more likely to absorb moisture, and preheating the welding rod can reduce the impact of moisture on the weld quality.​
2.4 Welding structure and stress state​
For welded structures with high restraint, such as rigid joints or closed structures, preheating is essential. The restraint stress generated during welding, combined with the cold shrinkage of the weld, can easily lead to cracks. Preheating can reduce the temperature difference between the weld and the base metal, thereby reducing the restraint stress. For example, in the welding of box-shaped structures, the restraint degree is high, and preheating with 7018 welding rods can effectively prevent cracking.​
3. How to correctly preheat 7018 welding rods and base metal?​
3.1 Preheating of the welding rod​
7018 welding rods must be stored in a dry environment after purchase. If they are exposed to humid air for a long time (such as storage in a warehouse with high humidity), they need to be baked before use. The standard baking temperature is 350 - 400°C, and the baking time is 1 - 2 hours. After baking, the welding rods should be stored in a holding oven at 100 - 150°C to prevent re-absorption of moisture. When taking them out for use, they should be placed in a portable electrode holder to avoid contact with humid air for a long time. Generally, if the welding rod is taken out of the holding oven for more than 4 hours, it needs to be re-baked.​
3.2 Preheating of the base metal​
The preheating temperature of the base metal should be determined according to the material, thickness, and welding specifications. Common preheating methods include flame heating (using an oxy-acetylene torch), electric resistance heating, and induction heating. During preheating, a temperature measuring instrument (such as an infrared thermometer or a thermocouple) should be used to monitor the temperature to ensure that the preheating temperature meets the requirements and is evenly distributed. For example, when welding a 20mm thick high-strength steel plate with 7018 welding rods, the preheating temperature can be controlled at 200 - 250°C, and the heating range should be at least 75mm around the weld.​
3.3 Post-heating and heat preservation​
After welding, if the thickness of the workpiece is large or the material has high hardenability, post-heating and heat preservation measures should be taken. Post-heating can promote the diffusion of hydrogen in the weld and reduce the residual stress. The post-heating temperature is usually 250 - 350°C, and the heat preservation time is calculated according to the thickness of the workpiece (generally 1 hour per 25mm thickness). For example, after welding a 30mm thick workpiece with 7018 welding rods, post-heating at 300°C for 1.5 hours can effectively reduce the risk of hydrogen-induced cracks.​
4. What happens if preheating is neglected?​
4.1 Weld defects​
Neglecting preheating of 7018 welding rods may lead to a series of weld defects. If the welding rod is damp and not preheated (baked), pores and slag inclusion will appear in the weld, and hydrogen-induced cracks may occur after welding. For the base metal, insufficient preheating will cause incomplete fusion, insufficient penetration, and cold cracks in the heat-affected zone. These defects will reduce the strength and tightness of the weld, and in severe cases, may cause the welded structure to fail during use.​
4.2 Safety risks​
In critical applications such as pressure vessels and bridge structures, weld cracks caused by lack of preheating may lead to serious safety accidents. For example, if a pressure vessel welded with 7018 welding rods without preheating has hidden cracks, the cracks may expand under internal pressure, resulting in leakage or even explosion, endangering personal safety.​
5. Conclusion: preheating is a key step that cannot be ignored​
In summary, whether 7018 welding rods need to be preheated depends on factors such as base metal material, thickness, environmental conditions, and structural stress. In most cases (especially for thick workpieces, high-strength steels, low-temperature environments, or high-restraint structures), preheating is not only a recommended operation but also a mandatory requirement to ensure weld quality and safety. Correct preheating of the welding rod (baking and heat preservation) and the base metal can effectively prevent hydrogen-induced cracks, improve weldability, and ensure the mechanical properties of the welded joint. Therefore, welders must not ignore the preheating step when using 7018 welding rods-only by following the standard preheating procedures can reliable welding results be achieved.