Wear steels such as bucket steel / cutting edge and wear plate are more difficult to weld than ordinary weldable structural steels, such as S355. This is because wear steel has good hardenability. Your biggest enemies when welding wear steel are moisture and too short transitions between unhardened and hardened steel at your weld. Therefore, there are some things to keep in mind when welding wear steel such as bucket steel / cutting edge and wear plate.
The mechanical qualities of wear steel change a lot with temperature changes in the steel. The hardened steel has a very high yield strength and low impact resistance. When the same steel is normalized (hardened), it shrinks slightly in size and has a low yield strength. The difference is due to the fact that the structure of the steel inside has changed from martensite (hardened) to ferrite (unhardened). If the transition from martensite to ferrite in the wear steel is short, there will be a crack-prone "scarring" that can crack immediately after the weld has cooled, a few days later or when it begins to load.
Soft hardness transitions at the weld - let the heat spread
Directly at the weld itself, the bucket steel or wear plate becomes unhardened and mostly ferritic. In order for the transition to unhardened to not be too short, the heat from the joint must be spread in the wear steel, ie do not weld too quickly and do not let the steel cool too much before you lay the next joint. But at the same time keep in mind that the wear steel loses its wear resistance when it softens so you must not burn too hard either. The balance with it is a craft that every welder must learn and it is a little different for different types of wear steel (depending on carbon content and alloy). A rule of thumb, however, is that the steel about 7-10 cm from the weld should be at about 150-200 degrees when you weld.
In order for the transition from hardened to unhardened to be smooth, the surrounding steel cannot be too cold. Thinner steel heats up quickly by the weld, but the thicker and larger it is, the more the steel cools against the weld and the more you may need to preheat the entire steel before welding. Also keep in mind that the steel can quickly cool below 150 degrees, for example when the bucket has to be turned, the steel may need to be preheated again before further welding.
How quickly the wear steel loses hardness with increasing temperature depends on its carbon content and alloy, but roughly it can be said that weldable wear steels begin to soften when they become above 200 degrees. The hardness does not disappear immediately and completely above 200 degrees, but the softening of eg wear steel with 0.27% carbon content begins approximately there and increases with increasing temperature and the time that the steel is above 200 degrees. At 900 degrees, the steel is red annealed and then the steel is as soft as it can be before it melts. Even if reduced hardness reduces the wear resistance of the steel, you should primarily prioritize a soft hardness transition so that you get a strong weld. Reduced wear resistance of the wear steel around the welds is usually preferable to cracked welds.
Moisture is the enemy of welding - keep it dry and clean
Your second enemy when welding, moisture, is everywhere present when you are going to weld. Moisture, ie water, is present in the air, on the steel, in dirt on the steel and in welding rods. Water consists of oxygen and hydrogen, it is the hydrogen that is the enemy of steel. When the steel melts, the hydrogen is drawn into the steel and when the steel solidifies, it can cause the steel to crack.
Things to keep in mind to minimize moisture when welding:
- Store welding rods in tight and closed packaging,
- Clean the welding surfaces from slag, paint and embers,
- Preheat the bucket steel and wear plate with gas to drive away the moisture that is present on the surface,
- Have low humidity in the room, ie ventilate (especially important if you often drive wet/snowy tools and machines into the workshop).
Wear steel and carbon equivalent
Manufacturers of bucket steel and wear plate generally state in their data sheets instructions for welding specific types of wear steel. How the steel reacts in heat, which directly affects the weldability, depends, as stated above, on its hardenability. The higher the carbon content and the higher the content of alloying elements, the more reactive in heat the steel is and the more difficult it is to weld. The data sheets usually state a value for weldability, the "carbon equivalent tent", a mathematical formula for estimating the weldability of steel. The higher the value of the carbon equivalent, the worse the weldability. The higher the carbon content and the higher the content of alloys the higher the carbon equivalent. For structural steel, the norm is that the carbon equivalent may be a maximum of 0.45. But the thicker the steel, the lower the carbon equivalent should be to have good welding properties. The carbon equivalent is not an exact science and therefore there are also different ways to calculate it. However, it is the carbon that mainly impairs the welding qualities, the alloying elements have less effect. The harder the wear steel is, the more carbon it usually contains and the deeper it is hardened, the greater the amount of alloying elements it usually contains. "Hardened" wear steels with high hardness are good for wear resistance, but for welding it is more difficult and becomes increasingly difficult with increasing thickness. To maximize both wear resistance and weldability in thicker steel, it is better with lower carbon content and higher alloy content than the other way around.
No standard for designations - look in the data sheets
For structural steels, there are standards and general designations for the steels that mean the same thing from different manufacturers. This is not the case with bucket steel and wear plate. What manufacturers call their wear steels and what they state about the hardness of the steel are different between the manufacturers, especially in thicknesses over about 35 mm. Some manufacturers, for example, have very high carbon contents in their 400 Brinell wear plates in thicker dimensions. This is to be able to achieve high hardness, but it destroys the weldability. The same manufacturer may also have wear plates called "450" which in the same thickness have a lower carbon content than their own "400". In order to compare the wear steels of different manufacturers regarding weldability, one should therefore look at the carbon and alloy contents in the data sheets.
Choice of wear steel
At Partrex, we are careful and conscious when we choose bucket steel / cutting edge and wear plate to be welded, especially in thicker dimensions. For cutting edge in 60 mm, for example, in which tooth adapters are to be welded, a carbon content of a maximum of 0.22% can be a good choice. Feel free to ask us for advice on what can be a good choice for how you should use the wear steel. Regarding the choice of welding wire and welding rods and recommended preheating temperatures, we recommend that you start from the manufacturer's data sheet for the type of steel in question.
Partrex wear steel products: