Welding often makes the steel lose its strength. So, you might be wondering how to harden steel after welding to make it stronger. Think of hardening steel after welding as giving your creation a finishing touch, like adding frosting to a cake. Hardening can make your welded piece look good and last longer, like frosting.
Don’t worry; we’ll cover everything from the different methods of hardening steel to the best techniques for achieving the perfect hardness. Trust us, after reading this blog, your welded steel will be ready to take on anything life throws its way!
Types of Steel
Knowing the steel type is important before attempting to strengthen it because each has distinctive characteristics, such as melting temperature, carbon content, and hardness, that determine its reaction to different welding processes and post-heat treatments.
Understanding these unique steel properties will help you select the appropriate welding technique and heat treatment methods to achieve the desired hardness and strength. Failing to follow the recommended guidelines for each steel type can result in issues such as cracking, warping, and decreased hardness.
I have discussed some of the necessary steel types to clarify your concepts.
1: Low Carbon Alloy Steel
This type of steel has low carbon content, typically ranging from 0.05% to 0.30%. Adding other alloying elements, such as nickel, chromium, or molybdenum, adds strength and corrosion resistance to the steel.
However, the low carbon content also allows for greater flexibility and ductility, making it easier to bend and shape without breaking. It is often used in applications where a combination of strength and flexibility is required, such as automotive parts, pipelines, and construction materials.
2: Medium Carbon Steel
This type has a carbon content that ranges from 0.30% to 0.60% and offers a good balance of strength and toughness, making it suitable for various applications.
It is often used for parts that require high-impact resistance, such as springs, gears, and structural components. The increased strength compared to low-carbon alloy steel makes it more suitable for applications that require a higher level of wear resistance.
3: High Carbon Steel
Carbon content ranges from 0.60% to 1.5% in this type and is known for its high hardness and strength, making it ideal for applications requiring high wear resistance and durability. High Carbon Steel is commonly used to produce cutting tools, knives, and other wear-resistant components.
However, its strength and hardness make it more brittle and susceptible to cracking. Therefore, extra care must be taken during welding and other fabrication processes to avoid damaging the steel.
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The Effect of Welding on Steel Composition
Work- or Strain Hardened Metals
When these metals are exposed to intense heat, they lose their hardness and become soft in the HAZ (Heat affected area). The only area that experiences this change is the HAZ, as the filler metal and admixture used in the welding process do not undergo recrystallization and remain as strong as the base metal. This is why failures in these types of steel typically occur in the HAZ near the weld joint rather than directly in the joint.
So, you need to consider the design of the weld joints, especially when welding metals such as wrought iron, rolled aluminum, and cold-rolled steel. Also, don’t forget to factor in the stress the final weldment will experience in the welding process.
Precipitation- Hardened Metals
Precipitation-hardened metals experience more complex changes when compared to work-hardened metals. However, the result is similar, as the HAZ becomes soft because of the annealing cycle.
This is because the precipitate that provides the metal with its strength grows and agglomerates when exposed to heat, leading to over-aging and reducing precipitation hardening effects. The higher the heat, the faster the metal reaches this weakened state.
However, you can overcome this issue with the help of the post-weld heat treatment provided if the filler metal matches the aging characteristics of the base metal.
On the other hand, metals that have been solid-solution-hardened experience the least amount of change during welding, with only a small amount of grain growth at the fusion line that typically does not affect the metal’s properties.
Transformation- Hardened Metals
When these metals go through the welding process, their reaction is similar to solid-solution-hardened metals if they can form martensite through heat treatment or have already formed it in previous treatments.
The temperature during the welding process can be divided into four regions in the heat-affected zone (HAZ), and the amount of heat used affects both the size of the HAZ and each region.
The more heat used, the wider the HAZ becomes and the slower the cooling process is. A slower cool makes it less likely to form martensitic regions, which can cause brittleness after welding. To prevent this, preheating can be done to slow down the cooling rate and reduce post-weld brittleness.
In some cases, it may also be necessary to perform post-weld heat treatment to slow the cooling even further. Keep in mind the harder the HAZ, the more martensite is formed and the higher the risk of cracking.
Heat Treatment Methods for Steel Hardening
It’s crucial to control the metal temperature when heating and cooling to get the desired outcome. Metal can be heated using ways such as using oxyfuel or fuel-air torches, temperature-indicating crayons, furnaces, induction heating, electrical resistance heating, natural gas, or electrically heated salt baths.
Similarly, there are different methods for cooling, such as gradual furnace cooling, still air cooling, agitated air cooling, fan cooling, water cooling, or cooling buried in the sand.
The success of the heating and cooling process lies in controlling the heating rate, temperature, duration of heating, and rate of cooling. These specifications vary based on the type of metal and the desired outcome of the heat treatment.
I’m going to discuss some of the methods you can use to increase the strength of the steel after welding.
Annealing is a heat treatment process often used to improve the mechanical properties of steel after welding by changing the steel microstructure and reducing internal stresses witnessed during the welding.
The process starts by heating the steel to a specific temperature range (600 to 650 degrees °C), then holding or soaking it at that temperature for one hour according to per inch thickness, and then slowly cooling it down in a furnace (below 600°C) and finally let it cool down to room temperature.
The heating, soaking, and slow cooling grow the grain larger, leading to softness and ductility.
There are different types of annealing processes, including full annealing, process annealing, and stress-relief annealing. The annealing process type depends on the steel type and the desired outcome. For example, full annealing is typically used for low-carbon steels, while process annealing is used for high-carbon steels. Stress-relief annealing reduces internal stresses in steel subjected to cold working or welding.
When annealing steel after welding, it’s important to control the heating, temperature, and cooling rate. Quickly heating steel could distort the grain structure, resulting in low strength. Similarly, if the steel is cooled too quickly, it can cause the formation of hard, brittle regions within the steel that can lead to cracking.
Normalizing is another heat treatment process that changes the steel microstructure resulting in a more uniform grain structure. This uniformity in the grain structure leads to improved strength and toughness of the steel.
The normalizing process involves heating the steel to a temperature above its critical range, typically higher than 650 degrees °C, unlike in annealing, and then held at this temperature for a set period, typically 30 minutes to 1 hour, depending on the type of steel and the desired outcome.
After this time, the steel is removed from the heat source and cooled in still air at room temperature. However, this doesn’t makes soft as annealing.
Hardening or Quenching
The process involves heating the steel to a high temperature, typically above its critical temperature, and then dip in oil or water for sudden cooling, transforming the steel’s microstructure and causing it to harden.
It is carried out by pre-heating previously quenched or normalized steel to a temperature below the critical range, holding and then cooling it down to get the desired mechanical properties.
Benefits of Post-Weld Heat Treatment
I have listed some of the benefits of heat treatment
- Heat treatment increases the strength and toughness of the welded joint, making it more resistant to stress and cracking.
- Residual stress is the stress that remains in a material after it has been subjected to deformation Heat treatment can reduce residual stress in the welded joint, preventing cracking and improving the structure’s overall stability.
- Fatigue is the gradual failure of a material due to repeated loading and unloading. Heat treatment can improve the fatigue resistance of the welded joint, increasing its ability to withstand repeated cycles of stress.
- Heat treatment can improve the corrosion resistance of the welded joint by reducing the concentration of impurities and enhancing the metal structure.
- It reduces warping and distortion in the welded joint, improving its dimensional stability and accuracy.
- It improves the properties of the base metal, such as strength, toughness, and ductility in the heat-affected zone (HAZ).
Overall, heat treatment in welding can greatly improve the quality and performance of the welded joint, making it stronger, more stable, and more resistant to various types of stress and damage.
Strengthening steel after welding is a crucial step in ensuring the longevity and durability of the final results. Using techniques such as post-weld heat treatment, normalizing, annealing, quenching, and tempering can significantly increase the strength of the welded steel.
To make the most of these techniques, it’s important to choose the right process for your specific project and steel type and follow proper heating and cooling procedures. Following the right approach can help you produce strong and reliable welds that meet even the toughest conditions.
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