Carbon steel, a widely-used material in various industries, offers strength, durability, and versatility. While its alloy composition enhances its mechanical properties, welding carbon steel comes with challenges, particularly as the carbon content increases. In this guide, we’ll explore the nuances of welding low, medium, and high carbon steels and provide essential tips for successful outcomes.

Understanding Carbon Steel:

Carbon steel is an alloy composed primarily of iron and carbon. It’s the most common type of steel and typically contains low amounts of other alloying elements. It is one of the most widely used materials in construction, manufacturing, and numerous other industries due to its strength, durability, and versatility.

Keep in mind that all types of steel contain more than just iron and carbon. Modern steels only gain some of their strength from simple carbon additions but also from the added effect of small amounts of other alloying elements. These include  manganese, silicon, chromium, molybdenum, vanadium, nickel, and copper. The amount of these elements is represented by the carbon equivalency formula. In addition, this formula will help you understand how the different alloying elements affect the hardness of the steel being welded.

Types Of Carbon Steel

As the amount of carbon in the steel increases, the steel becomes harder, which is desirable when a piece is exposed to high wear and tear. As a result, we’re seeing three different types of steel:

• Low-carbon steel (less than 0.30% carbon)
• Medium carbon steel (0.30%-0.60% carbon)
• High carbon steel (0.60%-2% carbon)

Welding Low-Carbon Steel (Mild Steel):

Low-carbon steel is characterised by a low ratio of carbon to iron. It contains minimal amounts of carbon, typically less than 0.30%, and it is known as mild steel. The low amounts of carbon increase the ductility of the metal and make it cheaper to produce, which makes this type one of the most widely used. This makes low-carbon steel ideal for beginners. Plus, methods such as Gas Metal Arc Welding (MIG), Tungsten Inert Gas (TIG), Stick, or Flux Core can be used without the need for extensive pre or post-weld heat treatment. 

Welding Medium Carbon Steel:

Medium carbon steel typically contains 0.30%-0.60% carbon. This balances the ductility and formability of low-carbon steel and the strength and hardness of high-carbon steel. With added chromium, nickel, and molybdenum, this type of steel is useful when creating items such as studs and gears that will encounter a lot of wear and tear. Due to higher carbon content, medium carbon steels are somewhat more challenging to weld.

High carbon steel 

High carbon steel contains a carbon content ranging between 0.60% – 2%. This makes it highly corrosion-resistant, hard, and resistant to wear and tear. They are often referred to as tool steel since it is widely used to make cutting tools, punches, dies, injection molding tools, ball and roller bearings for engines, and more. However, the higher carbon content also makes these steels more brittle and less ductile, causing cracking in the weld joint.

Challenges of Welding Carbon Steel

When welding carbon steel, the heat from the welding process affects the surrounding material. This creates what’s known as the heat-affected zone (HAZ). Due to a higher carbon equivalent, this zone can experience changes in microstructure when welded, leading to potential weaknesses or alterations in properties.

Firstly, high carbon content can lead to increased susceptibility to weld cracking. The carbon content in the metal can cause brittleness, leading to cracking during the cooling process. Rapid cooling during the welding process can result in martensite formation, a very hard but brittle microstructure in the weld area.

The huge carbon content in the metal is also the leading cause of warping. Warping is the deformation of the metal during the cooling process, and it can happen because of the high carbon content in the metal. Traditionally, weld discoloration has emerged as an esthetic issue in carbon steel welding. However, it can also indicate other problems, such as a lack of fusion, porosity, and more.

Therefore, welding carbon steel can be a daunting task, especially once the carbon content is high. However, there are certain precautions and procedures that can help you avoid cracking and other issues when welding carbon steel, so let’s check them out. 

Welding Mild Steel (Low Carbon steel)

As noted, you can weld mild steel with one of the four most popular methods, such as Gas metal arc welding (MIG), TIG, Stick, or Flux Core.

MIG welding mild steel is a common choice for beginners, as you need little to no prior experience to do it. You can choose matching filler metal according to the composition of the weld metal. Plus, there is no pre or post-weld heat treatment required. Stick welding low-carbon steels is often preferred for quick on-field and outdoor repairs, and like with MIG, there are no specific precautions. You can use Flux core welding as a wire welding alternative for SMAW.

TIG welding is often preferred when welding exotic or non-ferrous metals since it offers excellent heat control and provides high-quality welds. Still, you can use it to weld mild steel or any similar welding project with thinner material, but it requires more skill and experience. 

Welding Medium/High Carbon Steel

Things get tricky once the carbon is over 0.30% in the steel. The carbon reduces ductility and increases the strength so that the welds can become highly brittle and susceptible to cracking. As a result, welding high-carbon steel can be challenging and requires specific welding procedures. To successfully weld steel with medium to high carbon equivalency, you will need the right filler metal, heat input, and heat treatment.

Filler Metal For High Carbon Steels

Medium/high carbon steels are exceptionally hard, and their tensile strength can reach 35900 – 484000 psi. When welding such a high-strength steel material, there are two ways of thinking in selecting the welding consumables. You can either choose a filler that matches the strength of the base metal or consider a cracking issue and choose a lower-strength filler metal.

However, of these two, it is more important to pay attention to the cracking risks of the welding joint. The harder the joint is, the bigger the chance that it will crack. So when possible,  you will want to select the lower-strength filler metals. As the joint has higher crack resistance, it is considered stronger, even if its overall strength is lower than two separate plates.

Additionally, when selecting filler for steel with higher carbon quantities, you should avoid using electrodes with excessive hydrogen content. Diffusible hydrogen can cause cold cracking, so low-hydrogen electrodes are typically the best choice. 

In specific cases, 309−type austenitic stainless steel welding consumables are recommended when welding high-carbon steels. Once again, the reason is low diffusable hydrogen content, which can cause cracking. The absence of hydrogen in this filler contributes to crack resistance together with the stable structure of the weld metal.

Heat Treatment

Medium and high carbon steels require a specific pre and post-weld heat treatment to avoid cracking. Welding carbon steel in a room-temperature environment will cause rapid cooling. This then leads to hardening and eventually cracking, so you will need pre- and post-heating processes. These processes are also known as the stress relief annealing process. This is the precise heat treatment in which carbon steel is heated to a particular temperature and cooled down slowly to make it ductile and malleable, increasing the overall weldability.

Preheating the carbon steel before welding will promote slow cooling, remove moisture and hydrogen, and reduce the thermal stress into pieces. The preheat temperature will vary on the thickness and grade of the steel, and it is typically 150°C to 260°C. A high level of accuracy generally is not required for preheating carbon steels, but you don’t want to go over 540°C. 

Post Heating Treatments

Post-heating processes are much more critical when welding medium and high-carbon steels. With the post-heat treatment right after welding, you will prolong the cooling rate, which increases the cracking resistance and decreases the hardness of HAZ. Carbon steel pieces can be post-heated through full annealing and soft annealing.

• Full annealing includes slowly heating carbon steel to a holding temperature, which is 50 C (122 F) above the austenitic temperature (between 750-900 °C / 1320-1652 °F). After that, the pieces are cooled down slowly to the ambient temperature. The recommended cooling rate is 20 °C (68 °F) per hour. Once the parts reach 50 °C (122 °F), they can further be cooled to room temperature using air with natural draft.
• Soft annealing is often used with high-carbon steels that require slower cooling rates. This process can last 12 to 48 hours. After welding, pieces are heated to the temperature of 800 °C (1472 °F). The metal is held at this temperature for 2-4 hours so that the structure fully converts into austenite. Next, the temperature is quickly brought down to 790 C (1454 °F), and further cooling is controlled at a steady rate of 10 °C (50 °F) per hour until the temperature reaches 690 °C (1274 °F). The metal is then cooled to room temperature.
• Welding carbon steel can also cause stress on the pieces, so that you can opt for stress-relief annealing. Welded components are heated up to the temperature of 600 to 650 C (1112 to 1202 F) and kept in this condition for an hour or more. Next, they are allowed to cool in the furnace till the temperature drops to 426 °C (800 °F), then cooled to ambient temperature slowly in the still air. 

Welding Methods:

You can successfully weld carbon steel by using MIG welding, TIG welding, or Stick welding. Among these, MIG welding is a versatile option, but caution is advised to avoid excessive heat. When MIG welding high carbon steels, make sure you reduce the heat and use a smaller wire diameter. High amperage can cause hot cracks in the weld joint, so avoid using too much heat.

Stick welding high-carbon steels suits thicker pieces, with a focus on low-hydrogen electrodes. When using stick welding, make sure you opt for low-hydrogen electrodes. Hydrogen will cause cold cracking or delayed cracking, and even though post-heating will reduce the levels, try to avoid fillers with hydrogen.

TIG welding carbon steel is often preferred on thinner pieces, where you want to avoid high heat that can cause brittleness and cracking. The key to successful welding is undermatching the filler metal, which means using a filler rod that is one or two grades lower than the base metal. 

Just as a quick recap, here are some key takeaways on welding carbon steel. 

Key Takeaways:

1. Weld low-carbon steels at room temperature.
2. Increase pre and post-heat treatment as carbon content rises.
3. Preheat to 300-500°F to promote slow cooling and reduce stress.
4. Post-heat to prevent martensite formation and ensure a ductile microstructure.
5. Undermatch filler metals and choose low hydrogen rods.
6. Avoid excessive heat to prevent thermal stress and defects.

Final Thoughts:

Mastering the art of welding carbon steel requires a special approach. Its carbon content and using the proper methods and precautions make welding carbon steel a tedious and challenging task. Whether you’re a novice or an experienced welder, understanding the intricacies of carbon steel welding can help ensure a successful and durable outcome.