Welding Cast Steel: A Helpful Guide For Beginners

ByAndrew Valsamis Updated on Guides

If you are a beginner welder, you might wonder about the weldability of cast steel. For most applications, it isn’t as difficult as someone may believe. But there are several factors to consider in order to avoid costly mistakes.

This article will help you learn the basics of cast steel welding and how different it is compared to typical wrought steel.

Table of Contents

A photo of a cast steel component
cast steel

What is cast steel?

Cast steel is carbon or low-alloy steel that has been melted, poured into a mold to flow in the desired shape, and solidify to produce the desired component.

Casting is beneficial when you want to create complex shapes of various sizes with uniform joints. You avoid cutting, machining, fitting, welding, and a lot of wasted material with the casting method (Wikipedia).

But many times, cast steel wears off, cracks or breaks, and some welding must take place.

Can you weld cast steel?

You can weld cast steel with great success if it has a typical low-carbon composition. However, it’s tough to weld if the casting is medium, low-alloy, or even worse, high-carbon steel.

The chemistry of cast steel is similar to typical rolled steel, but some differences make it a bit harder to weld. Also, the service conditions of cast steel parts are often more demanding, making it even harder to weld.

But are you certain it is cast steel?

Is it cast steel or cast iron?

You must be certain that your casting is made of steel because it’s very different to weld than cast iron.

Most castings are made of cast iron because:

  • It’s easier and cheaper to produce.
  • Can absorb vibrations to a high degree.
  • Keeps precise dimensions under stress.
  • Has high compression strength.
  • Is corrosion-resistant compared to typical cast steel.

However, there are service conditions where cast steel is preferred because it has high ductility, tensile strength, and impact resistance. Ductility is how much it can bend without breaking. The most common cast iron types almost don’t bend and break under stress.

Welding cast steel is much easier than cast iron because the last one contains huge amounts of carbon. The higher the carbon content, the more prone to cracking the casting is.

Weldpundit has an introductory article on cast iron welding.

How to tell the difference between cast iron and cast steel

Let’s see a quick overview of how to tell between the two metals:

  • The color of the surface or a broken part is a good indicator. Typical cast iron looks dark gray because it contains high amounts of carbon. Cast steel has a typical metallic grayish color.
  • You can grind the casting and look at the sparks. If the sparks are long and yellowish, it is cast steel. If they are short and reddish, it is cast iron.
  • You can slightly drill, file, or use a chisel on the casting. If the shavings are continuous, it is cast steel. If they look like dust or small pieces chip off, it is cast iron.
  • The service conditions can indicate the metal. Cast steel is used if the workpiece must have ductility and impact resistance. Cast iron is used for high vibration absorption and compression resistance.
  • You can form a small puddle with a TIG or oxyfuel torch. If the puddle is reddish, it is cast iron. If it looks yellowish, you have cast steel. Keep in mind, that this method will locally harden cast iron and high-carbon steel castings.
  • Finally, if the casting comes from a well-known manufacturer, call them for information. The casting composition of popular equipment might change through the years, and the manufacturer can give precise information.

Identify the cast steel composition

But that is not all. After figuring out that you indeed have cast steel, you must know what type you have.

Most cast steels are low or medium-carbon steels. But sometimes, it may be low-alloy, or high-carbon, depending on the service conditions.

Identification is the most important thing to get right to select the correct welding procedure and filler metal, if preheating is necessary, etc.

Weldpundit has an in-depth article on how to identify metals for welding.

Is welding cast steel hard?

Welding cast steel is slightly harder than the equivalent wrought steel types. You use the same filler metals and follow the same procedures, joint design, etc. However, you must take into consideration the casting’s service conditions and focus on preventing warping issues.

What to consider before welding cast steel?

1. Service conditions

The service conditions of the casting play a big role in how and if you should attempt to weld it.

For example, it will be hard to weld it with success if the casting endures constant heating and cooling or endures high pressure or impact.

These castings have a high-carbon content and/or other alloying elements. Furthermore, if you cause warping or metallurgical changes, the joint may fail, resulting in damages or injuries.

On the other hand, if the casting endures low loads at room temperature, it would be easy to weld. Even if the weld is not the best and fails at some point, the consequences would not be serious.

If the casting has demanding service conditions or is irreplaceable, it would be best to give it to a professional for welding.

2. Distortion

A big problem with castings is avoiding distortion (warping) caused by the welding heat.

If the workpiece expands or bends even slightly, it might not be able to serve its original purpose anymore. Furthermore, cast steel with distortion is very hard, if not impossible, to correct.

The key to prevent distortion is to avoid fast and uneven temperature changes in the workpiece. Some basic measures to avoid distortion are:

  • Tack weld frequently.
  • Preheat the casting according to the metal type and thickness.
  • Use the lowest amperage that will penetrate the root and offer good overall fusion.
  • Use the fastest traveling speed possible.
  • If the casting is too thin or complex, use techniques such as backstep welding that distributes the heat more evenly.
  • Use straight beads with minimal side-to-side movement.
  • Use the fewest passes possible.
  • After welding, ensure the casting will cool down at a slow rate. You can do this by wrapping it with welding blankets or by burying it in sand.

3. High amounts of silicon and manganese

Another thing you must know is that cast steels always contain higher amounts of silicon than their rolled steel equivalents. Silicon helps the molten metal to flow easier in the mold and also prevents oxidation. Furthermore, the amount of manganese that cast steel has can also be higher.

The increased amounts of these elements make cast steel a bit harder to weld because they:

  • Make thick metals hard after welding. If that happens, the casting will be prone to cracking caused by trapped hydrogen in the weld area. This is called cold cracking and can happen days, weeks, or even longer after welding.
  • Increase the chances of hot cracking. That is when the weld cracks longitudinal as it solidifies after you stop the arc. Hot cracking happens when added elements and impurities concentrate in the middle of the weld, making it weaker.

4. Preheating difficulties

Another issue is that castings often have irregular and complex shapes. This can be troublesome when preheating because the thinner areas will get hotter much faster.

As a result, they will suffer from expansion and contraction forces far more than the rest of the casting. This can lead to distortion or cracking.

To counter this, concentrate the heat more on the thicker areas and preheat the workpiece a bit slower so the heat can spread out more evenly.

5. Dirty surface

Cast steel often comes into contact with oil. Castings are porous, and oil can penetrate the surface. Trapped oil will result in porosity and hydrogen in the weld metal. After you clean the casting, you can preheat it to check if any remaining oil appears.

6. Reaching the metal’s fatigue limit

If the casting endured high mechanical stress or high temperatures for a long time, it might have reached its fatigue limit.

If so, it becomes so weak that multiple cracks start to appear. In cases like these, it might not be economical to repair the part all the time. It would be better to replace it with a new one.

High heat can damage cast steel so much that, after a point, the arc cannot form a healthy puddle to fuse the casting with the filler metal.

Cast steel types and welding difficulty

Now let’s see the cast steel types and how easy or hard it is to weld them.

Low-carbon cast steel

This cast steel type is the most common and contains a low amount of carbon, up to 0.30%. It’s equivalent to typical mild steel and is the easiest to weld. A common cast steel grade with low carbon is the A27.

Low-carbon cast steel is not prone to cold cracking except if it’s very thick, for instance, 3/4″ (19 mm). Therefore, you do not need to follow special procedures, such as preheating.

However, if the risk of distortion and porosity is high, you can use preheating to prevent them.

Hard-to-weld cast steel types

Every cast steel type that has a carbon content higher than 0.40% or contains added alloy elements is hard to weld.

These metals can get very hard when they cool down after welding. If there is hydrogen trapped inside them, they may crack. But only if there is high residual stress from shrinkage or high external stress from demanding service conditions.

One easy way to tell if the casting belongs to this category is by doing a hardness test. For example, you can use a TIG welder and do a quick tack weld without filler metal at a safe place. Then use a file to check the hardness of this spot. If it got harder than before, this casting is hard to weld.

Medium-carbon and low-alloy castings

Medium-carbon cast steel contains 0.30-0.50% carbon. Low-alloy cast steel contains low carbon, but it also has small amounts of added elements for higher strength and added corrosion resistance.

You can weld these metals and make them crack-free if you follow a precise low-hydrogen procedure:

  • Select matching electrodes. Low-hydrogen stick rods or flux-cored wires must be completely dry.
  • Perfectly clean the joint from all types of dirt, oil, rust, moisture, etc.
  • Preheat the casting according to the metal type, and thickness.
  • Use flawless welding technique.
  • Allow the casting to cool down slowly.

High-carbon castings

High-carbon cast steel contains at least 0.50% carbon. This metal is exceptionally hard to weld.

This is because too much carbon hardens the metal to a high degree when it cools down. The shrinkage forces are enough to crack it.

These metals need specific and flawless low-hydrogen procedures with preheating and additional post-welding heat treatments. Even slight mistakes can result in cracking and other defects.

The easiest way to join hard-to-weld cast steels

The easiest way to join or repair hard-to-weld cast steels is with austenitic stainless-steel rods (or wires) such as the 309 or, even better, the 312. The 312 is also your best choice if you cannot figure out the composition of your casting.

These consumables don’t need pre and post-heating and their weld metal has superb ductility and strength. Furthermore, they do not absorb impurities from the casting, but they can absorb hydrogen from it. All these help to avoid hot and cold cracking.

However, they are not suitable if further post-weld heat treatment is necessary.

Tips for welding cast steel

Some helpful tips to weld cast steel are:

  • Clean the edges to be welded down to bare metal. If there is oil, you can clean it fast by using brake cleaners that specifically mention they are not chlorinated. All products that contain chlorine are very unhealthy to breathe. They become far more dangerous if you weld on a surface they cleaned or if their vapors are in the air. Read every product’s safety instructions.
  • You have to preheat cast steel if it’s thicker than 0.75″ (19 mm), it has more than 0.40% carbon, or it is low-alloy steel. Preheat to 250 °F (120 °C) to prevent hydrogen in the weld. After welding, let it cool down at a slow rate for a couple of hours to avoid hardening and strong contraction stress.
  • In a cold environment, you need to preheat castings at least to 75 °F (24 °C).
  • As mentioned earlier, you must avoid distortion if you want to retain the casting’s original functionality. If you are not confident, it would be better not to weld it.
  • If you want to repair a crack, always drill both ends (1/8″) and remove the crack by grinding, gouging, etc.

Now, let’s see how each welding process performs.

Can you stick weld cast steel?

Stick welding is a great process to weld cast steel because you have high heat control. It’s great for quick repairs and fieldwork. Furthermore, it’s the best process to weld metals with dirty surfaces.

One common question is what welding rod to use?

The best welding rod for cast steel is the E7018, especially if the casting is large, has a complex joint, or you suspect it has impurities that cause hot cracking. This rod offers higher ductility and can clean the weld metal. An E7018 is also low-hydrogen, something vital if you want to weld cold crack-sensitive metal.

An E7018 rod offers low penetration and a thick slag covering that helps against warping the casting.

When using E7018s, you need to keep a short but steady distance from the joint, no more than the wire’s diameter. That’s because E7018 rods don’t generate a lot of shielding gasses. This can cause porosity and let hydrogen in the metal, especially when you start the arc.

You can use rutile rods such as the E6013 for common cast steel, especially if it’s small or thin. The E6013 is the easiest rod to use.

You can also use cellulosic rods such as the E6011. But keep in mind that these rods create an aggressive arc that increases the chances of warping.

Can you MIG weld cast steel?

MIG welding can weld cast steels with great success, especially if you weld long joints or thin castings. In addition, it’s the easiest process to use.

The common ER70S-6 wire is more than enough. You can also use the ER70S-2 wire that contains additional deoxidizers if the casting is somewhat dirty. However, this wire is quite expensive.

For shielding gas, you can use the affordable 100% CO2. Or the 75% argon and 25% CO2 mix (C25 or 75/25) for lower penetration welds and fewer spatter.

Can you weld cast steel with flux-cored wire?

Flux-cored wires, both self-shielded and gas-shielded, can weld cast steel with results comparable to rolled steel welding.

When using flux-cored wire, remember to use serrated rollers and change the polarity to DC- if it’s self-shielded.

You can use the typical E71T-11 self-shielded wire, which is excellent if you want to weld outside.

If you have a 240V high-amperage machine, the E71T-1 gas-shielded wire offers superior welding results and higher productivity than a self-shielded wire. It uses 100% CO2 or 75/25 shielding gas.

But it’s hard to find thin diameters or small spools, and is more expensive.

Can you TIG weld cast steel?

TIG welding is excellent for cast steel because you have precise heat control, which helps to eliminate distortion.

But TIG is the hardest process to use and needs a perfectly clean metal to weld. If the casting is even slightly dirty, you will end up with:

  • An erratic arc.
  • A contaminated electrode.
  • A narrow and tall bead.
  • A bead with porosity, and other defects.

TIG uses the ER70S-6 and ER70S-2 filler metals in the form of rods.

Conclusion

Welding cast steel is like welding typical rolled steel but somewhat harder because of the demanding service conditions, slightly different compositions, and surface impurities.

Perhaps, the best way to weld most cast steel is with stick welding and E7018 rods or 312 stainless-steel rods for hard-to-weld or unknown castings.

Using TIG welding is excellent for eliminating warping on thin steel castings.

Related questions

Can you weld cast steel to mild steel?

You can weld cast steel to mild steel with matching filler metal and the correct procedure if the casting is a hard-to-weld metal. Or you can use the stainless-steel 312 rod if the cast steel composition is unknown. The concern here is to avoid warping the casting part to keep its original dimensions and shape.

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