8 Tips for How To MIG Weld Mild Steel

If you haven’t welded mild steel before or welded anything, mild steel is a great place to start. Compared to welding other metals such as Aluminium and stainless steel, mild steel can be quite forgiving, but that doesn’t mean that it’s going to be a walk in the park.

Generally, mild steel is welded by MIG or stick welding and is used in most construction applications. Preheating of mild steel is necessary for material with a thickness of more than 12 mm. The most common mild steel Welds are fillet and butt welds. The best alternative to MIG welding mild steel in harsh conditions is stick welding.

So this article is for everyone who is looking for a guide on how to get started with MIG welding mild steel if you have been struggling to MIG weld steel or just looking for some extra tips on steel welding!

Understanding Mild Steel?

Mild steel is a specific type of carbon steel that has a reduced amount of carbon, and some people may refer to mild steel as simply ‘steel’ or ‘low carbon steel.’ Generally, the amount of carbon found in mild steel varies from 0.05% – 0.25%. Also, because mild steel isn’t an alloy, you won’t find large amounts of other elements except for iron.

Generally, mild steel has a melting point of 1350.C-1530.C (2462.F-2786.F) depending on the grade of the steel, which is determined by the amount of carbon present in the steel. Therefore, mild steel is one of the most used metals globally, and you can find mild steel in almost every large construction project.

Like with any welding process, you need the correct equipment to get started welding mild steel. You have two main options for welding mild steel:

• GMAW/MIG Welding
• MMAW/Stick or Electrode Welding

In this guide, we’re going to focus on MIG welding mild steel. However, if you are doing a lot of remote welding or welding in windy conditions and wet conditions, then MMAW welding of mild steel may be preferred.

What Do You Need To MIG Weld Mild Steel?

In this section, we’ll cover the basics of what you need to get started with MIG welding mild steel.

Personal Protective Equipment – Regardless of what type of metal you’re MIG welding, you need to ensure that you protect yourself with basic Personal Protective Equipment. Welding any metal can produce dangerous fumes which contain carcinogens that could cause cancer. Therefore, along with gloves, safety glasses, welding screens, hoods, and protective clothing, you should also consider wearing respiratory protection.

MIG Welding Machine – A MIG welding machine is required to MIG weld mild steel, and it should be equipped with either a push-pull gun or a mechanical wire feeding system. MIG welding machines typically have faster travel speeds, and higher deposition rates compared to a TIG or stick welding machine. There are various MIG welding machines available to choose from, each with its own advantages and disadvantages.

Grinder/Sander – A grinder or sander is essential for removing the scale or rust from mild steel as well as preparing any joints for welding. Mild steel can be much harder to grind or sand compared to softer metals and may require a little extra work, so be careful about your preparation before you weld, as grinding it out and redoing it can be quite time-consuming.

Wire Brush – A wire brush can be used to clean debris and scale from weld joints before welding and even to clean welds after welds.

Shielding Gas – There are two types of shielding gases that are commonly used for MIG welding mild steel:

1. 100% CO2 
2. 25-75 Carbon-Argon mix.

The most commonly used by hobbyists and home welders is the 25-75 Carbon-Argon mix. It provides you with a good balance between aesthetic appearance, quality of the finished weld, and cost-effectiveness.

What Influence Has Material Thickness?

Material thickness plays a huge role in determining what heat settings you use when welding any mild steel. Unlike alloys, such as aluminium, mild steel absorbs much more heat and is more forgiving when you’re welding it. As a result, the heat from the weld doesn’t travel or spread through the metal as quickly and allows you to weld larger areas without as much distortion.

Mild steel can be MIG welded all the way from 2-4 mm up to 100mm and above with ease. However, as the thickness of the material increases, so to will the amount of passes or runs it needs. Thicker mild steel will also require more joint preparation to ensure that your weld penetrates to the root of the material.

In extreme cases, when you are welding extremely thick mild steel, you may have to pre-heat the material that is being welded to help with those first few passes or root runs.

How To Best Prepare Mild Steel For Welding

Preparing your mild steel before welding is critical to the strength of the finished weld and to ensure that you get good penetration. In this section, follow the steps below to prepare your mild steel plate for welding:

1. Clean – If the mild steel plate is new and rust-free, then it isn’t going to require extensive preparation before welding. However, if you are welding old mild steel or a plate that has been stored incorrectly or exposed to the elements, then some surface preparation will be required.
2. Sand/Brush – Light sanding or grinding is the best way to remove any surface rust or scale from your mild steel plate before welding. Try not to overdo any grinding as it can weaken the structural integrity of the mild steel and also ruin the aesthetic of the finished piece if there are large grinder marks.
3. Pre-Heating – Pre-heating of mild steel plate is generally only required for any plates that are 10-12mm thick or above.
4. Plate Preparation – If you are butt welding to pieces of the mild steel plate together, you should always put a 60. degree bevel on the edge of both plates. However, this preparation can vary greatly depending on the thickness of the plate and engineer requirements. There should be a 1-2 mm root gap, and run-off and run-on plates can be used at both ends of the weld to avoid impurities in starts and stops.

Finding The Best Speed and Temperature Setup

The speed of travel and temperature when MIG welding mild steel is determined by the thickness of the material or the type of joint that is being welded. Mild steel is quite forgiving when it comes to the amount of heat that you can pour into it when welding but always practice on some pieces of scrap first to determine the temperature settings required.

Keeping a diary or a few notes of what you’re welding, how thick it is, and your temperature and wire speed settings is a great way to track temperatures and settings when you first get started welding mild steel, especially if you must weld multiples of the same item or have recurring items that you need to consistently weld.

When welding mild steel, you should always push the weld pool rather than pull it, although both options will work. Pushing your weld pool allows you to clearly see what is happening as it happens rather than watching the outcome. One consistent motion is recommended, without any pauses or steps. A good way to extend how far you can weld is to set up a slide bar to slide your glove along.

Your travel speed needs to remain consistent so that equal amounts of the weld are on both plates, but not too fast or too slow. Smooth and consistent is the key to a good-looking weld.

Recommended Nozzle Ange and Travel Direction

You should have a 10-15-degree angle on your nozzle when MIG welding mild steel and point your nozzle in the direction of travel. This is known as pushing the weld and will give you a good field of vision and allow you to make minor adjustments as you weld to angles, travel speed, and gas coverage.

While you are welding, you should be able to clearly see the area of the weld that is being protected by the shielding gas around your weld pool. However, if you travel too fast or have the wrong angle on your nozzle, your weld pool won’t be protected by the shielding gas. This can introduce impurities into your weld.

Never drag your nozzle along with the workpiece, and try to maintain a wire stick-out length of approximately 10mm. Once you pull the trigger, allow your weld pool to form before beginning to move the nozzle. If you find the weld pool getting too small, try slowing down your travel speed.

If the weld pool of molten metal is getting too big or you notice the plate beginning to overheat, try increasing your travel speed or making a small temperature adjustment.

As with any type of welding, it can take some time to figure out the appropriate temperature settings, nozzle angle, and travel speed for welding mild steel. However, practice makes perfect, and doing some practice welds on scrap mild steel plate is a great way to get your settings dialed in.

Experiment with different settings and travel speeds until you find the desired result for you or your project.

Identifying Different Types of Mild Steel Welds

There are two common types of mild steel welds:

• Fillet Welds – Fillet welds can be used to join two pieces of mild steel like a ‘T.’ You can do horizontal, vertical, or overhead fillet welds on mild steel. Fillet welds are more forgiving as the thickness of the material being welded is effectively doubled, and they don’t require 100% penetration to be effective.

Fillet welds are usually great for beginner welders. When welding a vertical fillet with mild steel, you can either choose to do a vertical up weld or a vertical down, but most welders believe that welding in an upwards direction gives better penetration. However, mastering vertical up welds is a lot harder than vertical down. 

• Butt Welds – Butt welds are where you join two pieces of the plate together on the same plane or level. However, plates of varying thicknesses can be butt welded together. With butt welds, you are usually required to weld both sides of the butt, and a 45-60-degree prep is placed on one side of the butt to ensure root penetration.

The process of butt welding involves laying a root run first to penetrate into both pieces of the mild steel plate and then a capping run. Next, the plate is usually flipped, the root is ground into from the reverse side, and the root run and capping run is placed on the reverse side to complete the weld.

Butt welding any metal, including mild steel, takes some practice and skill and isn’t recommended for beginner welders.

Trouble Shooting Common Problems

Below are some of the common problems that you will encounter when welding mild steel and also how to troubleshoot them:

Burn Through/Over Penetration – Caused by excessive heat or incorrect travel speed.

• Increase your travel speed.
• Reduce the temperature setting on the machine.
• If you are welding something small, allow the material to cool.
• Reduce gaps in the material being welded.
• Consider another joint preparation or design.

Dirty Looking Welds – Caused by weld contamination/inadequate gas coverage.

• Ensure that your plate is clean.
• Try ‘pushing’ the weld rather than pulling it.
• Increase the gas flow to ensure that you have adequate gas coverage.
• Ensure that your nozzle and tip have been cleaned before welds and any spatter has been removed from inside the nozzle. If your nozzle is dirty, it can block shielding gas.

Incorrect Welder Settings

• Refer back to the manual that came with your welder to ensure that you are using the correct functions and settings that are recommended for welding mild steel.

Weld Looks Cold/Lumpy

• Increase the temperature of your welder settings. As the material thickness increases, your temperature will also need to increase.
• Slow down your travel speed and ensure that you are not pushing your weld pool faster than it can fuse the parent material together.

Tips for How To MIG Weld Mild Steel – Conclusion

Hopefully, now you have a much clearer idea about how to MIG weld mild steel, the equipment needed, the processes to follow, and some of the tips and tricks to help improve your MIG welding skills.

Welding anything, from mild steel to aluminium, can appear quite intimidating when you first get started. There’s a lot of noise, heat, and bright light, but with the right equipment and knowledge, you’ll be able to lay down some decent welds in no time at all.

Like anything, welding takes a lot of practice and repetition. Once you have the technique down, you’ll be in a much better position to start tackling more complicated jobs. Start slowly, take your time, and get the basics right.

If you have any questions or would like to learn more about MIG welding mild steel, please don’t hesitate to contact me directly or consider subscribing for my newsletter for regular bite sizes tips and tricks about welding.