Welding Carbon Steel

Carbon steel is the most extensively used metal in fabrication industry. This article discusses some hands-on advice on how to weld carbon steel. Three short sections focusing on welding of low carbon steels, medium carbon steels and high carbon steels have been included. This advice is equally useful for a welder as well as a welding engineer.

welding carbon steel
Carbon Steel

Welding Low Carbon Steel

When carbon steel is welded with metal-arc welding processes, the electrodes can be of covered electrode type or bare electrodes. The bare electrodes come with a thin copper coating that prevents rusting of the rod during storage.

The carbon content of these filler metal is kept low (between 0.10 to 0.14), because welding with high carbon rods makes the weld susceptible to cold cracking.

Stick electrodes are a highly versatile method of welding that can be used to weld any composition, in any position.

Use Preheat If Welding In Cold Conditions

When the welding is being done in sub-zero temperatures, the metal should be preheated to a temperature > 15°C. When welding in subzero temperatures, preheating the metal up to 15°C-20°C avoids the risk of weld cracking up.

How To Prepare Metal For Welding

Regarding edge preparation in welding of carbon steel, in welding sheet metal up to 1/8 in. (3.2 mm) in thickness, the usual bevelling of edge is not required. The welding ends of the two sheets can be simply butted together with some gap. And welding can be done wither autogenously or with a filler rod. This type of edge preparation is called plain square butt joint.

For higher thickness, the plates are required to be bevelled to an angle. A 60° bevel may be used.

Give Allowance For Warpage

When high section thicknesses are being welded, allowance should be given for shrinkage to occur. When the molten metal cools down, it tends to shrink in all directions. Due to this shrinkage, it creates a pull on the adjoining base metal. More the amount of weld metal, more is the pull created due to shrinkage.

Hence a small gap should be kept during fit-up to account for this shrinkage. Generally a gap of 2 to 3 mm is considered enough. This shrinkage is more severe in gas welding than in arc welding.

Besides shrinkage, the joint designer and welder must also make allowance for distortion and warpage of the joint. The back step welding technique or skip welding technique can be used to prevent warpage or distortion.

Further, in a double welded joint – alternating the welding between the front side and the back side distributes the welding heat. This helps in arresting distortion as well.

When welding heavy sections that have been beveled from both sides, the weave beads should be deposited alternately on one side and then the other. This will reduce the amount of distortion in the welded structure.

Welding On Heavy Sections

When welding thick sections, an edge preparation should be done to keep an included angle of 60°C. Before the welding commences, tack welding should be done at regular intervals of the seam to arrest the assembly in place.

Use Lower Electrode Sizes For Root And Higher Sizes For Filling

The root pass in carbon steel welding is generally done with a lower diameter electrode. This ensures that excess penetration is avoided, which is a problem with high diameter electrodes that come with high heat input. A lower diameter electrode has a better reach to the root of thick joints. Thus full root fusion can be achieved.

For root passes, an electrode diameter of 1/8’’ or 5/32’’ (3.2 mm or 4.0 mm) can be used. For remaining thickness of the weld, electrode diameters of 4.8 mm can be used.

Though, it must be said – obtaining good root fusion while avoiding root concavity or excess penetration requires a skilled welder.

Inter-pass Cleaning

Each bead should be cleaned thoroughly to remove all slag, scale, or any other oxides, before the next weld pass is deposited. Proper cleaning is very important, it cannot be stressed enough, to obtain a quality weld.

Welding Medium Carbon Steel

For medium carbon steel welding, the edge preparation of the base plates and fit-up is similar to that of low carbon steel. A few guidelines, as described in the paragraphs below, should be kept in mind when welding medium carbon steels.

Control The Heat Input

Medium carbon steels are also welded with low carbon content electrodes. While welding, overheating the molten metal should be avoided. The heat of the flame/arc should be directed at the already deposited beads instead of on the base metal sidewalls.

This prevents the dilution of weld metal by the base metal, thus preventing influx of carbon from base metal into the weld metal. Less carbon is desired in the weld metal to keep cracking related problems at bay. More carbon makes the structure hard, brittle, and therefore more prone to cracking.

Directing the heat on previously deposited bead helps avoid melting of the base metal. The molten metal produced by melting of the filler rod is made to wash up against the side walls, where it fuses with the side walls, without much dilution.

In order to produce less dilution, the weld beads should be small sized, and should be of stringer type. The size of the weld puddle should be small.

In thick sections, the top layers (especially the passes in middle) can be deposited with weaving beads too. The width of the weaving motion should not exceed three times the diameter of the electrode.

Heat Treat When Possible

Post weld heat treatment plays an important role in ensuring the ultimate quality of weldment. Especially, when the base metal thickness is high, and the carbon content of base metal is on the higher side, the importance of PWHT goes still further up.

Welding High Carbon Steel

Good control over the heat of the arc/flame is still more important in welding high carbon steels. The heat input should be adjusted to avoid too much dilution of the weld metal from the side walls, while also ensuring adequate fusion with side walls, and adequate root penetration.

Stringer beads in high carbon steels becomes all the more important, in comparison to medium and low carbon steels.

The puddle size should be kept small. This is because a larger puddle near the side walls will attract more dilution from the side walls. This results in influx of more carbon into the weld metal from the base metal. The carbon picked up from the base metal renders the weld hard and brittle, and therefore prone to solidification cracking.

This is why, the welder should aim to obtain a narrow fusion zone so that the dilution from base metal is minimized.

The remaining guidelines for edge preparation, preheat, interpass cleaning, bead sequencing for distortion control remain same for high carbon steel as well.

Sometimes, high carbon steel parts are ‘softened up’ by heating to red hot condition followed by slow cooling (annealing). This makes the metal amenable to welding and machining. The part can then be welded using a low carbon electrode.

After the necessary fabrication, the part is then given another appropriate heat treatment to restore its’ original properties. This procedure is especially used when it is required to repair by building up a worn out surface of a part made up of high carbon steel.

Then the piece should be welded or built up with medium carbon or high strength electrodes and heat-treated after welding to restore its original properties.

Extra Reading

Can you weld carbon steel to stainless steel? Yes, you can. Welding carbon steel to stainless steel requires understanding the welding conditions, choosing the right welding consumables, and the right welding procedure.

More importantly, welding stainless steel to carbon steel also requires understanding intended application of the joint. Here is an article containing some useful advice on the subject.

So this was a small discussion on carbon steel welding or CS welding, in general. Thanks for reading. Do leave your thoughts in the comments section below.

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