Welding High Carbon Steels

High carbon steels are those with a carbon content exceeding 0.55 percent. These are very difficult to weld because they are prone to cracking which is the most usual problem in carbon steels. Special measures are required to be incorporated in the welding procedure in order to avoid a hard microstructure that is brittle, and does not have the intended properties.

However, welding high carbon steel is easy if one just understands its’ properties and behavior and necessary steps are taken during welding. This article offers some hands-on advice about what must be borne in mind when dealing with high carbon steel. The article begins with few basic properties of HCS, then moves on to its weldability, followed by a few pieces of welder-oriented advice on welding of HCS.

How Is High Carbon Steel Different From Low Carbon Steel?

High carbon steel is different from mild steel in appearance, its’ properties, and of course its’ end uses.

Appearance

Un-machined surface of high carbon steel looks dark gray in color, similar to other steels. The fractured surface produces a white colored surface – whiter than low carbon steels.

In appearance, molten high carbon steel has a brighter surface than low carbon steel, and the melting surface has a cellular appearance. It sparks more freely than low carbon steel, and the sparks are whiter in color.

Properties

The ductility of high carbon steel is lower than low or mild carbon steel, owing to its high carbon content. The tensile strength is around 99 ksi (692 MPa).

At a given cooling rate, the chances of hard martensitic structure formation are higher for high carbon steel than low carbon steel. If care is not taken to keep the cooling rate low enough, the HAZ and weld metal become very hard and brittle.

The hardness of high carbon steel is higher in comparison to other steels with lower carbon content. But this hardness is still lower than that of tool steels. Tool steel is supposed to be harder, considering its intended use.

When required high hardness can be obtained on high carbon steel too by heating it to red hot condition, followed by quenching in water. This kind of hardening cannot be achieved in low carbon steel, wrought iron, and steel castings.

This hardening property of HCS is advantageous for some end applications, but quite a nuisance when welding such steels.

Uses

Due to its’ hardening property, high carbon steel is used to manufacture tools. The main feature expected from tools is to have a hard structure capable of withstanding high stress without much wear and tear. In addition, high carbon steel is used for manufacture of drills, taps, springs, and dies.

These tools are heat treated after fabrication to develop the necessary hard structure.

The raw material for making tool steel is subjected to annealing or normalizing-and-annealing. This is done to make the material suitable for machining. Subsequently, it is again subjected to a final heat treatment necessary to develop hard microstructure.

Difficulties With Welding Of High Carbon Steel

The high carbon steel is difficult to weld mainly because of its’ hardening property. When heat is applied at the joint, the base metal gets hardened quickly – owing to its’ high carbon content. This hard structure if susceptible to cracking.

The heat of the welding arc (or flame) negatively affects the property of base metal lying adjacent to the welding joint. To restore its’ properties, a heat treatment is necessary.

In brief, following are a few challenges associated with welding of high carbon steel:

Cracking

High carbon steel, owing to its hard microstructure is highly susceptible to cold cracking. The strong microstructure does not permit dissolved hydrogen to escape from the cooling weld metal. The entrapped hydrogen then tears (or cracks) the metal to escape, leading to cracking.

Moreover, residual stresses are produced in any welding. If these stresses are more than the yield strength of the material, the material yields to relieve the residual stress, and plastic deformation occurs.

However, yield strength of high carbon steel is quite high which does not allow the metal to yield. Under conditions of high restraint or external loading, the metal cracks – since it is not allowed to yield.

Softening Of Base Metal

High carbon steels are often subjected to high preheat temperatures in an effort to bring down the cooling rate, and thus prevent cracking. However, this sometimes has an unintended effect of softening up the base metal. This is undesirable from the point of view of its intended application.

How To Weld High Carbon Steel?

Any strategy for successfully welding high carbon steel without cracking must involve a combination of correct welding procedure, right filler metal, and a few imperatives that the welder must bear in mind. The following paragraphs have been written keeping arc welding in mind, although the general principles are applicable to all welding processes. Let us see in some detail:

Right Welding Procedure

Use Preheat

A preheat of 500° F to 800°F (260°C to 427°C) is essential to welding of high carbon steel. The preheat should be applied to a sufficiently wide zone from the joint. The preheat is checked using a thermal chalk. These chalks come with various readings, beginning with 100°C to almost 400°C.

When the chalk is applied on the preheated surface, the chalk melts and evaporates. This indicates that the temperature has been reached. If the chalk does not melt, it indicates that temperature has not yet reached the desired value.

Use Low Hydrogen Electrode

Use low-hydrogen electrodes to prevent cracking. Cracking is a result of multiple factors such as hard microstructure, degree of restraint, diffusible hydrogen in the weld metal, and thickness of weld. With high carbon steel, hard microstructure is a given – there is no escaping from it.

But if we use low hydrogen electrodes, one of the contributors of diffusible hydrogen gets eliminated, thus helping us avoid hydrogen-induced cracking. SFA 5.1 and SFA 5.5 of ASME BPVC Section II Part C provide for low-hydrogen electrodes that have a hydrogen-designator in their name such as H2, H4, H8 etc.

H2 indicates that the weld metal will contain just 2 ml/100 gram of diffusible hydrogen in the weld metal deposited with that electrode. For most high carbon steels though, a H4 electrode is good enough.

Right Heat Input

The heat input should not be too high. Higher heat input would involve a larger puddle, which in turn means a higher dilution from base metal. This, as we saw above, is not desirable because it brings more carbon to the weld metal.

Heat input should not be very low either because low heat input is associated with a higher cooling rate. High cooling rate is also, as we saw above, not desirable for high carbon steel.

The heat input should be just right. Whatever value has been qualified in procedure qualification should be adhered to. ASME Section IX, at QW 409.1, does not permit increase in heat input beyond that qualified.

Use Under-matching Filler Metal If Possible

If design permits, an under-matching filler metal should be used for welding high carbon steel. A very crude rule of thumb says that ductility of metal decreases as strength increases. The idea behind using an under-matching filler metal is that a metal of lower strength would have enough ductility to yield when subjected to residual stresses, thus avoiding cracking.

Any loss of strength incurred by use of an under-matching filler can be compensated by using a bigger weld (for example, a larger fillet size in case of a fillet weld).

When the design requires a partial penetration joint, using an under-matching filler is not a bad idea. Although, if the joint involved is a full-penetration joint, an under-matching filler would probably not be permitted.

If two base metals of dissimilar strength are being joined together, most codes allow use of a filler metal matching the strength of the weaker base metal.

Once a welding procedure has been developed, document it carefully so that it can be repeated again.

Avoid Deep Penetration

Generally, filler metals with low carbon content are used for welding of carbon steels containing high amounts of carbon. The strength is compensated by alloying the filler with other elements such as chromium, manganese, molybdenum, etc. Thus, low carbon content in weld metal keeps the microstructure from the risk of turning martensitic.

However, this can come undone if the welder uses high voltage and aims for deep penetration into the base metal. Deep penetration results in a larger fusion zone, and more carbon from the base metal enters the weld metal, which defeats the whole point of using low carbon filler.

So, when welding high carbon steel, the welder must aim for just the right amount of side-wall fusion. Fusion between the filler metal and the side walls should be confined to a narrow zone. This can be achieved by depositing small-sized string beads .

Special care is required at the root so that full root fusion is attained while excess penetration into the base metal is avoided too.

Ensure Cleanliness

Ensure thorough cleanliness before and during welding. This eliminates grease, moisture, etc. on the base metal surface from contributing to porosity.

Anneal Before Welding

A less commonly seen practice is to anneal the metal before welding. This ‘softens’ up the metal and makes it amenable to welding without getting cracked. The reduced hardness has lower susceptibility to cracking. After welding, an appropriate heat treatment can be again given to the part for restoration of original mechanical properties.

This is sometimes seen in repair of worn out parts made up of high carbon steel.

Small, high carbon steel parts are sometimes repaired by building up worn surfaces. When this is done, the piece should be annealed or softened by heating to a red heat and cooling slowly.

Then, a build up can be done on the work-piece using medium carbon or high strength electrodes. Thereafter, the piece can be again given an appropriate heat-treatment after welding to restore the original properties.

Surfacing

Sometimes, surfacing or buttering technique can be employed to counter the problem of cracking in HCS. In this, the base metal is buttered with a low carbon filler metal. The carbon content of filler is much lower than the base metal. As a result the carbon content of the weld metal, which is a function of electrode chemistry and dilution from base metal, would have a relatively low carbon too.

The weld is then made between this buttered surface and the second base metal. This avoids carbon pick up from the base metal in the bulk weld metal This results in a more ductile weld metal, that is not susceptible to cracking. However, the fillers must be so chosen such that the strength requirement is fulfilled.

PWHT After Welding

After the welding, a stress relieving at 1200°F to 1450°F (650°C-788°C) is necessary. The soaking time is decided based on the thickness. Generally, the time is 1 hour per inch (25 mm) of weld thickness. This heat treatment is followed by deliberate slow cooling. Fast cooling should be steadfastly avoided.

If the parts to be joined can be softened by giving a annealing treatment to the entire part, then a high carbon welding rod can be used. Subsequent to the welding, the entire part must then be given a suitable heat treatment to restore the original properties of the base material.

Oxy-fuel Welding For High Carbon Steel

Since the melting point of high carbon steel is lower than low carbon steel and medium carbon steels, caution should be exercised to not overheat the metal. If the molten metal shows excessive sparking, it is an indication of overheating.

The amount of sparking is used as a gage to keep under check overheating of the metal.

The welding should be completed without unnecessary delay, that is – it should not be prolonged.

The flame should be adjusted to carburizing in oxy-fuel welding torch. This type of flame produces good welds in high carbon steels.

The welding rod used to deposit weld metal should have medium to high carbon content.

Brazing

Repairs to high carbon steel are difficult as well, due to the same reasons as discussed above. However, minor repairs can be made by brazing. Brazing does not involve as high temperatures as welding, so the problems described above are not as acute. However, a brazed joint does not have the same strength as a welded joint. Hence, brazing is made use of only sparingly.

So this was about welding of high carbon steel. Would you like to share your experiences in dealing with this metal. Please feel free to share in the comments section below.

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