Carbon Content in Steel

Carbon steel can be classified into :

- Low carbon: to 0.30 %C (very low 0.15 %C and mild steel 0.15-0.30 %C)

- Medium carbon: 0.30 - 0.45 %C

- High carbon: more than 0.45 %C

• Low Carbon Steels

- In general, steels with carbon contents to 0.30% are readily joined by all common arc welding processes. These grades account for the greatest tonnage of steels used in welded structures. Typical applications include tanks, structural assemblies, vessels, machine bases, earth moving and agricultural equipment, and general weldments.

- Steels with very low carbon contents to 0.13% are good welding steels, but they are not the best for high speed production welding. The low carbon content and the low manganese content (to 0.30%) tend to produce internal porosity. This condition is usually corrected by modifying the welding procedure slightly - usually by using a slower speed.

If the presence of some internal porosity has no detrimental effect on service requirements of the assembly, standard high speed welding procedures can be used.

Steels with very low carbon content are more ductile and easier to form than higher carbon steels. They are used for applications requiring considerable cold forming, such as stampings or rolled or formed shapes.

- Steels with 0.15 to 0.20 % carbon content have excellent weldability. They seldom require anything beyond standard welding procedures, and they can be welded with all types of mild steel electrodes. These steels should be used for max. production speed on assemblies or structures that require extensive welding.

- Steels at the upper end of the low carbon range 0.25 to 0.30 % carbon content have very good weldability, but when one or more of the elements is on the high side of permissible limits, cracking can results, particularly in fillet welds. With slightly reduced speeds and currents, any of the standard electrodes can be used for these steels.

If some of the elements - particularly carbon, silicon or sulfur - are on the high side of the limits, surface holes may form. Reducing current and speed minimizes this problem.

Although most welding applications of these steels require no preheating, heavy sections (2" or more) and certain joint configurations may require a preheat. Less preheating is required when low hydrogen processes are used.

In general, steels in the 0.25 to 0.30% carbon range should be welded with low hydrogen electrodes or with a low hydrogen process if the temp. is below 50 ^oF.

• Medium and High Carbon Steels

Because hardenability of steel increases with carbon content, the medium and high carbon steels serve where hardness, wear resistance or higher strength are needed. Important uses for medium carbon steels (to 0.45%) include wear plates, springs and components for railroad, agricultural, and earth moving and materials handling equipment.

Unfortunately, the same characteristics that make these steels so suitable for use in rugged parts and structures make them more difficult and costly to weld. The medium carbon steels can be welded successfully, however, provided proper procedures and preheat and interpass temperatures are used. Sometimes, postweld stress relief may be required.

The high carbon steels are almost always used in a hardened condition. Typical applications are for metalworking and woodworking tools, drills, dies, and knives, and for abrasion resistant parts such as plowshares and scraper blades. Some farm equipment is built from rerolled rail stock (0.65 %C), which is welded in the as-rolled condition, using preheating, interpass heating, and postweld stress relief.

Hardness of these steels can range from dead soft in the annealed condition to Rockwell C 65 (with rapid quench treatment) for the higher carbon grades. Although an AISI 1020 steel can be made as hard as Rc 50, hardness is very shallow. Increased carbon content increases depth of hardening and max. attainable hardness to about Rc 65.

Alloying elements increase depth of hardening but have little effect on max. hardness possible.

It is advisable to make sample weld tests to determine cracking tendencies of steels containing 0.30% or more carbon. If such tendencies are apparent, preheating of the steel may be necessary to retard the cooling rate from the welding temp. Required preheat temp. varies with analysis, size, and shape of the steel and with the amount of heat input from the welding process.

In general, the higher the carbon or alloy content and the thicker the plate, the higher the preheat temp. needed to provide the slow cooling rate required to prevent hardening.

Use of low hydrogen processes can minimize the degree of preheating necessary and can eliminate the need for preheating entirely in thinner materials. As a rule of thumb, preheat temp. used with low hydrogen electrodes can be 100 to 200 ^oF lower than those needed for electrodes other than low hydrogen.

• Carbon Equivalent & Amount of Preheat Required

There are many equations (differed by the used code) to calculate the carbon equivalent C_eq used to determine the amount of preheat for carbon steel material,

C_eq = %C + %Mn/6 + %Ni/15 + %Mo/4 + %Cr/4 + %Cu/13 (the ASME code may be have another eqn.)

- This formula is valid only if the alloy contents are less than the following :

0.50% C, 3.5% Ni, 1.00% Cr, 1.60% Mn, 0.60% Mo & 1.00% Cu

- Approximate preheat and interpass temperatures, based on carbon-equivalent values for steels, are :

C_eq up to 0.45% . . . . . . . . . . . . preheat is optional

C_eq = 0.45 to 0.60% . . . . . . . . . . 200 to 400 ^oF

C_eq over 0.60% . . . . . . . . . . . . . 400 to 700 ^oF

- Since all of the welding heat input at the arc does not enter the plate, the following heat inefficiencies are suggested for use with the formula :

75 - 80% for manual welding

90 - 100% for submerged-arc welding