Plain Steels

Just plain carbon steel — What is it?

Is there any such material as plain carbon steel? Is so, what is it, what’s its chemical composition, and which applications are best-suited for its use?


Low-carbon Steel Expansion Joint for Power Plant; Small Amount of Copper Added to Enhance Corrosion Resistance.

Those of us in the welding and job shop industry have seen and heard the term just plain carbon steel many times. Is there really any such thing as plain carbon steel? A term used often in the past to describe plain carbon steel is mild steel, a metallurgical descriptor that defines the material’s capability for end use.

You must study several specifications to determine if a metal is fit for its intended use. In most cases, the egg comes before the hen. Engineers establish requirements for the end use and then seek the proper material to achieve the desired result.

The term plain carbon steel usually refers to steel such as ASTM A36/ASME SA36, or SAE 1020. These materials, which have relatively low carbon and other alloy content, are used most often in noncritical structural fabrications. A36/SA36 is available in plate and shapes such as beams, channels, angles, and round and flat bars.

If a job specification does not specify a material, you must decide which to use based on the following criteria:

1. Strength 7. Shape (plate, beam, angle, T, channel, etc.)
2. Wear resistance 8. Surface condition
3. Corrosion resistance 9. Available thicknesses, widths, and lengths
4. Ductility 10. Producer’s reputation
5. Form-ability 11. Supplemental test requirements
6. Hardness, harden-ability 12. Delivery availability
13. Cost


For A36/SA36 material, the tensile and yield strength are moderate. The yield is approximately 36,000 PSI, and the tensile strength varies from 58,000 PSI to 80,000 PSI. Variations in tensile strength allow for the difference in carbon, manganese, and silicon content in different thickness. Also the production method—rolling, hot or cold, or extruding—can affect tensile strength.

Additional carbon and manganese produce a higher strength but lower ductility. For example, the minimum elongation in 2-inch plate is 23 percent, and for shapes, 21 percent.

Wear and Corrosion Resistance

If wear resistance is required, then A36/SA36 is not a good choice. Wear resistance is related to hardness and hardenability. It is common to say that a metal containing less than 0.30 percent carbon is not readily hardenable, although it may be case-hardened in an atmospheric carbon-enhanced furnace.

If corrosion resistance is a must, A36/SA36 is not a good choice. This material does not contain the elements, such as copper, chromium, or nickel, which produce corrosion resistance. In the past this material was available in a pickled and oiled form condition that was slightly more corrosion-resistant.

Ductility and Form-ability

Ductility and form-ability are directly related. These properties are determined by tests for tensile, yield, elongation (how much the material will stretch before failure), and reduction (shrinkage in cross-sectional area before failure). If the material is to be bent or rolled, ductility is very important, and A36/SA36 is a good choice. A 90-degree bend is not at all uncommon for this material. It also can be rolled easily and has practically no springback (opening up after rolling).

The nominal chemical requirements for these materials are:

Metal Thickness (In.) and Chemical Values (Percent)


Up to 34

34 – 112

112 – 212

212 – 4


Carbon (max.)







No requirement

0.80 – 1.20

0.80 – 1.20

0.85 – 1.20

0.85 – 1.20

Phosphorus (max.)






Sulfur (max.)







0.40 (max.)

0.40 (max.)

0.15 – 0.40

0.15 – 0.40

0.15 – 0.40

Copper (min. when copper steel is specified)







Some relatively new specifications relating to ASTM A36 material are available. The ASTM A1011 Grade 36 and ASTM A1018 Grade 36 specifications are for the sheet and strip forms of A36. The A709 Grade 36 is denoted as bridge material with the same basic chemical, mechanical, and physical properties.

Carbon adds to strength and hardenability of steel. A36/SA36 is less than 0.30 percent carbon and will not harden appreciably. The rule of thumb is that 0.30 percent is the lower limit for quench and temper through hardening.

Manganese contributes to work hardening in an iron matrix. Work hardening is also used to increase tensile strength. Manganese usually is alloyed in A36/SA36 at 1 percent to 1.60 percent.

Phosphorus is not considered a beneficial element in most cases. While this chemical can increase harden-ability, any amount over about 0.04 percent may cause embitterment.

Sulfur is detrimental for welding but enhances machinability. Materials sometimes are treated in a sulfur atmosphere if they are to be machined. In some cases, sulfur combines with manganese (creating manganese sulfides) to cause lamellar inclusions, which promote laminations or lamellar tearing. Sulfur is normally limited to 0.05 percent or less.

Silicon, a metalloid, is an excellent deoxidizer and scavenger for floating out contaminants. It is also very valuable for increasing hardenability.

Copper is added to some low-carbon steels for atmospheric corrosion resistance. It is used predominantly at 0.20 percent to 0.40 percent for high-strength, low-alloy materials and weathering steels.

Iron is abundantly available and an extremely valuable material. The greatest advantage of iron is that it may be alloyed for various usages. The alloying elements noted previously may be used to transform the melting point (about 2,780 degrees F), the tensile strength, formability, and, ultimately, the end use of the material.

Perhaps black iron is a good slang term for A36/SA36 material. Consider the math for the chemistry for 1-in.-thick A36/SA36 plate: Carbon (0.25%) + Manganese (1.00%) + Phosphorus (0.04%) + Sulfur (0.05%) + Silicon (0.40%) = 1.74% total alloys other than iron. 100% – 1.74% = 98.26%. The 98.26% represents the balance of the chemistry, which is Iron.

This is what most call just plain low-carbon steel!


By Carl Smith







Alloy Steel


What is Alloy Steel?



Steel is a metal alloy consisting mostly of iron, in addition to small amounts of carbon, depending on the grade and quality of the steel. Alloy steel is any type of steel to which one or more elements besides carbon have been intentionally added, to produce a desired physical property or characteristic. Common elements that are added to make alloy steel are molybdenum, manganese, nickel, silicon, boron, chromium, and vanadium.

Alloy steel is often subdivided into two groups: high alloy steels and low alloy steels. The difference between the two is defined somewhat arbitrarily. However, most agree that any steel that is alloyed with more than eight percent of its weight being other elements beside iron and carbon, is high alloy steel. Low alloy steels are slightly more common. The physical properties of these steels are modified by the other elements, to give them greater hardness, durability, corrosion resistance, or toughness as compared to carbon steel. To achieve such properties, these alloys often require heat treatment.

If the carbon level in a low alloy steel is in the medium to high range, it can be difficult to weld. If the carbon content is lowered to a range of 0.1% to 0.3%, and some of the alloying elements are reduced, the steel can achieve a greater weld ability and formability while maintaining the strength that steel is known for. Such metals are classified as high strength, low alloy steels.

Perhaps the most well-known alloy steel is stainless steel. This is a steel alloy with a minimum of 10% chromium content. Stainless steel is more resistant to stains, corrosion, and rust than ordinary steel. It was discovered in 1913 by Harry Brearley of Sheffield, England, but the discovery was not announced to the world until 1915. Stainless steel is commonly used in table cutlery, jewelry, watch bands, surgical instruments, as well as in the aviation industry. Its familiar luster has also been appropriated for many famous architectural designs, such as the Gateway Arch in St. Louis, Missouri, and the pinnacle of the Chrysler Building in New York City.

In all types of alloy steel, the alloying elements tend to either form carbides or compounds, rather than simply being uniformly mixed in with the iron and carbon. Nickel, aluminum, and silicon are examples of the elements that form compounds in the steel. Tungsten and vanadium will form carbides, both of which increase the hardness and stability of the finished product.

Alloy Steel Properties

  • However, most agree that any steel that is alloyed with more than eight percent of its weight being other elements beside iron and carbon, is high alloy steel. Low alloy steels are slightly more common. The physical properties of these steels are modified by the other elements, to give them greater hardness, durability, corrosion resistance, or toughness as compared to carbon steel.
  • Each alloy presents different characteristics, so these qualities should be checked before using this steel for any purpose. For example, if low alloy steel with poor corrosion resistance is used for piping, then the pipes may quickly wear away.

Effects of Alloying Elements in Steel

Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt. Listed below is a summary of the effects various alloying elements in steel.


The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength by heat treatment but the addition of carbon enables a wide range of hardness and strength.


Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304)


Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. ‘Stainless Steel’ has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength.


Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.


Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.


The main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon combines with the titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.


Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.


When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present, the weak spots at the grain boundaries are greatly reduced during hot working.


Selenium is added to improve machinability.

Niobium (Columbium)

Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.


Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenite forming element. Yield strength is greatly improved when nitrogen is added to austenitic stainless steels.


Silicon is used as a deoxidising (killing) agent in the melting of steel, as a result, most steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminium killed steels.


Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually aproximately 0.2% maximum. This problem is emphasised because there is residual cobalt content in the nickel used in producing these steels.


Chemically similar to niobium and has similar effects.


Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to produce precipitation hardening properties.







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