Thursday, November 6, 2008

Chemical Composition of Carbon Steel

The chemical compositions of the materials are also established by the material specifications for each type or grade of material. The elements that are not identified should not be present in more than trace amounts except iron, of course, the primary constituent of carbon steels.
Single values are minimums unless otherwise identified, and ranges are given for other elements. The UNS number is listed again for convenience and because the main criteria used to establish that identification is the chemical composition.

The heat analysis is given unless otherwise noted। Although this is the analysis taken from the molten heat and given on the certified material test report, the actual composition of the end product might vary in excess of the heat analysis due to fluctuations that occur during solidification and processing. The limits on the product analysis are therefore somewhat less restrictive than those of the heat analysis.

As previously discussed, the alloying that is used for the materials covered by this report is limited primarily to carbon, manganese, and silicon added in limited and varying percentages to the iron base. In spite of this limited alloying, the properties of the materials are wide-ranging.
The metallurgical structure and the carbon content are major contributors to the overall properties of the different carbon steel materials। Materials classified as carbon steel might also contain small amounts of other elements, such as chromium, nickel, molybdenum, copper, vanadium, niobium (columbium), phosphorous, and sulfur.

Each element that is added to the basic constituent of iron has some effect on the end properties of the material and how that material reacts to fabrication processes. The alloying additions are responsible for many of the differences between the various types or grades of carbon steels.
Following is a list of the elements commonly added to iron and their effects on the material:
  • Carbon: Carbon is the most important alloying element in steel and can be present up to 2% (although most welded steels have less than 0.5%). The carbon can exist either dissolved in the iron or in a combined form, such as iron carbide (Fe3C). Increased amounts of carbon increase hardness and tensile strength as well as response to heat treatment (hardenability). On the other hand, increased amounts of carbon reduce weldability.
  • Manganese: Steels usually contain at least 0.3% manganese, which acts in a three-fold manner: it assists in deoxidation of the steel, prevents the formation of iron sulfide inclusions, and promotes greater strength by increasing the hardenability of the steel. Amounts up to 1.5% are commonly found in carbon steels.
  • Silicon: Usually, only small amounts (0.2%, for example) are present in rolled steel when silicon is used as a deoxidizer. However, in steel castings, 0.35–1.0% is common. Silicon dissolves in iron and tends to strengthen it. Weld metal usually contains approximately 0.5% silicon as a deoxidizer. Some filler metals can contain up to 1.0% to provide enhanced cleaning and deoxidation for welding on contaminated surfaces. When these filler metals are used for welding of clean surfaces, the resulting weld metal strength will be markedly increased. The resulting decrease in ductility could present cracking problems in some situations.
  • Sulfur: This is an undesirable impurity in steel rather than an alloying element. Special effort is made to eliminate or minimize sulfur during steelmaking. In amounts exceeding 0.05%, it tends to cause brittleness and reduce weldability. Additions of sulfur in amounts from 0.1% to 0.3% will tend to improve the machinability of steel but impair weldability. These types of steel can be referred to as freemachining.
  • Phosphorus: Phosphorus is also considered to be an undesirable impurity in steels. It is normally found in amounts up to 0.04% in most carbon steels. In hardened steels, it tends to cause embrittlement. In low-alloy, high-strength steels, phosphorus can be added in amounts up to 0.10% to improve both strength and corrosion resistance, although it is not generally added for this reason in carbon steels.
  • Chromium: Chromium is a powerful alloying element in steel. It is added for two principal reasons: first, it greatly increases the hardenability of steel; second, it markedly improves the corrosion resistance of iron and steel in oxidizing types of media. Its presence in some steels could cause excessive hardness and cracking in and adjacent to the weld. Stainless steels contain chromium in amounts exceeding 12%.
  • Molybdenum: This element is a strong carbide former and is usually present in alloy steels in amounts less than 1.0%. It is added to increase hardenability and to elevate temperature strength.
  • Nickel: Nickel is added to steels to increase their hardenability. It performs well in this function because it often improves the toughness and ductility of the steel, even with the increased strength and hardness. Nickel is frequently used to improve steel toughness at low temperatures.
  • Vanadium: The addition of vanadium will result in an increase in the hardenability of steel. It is very effective in this role, so it is generally added in minute amounts. In amounts greater than 0.05%, there can be a tendency for the steel to become embrittled during thermal stress relief treatments.
  • Columbium: Columbium (also called niobium), like vanadium, is generally considered to increase the hardenability of steel. However, due to its strong affinity for carbon, it can combine with carbon in the steel to result in an overall decrease in hardenability.
  • Other alloying elements; Some carbon steel specifications allow additions of certain other elements, but they are not deliberately added। Other specifications might list these elements as a specified addition to the steel, but the addition would be minor in carbon steels.

No comments: