Low alloy piping materials are often called chrome moly materials because of the chemical makeup of Molybdenum (Mo) and Chromium (Cr). Chromium increases the hardness and strength and only minimally reduces elasticity. Molybdenum improves tensile strength and especially heat resistance.
When you add various metallic and non-metallic elements in a specific amount to carbon steel, it will change the properties of carbon steel. We can manipulate these percentages of alloying elements in steel to achieve better properties than plain carbon steel.
Alloy steel can further classify as
- Low alloy steels: in which the sum of total Alloying elements is < 5%
- High alloy steels: in which the sum of total Alloying elements is > 5%
Alloying Elements
Commonly used alloying elements and their effects are listed in the table given below.
| Alloying Elements | Effect on the Properties |
|---|---|
| Chromium | Increases Resistance to corrosion and oxidation. Increases hardenability and wear resistance. Increases high temperature strength. |
| Nickel | Increases hardenability. Improves toughness. Increases impact strength at low temperatures. |
| Molybdenum | Increases hardenability, high temperature hardness, and wear resistance. Enhances the effects of other alloying elements. Eliminate temper brittleness in steels. Increases high temperature strength. |
| Manganese | Increases hardenability. Combines with sulfur to reduce its adverse effects. |
| Vanadium | Increases hardenability, high temperature hardness, and wear resistance. Improves fatigue resistance. |
| Titanium | Strongest carbide former. Added to stainless steel to prevent precipitation of chromium carbide. |
| Silicon | Removes oxygen in steel making. Improves toughness. Increases hardness ability |
| Boron | Increases hardenability. Produces fine grain size. |
| Aluminum | Forms nitride in nitriding steels. Produces fine grain size in casting. Removes oxygen in steel melting. |
| Cobalt | Increases heat and wear resistance. |
| Tungsten | Increases hardness at elevated temperatures. Refines grain size. |
Alloy pipes comprise 1 to 5 percent of a particular steels content and are added to provide the steel with a specific attribute.Low alloy steel pipes contain a few percent (typically between 1 and 7%) of elements such as Cr, Ni, Mo and V. This category includes chromium steels (containing up to 5% Cr and 1% Mo) and nickel steels (containing up to 5% Ni).
For many low-alloy pipes, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.
What’s Low Alloy Steel Pipe?
As with steels in general, low-alloy steels can be classified according to:
1)Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels
2)Heat treatment, such as quenched and tempered, normalized and tempered, annealed.
Applications for low-alloy steel pipe range from military vehicles, earthmoving and construction equipment, and ships to the cross-country pipelines, pressure vessels and piping, oil drilling platforms, and structural steel pipe.
GB8163-1999 and GB6479-2000 are the main standards for low alloy steel pipe in China.That is 16Mn(referred as Q345B) ,among them 16MnQ are Special grade used for Bridges ,16MnL special for automobile beam,16MnR special for pressure vessel.JIS3466,S355 and EN10020 are the international standards for low alloy steel pipe.
Which steel Can Be Defined As Low Alloy Steel?
Among alloy steels,when additions of alloying elements such as nickel, chromium, and molybdenum total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr, are defined as low alloy steels. As with steels in general, low-alloy steels can be classified according to:
Chromium- Molybdenum Steel (High heat resistance steel )
This low alloy steel series contains 0.5% to 9% Cr and 0.5% to 1% Mo. Its carbon
content on average is lower than 0.20%. The Cr content improves its anti-oxidization and anti-corrosion ability, and Mo enhances its strength in high temperature condition; The steel supplying conditions are generally gone through annealing or normalizing and tempering processes. Chromium-Molybdenum Steel has been widely used in the areas such as oil and gas industry, power plants, and other high temperature application.
Available standards:AISI 4140/ AISI 4130
Weathering Steel (low-alloy low-carbon steel)
Weathering steel is a high strength steel between ordinary steel and stainless steel, best-known under the trademark Cor-ten steel, is a group of steel alloys which were developed to obviate the need for coating, and form a stable rust-like appearance if exposed to the weather for several years. Weathering steel has increased resistance to atmospheric corrosion when compared to other steels. In simple terms the steel is allowed to rust and that rust forms a protective coating that slows the rate of future corrosion.
What steel can be defined as low-alloy steel?
Among alloy steels, when additions of alloys such as nickel, chromium and total molybdenum alloy content can range from 2.07% to levels immediately below those of stainless steel, containing a minimum of 10% Cr, they are defined as low-alloy steels. As with steels in general, low-alloy steels can be classified according to:
Chromium-Molybdenum Steel
This series of low alloy steel contains 0.5% to 9% Cr and 0.5% to 1% Mo. Your carbon
The average content is less than 0.20%. The Cr content improves its antioxidant and anticorrosive capacity, and Mo increases its resistance in high temperature conditions; Steel supply conditions are usually eliminated by annealing or standardization and tempering processes. Chromium molybdenum steel has been widely used in areas such as oil and gas industry, power plants and other high-temperature applications.
- Available standards: AISI 4140 / AISI 4130
Outdoor steel (low carbon low-alloy steel)
Weathering steel is a high-strength steel between ordinary steel and stainless steel, better known under the brand name Cor-ten steel, is a group of steel alloys that were developed to obviate the need for coating and form a stable appearance of rust if exposed to the weather for several years. Steel weathering has a higher resistance to atmospheric corrosion compared to other steels. In simple terms, the steel is allowed to rust and the rust to form a protective layer that slows down the rate of future corrosion.
Available standards:
- Boards: ASTM A242 / ASTM A606 Type 4 / ASTM A588
Nickel steel (Low temperature and low alloy steel)
Low alloy service low temperature steel is formed by adding 2.5% to 3.5% Ni in carbon steel to improve its low temperature strength. Ni can strengthen the ferrite matrix while reducing the Ar3 (temperature of the third transformation), which helps the formation of fine grains. In addition to the normalization treatment during the production process. Average steel in a low temperature environment will have higher strength but lower elongation and toughness, therefore increasing the chance of brittle fracture. If steel is needed in a low temperature environment, it is essential to have superior resistance at low temperature. Any steel suitable for this purpose is called low-temperature service steel or nickel steel.
- Available standards: ASTM A265 / ASTM A203
High performance steel and high tensile strength
This series of low-alloy steel is added Mn, Ni, Cr and Mo, etc., it can increase the strength of ferrite matrix; improve the tendency to harden; and allows better control of grain size. This kind of steel in welded conditions can meet the demand for high strength, corrosion resistance or improve notch strength and other mechanical properties. This type of steel has a good welding capacity with a yield strength of 70 to 120 ksi and a tensile strength of 90 to 150 ksi.
Available standards:
- Plates: ASTM A204 Gr. A, B, C / ASTM A336 / ASTM A387
- Tube and tube: ASTM A209 / ASTM A335 / ASTM A213 / ASTMA182