Steels used in Process Piping Industry
Different types of steel are produced according to the properties required for their application, and various grading systems are used to distinguish steels based on these properties. Steel is basically an alloy of iron and carbon with a small percentage of other metals such as chromium, aluminum, cobalt, molybdenum, nickel, tungsten etc. The carbon content in steel can range from 0.1-1.5%, but the most widely used grades of steel contain only 0.1-0.25% carbon.
According to the American Iron and Steel Institute (AISI), steel can be broadly categorized into four groups based on their chemical compositions:
Ferrous material is designated as carbon steel when its core makeup is specified to include no more than 1.65 percent manganese, 0.60 percent silicon and 0.60 percent copper and when no minimum content is specified for other alloying elements. Carbon steel pipe enjoys wide use across many industries due to its strength and ease of workability. Because it contains relatively few alloying elements and in low concentrations, carbon steel pipe is relatively inexpensive. Carbon steels account for approximately 90% of total steel production.
Carbon steels can be further categorized into three groups depending on their carbon content:
High Carbon Steel – Commonly known as “carbon tool steel” it typically has a carbon range between 0.61% and 1.50%. High carbon steel is very difficult to cut, bend and weld. Once heat treated it becomes extremely hard and brittle.
Medium Carbon Steel – Typically has a carbon range of 0.31% to 0.60%, and a manganese content ranging from .060% to 1.65%. This product is stronger than low carbon steel, and it is more difficult to form, weld and cut. Medium carbon steels are quite often hardened and tempered using heat treatment.
Low Carbon Steel (Mild Steel) – Typically contain 0.04% to 0.30% carbon content. This is one of the largest groups of Carbon Steel. It covers a great diversity of shapes; from Flat Sheet to Structural Beam. Depending on the desired properties needed, other elements are added or increased. For example: Drawing Quality (DQ) – The carbon level is kept low and Aluminum is added, and for Structural Steel the carbon level is higher and the manganese content is increased.
Low Alloy Steel (Temperature Range from -29ºC to 593ºC)
Low Alloy Steels contain one or more alloying elements to improve mechanical or corrosion resisting properties of carbon steel. Nickel increases toughness and improves low temperature properties & corrosion resistance. Chromium and silicon improve hardness, abrasion resistance, corrosion resistance and resistance to oxidation. Molybdenum provides strength at elevated temperatures. Some of the low alloy steels are listed below:-
· Carbon 1/2% Moly and Manganese 1/2% Moly: These low alloy steels are used for higher temperature services and most frequently for intermediate temperatures for its resistance to hydrogen attack. They have the same maximum temperature limitation as killed steel (ASME Code 1000ºF) but the strength above 700ºF is substantially greater.
· 1% chrome 1/2% Moly and 11/4 % Chrome 1/2% Moly: These alloys are used for higher resistance to hydrogen attack and sulphur corrosion. They are also used for services where temperatures are above the rated temperature for C 1/2 Mo steel.
· 2 1/4 Chrome 1% Moly and 3% chrome 1% Moly: These alloys have the same uses as 11/4% Cr, but have greater resistance to hydrogen attack and higher strength at elevated temperature.
Naturally, stainless steel pipe is used in any industry where corrosion protection is necessary. While stainless steel pipe is essentially alloy pipe by another name, it is not well suited for extreme service unless it’s been appropriately heat treated to increase strength and impact resistance.
Due to its aesthetic appeal, stainless steel is often chosen if pipe must be visible in public or professional settings.
Stainless steels can be divided into three groups based on their crystalline structure:
Austenitic – Austenitic steels are non-magnetic and non heat-treatable, and generally contain 18% chromium, 8% nickel and less than 0.8% carbon. Austenitic steels form the largest portion of the global stainless steel market and are often used in food processing equipment, kitchen utensils, and piping.
Ferritic – Ferritic steels contain trace amounts of nickel, 12-17% chromium, less than 0.1% carbon, along with other alloying elements, such as molybdenum, aluminum or titanium. These magnetic steels cannot be hardened by heat treatment but can be strengthened by cold working.
Martensitic – Martensitic steels contain 11-17% chromium, less than 0.4% nickel, and up to 1.2% carbon. These magnetic and heat-treatable steels are used in knives, cutting tools, as well as dental and surgical equipment.
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