Most Common types of Steel in Process Piping Industry
Steel is basically an alloy of iron and carbon with a small percentage of other metals such as nickel, chromium, aluminum, cobalt, molybdenum, 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. 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.
According to the World Steel Association, there are over 3,500 different grades of steel, encompassing unique physical, chemical, and environmental properties. According to the American Iron and Steel Institute (AISI), steel can be broadly categorized into four groups based on their chemical compositions:
- Carbon Steel
- Alloy Steel
- Stainless Steel
- Tool Steel
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.
However, it isn’t suited for extreme temperature or high-pressure service because the lack of alloying elements makes it less resistant to the accompanying stresses.
Carbon steels can be further categorized into three groups depending on their carbon content:
- 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.
- 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.
- 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.
Alloy steels include specified amounts of alloying elements in order to manipulate the steel’s properties, such as its hardness, corrosion resistance, strength, formability, weldability or ductility. While the most common alloying elements include nickel, chromium, molybdenum, manganese, silicon and copper, many others are used in the production of steel.
There are countless combinations of alloys and concentrations in use in industry, with each combination designed to achieve specific qualities.
High-alloy types of steel are favored in the piping industry for service in extreme conditions, whether it be in hot or cold conditions or subject to rough use. That’s because the combination of chemistry and proper heat treating can yield strong yet ductile pipe that can take a beating. The oil & gas and power generation industries often favor alloy pipe due to its toughness.
Alloying elements also impart increased corrosion resistance to steel pipe. That makes it a leading choice for chemical companies as well.
The term “stainless steel” is a bit of a misnomer. There’s no one combination of iron and alloying elements that makes stainless steel what it is. Instead, stainless steel refers to the fact that products made from it do not rust.
Stainless steels generally contain between 10-20% chromium as the main alloying element and are valued for high corrosion resistance. Other alloys in stainless steels can include manganese, silicon, nickel and molybdenum. These alloys work together to interact with oxygen in water and air to quickly form a thin but strong film over the steel that prevents further corrosion.
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.
Tool steels are what turn other types of steel into products or equipment used in industry. They must be incredibly strong, tough, ductile and resistant to corrosion. They also must be able to retain cutting edges and maintain their shape in high temperatures. To achieve those qualities, these steels contain very high concentrations of alloying elements and are precisely heat treated.
Tool steels contain tungsten, molybdenum, cobalt and vanadium in varying quantities to increase heat resistance and durability, making them ideal for cutting and drilling equipment.
Sometimes called super-alloys, tool steels are not well-suited for piping products. For one thing, incorporation of higher quantities of alloys makes tool steels more expensive to produce. For another, the amount of alloying elements present in tool steels make them harder to form into piping products. Finally, pipes don’t need cutting edges.
It’s cheaper and easier to use comparatively softer, lower-alloy steels to form pipe and then heat treat up to a specified hardness.
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