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What is Alloy Steel? and What is Chromoly?

Publisher:TianchengTime:2018-06-29Number:94

What is Alloy Steel?

Alloy steel is a steel that has had small amounts of one or more alloying elements (other than carbon) such as such as manganese, silicon, nickel, titanium, copper, chromium and aluminum added. This produces specific properties that are not found in regular carbon steel. Alloy steels are workhorses of industry because of their economical cost, wide availability, ease of processing, and good mechanical properties. Alloy steels are generally more responsive to heat and mechanical treatments than carbon steels.

Technically, every steel is an alloy, but not all of them have the “alloy steel” designation. To be called alloy steel, other elements must be intentionally added to the iron and carbon composition. A small percentage of alloying elements — typically, no more than 5% — is added to the mix, and these metals can improve corrosion resistance, machinability, and many other properties.

What’s the difference between high and low alloy?

Most people say that high alloy is any steel with alloying elements (not including carbon or iron) that make up more than 8% of its composition. These alloys are less common, because most steel only dedicates a few percent to the additional elements. Stainless steel is the most popular high alloy, with at least 10.5% chromium by mass. This ratio gives stainless steel more corrosion resistance, with a coating of chromium oxide to slow down rusting.

Meanwhile, low alloy steel is only modified slightly with other elements, which provide subtle advantages in hardenability, strength, and free-machining. By lowering the carbon content to around 0.2%, the low alloy steel will retain its strength and boast improved formability.

What are some common alloying elements?

  • Manganese: Used in tandem with small amounts of sulfur and phosphorus, the steel alloy becomes less brittle and easier to hammer.

  • Chromium: A small percentage (0.5% – 2%) can help to harden the alloy; larger percentages (4% – 18%) have the added effect of preventing corrosion.

  • Vanadium: With only .15%, this element can boost strength, heat resistance, and overall grain structure. Mixed together with chromium, the steel alloy becomes much harder, but still retains its formability.

  • Nickel: Up to 5%, this alloying element will improve the steel’s strength. In excess of 12%, it provides impressive corrosion resistance.

  • Tungsten: Boosts heat resistance, so the melting point is higher. Also improves the structural makeup of the steel.

WHAT IS CHROMOLY?

    Chromoly steel is a type of low alloy steel that gets its name from a combination of the words “chromium” and “molybdenum” – two of the major alloying elements. Chromoly steel is often used when more strength is required than that of mild carbon steel, though it often comes at an increase in cost. Chromoly falls under the the AISI 41xx designations, and it is important to be aware of the different properties and applications of chromoly steel when deciding which material you need.

    AISI 4130

    Chromoly steel is actually alloy steel grade 4130. The “30” at the end of the grade number designates that it has approximately 0.30% carbon by weight. The added chromium and molybdenum help to give the steel different properties from its mild steel counterpart, AISI 1030, even though they have the same percentage of carbon. The alloying elements found in AISI 4130 help to increase the strength to a level higher than that of AISI 1030, which can be increased even more using a proper hardening procedure. The added chromium helps to increase the steels hardenability and also the corrosion resistance; although other types of materials with more corrosion resisting properties should be considered if the material is to be used in a corrosive environment. The added molybdenum helps to increase the toughness. Other important benefits of AISI 4130 include the ability to be easily hardened by heat treating or work hardening, and the ability to be case hardened using a process called carburizing.

    In its annealed state, AISI 4130 has good formability, machinability, and is quite weldable. For thick sections or low heat input welding processes, preheat may be necessary and is almost always a good idea.

    Common Applications

    The applications of Chromoly steels can be found in a variety of industries. They are very commonly used in automotive, bicycle, and heavy equipment parts, the oil and gas industry, metal production and forming equipment, and many other industries. Here are some more specific examples:

    • Molds

    • Pins

    • Bicycle tubing

    • Furnace equipment

    • Crank shafts

    • Chain links

    • Drill collars

    • Machine shafts

    • Conveyors

    • Tie rods

    • Miscellaneous tooling

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