Alloy steel can be used in a wide variety of applications because there are so many different combinations of alloying elements that achieve different properties. With such a wide variety of alloying elements and combinations, choosing an alloy steel grade can be difficult. This article will explain some considerations when choosing an alloy steel for your next project.
7 things you should consider when choosing an alloy steel grade include:
- Does it need to have good formability?
- Does it need to be welded?
- Does it need to be machined?
- Does it need to have corrosion resistance?
- Does it need to be heat treated?
- What strengths are required?
- What are the typical applications?
Does the alloy steel need to have good formability?
Some elements, such as chromium and boron, increase the steel’s hardenability. Since most alloy steels are able to be hardened, forming is typically done in the annealed state of the material. Alloy steels that are annealed and have lower amounts of alloys and carbon are typically more formable than those with higher amounts of carbon and other alloying elements. AISI 4130 is a good example of an alloy steel that when annealed has relatively good formability when compared with other alloy steels.
Does the alloy steel need to be welded?
Similar to formability, some of the elements added to alloy steels can be detrimental to welding. Be wary of additions of sulfur and boron if you require an alloy steel to be welded as both can induce cracking. Also, if the carbon content in the alloy steel is high, care must be taken to slow the rate of cooling to avoid cracking. The welding of most alloy steels should be performed in the annealed state. AISI 4130 is an example of a very weldable alloy steel. AISI 6150 on the other hand, while weldable, must follow strict preheat and post heat procedures to avoid weld cracking.
Does the alloy steel need to be machined?
The addition of sulfur and phosphorous can help increase the machinability of an alloy steel, while the addition of boron and chromium can decrease machinability. Machining of alloy steels is typically performed in the annealed state because the reduced hardness helps the machining process, but it can also be done after heat treatment. In the annealed condition, AISI 4130 and AISI 8620 are examples of alloy steels that can be machined readily. AISI 4340 is more difficult to machine than AISI 4130 and AISI 8620, even in the annealed state.
Does the alloy steel need to have corrosion resistance?
If corrosion needs to be inhibited (and coating is not an option) then it is important to find an alloy steel grade that has increased corrosion resistance. Alloy steel grades with a higher amount of elements such as chromium, copper, nickel, and molybdenum will generally have a greater resistance to corrosion.
Does the alloy steel need to be heat treated?
One of the main benefits of alloy steel is that it can be easily heat treated. Heating and quenching alloy steels such as AISI 4340 and AISI 6150 can result in very high tensile strengths and hardnesses throughout the thickness of the material when compared with low carbon steel. AISI 8620 is a steel that is commonly carburized which is a form of case hardening. Once carburized, it is very hard and abrasion resistant on the outside layer of the material, but inside the outer “case” it is still ductile and tough.
What strengths are required of the alloy steel?
Depending on the alloying elements used, very high tensile strengths can be achieved. The strength achieved with alloy steel depends on what elements the steel is made up of as well as the heat treated state the alloy steel is in. Alloy steels that are annealed or normalized will have lower strengths than the same ones that are heated and rapidly quenched. It is best to look at a material data sheet to determine what strengths a particular alloy steel can achieve.
Here are some common types of alloy steels and how they are used:
- AISI 4130: Aircraft parts, machine tools, hydraulic tools, ball bearings
- AISI 4140: Shafts, gears, machine tools, hydraulic tools, spindles, sprockets
- AISI 4340: Landing gear, axles, oil and gas extraction, gears, sprockets, spindles
- AISI 6150: Gears, shafts, spindles, tools
- AISI 8620: Fasteners, axles, gears, pins, shafts, springs
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