How to Select the Right Steel Grade
Steel is produced in thousands of grades with widely varying properties. Choosing the wrong steel grade for your application could be an expensive mistake. Either it’s harder and tougher than needed, and you pay a price premium, or the steel doesn’t have the right properties and prematurely fails.
“Steel grade” refers to chemical composition. The steel grading system used most widely in the United States is maintained by the SAE. The American Society for Testing and Materials (ASTM) has a broader system covering ferrous and nonferrous metals. The ASTM and SAE systems are linked by the Unified Numbering System (UNS), which is sometimes used as an alternative to SAE references.
The SAE system uses four or five-digit codes to indicate composition and can be used to look up the steel’s properties. For example, steel primarily composed of carbon will have a first digit of ‘1,’ while chromium-based steel will have an SAE designation starting with a ‘5.’ The following numbers in the SAE designation are based on different alloying elements within that steel.
Because there are many different types of steel grades, it can be challenging to select the best one for your application. This article will cover the primary application requirements that should be considered when selecting a steel grade.
Steel strength is defined by yield and ultimate tensile strength (UTS). Fatigue strength is a third parameter to consider when fluctuating loads are likely.
Yield strength is the load at which steel takes on a permanent deformation and won’t return to its original size. UTS is the load at which the steel breaks and is greater than the yield strength.
Fatigue strength is the load at which a steel will fail after a defined number of cycles. For simplicity, it’s sometimes defined as a proportion of the UTS for a specific type or grade.
Carbon content has a significant effect on strength. In general, higher carbon steel is stronger, although other alloying elements, particularly manganese, copper, nickel, niobium, vanadium, and titanium, have an influence too.
Toughness is the ability of steel to withstand an impact. It’s related to strength and ductility, although a material that’s strong but lacks ductility will be brittle and, therefore, not tough.
High-carbon steels are strong but usually have low ductility, resulting in a lack of toughness. Adding manganese, nickel, chromium, and molybdenum increases toughness by raising ductility without reducing strength.
Toughness can also be modified through heat treatment processes like quenching and tempering that alter the microstructure.
Hardness is measured by pushing a shaped probe against the surface with a set force. The size of the indentation is inversely proportional to the hardness of the steel.
Higher carbon content creates harder steel, although other alloying elements can contribute hardness, too. Chromium, vanadium, tungsten, and molybdenum are the main alloying elements to improve hardness.
Quenching and tempering heat treatment processes also raise or lower hardness.
Iron in steel bonds with oxygen in the air, which causes corrosion. Most steels need corrosion protection, but stainless steels are the exception.
Stainless steel contains at least 10.5% chromium, which reacts with oxygen to form a protective layer inhibiting corrosion. Nickel and molybdenum, as found in “alloy” steels, increase corrosion resistance.
Stainless steel is produced in austenitic, ferritic, and martensitic grades. These terms refer to the different forms of crystal structure. Austenitic stainless has excellent corrosion resistance thanks to its high levels of chromium and nickel.
Low-carbon steel melts at around 2,750 degrees Fahrenheit. However, yield strength declines quickly above 750 degrees Fahrenheit, and corrosion accelerates at temperatures above 550 degrees Fahrenheit.
Higher carbon-content steels have higher practical working temperature limits. Adding chromium, nickel, molybdenum, vanadium, and tungsten increases temperature resistance. Austenitic stainless steels perform exceptionally well at elevated temperatures.
How the steel will be processed also influences grade selection. Some grades weld readily, while others do not. Some are difficult to machine or forge, while others cut or deform easily. Thus, the expected manufacturing method should be part of the steel selection process.
Low-carbon steels have good machinability, and their ductility permits forming operations like forging. As carbon content rises, increased toughness makes processing more difficult, although adding lead improves machinability. Tempering and normalizing heat treatment processes improve machinability by reducing hardness.
Generally, steels with alloying elements are difficult to machine, and some don’t weld well. Austenitic stainless steels are an exception, as these have good weldability.
Speak With an Expert
Rather than risk a mistake, the safest approach when selecting steel is to work with an experienced steel supplier like King Steel.
Founded over 50 years ago, King Steel is a trusted supplier of hot rolled and cold drawn steel bar, wire, and rod products. We also offer a wide range of processing capabilities, including heat treatment, turning/polishing, machining, descaling, shot blasting, and eddy current testing, to ensure we provide the exact specification and finish you need.
Contact us today with your application requirements, and one of our experts will assist you.