What is steel grade classification?
Steel is the powerhouse metal for engineering applications because of its high tensile strength. In addition, steels are econominal for industrial use because of their inexpensive cost and supply stocks. The steel manufacturing technology is one of the most studied in the human history. In this article we will discuss about steel, steel composition, and steel grade. Check our previous article about precision casting process! This article helps understanding : What is steel grade?
Let us answer the most basic question, “What is the difference between pig iron and steel?”. Their similarity is that their main component is iron. The difference among the two lies in the carbon composition of the material. Pig iron (sometimes also called as cast iron) contains more than 2% of carbon, while steel contains less than 2% of the element. Other than iron and carbon, steels also consist of several impurity elements, namely: silicon, sulfur, phosphor and so on.
In addition to iron and carbon, alloy steels contain alloying elements to modify the properties of the steel. Alloying elements generally influence the mechanical properties of the material, such as tensile strength, ductility, et cetera. Some alloying elements also change the phase thermodynamics, for instance: chromium and nickel stabilize austenitic phase in room temperature. This modification usually follows a certain purpose or application, for example: stainless steel for anti-corrosion application and tool steel for manufacturing purpose.
Elements within steel
Carbon is the most common alloying element in steel. In general, carbons increase the tensile strength and hardness of the steel, because they act as interstitial atoms and form intermetallic phase precipitates (Fe3C). Steels can be surface hardened if the application requires hard and wear resistant surface. Carbon contributes to two surface hardening approaches: carburizing and martensitic transformation. Carburizing simply increases the carbon content only on the surface up to a certain depth. Martensitic transformation changes the phase on the surface. It is also important to note that high carbon content decreases the thermal shock resistance, ductility, and weldability.
Chromium is the most important alloying element in stainless steel grade. It improves the corrosion and oxidation resistance. An addition of 12% chromium does wonder to steels, making them significantly more corrosion resistant; hence the name stainless steel. Stainless steels form a passive protective layer on the surface to protect the metal underneath. Moreover, chromium increases wear resistance, hardness and hardenability of steel. A drawback of chromium addition is the steel become more susceptible to temper embrittlement during a tempering heat treatment between 375-575°C.
Manganese presents in the steel because of the desulfurization and deoxidation processing during prior steelmaking. From the manufacturing perspective, manganese increases the workability of the alloy because it prevents the iron sulfide FeS precipitate formation.
Nickel is a solid solution atom that increases the strength and toughness (especially at low temperature) of the steel alloy. In addition, nickel also increases the hardenability and atmospheric corrosion resistance.It is an austenite stabilizer element and it hinders the pearlite and bainite formation. Within an alloy with high carbon content, nickel can graphitize carbons.
In stainless steel grades, molybdenum increases the corrosion resistance of the alloy in the acidic environment, specifically upon chloride exposure. In high-strength low-alloying steel grades, molybdenum promotes acicular structure formation. Molybdenum also increases hardenability.
Sulfur is not added in purpose, but rather exists within the alloy as an impurity element which presence is undesirable. It embrittles steel and reduce weldability. Sulfur increases the steel susceptibility to cracking, such as stress corrosion cracking and hydrogen induced cracking. During the steelmaking, usually desulfurization stage drastically reduces the sulfur content up to a beneficial or tolerable concentration. In low amount sulfur can benefit the machinability of the steel. In case that a little amount of sulfur is needed, a controlled resulfurizing can be done.
There are thousands of steel grade, each with its own combination of chemical composition, processing method and material properties. With such broad range of variation, a steel classification method is required to help engineers design products, especially during the material selection. Steel grades assist steel consumers specifying the correct material they need.
A steel may have more than one name because within the steel industry, there are several numbering systems. Different institutions develop them such as Alloy Casting Institute (ACI), American Standard for Testing and Materials (ASTM), Society of Automotive Engineers (SAE), Japanese Industrial Standards (JIS), Deutsches Institut für Normung (DIN), and so on. For example: The chemical composition of ACI CF-8 steel is equivalent to SAE 304 and ASTM A 351.
However, please note that these different numbering systems set different chemical compositions for their grade, so comparing one steel grade to another is more than simply a conversion or translation. Slight differences in chemical composition for two similar grades often happens. The composition specification for a steel grade is hardly an exact value, rather it is a range in which acceptable to classify the steel grade.
The most important benefit of this standardized system is the practicality for an engineering material selection. In general, a material grade possesses certain material properties, such as yield strength and tensile strength, ductility, response to heat treatment and mechanical working, and so on. In practice, this simplify the work of engineers, because they just need to compare the standardized steel properties with their applications, rather than performing trial-and-error experiments. Taking the example of the previous SAE 304 SS: The tensile strength would be around 500-700 MPa, the elastic modulus would be around 193 GPa, et cetera.
Study case: Bone Implant Steel Grade Material Selection
Bone implant supports broken bone load-bearing function. The alloy should be able to carry loads during daily activities, including repeated cycle loadings. This application requires the steel to be corrosion resistant in the biological environment. The alloying elements in the steel have to also be non-toxic to the human body. Biological environment is corrosive, because it exposes dissolved oxygen (via blood) and chloride ion (via body fluid) to the implant. The alloy also has to exhibit a certain degree of ductility to prevent a brittle failure inside the human body. Stainless Steel (SS) is suitable for the bone implant application because it is corrosion resistant and its mechanical properties can bear the loading.
SS grade 304 and 316 contain around 18% chromium. Thus, they are known to be particularly corrosion resistance, even among other the stainless steel grade. SS 316 is preferred than a SS 314 because the molybdenum content is higher, so the chloride resistance is higher. Between SS 316 and SS 316L, the carbon and inclusion (for example: sulfur) contents are lower in SS 316L. Within the medical application, engineers always take an extra effort to choose the cleaner steel grade.
From the mechanical properties perspective, the tensile strength (485 MPa) of SS 316L are sufficient to carry load, for example 40 MPa loading stress of cortical bone. The ductility of SS 316L (40% in 2″) also prevents brittle failure inside human body.
SS316L steel grade satisfies requirements from both corrosion and mechanical properties. In addition, SS 316L is non-magnetic, so this allows the patient to undergo medical treatment such as MRI. So, from these discussion, the bone implant application selects SS 316L as the best steel grade.
Zenith Allmart Precisindo has penetrated into the medical industry after the development of bone implant. We are the first company in Indonesia to achieve this. Check our medical products catalogue here!
Steel investment casting in Zenith Allmart Precisindo
Zenith Allmart Precisindo specializes in steel investment casting. We are capable of casting a wide variety of steel alloy, for instance: SAE 316, SAE 316L, ASTM A297, and the likes. With our experience in steel casting, we are confident to cast for your industrial needs. Please check our portfolio here to learn more about our company products!
Zenith Allmart Precisindo is more than merely your metal casting supplier or vendor. We want to grow together with you, and we will solve your manufacturing challenges using our advanced technology, professional practice, and manufacturing knowledge. Contact us here for more information!
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