Since 2000, steel casting has become an increasingly important part of DaWang’s business. Our steel casting foundry plant in Dandong, China, has excellent design, good project management, and quality assurance. Compared with other peers, the advantages of low cost, high quality, and increased production capacity have made us stand out in the steel casting industry and won many famous steel casting projects.
The factors affecting Steel Casting Grades
The division of steel casting grades is affected by the chemical composition, handling, and mechanical properties of steel casting. In addition to the actual percentage of carbon and other alloys in the material, the microstructure also has a significant impact on the mechanical properties of steel casting, thereby affecting the classification of steel casting grades.
It is vital to understand the microstructure of steel casting, the use of hot and cold forming, and how to deal with steel casting after manufacturing. In addition, manipulating the composition and microstructure of steel casting will result in changes between different properties. For example, more complex steel casting may eventually reduce the strength grades.
Microstructure influence steel casting grades
The microstructure of steel casting refers to how the forces between molecules connect. Heating and cooling processes are used to change the microstructure from one form to another, thereby affecting steel casting grades.
The microstructure of steel casting cannot be observed with the naked eye but can be studied under a microscope. Steel casting can use several different microstructures-ferrite, pearlite, martensite, cementite, and austenite.
Ferrite
Generally, steel casting with shallow carbon content adopts this kind of microstructure. The characteristic shape of ferrite is a body-centered cubic (BCC) crystal structure. Visually, it is a cube with a molecule at each corner and a molecule in the cube’s center. Compared with the microstructure of other steel casting grades, the molecules in BCC are more loosely packed. However, the amount of carbon added to steel casting without changing the ferrite microstructure is meager, only 0.006% at room temperature.
Austenite
Austenite is a microstructure formed when steel casting iron-based alloys are heated to above 1500°F but below 1800°F. If the proper alloy is present in steel casting, such as nickel, the steel casting material will maintain this microstructure even when it is cooled. The characteristic shape of austenite is the face-centered cubic (FCC) crystal structure. Visually similar to a cube, each corner has a molecule, and each side of the cube has a molecule in the center. The molecules in the austenite structure are denser than the steel casting molecules in the ferrite structure. Austenite can contain up to 2% carbon and is a common microstructure of steel casting grades.
Cementite
When steel casting is heated to the austenite range, it needs to be cooled in the absence of any alloy to maintain the austenite shape. That is, the microstructure of steel casting is restored to the form of ferrite. However, if the carbon content in steel casting is more excellent than 0.006%, the excess carbon atoms will combine with iron to form a compound called Fe3C or cementite. Under normal circumstances, cementite does not appear alone because some steel casting grades will maintain ferrite.
Pearlite
Pearlite is a layered structure formed by alternating layers of ferrite and cementite of steel casting grades. When steel casting grades are cooled slowly, a eutectic mixture is formed. The eutectic mixture is a mixture produced by the simultaneous crystallization of two molten materials. Under these conditions, steel casting grades of ferrite and cementite are formed simultaneously, thereby forming alternating layers within the microstructure.
Martensite
Martensite has a body-centered tetragonal crystal structure. This microcrystalline form is obtained by rapid cooling steel casting, which causes carbon atoms to be trapped within the iron lattice. In the end, steel casting grades with very hard needle-like structures can be achieved. Steel casting with a martensite microcrystalline structure is usually a
low-carbon steel alloy containing about 12% chromium.
Steel casting manufacturers and consumers need to understand the microstructure of steel casting and steel casting grades. Carbon content, alloy concentration, and finishing methods will all affect steel casting grades to control the performance of the finished product. Depending on the finishing method and heat treatment used, two steel castings with the same alloy content may have different microstructures.
Hot forming and cold forming affect steel casting grades
Steel casting is formed after molten steel is cast. Then the steel casting is finished to prevent corrosion. Steel casting is usually thrown into existing models: blooms, billets, and slabs. Then, a mold is formed by rolling. According to steel casting grades, material, and target application, it can be hot rolled, warm rolled, or cold rolled. In the rolling process, the compression deformation is completed by using two work rolls. The rolls rotate rapidly while pulling and squeezing the steel between steel castings.
The influence of cold-forming on steel casting grades
The microstructure of steel casting can be changed by controlling heating and cooling. This has led to the development of various steel casting grades. Changing the microstructure can change the steel casting grades.
Cold forming is a process of rolling below the recrystallization temperature of steel casting. The pressure exerted by the rollers on the steel casting will cause dislocations in the material microstructure, thereby affecting the steel casting grades. With the accumulation of these dislocations, steel casting becomes harder and harder to be deformed. Cold rolling will also change the steel casting brittleness grades, which can be overcome by heat treatment.
The influence of hot forming on steel casting grades
Heat treatment includes a series of processes, including annealing, quenching, and tempering. In steel casting grades, flexibility and strength are inversely proportional. Heat treatment can increase flexibility at the expense of steel casting strength grades and vice versa.
The microstructure of steel casting undergoes a phase change at a specific temperature. Heat treatment is based on the understanding and operation of particular transition points:
Normalizing temperature: Austenite is the phase that forms other structures. Most heat treatments first heat the steel casting to a uniform austenite phase of 1500-1800°F.
Upper critical temperature: The upper critical temperature is the temperature point at which cementite or ferrite begins. This happens when steel casting cools from the normalizing temperature. Depending on the carbon content, this point lies between 1333–1670°F.
Lower critical temperature: The lower critical temperature is the point of transformation from austenite to pearlite. Austenite cannot exist below the lower critical temperature of 1333°F.
Cooling rate: From normalizing temperature to upper and lower critical temperature will determine the microstructure of steel casting grades obtained at room temperature.
Type of heat treatment
Spheroidization
Spheroidization occurs when steel casting is heated to approximately 1290°F for 30 hours. The cementite layer in the pearlite microstructure transforms into a spheroid, thus forming the softest and most ductile steel casting.
Fully annealed
The steel casting is annealed by first heating it to slightly above the critical temperature and then cooling it at a rate of approximately 36°F per hour. This process produces a coarse pearlite structure with flexibility and no internal stress.
Process annealing
Process annealing can reduce the stress of cold working steel casting grades. The steel casting is heated to 1025-1292°F. Dislocations in the microstructure are repaired by cooling the pre-crystal reforming of steel casting.
Isothermal annealing
The steel casting is first heated to the upper critical temperature. Then it is cooled to a lower critical temperature and maintained. Then gradually cool to room temperature. This process ensures that the steel casting material reaches a uniform temperature and microstructure before the next cooling step.
Normalized
Heat the steel casting to the normalizing temperature for one hour. At this time, steel casting ultimately enters the austenite phase. Then the steel casting is air-cooled. Normalizing produces a fine pearlite microstructure with high strength and hardness.
Quenching
Steel casting is heated to normalizing temperature and then quenched (through rapid cooling by immersion in water, salt water, or oil) to the upper critical temperature. The quenching process will produce a martensitic structure. At this time, the steel casting grades are challenging but very brittle.
Tempered hardened steel
Tempering hardened steel is the most common heat treatment because its steel casting grades can be accurately predicted. The quenched steel is reheated to a temperature below the lower critical point and then cooled. The climate varies depending on the expected result-the 298-401°F range is the most common. This process restores some of the toughness of brittle steel casting grades by allowing some spheroids to form.
Mechanical properties affect steel casting grades
The mechanical properties that affect steel casting grades include the following aspects according to international standards. The main mechanical properties of steel casting
Hardness
Hardness is the ability of steel casting materials to withstand wear. The increase in hardness can be achieved by increasing the carbon content and quenching to form martensite.
Power
Strength is the force required to deform a steel casting material. Normalizing a piece of steel casting will improve steel casting grades by forming a consistent microstructure throughout the material.
Malleability
Ductility is the grades of steel casting deformed under tensile stress. Due to dislocations in the microstructure, cold-formed steel has low elasticity. Process annealing will improve this by reforming the steel casting crystal and thus eliminating some dislocations.
Toughness
Toughness is the ability of steel casting to withstand pressure without breaking. Hardened steel can be made more rigid by tempering because of the addition of spheroids to the steel casting microstructure.
Machinability
Machinability refers to the problematic and easy steel casting grades formed by cutting, grinding, or drilling. Machinability is mainly affected by the hardness of steel casting grades. The more complex the material, the more difficult it is to process.
Solderability
Solderability is the ability to weld steel casting. It mainly depends on chemical composition and heat treatment. Melting points, as well as electrical and thermal conductivity, will affect the solderability of steel casting.
Quality description based on steel casting grades
The quality descriptor is a symbol used when evaluating a wide range of steel casting products using the steel casting grades system. For example, describe the commercial, industrial or structural quality of steel casting. These steel casting grades systems divide steel into specific applications and manufacturing processes into certain areas, facilitating consumer decision-making. According to the steel casting grades system, the quality of steel is judged by the description of the following factors:
- Internal health
- Chemical composition and uniformity
- Surface defect degree
- Scope of testing in the manufacturing process
- Number, size, and distribution of inclusions
- Hardenability
Steel casting grading system
ASTM, AISI, and SAE have all issued their steel casting grades classification system, which provides engineers, manufacturers, and consumers with a reference standard to judge the characteristics of steel. Steel casting grades are usually particular, describing many aspects such as the chemical composition, physical properties, heat treatment, manufacturing process, and form of the steel.
ASTM’s steel casting grades system
ASTM’s descriptive system of steel casting grades takes the form of letters followed by serial numbers. For example, use “A” for ferrous metal and “53” for the number of galvanized carbon steel. The meaning of ASTM A53 includes the following:
chemical composition:
Carbon: 0.25 (Grade A), 0.30 (Grade B)
Manganese: 0.95 (Grade A), 1.20 (Grade B)
Phosphorus: 0.05
Sulfur: 0.045
Mechanical behavior:
Tensile strength: 330 MPa or 48,000 psi (Grade A), 414 MPa or 60,000 psi (Grade B)
Yield strength: 207 MPa or 30,000 psi (Grade A), 241 MPa or 35,000 psi (Grade B)
Form and processing:
Pipe NPS 1/8 – NPS 26
Galvanized steel
Black and hot-dip
Galvanized
Welding seamless
SAE’s steel casting grades system
AISI/SAE’s steel casting grades system uses four digits to classify steel casting. The first two digits indicate the concentration of steel grade and alloying elements, and the last two digits indicate the concentration of carbon contained in steel casting. For example, SAE 5130 describes a steel casting with 1% chromium and 0.30% carbon. The letter prefix indicates the quality of the merchant.
DaWang specializes in casting popular steels of different steel casting grades. For steel casting, we refer to the steel casting grades standards of WCB and WCC. Steel casting has become DaWang’s professional field, which can satisfy many applications. We welcome customers to contact us at any time and tell us your needs so that we can make suggestions for you.
