Heat Treatment of Carbon Steel Castings
Common heat treatment methods for carbon steel castings include annealing, normalizing, or normalizing + tempering. Normalized cast steel exhibits slightly higher mechanical properties than annealed steel. Due to the greater degree of undercooling during microstructural transformation, the hardness is also slightly higher, resulting in better machinability. Currently, normalizing is the most common treatment method for steel castings in production.
Carbon steel castings with high carbon content and complex shapes can be tempered after normalizing to eliminate residual stresses and improve toughness. The tempering temperature is preferably between 550°C and 650°C, followed by cooling in still air.
Casts with a carbon content of 0.35% or greater can also undergo quenching and tempering (quenching followed by high-temperature tempering) to improve their overall mechanical properties. Small carbon steel castings can be tempered directly from the as-cast state; large or complex-shaped carbon steel castings should be tempered after normalizing.
Heat Treatment of Medium- and Low-Alloy Steel Castings
Medium- and low-alloy cast steels contain small amounts of alloying elements such as silicon, manganese, chromium, molybdenum, nickel, copper, and vanadium (the total mass fraction of these alloying elements is less than 8%). They exhibit good hardenability and can achieve excellent overall mechanical properties after appropriate heat treatment.
The heat treatment characteristics of medium- and low-alloy cast steels are as follows:
1. Medium- and low-alloy steel castings are primarily used in critical components in the automotive and tractor industries, where excellent strength and toughness are required. Generally speaking, for castings with a tensile strength requirement of less than 650 MPa, normalizing and tempering are performed; for castings with a tensile strength requirement of greater than 650 MPa, quenching and high-temperature tempering are used. The resulting microstructure is tempered bainite. This structure exhibits higher strength and good toughness than the pearlite and ferrite obtained by normalizing or annealing. This heat treatment is commonly referred to as quenching and tempering.
However, when the shape and size of the casting are not suitable for quenching, normalizing and tempering is recommended instead of quenching and tempering, but the resulting mechanical properties are slightly lower than those of quenched steel.
2. Medium- and low-alloy steel castings should ideally undergo a normalizing or normalizing + tempering pretreatment before quenching and tempering to refine the grain size, homogenize the microstructure, and enhance the effectiveness of the final quenching and tempering treatment. This also helps minimize the impact of the as-cast microstructure on the properties of the quenched and tempered steel and prevents internal casting stresses that may cause deformation or cracking during quenching. Normalizing pretreatment is recommended for low-carbon, low-alloy steel castings with a carbon content of less than 0.2% by mass before quenching and tempering.
3. Quenching of medium- and low-alloy steel castings requires the achievement of a martensitic microstructure whenever possible. To this end, the quenching temperature and cooling medium should be selected based on the steel grade, hardenability, wall thickness, and shape.
4. Medium- and low-alloy steel castings should be tempered immediately after quenching to adjust the quenched microstructure to achieve the desired overall mechanical properties, eliminate quenching stresses, and prevent deformation or cracking of the quenched casting.
5. Toughening is a treatment process that improves the plasticity and toughness of the steel without reducing its strength. It is suitable for medium-carbon, low-alloy, high-strength steel castings.
① High-temperature quenching process.
After quenching medium-carbon, low-alloy steel at normal temperatures, its microstructure is primarily composed of lamellar martensite. Increasing the quenching temperature results in a microstructure dominated by lamellar martensite. This process is characterized by high strength and excellent toughness, and eliminates the adsorption of harmful impurities at grain boundaries, which improves the steel's toughness.
② Subcritical quenching process.
Low-carbon, low-alloy cast steel is generally fully quenched, but the quenched microstructure often contains eutectoid ferrite that precipitates along the grains, reducing the steel's toughness. The quenched microstructure is a duplex structure of martensite and evenly distributed fine ferrite, reducing the risk of temper brittleness in conventional quenched cast steel, significantly improving its toughness, and lowering its low-temperature brittle transition temperature. Low-carbon steel quenched in the dual-phase region and possessing a ferrite + martensite microstructure is called duplex steel. It is worth noting that as the carbon content in the steel increases, the strength of duplex steel increases and its elongation decreases. Compared to conventional duplex steels, secondary quenching in the duplex region improves both strength and ductility, with the improvement in ductility being particularly significant.
③ Refining or Ultra-Refining Treatment.
For high-carbon, low-alloy hypereutectoid cast steel, refining carbides and improving their distribution characteristics are effective methods for increasing the toughness of this type of steel. This process utilizes high temperatures to dissolve excess carbides in the steel, followed by oil quenching to produce a martensite + retained austenite structure. Tempering at 350-450°C produces bainite and tempered martensite, along with extremely fine carbide particles. This pretreatment, used as a pretreatment before the final heat treatment, produces a finer carbide structure and improves the properties of the cast steel.
Post time: Aug-28-2025