Ordinary Normalizing
The purpose of ordinary normalizing is to obtain pearlitic or sorbitic ductile iron castings, as per ISO 800-2, ISO 700-2, and ISO 600-3.
When the as-cast microstructure lacks free cementite, metastable ternary or complex phosphine eutectics, the normalizing process without cementite can be used. When the volume fraction of free cementite in the as-cast microstructure is greater than or equal to 3%, and metastable ternary or complex phosphine eutectics are present, a process of high-temperature decomposition of free cementite, furnace cooling to a lower austenitizing temperature, and holding before normalizing should be adopted.
To ensure that thick-section castings obtain a completely pearlitic matrix after normalizing, pearlite-stabilizing elements such as copper, molybdenum, nickel, and vanadium can be added, thereby increasing the hardness of thick-section castings.
After normalizing, ductile iron must be tempered to improve toughness and relieve stress. The tempering temperature is 550~600°C.
1. Partial Austenitizing Normalizing
The purpose of partial austenitizing normalizing is similar to ordinary normalizing, namely, to obtain a pearlitic matrix structure. However, the difference lies in controlling the amount of dispersed ferrite to improve toughness. Therefore, the austenitizing temperature used is not above the eutectoid transformation temperature, but within the eutectoid transformation temperature range, that is, between the upper and lower critical temperatures. At this point, only partial austenitizing occurs. Consequently, dispersed ferrite forms along the grain boundaries, the amount of which depends on the austenitizing temperature and the holding time. The closer the temperature is to the upper limit of the eutectoid transformation temperature, the less dispersed ferrite there is, resulting in higher strength but lower toughness. Furthermore, the same effect occurs if the holding time is too short.
When there is no free cementite, metastable ternary or complex phosphorus eutectic in the as-cast structure, a cementite-free partial austenitizing normalizing process can be used. When the volume fraction of free cementite in the as-cast microstructure is greater than or equal to 3%, and metastable ternary or complex phosphorus eutectics are present, high-temperature decomposition should be used first, followed by furnace cooling to the eutectoid transformation temperature range for partial austenitizing normalizing. It should be noted that the partial austenitizing temperature is closely related to the silicon content.
Quenching and Tempering
Castings without free cementite, metastable ternary or complex phosphorus eutectics, and with fine, uniform eutectic clusters can undergo quenching and tempering. Castings with a volume fraction of free cementite greater than or equal to 3%, containing metastable ternary or complex phosphorus eutectics, coarse eutectic clusters, and inhomogeneous microstructure should first undergo high-temperature graphitization annealing and normalizing to achieve a uniform pearlitic microstructure before quenching. The quenching and tempering treatment (also known as tempering) aims to obtain comprehensive mechanical properties with good strength, plasticity, and toughness.
1. Quenching Process
After austenitization, quench in a quenching medium at 860-880°C (holding time depends on the casting wall thickness, generally 1 hour for every 25mm) to obtain a martensitic matrix structure.
Due to the good hardenability of ductile iron, milder quenching media can be used, such as No. 10 or No. 20 spindle oil or diesel oil. Caution must be exercised when using water or brine as the quenching medium to prevent cracking of the casting.
2. Tempering Process
Low-temperature tempering: Temper at 140-250°C for 2-4 hours, followed by air cooling, wind cooling, oil cooling, or water cooling. For thick castings, the tempering time can be extended to obtain tempered martensite and retained austenite structure, achieving a hardness of 46-50 HRC, with good strength and wear resistance. Low-temperature tempering eliminates quenching stress and reduces brittleness. The tempering temperature should not exceed 250°C. Tempering at 250-300°C will result in low-temperature temper brittleness.
Medium-temperature tempering (350-450°C), followed by air cooling, air cooling, oil cooling, or water cooling after 2-4 hours, yields tempered troostite and retained austenite, with a hardness of 42-46 HRC. This provides good wear resistance while maintaining a certain level of toughness. Tempering at 450-510°C or slow cooling may result in high-temperature temper brittleness. Reheating to above this temperature range, holding at that temperature, and then rapid cooling can eliminate this high-temperature temper brittleness.
High-temperature tempering (high-temperature tempering after quenching, also known as tempering treatment): Tempering at 550-600°C, followed by air cooling, air cooling, oil cooling, or water cooling after 2-4 hours, yields tempered austenite and retained austenite, with a hardness of 250-330 HBW. This provides a combination of high strength and good toughness in terms of overall mechanical properties.
Isothermal quenching
Isothermal quenching, also known as austenitic isothermal quenching, involves quenching in the acicular ferrite transformation zone (230–450°C) and in the martensitic transformation zone (Mf). The acicular ferrite transformation zone is divided into two parts. If, after austenitization, the quenching proceeds to the acicular ferrite transformation zone and is held, an acicular ferrite matrix is obtained. If, after austenitization, the quenching proceeds to the martensitic transformation zone and is held, a martensitic matrix is obtained. For ductile iron, isothermal quenching typically refers to quenching in the acicular ferrite transformation zone.
Generally, 350–450°C is considered a higher temperature range for the isothermal transformation zone, while 350–230°C is considered a lower temperature range. Isothermal quenching in the first transformation zone (generally 350~380°C) aims to obtain acicular ferrite and a high-carbon stable austenite structure with a volume fraction of 25%~40%. Its mechanical properties can reach: tensile strength ≥1000MPa, elongation after fracture ≥10%, unnotched impact toughness ≥80J/cm, hardness ≥30HRC, exhibiting good impact toughness and fatigue strength. Isothermal quenching in the second transformation zone (generally 270~330°C) aims to obtain a fine acicular ferrite structure (often accompanied by a small amount of retained austenite and martensite). Its mechanical properties can reach: tensile strength ≥1200MPa, elongation after fracture ≥2%, unnotched impact toughness ≥30J/mm², hardness ≥38HRC, exhibiting good wear resistance and high fatigue strength.
Before isothermal quenching, the casting should have a well-spheroidized microstructure (spheroidization grade 1-2), fine eutectic clusters (graphite size less than or equal to grade 6), and no free cementite. If the volume fraction of free cementite in the as-cast microstructure is greater than 1%, high-temperature graphitization annealing should be performed beforehand.
Alternatively, treatment can be performed at a lower austenitizing temperature (≤850°C) to retain a small amount of dispersed ferrite. This partial austenitizing isothermal quenching improves toughness.
Salt bath isothermal quenching is generally used. To shorten the isothermal time, two-stage isothermal quenching can be used: first, a short cooling time in a 200°C low-temperature salt bath or warm water bath, followed by immersion in a pre-determined temperature isothermal salt bath. This allows for the formation of acicular ferrite in the core of castings with larger cross-sections. It is crucial to avoid over-cooling.
Post time: Nov-21-2025