Understanding the heat treatment of ductile iron and doubling the strength and toughness of castings is not a dream!

In the field of casting, ductile iron has become a versatile tool for industrial applications due to its unique spherical graphite structure. And heat treatment, as a key step in tapping into its performance potential, is particularly important.

So, how to achieve the optimal matching of strength, toughness, and wear resistance through process control? Today, we will combine practical applications to summarize the core processes and operational points of heat treatment for ductile iron.

Low temperature graphitization annealing requires heating the temperature to 720-760 ℃, cooling it in the furnace to below 500 ℃, and then air cooling it out of the furnace. The core function of this process is to promote the decomposition of eutectoid carbides, thereby obtaining ductile iron with a ferrite matrix.

Due to the formation of the ferrite matrix, the toughness of the material can be significantly improved. This process is particularly suitable for scenarios where a mixture of ferrite, pearlite, cementite, and graphite is prone to occur in thin-walled castings due to chemical composition, cooling rate, and other factors. Low temperature graphitization annealing can effectively improve the toughness of such castings.

02 High temperature graphitization annealing

High temperature graphitization annealing first requires heating the casting to 880-930 ℃, then transferring it to 720-760 ℃ for insulation, and finally cooling it in the furnace to below 500 ℃ and leaving the furnace for air cooling.

The main goal of this process is to eliminate the white cast structure in the casting, by fully heating and holding at high temperatures, decomposing the cementite in the white cast structure, and ultimately obtaining a ferrite matrix. After high-temperature graphitization annealing treatment, the hardness of the casting decreases, and the plasticity and toughness significantly increase. At the same time, it is convenient for subsequent cutting and is suitable for ductile iron parts that need to improve processing performance or enhance plasticity and toughness.

Strength and comprehensive performance regulator

02 Incomplete austenite normalizing

The heating temperature for incomplete austenitization normalizing is controlled at 820-860 ℃, and the cooling method is the same as that for complete austenitization normalizing, supplemented by a tempering process of 500-600 ℃. When heated within this temperature range, some of the matrix structure transforms into austenite, and after cooling, a structure consisting of pearlite and a small amount of dispersed ferrite is formed.

This organization can endow castings with good comprehensive mechanical properties, balancing strength and toughness, and is suitable for structural components with high requirements for comprehensive performance.

Creating high-performance 'hardcore' components

01 Quenching and tempering treatment (quenching+high temperature tempering)

The process parameters for quenching and tempering treatment are heating temperature of 840-880 ℃, quenching with oil or water cooling, and high-temperature tempering at 550-600 ℃ after quenching. Through this process, the matrix structure is transformed into tempered martensite while retaining the spherical graphite morphology.

The tempered martensite structure has excellent comprehensive mechanical properties, with a good match between strength and toughness. Therefore, quenching and tempering treatment is widely used in diesel engine crankshafts, connecting rods and other shaft components, which require both high strength and toughness to adapt to working conditions.

02 isothermal quenching

The process steps of isothermal quenching are heating to 840-880 ℃, followed by quenching in a salt bath at 250-350 ℃. This process can achieve a microstructure with excellent comprehensive mechanical properties in castings, usually a combination of bainite, residual austenite, and spherical graphite.

Isothermal quenching can significantly improve the strength, toughness, and wear resistance of castings, especially suitable for parts with high requirements for hardness and wear resistance, such as bearing rings.

Local performance 'precise upgrade'

01 Surface quenching

High frequency, medium frequency, flame and other methods can be used for surface quenching of ductile iron castings. These surface quenching techniques form a high hardness martensitic layer on the surface of castings by locally heating and rapidly cooling them, while the core maintains its original structure.

Surface quenching can effectively improve the hardness, wear resistance, and fatigue resistance of castings, and is suitable for parts with high local stress such as crankshaft journals and gear tooth surfaces. Through local strengthening, the service life of parts can be extended.

02 Soft nitriding treatment

Soft nitriding treatment is a process of forming a compound layer on the surface of castings through nitrogen carbon co diffusion.

This process can significantly improve the hardness and corrosion resistance of the casting surface, and greatly enhance the surface wear resistance without significantly reducing the toughness of the substrate. It is suitable for ductile iron parts with high surface performance requirements, such as mechanical components that need to withstand friction for a long time.

Key points of heat treatment operation

1. Furnace temperature control

The temperature of castings entering the furnace generally does not exceed 350 ℃. For castings with large size and complex structure, the temperature entering the furnace should be lower (such as below 200 ℃) to avoid cracking due to thermal stress caused by excessive temperature difference. 2. Selection of Heating Rate

The heating rate needs to be adjusted according to the size and complexity of the casting, usually controlled at 30-120 ℃/h. For large or complex parts, a lower heating rate (such as 30-50 ℃/h) should be used to ensure uniform heating of the casting and reduce the risk of thermal deformation. 3. Determination of insulation time

The insulation time is mainly determined based on the wall thickness of the casting, generally calculated as insulation for 1 hour every 25mm wall thickness, to ensure that the matrix structure can fully transform during the heating process and achieve the expected heat treatment effect.

From the "softening" of annealing to the "hardening" of quenching, from overall strengthening to surface optimization, each process needs to be designed comprehensively based on material composition, part structure, and service conditions. It is recommended that enterprises establish a "process performance" database and dynamically optimize solutions through metallographic analysis (such as pearlite ratio, graphite spheroidization grade) and mechanical testing (tensile/impact testing), truly making heat treatment the "core engine" to enhance product competitiveness.