Quenching and tempering technology of ductile iron castings
Austenitizing is the first step in the quenching of Ductile Iron Castings. The austenitizing temperature is 860-880 °C, and the holding time is 1 hour for every 25mm thick casting, which is the same as that of normalizing and annealing. After the insulation is finished, it cools quickly in the flowing air. After quenching, the castings need to be tempered in time to reduce the quenching stress and improve the plastic toughness of the workpiece.
The casting should have a proper original matrix structure before quenching. The most important thing is that there should not be too much carbide and phosphorus eutectic in the Matrix. If the carbides do not dissolve completely at the quenching temperature, the interface between carbides and austenite is usually the place where the crack initiation occurs. The presence of these carbides significantly increases the probability of quenching cracks. Before quenching, the castings containing carbides must be heated to high-temperature graphitization annealing or normalizing temperature, and heat preservation to remove the carbides. The most suitable Matrix for quenched castings is the fine and homogeneous eutectic structure with pearlite as the main component. Such microstructure is easy to be austenitized, and a uniform quenched structure is obtained.
The hardness of nodular cast iron is related to the carbon content of austenite. Table 8-4 shows the relationship between austenitizing temperature and carbon content of ductile iron castings with 3.32% carbon, 2.52% silicon, and 0.29% manganese.
Increasing the austenitizing temperature will increase the carbon content of austenite, and more retained austenite will appear after quenching, which will make the Martensite coarsening and reduce the quenching hardness. If the lower quenching temperature is chosen, the carbon content of austenite can be kept at a lower level. After quenching, fine needle-like martensite can be obtained, but due to incomplete austenitization, the quenching hardness can not be achieved. Due to the combined effect of High Carbon Martensite and retained austenite, the maximum quenching hardness appears at different heating temperatures.
In addition, in order to eliminate most of the possible existence of Ferrite and make austenite obtain appropriate carbon content. The quenching temperature is 25 ° – 40 °C above the upper limit of eutectoid transformation.
The range of eutectoid temperature of nodular cast iron is changed by silicon. Therefore, the quenching temperature of ductile iron castings is affected by the silicon content of the castings. The upper limit temperature of eutectoid transformation is 835-845 °C when the silicon content is at the upper limit of the conventional content. Therefore, the quenching temperature can be selected as 860 ° 880 °C, and when the silicon content is less than 2% , the quenching temperature can be reduced to 840 ° 860 °C. The quenching hardness of nodular cast iron can reach 60-62(HRC) .
The effect of some alloying elements on the EUTECTOID transformation critical point of ductile iron should be taken into account when quenching.
The casting needs to be tempered in time after quenching. The quenching stress is reduced, the toughness of castings is improved, and the mechanical properties of materials are improved. Ductile iron tempering stability is high, so ductile iron tempering temperature and time should exceed steel.
In order to reduce phase transformation stress and brittle tempering temperature of 150-260 °C, 2s-3 hour air-cooling (also air-cooling and oil-cooling) is introduced. After tempering, the Matrix is tempered martensite and retained austenite—hardness HRC50 or so. The low-temperature tempering temperature should not exceed 250 °C. Avoid Temper embrittlement of castings.
It is required that ductile iron castings with good strength and toughness can be tempered at 350 ° 400 °C to obtain tempered martensite and retained austenite. This type of tempering is known as high-temperature tempering or quenching and tempering. After tempering, the casting can obtain high strength and high toughness to match it. It is a common heat treatment method for Ductile Iron with good comprehensive mechanical properties.
The effect of tempering temperature on mechanical properties of re MG ductile iron after oil quenching at 880 °C is shown in Fig. 8.7. The compositions of the castings are C 3.53, Si 2.05%, mn 0.75%.
AUSTEMPERING can be applied to ductile iron castings.