JAES Learning

We have almost reached the end of our journey within Zanardi Fonderie an Italian company that has been working in the cast iron section since 1931. In the previous episodes we have been able to observe in detail all the preparation from development to casting of a cast iron object.
This last episode of the series focuses on the operations carried out on freshly cast iron castings, such as deburring. Deburring is the mechanical operation, which can be carried out both manually and mechanically, through which the “burrs” present on the workpiece are removed. Before carrying out the deburring operations, all the cast iron castings exiting the foundry line undergo an initial quality check which includes a visual inspection and a hardness check. Once checked, the workpieces are divided into lots, collected within specific bins for initial handling and suitably identified with a code useful to perform all the subsequent phases. Painting is another operation that is carried out at the Zanardi Fonderie production plant. The painting of a cast iron object may be required both for an aesthetic factor and to protect its surface from possible damage related to the external environment, such as rust. There are several ways to paint, Zanardi Fonderie use the immersion in water-based paint technique. Zanardi Fonderie is equipped with an automatic painting line. The operators place the individual objects to be painted on poles hooked to a mobile chain. These first go through a tunnel where they are washed to remove impurities. Then they go through the painting booth where, by means of 2 lowerators, the workpieces are completely immersed in the tank; they are then manually finished where necessary. Then they go through 2 ventilated furnaces for the reticulation of the paint and then unloaded by the operators. Even after deburring and painting, the raw cast is not usually ready to be used by the customer. Workpiece mechanical and finishing processes are carried out with different machinery according to the type of operation required. Very often this phase is not carried out within a foundry but subcontracted to external mechanical workshops; instead, Zanardi Fonderie has developed its own mechanical workshop within the company, with 5-axes lathes for complex mechanical processing such as turning, toothing and drilling. Let us now continue our journey inside the Zanardi Fonderie production plant. A further and more important process present within Zanardi Fonderie is the austempering heat treatment. The austempering heat treatment is usually carried out by the companies specialised in heat treatments and not by foundries which subcontract this process. Instead, Zanardi Fonderie has been carrying out this process internally since the beginning of the new millennium, for both its own castings but also for the castings of other foundries. Being a foundry with in-house heat treatment makes it unique as a company in this sector. This treatment makes it possible to give the spheroidal graphite iron mechanical properties comparable or superior to those of quenched and tempered steel, maintaining the typical lightness of ductile iron and its natural capacity to adapt to the manufacture of complex moulds. The material obtained after the heat treatment is called Austempered Ductile Iron, also known as ADI. To obtain a casting in ADI, it is necessary to treat the spheroidal graphite iron casting suitably enriched by alloying elements such as copper, nickel and molybdenum. Let us now see how the Austempering cycle takes place: The operators load the treatment baskets with the cast iron castings which initially undergo a pre-heating in a specific furnace placed in front of the austempering furnace. The basket reaches the uniform temperature of about 600°C, it is then picked up by an automatic conveyor and transported first into the vestibule and then into the intermediate washing chamber, where all the oxygen and salt vapours are eliminated; then the basket with the workpieces moves to the austenisation chamber, where the heating phase takes place, reaching an optimal temperature for the diffusion of carbon in the austenite, typically between 800°C and 900°C, everything takes place in a controlled ENDOGAS atmosphere, composed of hydrogen, nitrogen, carbon monoxide and a compound of carbon dioxide, methane and water.


At the end of this phase the material passes from a generally pearlitic matrix to a completely austenitic one rich in carbon. Once austenitization is complete, the basket moves again into the intermediate washing chamber, without coming into contact with the external environment, then returns to the vestibule, and finally descends into the salt bath for the hardening operation. Hardening takes place in a tank containing a mixture of water, nitrates and nitrites, and the basket with the work pieces will remain immersed in the Austin tempering until the austempering reaction is completed. In the hardening phase, the temperature can vary, can vary between 240° and 400°C based on the desired ADI grade, and it can last from a few minutes to a few hours. At the end of this cycle the matrix of the material will have transformed into a matrix with an ausferritic microstructure, that is containing a mix of stable austenite and acicular ferrite.

The cycle is repeated continuously and in rotation, while one basket is in the preheating phase another is in the austenitizing chamber and another in the hardening phase in the salt bath.

At the end of the hardening phase, once the basket has risen in the vestibule, it is picked up by the automatic conveyor and taken to the water-washing system, where all saline residues present on the cast-iron pieces and on the basket will be removed; the salts will subsequently recovered by sending the wash water into the evaporation system and reintroduced into the salt tank.