Checking the quality of the cast iron

The concept of quality, for cast iron casting, is quite complex, since minimum requirements regarding the composition, microstructure, deflectology, and mechanical and technological characteristics must be achieved. The main quality control systems of cast iron castings are summarised below.

Chemical, microstructural and thermal analyses

Analysis of the chemical composition

The chemical composition, together with the production process, determines the microstructure and therefore the mechanical and technological behaviour of a cast iron. It is therefore essential to know, in almost real time, the content of “strategic” chemical elements for a cast iron, such as C, Si, Mg and the various alloying elements.

Microstructural investigations

The microstructure of a cast iron is the result of its composition and technological process, and subsequently determines its technological behaviour and mechanical performance. Knowing the microstructure of a cast iron is therefore essential in guaranteeing the quality and consistency of the castings produced.

Thermal analyses

The development of the microstructure during the solidification and cooling of a cast iron is always associated with heat development or absorption phenomena. By means of thermal analysis and the processing of its main output (the cooling curve), it is possible to monitor the microstructural evolution of a cast iron during its production cycle.

Mechanical tests

PStatic mechanical and hardness tests

The starting point for the design of any industrial component is naturally the knowledge of its mechanical characteristics. It is therefore essential to be able to determine tensile strength, yield strength, and elongation at break, by means of static testing.

Impact resistance tests

Impact resistance is the ability of a material to withstand shocks and impacts and, when a fracture process initiates, to counteract its propagation. There are specific tests to assess the impact resistance of a material and understand its toughness.

Dynamic mechanical tests

Material failure may be triggered by way of cyclic stresses able to induce cracks on the surface that may propagate to failure. Dynamic mechanical tests (the fatigue test is the most widespread) allow the laboratory assessment of a material’s resistance to cyclic mechanical stresses.

Tribology testing

Sliding, abrasion and contact phenomena may cause the surface of a material to progressively deteriorate and ultimately wear. Tribological tests allow the determination, almost always in a comparative manner, of an alloy’s wear resistance.

Non-Destructive Testing (NDT)

Ultrasonic testing

Among the various non-destructive testing techniques, ultrasonic testing stands out for its ability to detect defects such as inclusions and cavities, exploiting interactions between the ultrasound waves and the surfaces of said defects. The waves are reflected or deflected or attenuated, and special sensors allow the defect to be located.

X-ray testing

X-ray tests are quite common in foundries. They exploit the fact that the X-ray is attenuated to different extents depending on the material it passes through. In the presence of porosity, the radiation is not attenuated and, by means of suitable plates or screens, the porosity is identified. X-ray testing, in its conventional version, provides two-dimensional information (i.e., the X-ray), while in recent years, industrial tomography systems have been used to also acquire 3D images of the location of the defects.

Other types of non-destructive testing

Ultrasound and radiography are the types of non-destructive testing used in foundries. These are supplemented by other methods. These include penetrating liquids (to identify the presence of cracks or defects appearing on the surface), eddy-current and magnetic particle testing.

Checks in the production line

To ensure the quality of castings and the efficiency of the production cycle, it is essential to have real-time information on the consistency of the process parameters. Once optimised, these should not be subject to drifts or abnormal variations. Checks in the production line, for example of the temperature, allow the continuous monitoring of the process.

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