Cast iron casting quality controls
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
This is done by using a variety of methods which also depend on the chemical elements to be identified and quantified. The accurate identification of the presence and quantity of various elements is done using quantometers with spectrometric channels; the identification of elements such as carbon and sulphur is done by oxidising the sample and detecting the oxides obtained using infrared cells. An undoubtedly thorough, but also more time-consuming, procedure is the “wet method”. This type of analysis consists of dissolving the sample in acid and analysing the concentration of elements being investigated in the solution obtained using classic and instrument analysis techniques: spectrophotometry, gravimetry or titration; atomic absorption; optical emission spectroscopy; mass spectrometry.
They are carried out with the various metallographic procedures and instruments. Cast iron, after polishing and eventual chemical treatment, is observed under an optical microscope, highlighting and identifying the various microstructural constituents. Using image analysis systems interfaced with the metallographic microscope, it is possible to quantify the percentages of the constituents and the morphological parameters of graphite (nodularity, equivalent diameter, etc.). More specific and “local” information on microstructure and composition can be obtained by scanning electron microscopy (SEM), combined with an EMP microprobe.
Metallography – Macroscopic techniques
Metallography – Macroscopic techniques
Static mechanical and hardness tests
Static mechanical tests are essentially tensile tests, in which yield and ultimate tensile stress and elongation at break are determined under uniaxial stress conditions. Hardness tests, on the other hand, determine the resistance of the material to penetration by a point, of different shapes, depending on the type of test carried out.
Brinell hardness test
Vickers hardness test
Charpy tests (impact resistance)
The Charpy test used to determine the impact value of a metal alloy. A test specimen (55 mm long, cross-section 10 mm by 10 mm), notched if necessary, is subjected to an instantaneous impact by means of a pendulum with a “knife”. The total energy absorbed during the impact is the impact value. If the Charpy test with an instrument is performed, the instant energy absorption curve is determined, with the possibility of distinguishing between the energy absorbed during initiation and during crack propagation.
Charpy impact test
Dynamic mechanical tests
These are cyclic tests applied to metals (but also for other kind of materials), referred to as fatigue tests, as a result of alternating loads lead to the initiation and propagation of a crack, until the failure breaks, even after a very high number of cycles. By applying different load conditions (axial, bending, multi-axial, load ratios, etc.) the well known Wohler Curves can be drawn; in particular the fatigue limit, that is the level of stress amplitude below which, at least theoretically, the material exhibits infinite life, can be obtained.
In this case, the resistance of the material to wear by sliding or abrasion is assessed. The most common configuration is the so-called “pin-on-disk”: a pin (typically made of steel for tools or ceramic), subjected to a specific load, is slid (under dry or lubricated conditions) over a disc, consisting of the alloy to be assessed. The coefficient of friction during the test and the time-dependent consumption of the disk are determined. The test is mainly performed by way of comparison to compare the behaviour of different categories of materials.
Non-Destructive Testing (NDT)
Ultrasonic radiation propagates easily through metal. However, it is reflected if it encounters a cavity or an inclusion. With different types of probes, it is, therefore, possible to detect the presence of other more frequent classes of flaws in a foundry casting, such as shrinkage cavities, gases trapped or developed by the interaction between sand, binders and cast iron, slag inclusions and oxide particles.
It is well known that X-rays, depending on their intensity, can pass through a metal, which also depends on the intrinsic characteristics of the material. The X-ray resulting from this makes it possible to detect the presence of cavities (solidification shrinkage, trapped air and gases) within a casting. It is also possible to carry out a 3D reconstruction of the cavities with the Industrial computed tomography.
Other types of checks
There are also other types of non-destructive testing applicable to the cast iron foundry field. To this end, please note:
Liquid penetrant tests
With eddy currents
With magnetic particles