Pig Iron is the intermediate iron product between smelting iron ore and usable cast iron products. Grey Cast iron (CI) is made when pig iron is re-melted in small cupola furnaces (similar to the blast furnace in design and operation) and poured into molds to make castings. Cast Iron is considered as an alloy of iron with greater than 2% carbon, and usually with more than 0.1% silicon and there are are 3 other important types of cast iron (CI products) apart from grey cast iron.

The presence of flake graphite in the silicon-iron matrix make grey iron castings a unique material. The presence of graphite, the distribution pattern and the length of the flakes determine the strength and quality of cast iron.

Primarily there are 4 types of cast iron –

Grey Cast Iron
Ductile Iron
Malleable Iron
White Cast Iron

We will discuss more about Grey Cast Iron here –

In gray iron, slower solidification produces a larger graphite flake size. A slower cooling of the casting will produce a lower hardness in the metallic matrix.

Fatigue properties
Damping properties

The properties of grey iron can be determined by the amount of additional elements in the grey iron :

Cast iron contains small percentages of silicon, sulphur, manganese and phosphorus. The effect of these additional elements on the cast iron are as follows:

Silicon: About 3% of silicon can be present in cast iron. It provides the formation of free graphite which makes the iron soft and easily machinable.
Manganese: It makes the cast iron white and hard. It is often kept below 0.75%. It helps to exert a controlling influence over the harmful effect of sulphur.
Phosphorus: It aids fusibility and fluidity in cast iron, but induces brittleness. It is rarely allowed to exceed 1%. Phosphoric irons are useful for casting of intricate design and for many light engineering castings when cheapness is essential.

Quality control methodology has undergone a long period of change, involving a broadening of the subject to enhance many more aspects of the production process and product than were traditionally considered. These changes have had major effects in the field of internal management organization, as well as in the commercial relationships involved in the procurement of castings.

Casting defects can negatively impact the profit margin of the company. There are instances such as rework costs or casting scrap costs that not only affect financial state of the company but also affect customer services. However, in many cases, the casting defects may appear at the machining stage, at the assembly stage or during use of the component. The resultant value added costs and warranty costs may sometimes be passed on to the foundry by their customers.

Let’s take a look at the kind of defects and how they can be identified.

Tests for defects in the surface zone

Cracks and other flaws communicating directly with the surface of the casting can be detected using techniques including penetrant methods, chemical etching and oil and chalk testing. The penetrant methods are based on the use of fluids to permeate and highlight defects by increasing their contrast and are helpful in establishing the soundness and integrity of thin walled castings in cases where pressure testing is difficult to execute. If there is any seepage, then it can be easily observed.

Dye penetrant testing

The surface of the casting is treated with a dye solution based on a low viscosity solvent which is applied by brush or aerosol spray or the casting can be entirely immersed in a tank of solution; the casting is then thoroughly rinsed and dried. After a time, seepage of residual fluid occurs from cracks and other narrow surface defects to produce a visible stain.

Oil and chalk testing

This long established technique uses paraffin oil as the medium. The casting is treated, usually by immersion, with heated paraffin; it is then rinsed in detergent solution and water and dried. Using a low pressure air dispenser the surface is coated with dry powdered chalk, which becomes marked after a time by seepage of paraffin from cracks or porous zones.

Chemical etching

Very fine surface cracks and similar defects may be masked by a thin layer of metal resulting from superficial plastic deformation. This can occur in shot blasting, in fettling or even in the machining of a casting. The masking layer may consist of oxide scale formed in heat treatment.

A simple analysis of casting defects

For the characteristic criterion of quality level measuring, it is assumed quantity of defects in one year. The first step of analysis is the assessment of process by means of Ishikawa’s diagram. Ishikawa’s diagram – diagram of cause and effect, or “fish bone” diagram – makes possible identification of sources of problems formation, helps quality a sequence of causes and makes easy problem solution (SIEKANSKI K. 2002).

Ishikawa Diagram_BIC

Fig. 10.1 Part of Ishikawa´s diagram for problem of non-conformances formations in process of casting products.

Day: January 29, 2019

Cast Iron: CI Composition, Properties and Products

Cast Iron Products: Quality Control and Management

Tests for defects in the surface zone

Dye penetrant testing

Oil and chalk testing

Chemical etching

A simple analysis of casting defects

Source: https://tulip.co/blog/lean-manufacturing/fishbone-ishikawa-diagram-for-root-cause-analysis/