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