Castings: The beginning of everything

To the curious,

Lets brush up on some product tech, espcially if you’re a budding level 3 studying for your basic exam, casting exam questions will add a fair chunk to the overall exam!
Lets do a deep dive, a quick intro….
Castings in manufacturing involve pouring molten metal into a mold to create a solidified shape once cooled. This process is widely used for producing complex geometries that would be difficult or costly to machine. Common casting methods include sand casting, investment casting, and die casting, each suited for different materials and precision levels. Factors like shrinkage, porosity, and cooling rates must be controlled to ensure quality. Castings are commonly used in industries like automotive, aerospace, and heavy machinery due to their strength, versatility, and ability to produce intricate components efficiently.

Lets drill into the common casting methods, the three basic casting methods—sand casting, investment casting, and die casting—each have unique characteristics suited for different applications.

Sand Casting – This is the most common and cost-effective method, where molten metal is poured into a sand mold. It is highly versatile, allowing for large and complex shapes, but has a rough surface finish and lower dimensional accuracy. It is widely used for automotive parts, engine blocks, and heavy industrial components.

Investment Casting (Lost Wax Casting) – This method uses a wax pattern coated in ceramic to form a mold, which is then melted out before pouring metal. It produces highly detailed and precise components with a smooth surface finish, making it ideal for aerospace, medical, and intricate industrial parts. However, it is more expensive and time-consuming than sand casting.

Die Casting – In this process, molten metal is injected under high pressure into a steel mold (die). It is used for high-volume production of parts with excellent dimensional accuracy, smooth finishes, and fine details. This method is primarily used for non-ferrous metals like aluminum, zinc, and magnesium in automotive, electronics, and consumer goods. The downside is the high initial tooling cost, making it less suitable for low-production runs.

As an NDT tech, its especially important to know how the material you are testing was made. This will give you an indication as to what type of indications you might expect to find.
For example, welding, forged parts, cast parts are might have different indications…
So, what could we expect to find in cast materials. (Also, welding might have the same indications as you are essentially melting metal and cooling it, in the same way as casting.)

Shrinkage Porosity – Small voids caused by metal contraction during solidification.

Gas Porosity – Defects like shrinkage porosity and gas porosity are typically formed during the solidification process when the molten metal cools and contracts, creating voids or pockets of trapped gas.

Cold Shuts – Occur when two streams of molten metal fail to fuse properly due to premature cooling or contamination at the mold surface.

Hot Tears (Hot Cracks) – Irregular cracks caused by stress during cooling.

Inclusions – Usually form when non-metallic impurities or oxides are trapped during the pouring process, often due to poor mold preparation or material handling.

Below is an example of gas pores/porosity you could see in castings, RT would be the preferred method to detect such pores.

Nice, we know about likely indications we might encouter, lets dive into grain stuctures of cast iron. Different formations occur as it cools, each with its own distinct structure and properties. The primary crystalline structures of iron and their phases include:

Delta Ferrite (δ-Fe): At temperatures above 1,394°C, iron exists as delta ferrite, which has a body-centered cubic (BCC) structure. This phase is stable at high temperatures and has a relatively high solubility for carbon, but it is not commonly encountered in most cooling processes.

Austenite (γ-Fe): As iron cools down to around 912°C, it transforms into austenite, which has a face-centered cubic (FCC) structure. This phase is highly ductile and can dissolve more carbon than ferrite. It is a key phase for the formation of steel alloys, as carbon can be added to create different grades of steel.

Ferrite (α-Fe): Upon further cooling below 912°C, austenite transforms into ferrite, a BCC structure that is relatively soft and magnetic. Ferrite can only hold a small amount of carbon in solid solution (around 0.02% at room temperature). Ferrite is the phase found in low-carbon steels.

Cementite (Fe₃C): When the carbon content increases (above 0.8%), iron forms cementite, a hard, brittle iron carbide (Fe₃C). This phase is often found in steel alloys and contributes to hardness but reduces ductility.

Pearlite: When austenite cools slowly in carbon steels (especially around 727°C), it transforms into pearlite, a lamellar mixture of ferrite and cementite. Pearlite provides a balance of strength and ductility and is often seen in medium-carbon steels.

Martensite: When austenite is cooled rapidly (quenched), it doesn't have enough time to form pearlite or ferrite. Instead, it forms martensite, a very hard, needle-like structure that is a supersaturated solution of carbon in iron. Martensite is very strong but brittle, and it can be tempered to adjust hardness and toughness.

Heat treatment processes are used to alter the mechanical properties of iron and steel by changing the microstructure.