Steel Cleanliness: Non-Metallic Inclusion Testing

🔍 What are non-metallic inclusions?

Non-metallic inclusions are particles of a different nature from steel that may originate during the melting, refining and casting processes.

Their presence is unavoidable in any steelmaking process. However, their quantity, size and distribution must be carefully controlled.

In general, cleaner steel contains fewer inclusions and offers higher and more consistent performance.

In particular, this aspect becomes important in applications subjected to high mechanical stresses.

👉 Inclusions can affect:

• material toughness
• fatigue resistance
• machinability
• the uniformity of mechanical properties
• service life
• component reliability in service

🧪 Why check steel cleanliness?

Non-metallic inclusions can represent points of discontinuity within the steel matrix.

Under certain conditions, these particles can become initiation points for cracks, fatigue failures or premature failure.

Consequently, controlling them is particularly important for steels intended for demanding applications.

In these cases, the material must maintain consistent performance even under severe stresses.

👉 A high level of cleanliness contributes to:

• greater component reliability
• better toughness characteristics
• greater fatigue resistance
• more uniform mechanical properties
• longer service life

For this reason, inclusion testing is a fundamental part of the quality process for special steels.

⚙️ How is the test performed?

Steel cleanliness is assessed through a metallographic examination performed according to the ASTM E45 standard.

This is one of the most widely used international standards for the classification of non-metallic inclusions.

The process begins with the selection of a representative sample of the material.

The specimen is then prepared through:

• cutting
• mounting
• grinding
• polishing

The objective is to obtain a perfectly mirror-like surface suitable for microscopic observation.

Unlike other metallographic analyses, chemical etching is generally not performed when evaluating inclusions.

In fact, the objective is to directly observe the non-metallic particles present within the steel matrix.

Finally, the sample is examined under an optical microscope and compared with specific reference charts defined by ASTM E45.

These charts make it possible to classify the quantity, size and morphology of the inclusions.

📊 ASTM E45 inclusion types

ASTM E45 divides inclusions into different families according to their nature and microscopic appearance.

This classification makes it possible to distinguish the main types of inclusions and assess the cleanliness level of the material.

The following images show the reference classifications for Type A, B, C and D inclusions, distinguishing between thin and heavy series.

🅰️ Type A – Sulfides

Type A inclusions mainly consist of manganese sulfides.

They generally appear as elongated particles oriented in the rolling or forging direction.

Consequently, they can influence the anisotropic mechanical behavior of the material.

Furthermore, high values may indicate greater sensitivity to crack propagation in specific directions.

🅱️ Type B – Aluminates

Type B inclusions are generally aluminum oxides deformed during plastic working.

In many cases, they appear in groups or discontinuous chains and may act as stress concentration points.

Consequently, they can affect the fatigue resistance and toughness of the material.

🅲 Type C – Silicates

Type C inclusions mainly consist of silicates originating from refining and casting processes.

During mechanical processing, they also tend to take on elongated shapes.

Under cyclic loading conditions, they may therefore contribute to reducing material resistance.

🅳 Type D – Globular Oxides

Type D inclusions mainly consist of non-deformable oxides.

Unlike the previous types, they have a rounded or globular shape.

In general, they are considered less critical than elongated inclusions.

However, high quantities can still negatively affect mechanical performance and the quality of the finished product.

🧾 What do the values reported in the certificate indicate?

The results of ASTM E45 analysis are normally expressed through numerical values associated with each family of inclusions.

In general, as the value increases, the quantity or size of the inclusions observed in the sample also increases.

Conversely, low values indicate particularly clean steel.

For this reason, such values are generally required for high-performance applications.

👉 Applications where steel cleanliness may be particularly important include:

• die-casting dies
• aerospace components
• tools for demanding machining operations
• shafts and highly stressed mechanical components
• energy and petrochemical applications

In any case, the results must always be interpreted by taking into account the type of steel and its final application.

🔬 Why is steel cleanliness so important?

Steel cleanliness is an internal characteristic of the material and cannot be seen from the outside.

However, it can make the difference between a reliable component and one subject to premature failure.

In fact, non-metallic inclusions can act as initiation points for damage mechanisms such as cracks, fatigue failures and premature failure.

Conversely, steel with a higher level of cleanliness offers greater material uniformity.

Consequently, it can provide a more reliable response under the expected operating conditions.

For steels intended for demanding applications, inclusion testing is therefore a fundamental parameter for assessing the metallurgical quality of the material.

🏭 Inclusion testing in the BA context

Within Bonomi Acciai, steel cleanliness testing can be considered part of the qualification and verification process for special steels.

The assessment of non-metallic inclusions provides a better understanding of the internal quality of the material.

Furthermore, it helps verify its suitability for applications where reliability, uniformity and long-term performance are fundamental requirements.

👉 For the customer, this test provides tangible value because it makes it possible to:

• assess the metallurgical quality of the steel
• verify the presence and distribution of inclusions
• support the interpretation of data reported in the certificate
• better understand the expected behavior of the material
• reduce the risk of problems during service
• select steels better suited to critical applications

In this way, inclusion testing becomes a useful tool for linking the internal quality of the steel to the customer’s actual application requirements.

🔄 Connection to the series

Within the “Anatomy of Steel” series, inclusion testing represents a fundamental step in understanding the internal quality of the material.

After examining chemical composition, hardness, toughness, non-destructive testing and metallography, inclusion assessment adds another level of analysis.

👉 In particular, it makes it possible to connect:

• production process
• microstructure
• metallurgical cleanliness
• mechanical properties
• reliability in service
• final material quality

Steel cleanliness cannot be seen from the outside.

Nevertheless, it can significantly influence the behavior of the material over time.

🎯 Conclusion

Non-metallic inclusion testing is one of the most important metallurgical analyses for assessing steel cleanliness.

Through metallographic examination and classification according to ASTM E45, it is possible to objectively observe and measure the presence of non-metallic particles within the steel matrix.

👉 Consequently, this test makes it possible to assess the metallurgical quality of the material, support the selection of the most suitable steel and provide greater reliability in the most critical applications.

Ultimately, understanding steel cleanliness means recognizing that material quality does not depend only on what can be seen or measured at the surface.

In fact, it also depends on what is present within its internal structure.