Steel impact toughness: how the material reacts to impacts

Steel impact toughness measures the material’s ability to absorb energy during a rapid stress, such as an impact.

In practice, it makes it possible to understand how much energy the material can withstand before breaking.

👉 A steel with good impact toughness is a material that is better able to resist:

• crack formation
• fracture propagation
• sudden breakage
• the effects of impacts or shocks

This parameter is particularly important when steel must guarantee reliability and safety even under demanding operating conditions.

Impact toughness tests on materials are essential because they make it possible to evaluate the steel’s ability to resist impacts.

👉 In particular, they help determine whether the material is able to:

• absorb energy
• resist brittle fracture
• limit crack propagation
• maintain reliable behavior in service
• respond correctly to dynamic stresses

Impact toughness should not be confused with ductility.

Ductility indicates the ability of a material to deform plastically before fracture.

Impact toughness, on the other hand, measures the energy absorbed during an impact, up to the fracture of the specimen.

👉 They are two related properties, but they are not equivalent.

The most widely used method for evaluating impact toughness is the Charpy test.

In this test, a standard specimen is struck by a pendulum, called a Charpy pendulum, with a single blow.

👉 The objective is to measure the energy required to break the specimen.

The value obtained is expressed in Joules [J] and represents the material’s ability to absorb energy during impact.

The higher the measured value, the greater the energy absorbed by the material before fracture.

✔ The Charpy test therefore provides a concrete indication of the impact resistance of steel.

The test is carried out using a pendulum system.

The specimen is placed on a dedicated support and struck in the center by an oscillating mass.

👉 During the impact:

• the pendulum strikes the specimen
• the specimen breaks
• the instrument measures the energy absorbed during fracture
• the result is expressed in Joules

The difference between the initial energy of the pendulum and the residual energy after fracture makes it possible to calculate how much energy has been absorbed by the material.

This data is fundamental for evaluating the behavior of steel in the presence of impacts.

The specimens used in impact toughness tests can have different geometries and different types of notch.

The notch serves to concentrate the stress at a precise point, facilitating fracture initiation and making the test more controlled.

Among the main types we find:

• KV: specimen with a 45° V-notch, 2 mm deep
• KU: specimen with a U-notch, 5 mm deep
• KVW: specimen without notch, used for specific evaluations related to ductility

The standard specimen generally has:

• a length of 55 mm
• a square section of 10 x 10 mm

Specimens with reduced section can also be used, for example with a width of 7.5 mm or 5 mm, when the dimensions of the material do not allow a standard specimen to be obtained.

👉 The choice of specimen and notch affects the correct interpretation of the material’s behavior.

Steel impact toughness depends on several metallurgical and microstructural factors.

Among the main ones we find:

• chemical composition
• microstructure
• heat treatments
• grain size
• presence of non-metallic inclusions
• segregations in the material
• test temperature
• orientation of the sample with respect to processing

Non-metallic inclusions, such as sulfides, oxides and silicates, can act as initiation points for cracks.

👉 This means that greater steel cleanliness can help improve the material’s behavior in the event of impact.

For this reason, in particularly critical applications, steels with a high level of metallurgical purity, such as remelted steels, may be preferred.

Impact toughness can vary significantly depending on temperature.

A material that shows tough behavior at room temperature can become more brittle at lower temperatures.

👉 For this reason, some impact toughness tests are performed at controlled temperatures, even below room temperature, when the final application requires it.

This aspect is important for evaluating the behavior of steel in particular operating conditions, where impact resistance must also be guaranteed in severe environments.

In the Bonomi Acciai context, impact toughness tests represent an important control for evaluating the behavior of steel when subjected to impacts or dynamic stresses.

Through the Charpy test, it is possible to measure the energy absorbed by the material during the fracture of a standardized specimen.

At present, impact toughness tests are not yet performed internally in the BA laboratory. The instrument is expected to arrive in September. For the time being, BA relies on external laboratories such as AQM and KAIZEN for these checks.

An impact toughness test may be required or recommended in several cases.

This control represents one of the requirements included in the supply certificates for steels compliant with NADCA specifications, which are particularly used in the mould and high-performance applications sector.

The impact toughness test can be performed as part of quality controls, to verify the conformity of the material with the values required by technical specifications or supply certificates, especially after heat treatments.

In addition, this type of verification can be particularly useful in the case of failures or non-conformities detected during use of the component.

For example, in the event of a mould failure, the impact toughness test can help determine whether the cause of the problem can be traced back to the material’s characteristics or to other factors related to use or the production process.

KV-type specimens are normally used for impact toughness tests, in accordance with the main reference standards for notched Charpy tests.

Standard KV specimens have dimensions of 10 × 10 × 55 mm and feature a 2 mm deep “V” notch, with a 45° angle and a rounded root.

This geometry makes it possible to evaluate the material’s ability to absorb energy in the presence of a particularly severe stress concentration.

In some cases, KU-type specimens may also be required. These are also generally 10 × 10 × 55 mm in size, but feature a “U” notch, which is less pronounced than that of KV specimens.

This configuration produces a lower stress concentration during the impact test.

When the thickness of the material does not allow standard specimens to be obtained, reduced-size specimens may also be used, while still maintaining the same general geometry required by the standard.

Impact toughness tests can be performed both at room temperature and at controlled temperatures.

In the BA context, the tests normally required, in accordance with NADCA#207, are performed at room temperature.

The results of impact toughness tests are compared with the reference values required by NADCA #207.

This comparison makes it possible to verify the material’s conformity with the requirements needed for specific applications, particularly in the mould and high-performance applications sector.

The impact toughness test helps evaluate the reliability, safety and suitability of steel in critical applications.

For the customer, this means being able to rely on a greater guarantee regarding the suitability of the material for applications such as:

• moulds
• mechanical components
• parts subject to high operating stresses

Impact toughness, in fact, is not just a number shown on a certificate: it is a concrete indication of how steel can behave when subjected to an impact

In the path dedicated to steel quality, impact toughness represents a fundamental parameter for evaluating how the material reacts to impacts.

After analyzing chemical composition and hardness, impact toughness makes it possible to understand another essential aspect: the steel’s ability to absorb energy before fracture.

👉 In the next articles we will explore other elements found in controls and certificates, such as:

• grain size
• steel cleanliness
• microstructure
• ultrasonic testing
• radiometric testing

Each parameter contributes to building a more complete view of material quality.

Steel impact toughness is a fundamental property for evaluating the material’s resistance to impacts.

Through the Charpy test, it is possible to measure the energy absorbed during the fracture of a specimen and obtain a concrete indication of the material’s behavior.

👉 Understanding impact toughness means understanding how much a steel is able to withstand impacts, dynamic stresses and critical operating conditions.

It is not just laboratory data: it is an essential parameter for evaluating the safety, reliability and quality of the material.