Factors affecting the impact toughness of metallic materials

Impact toughness of metallic materials refers to their ability to resist damage and restore deformation when subjected to impact loading, and this performance index is of great significance to the practical application of materials. Impact toughness not only reflects the degree of toughness and brittleness of the material, but also determines the durability and reliability of the material under dynamic load. There are many factors affecting the impact toughness of metallic materials, including the nature of the raw material itself, the orientation of the specimen, the notch geometry and processing quality, the accuracy of the testing machine, the fit between the pendulum and the frame, the test temperature, the positioning of the impact specimen and so on.

1. The nature of the raw material itself

The impact toughness of metal materials is closely related to their own metallographic structure, chemical composition, physical properties, processing and heat treatment processes. For example, the chemical composition of the metal, especially carbon (C), phosphorus (P), sulfur (S) and other elements, when their content increases, usually leads to a reduction in the impact toughness of the material. This is because these elements tend to form brittle phases or inclusions within the material, increasing stress concentrations and reducing the toughness of the material. On the contrary, elements such as manganese (Mn) and nickel (Ni) can effectively improve the toughness of the material within a certain range; Mn can refine the grain and inhibit the precipitation of carbides along the grain boundaries, while Ni can increase the layering energy of ferrite and promote the cross-slip displacement of dislocations, which can help to improve the toughness of steel.

In addition, the phase composition of a metallic material also has a significant effect on its toughness. The higher the content of ferrite, a phase with low strength, good plasticity and toughness, the better the impact toughness of the material usually is. On the contrary, reticulated carburites deteriorate the toughness of the material, and the higher their amount, the worse the impact toughness of the material. Therefore, by adjusting the chemical composition of the material and the heat treatment process, the phase composition can be controlled and thus the impact toughness of the material can be optimised.
2. Orientation of specimens

The orientation of metallic materials affects their mechanical properties, including toughness. In the actual production and engineering applications, most of the metal materials are rolled, in the rolling process metal inclusions accompanied by metal grains along the main deformation direction is elongated, the formation of metal fibre tissue, seriously affecting the impact toughness of metal materials. Therefore, along the rolling direction sampling, that is, the long axis of the sample parallel to the rolling direction, notch open in the direction perpendicular to the rolling direction, so that the impact toughness of the sampling obtained by the larger; on the contrary, perpendicular to the rolling direction sampling, along the rolling direction of the opening notch, the impact toughness obtained by the sampling of the smaller.

3. Notch geometry and processing quality

Notch geometry and processing quality has an important impact on the impact toughness of the material. According to GB/T 229-2007 standard, the notch is mainly divided into U-type and V-type two kinds of V-type notch compared to U-type notch, the stress is more concentrated, so its impact toughness is usually lower. For the same metal material, the impact toughness of notched specimens is much smaller than that of unnotched specimens, because the notch will lead to stress concentration, which will reduce the toughness of the material. The notched impact specimens have a significant degree of stress concentration in descending order of I-type, V-type, U-type, and semi-circular impact specimens.

In addition, notch processing quality is also one of the important factors affecting impact toughness. Notch processing quality mainly through the impact of notch near the stress, strain concentration to affect the impact toughness of the material. Studies have shown that the impact toughness of the impact specimen notch depth with the increase in the gradual decrease, with the notch root radius increases, the impact toughness of metal materials gradually increased; impact toughness with the bottom of the notch machining scratches, hardening degree of the gradual decrease. Therefore, should be in strict accordance with GB/T 229-2007 impact notch specimen notch size of the provisions of the impact specimen processing.
4. The accuracy of the testing machine and the pendulum and frame with the coordination

Impact toughness of metal materials on the impact test machine precision has certain requirements, low precision test machine on the impact toughness of the greater impact. In addition, the impact toughness is also related to the error of the impact test machine reading device, so the test should be performed before the zero operation.

Pendulum with the frame is also critical. Impact testing is a one-time destruction test, so the fit of the pendulum to the frame must be precise. This includes the parallelism of the pendulum axis and the reference plane, the parallelism of the pendulum side and the swing plane, the radial and axial clearance of the pendulum axis, the distance from the pendulum axis to the strike centre, the relative position of the impact knife edge and the support span, etc., which should meet the requirements of the relevant standards. When the relative position of the impact knife edge and the support span centre does not meet the requirements, the impact knife edge and the specimen notch centre line can not coincide, resulting in inaccurate measurement results, impact toughness will be large.

5. Test temperature

Test temperature is also one of the important factors affecting the impact toughness of materials. In the impact toughness test process, to find the material is in the brittle zone temperature range, in the use of the process can be controlled to avoid the brittle zone temperature on the material. Different non-ferrous metal materials and their impact toughness by the impact of temperature are different, but the impact absorption work are related to the temperature of the temperature, temperature uniformity, the length of the insulation time. As the temperature decreases, the impact toughness of the material usually decreases, this is because the plastic deformation capacity of the material at low temperatures is reduced, the crack expansion rate is accelerated, which leads to a reduction in toughness.

6. Positioning of impact specimens

The positioning of the impact specimen is to ensure that the centre line of the impact specimen notch coincides with the edge of the impact knife on the pendulum to reduce the test operation error. If their relative positions do not coincide, can not meet the requirements of 0.5mm, the maximum impact force can not act on the impact specimen notch root minimum cross-section, ultimately leading to the impact toughness bias.

7. Other factors

In addition to the above factors, the internal defects and impurities of metal materials can also significantly affect their impact toughness. Defects and impurities increase stress concentration and reduce the toughness of the material. For example, internal defects such as inclusions and bubbles can lead to crack initiation and extension, thus reducing the impact toughness of the material. In order to reduce the impact of defects and impurities on the toughness of materials, the quality of raw materials and the conditions of the production process need to be strictly controlled during the preparation and processing of materials.