Impact Toughness and Brittle Fracture of High-Strength Bolts, Screws, and Studs

Impact Toughness and Brittle Fracture of High-Strength Bolts, Screws, and Studs

The inspection of high-strength bolts, screws, and studs is based on GB/T 3098.1-2010, Mechanical Properties of Fasteners – Bolts, Screws and Studs. Products leaving the factory are generally required to comply with the acceptance test requirements specified in this standard. However, based on current manufacturing practices and field applications, some aspects still require further improvement.

At present, brittle fracture is one of the major failure modes of high-strength bolts, screws, and studs used in mechanical structures. Brittle fracture occurs suddenly during service loading without significant macroscopic plastic deformation. Since there are often no obvious warning signs before failure, brittle fracture poses a considerable safety risk.

Impact toughness is used to characterize the toughness of a material. It depends not only on the chemical composition and microstructure of the material, but also on its metallurgical quality. In addition, impact toughness is closely related to the service temperature and the presence of notches or stress concentrators.

Traditional Methods for Preventing Brittle Fracture in Bolts

The selected material should possess adequate plasticity indicators, such as:

Elongation after fracture (A, %)

Reduction of area (Z, %)

It should also have sufficient impact toughness values, such as:

Impact absorbed energy KV (J)

Impact absorbed energy KU (J)

The ductile-to-brittle transition temperature of the material should be controlled. A common parameter used to determine the transition temperature associated with brittle fracture is FATT (Fracture Appearance Transition Temperature).

 

Relationship Between Material and Impact Toughness

The content of impurity elements in the chemical composition is one of the key factors affecting impact toughness. Strict control of phosphorus (P) and sulfur (S) is required in steel, and their combined content should generally not exceed 0.025%.

Phosphorus has a pronounced embrittling effect. It tends to form precipitates at grain boundaries, weakening the grain boundaries and thereby reducing impact toughness.

Carbon content also has a significant influence on impact toughness. When low-carbon alloy steel and medium-carbon alloy steel are heat-treated to the same strength level, the former generally exhibits much higher fracture toughness than the latter.

For example, when 20MnTiB and 40CrNiMo steels are heat-treated to achieve the properties of Grade 10.9 bolts, their strength levels are similar, but their fracture toughness values are approximately:

 

20MnTiB: 113 MN/m³⁄²

40CrNiMo: 78 MN/m³⁄²

However, in terms of impact toughness (KV and KU impact absorbed energy values), the latter is 20–45 J lower than the former.

Nickel (Ni) is one of the most effective alloying elements for improving toughness. It not only enhances fracture toughness but also effectively lowers the ductile-to-brittle transition temperature. Therefore, Ni is an important alloying element in steels intended for low-temperature applications.

Molybdenum (Mo) has a similar effect to Ni. Chromium (Cr) has relatively little influence on impact toughness. The addition of a small amount of boron (B) can improve fracture toughness during low-temperature tempering.

Bolts are often exposed to corrosive environments during service. Such conditions are among the external factors that contribute to delayed fracture. To improve fracture toughness in these steels, alloy design should focus on:

Improving hardenability

Enhancing microstructural stability

Refining grain size

Refining carbide particles and ensuring their uniform distribution

Preventing temper embrittlement

Lowering the ductile-to-brittle transition temperature

For low-alloy steels represented by 35CrMo, the addition of an appropriate amount of vanadium (V) can refine grain size and improve fracture toughness. Therefore, 35CrMoV steel is considered a suitable material for manufacturing Grade 12.9 high-strength bolts.

 

Impact Toughness and Brittle Fracture of High-Strength Bolts, Screws, and Studs

The inspection of high-strength bolts, screws, and studs is based on GB/T 3098.1-2010, Mechanical Properties of Fasteners – Bolts, Screws and Studs. Products leaving the factory are generally required to comply with the acceptance test requirements specified in this standard. However, based on current manufacturing practices and field applications, some aspects still require further improvement.

At present, brittle fracture is one of the major failure modes of high-strength bolts, screws, and studs used in mechanical structures. Brittle fracture occurs suddenly during service loading without significant macroscopic plastic deformation. Since there are often no obvious warning signs before failure, brittle fracture poses a considerable safety risk.

Impact toughness is used to characterize the toughness of a material. It depends not only on the chemical composition and microstructure of the material, but also on its metallurgical quality. In addition, impact toughness is closely related to the service temperature and the presence of notches or stress concentrators.

Traditional Methods for Preventing Brittle Fracture in Bolts

The selected material should possess adequate plasticity indicators, such as:

Elongation after fracture (A, %)

Reduction of area (Z, %)

It should also have sufficient impact toughness values, such as:

Impact absorbed energy KV (J)

Impact absorbed energy KU (J)

The ductile-to-brittle transition temperature of the material should be controlled. A common parameter used to determine the transition temperature associated with brittle fracture is FATT (Fracture Appearance Transition Temperature).

 

Relationship Between Material and Impact Toughness

The content of impurity elements in the chemical composition is one of the key factors affecting impact toughness. Strict control of phosphorus (P) and sulfur (S) is required in steel, and their combined content should generally not exceed 0.025%.

Phosphorus has a pronounced embrittling effect. It tends to form precipitates at grain boundaries, weakening the grain boundaries and thereby reducing impact toughness.

Carbon content also has a significant influence on impact toughness. When low-carbon alloy steel and medium-carbon alloy steel are heat-treated to the same strength level, the former generally exhibits much higher fracture toughness than the latter.

For example, when 20MnTiB and 40CrNiMo steels are heat-treated to achieve the properties of Grade 10.9 bolts, their strength levels are similar, but their fracture toughness values are approximately:

 

20MnTiB: 113 MN/m³⁄²

40CrNiMo: 78 MN/m³⁄²

However, in terms of impact toughness (KV and KU impact absorbed energy values), the latter is 20–45 J lower than the former.

Nickel (Ni) is one of the most effective alloying elements for improving toughness. It not only enhances fracture toughness but also effectively lowers the ductile-to-brittle transition temperature. Therefore, Ni is an important alloying element in steels intended for low-temperature applications.

Molybdenum (Mo) has a similar effect to Ni. Chromium (Cr) has relatively little influence on impact toughness. The addition of a small amount of boron (B) can improve fracture toughness during low-temperature tempering.

Bolts are often exposed to corrosive environments during service. Such conditions are among the external factors that contribute to delayed fracture. To improve fracture toughness in these steels, alloy design should focus on:

Improving hardenability

Enhancing microstructural stability

Refining grain size

Refining carbide particles and ensuring their uniform distribution

Preventing temper embrittlement

Lowering the ductile-to-brittle transition temperature

For low-alloy steels represented by 35CrMo, the addition of an appropriate amount of vanadium (V) can refine grain size and improve fracture toughness. Therefore, 35CrMoV steel is considered a suitable material for manufacturing Grade 12.9 high-strength bolts.