Fastener Material and Impact Test

1.Overview
The technological advancement of vehicles and mechanical equipment is progressing toward greater safety, comfort, economy, and environmental friendliness. To date, the research, development, and manufacturing technologies for fastener materials still require further improvement and advancement. Fasteners after heat treatment are required to have tensile strengths exceeding 700 to 1300 MPa, yield strengths exceeding 550 to 1150 MPa, percentage elongation after fracture of 8% to 15%, and a product of strength and plasticity generally greater than 10⁴ MPa%, indicating excellent comprehensive mechanical properties. Serving functions such as connection, fastening, and sealing, the technological development of fasteners relies on, on one hand, the development and application of new materials with higher strength and toughness to ensure greater durability and reliability of mechanical connections; on the other hand, through the enhancement of material strength and toughness, it aims to increase design stress and achieve lightweighting of product components.

Relationship Between Material and Toughness
• The inspection of high-strength bolts, screws, and studs is based on the standard GB/T 3098.1-2010 "Mechanical properties of fasteners - Bolts, screws and studs." Products leaving the factory should generally comply with the acceptance test items specified in the new standard. However, current production practices and usage indicate that this standard is not yet perfect.

Relationship Between Material and Toughness

• The inspection of high-strength bolts, screws, and studs is based on the standard GB/T 3098.1-2010 "Mechanical properties of fasteners - Bolts, screws and studs." Products leaving the factory should generally comply with the acceptance test items specified in the new standard. However, current production practices and usage indicate that this standard is not yet perfect.

• Currently, brittle fracture in mechanical components is one of the significant failure modes for high-strength bolts, screws, and studs. This type of fracture occurs suddenly during the loading of the fastener without significant macroscopic plastic deformation. Because there are no obvious signs beforehand, brittle fracture poses a considerable risk.

• Impact toughness is used to characterize the toughness of a material. It depends not only on its chemical composition and microstructure but also on factors such as the metallurgical quality of the material, and is closely related to the ambient temperature and the condition of the notch.

 

• Traditional methods for preventing brittle fracture in bolts include: ① Requiring the selected material to have certain plasticity indicators (percentage elongation after fracture A/% and percentage reduction of area Z/%) and a certain impact toughness value (impact absorption energy KV/J, KU/J); ② Using the transition temperature method to impose specific requirements on the material's ductile-to-brittle transition temperature. The transition temperature for brittle fracture is commonly expressed using FATT.

 

2.1 Relationship Between Material and Impact Toughness

• The amount of impurity elements in the chemical composition is one of the factors affecting impact toughness. Strict control of P and S in steel is required, with their sum generally ≤ 0.025%. P has an embrittlement effect on the material, forming precipitates at grain boundaries, which weakens the grain boundaries and leads to a decrease in impact toughness. Carbon content also significantly impacts impact toughness. When low-carbon alloy steel and medium-carbon alloy steel are heat-treated to the same strength level, the fracture toughness of the former is significantly higher than that of the latter. For example, when 20MnTiB and 40CrNiMo steels are processed into 10.9-grade bolts with similar strengths, their fracture toughness values are 113 MN/m³/² and 78 MN/m³/², respectively, while the impact toughness value (impact absorption energy KV, KU) of the former is 20–45 J lower than that of the latter.

 

• Ni is the most effective toughening element. It not only improves the fracture toughness of steel but also effectively lowers its ductile-to-brittle transition temperature. Therefore, Ni is an important alloying element in low-temperature steels. Mo has a similar effect to Ni. Cr has a relatively small impact on impact toughness. Adding a small amount of B can improve fracture toughness during low-temperature tempering.

 

• Bolt operating environments often contain certain corrosive media, which constitute one of the external conditions leading to delayed fracture. To improve the fracture toughness of such steels, the alloying principle still focuses on enhancing the steel's hardenability, improving microstructural stability, refining grains, refining carbides, ensuring their uniform distribution, preventing temper embrittlement, and lowering the ductile-to-brittle transition temperature.

 

• In low-alloy steels represented by 35CrMo steel, adding an appropriate amount of V can refine grains and improve fracture toughness. 35CrMoV steel is a material suitable for 12.9-grade high-strength bolts.

 

2.2 Influence of Microstructure on Impact Toughness

• The morphology of martensite in steel significantly affects impact toughness. There are two types of martensite morphology: lath martensite containing a high density of dislocations and plate martensite containing twins. The presence of twins reduces the slip system to one-quarter of the original and can induce microcracks. Therefore, the fracture toughness of plate martensite is lower than that of lath martensite. By reducing the amount of twinned martensite and increasing the amount of dislocated martensite in the steel through alloying (e.g., lowering carbon content) or heat treatment methods, the strength and toughness of the steel can be enhanced.

 

• Regardless of whether it is during normalizing, annealing, quenching, or tempering processes in heat treatment, steel with a finer prior austenite grain size will have superior final properties compared to steel with a coarser prior austenite grain size.

 

• Based on production practice, when the prior austenite grain size of steel is coarser than grade 1–4, the impact toughness value of bolts tends to decrease progressively, seriously affecting the mechanical performance indicators of the bolts. As the grain size number increases (i.e., grains become finer), the low-temperature impact absorption energy KV and KU increase. This is because finer grains mean more grains and more grain boundaries, which increase resistance to crack propagation and thus result in higher impact absorption energy values.

 

• A quantitative expression exists for the effect of grain size on the yield strength of a material. Finer grains lead to a higher yield ratio and require more energy consumption during crack propagation. Grain refinement strengthening is an effective means of simultaneously improving both the strength and toughness of a material. The actual grain size of raw materials for high-strength bolts should be controlled within grade 6–8, with grade 7–8 being optimal.

 

2.3 Influence of Heat Treatment on Impact Toughness

• Without sacrificing mechanical properties, appropriately lowering the quenching temperature can, to a certain extent, refine grains, increase grain boundary area, reduce P segregation, effectively curb the embrittlement effect of P on the material, thereby improving the material's impact toughness and lowering the steel's low-temperature brittle transition temperature (FATT). Production practice shows that increasing the cooling rate during quenching can also lower the brittle transition temperature, thus enhancing impact toughness. The quenching temperature has little effect on the yield limit and tensile strength. However, intercritical quenching (i.e., quenching after heating into the Ac1–Ac3 dual-phase region) can also improve low-temperature toughness and suppress temper embrittlement.

 

• Regarding the influence of tempering temperature, the general trend is that as the tempering temperature increases, the impact toughness value increases while the strength decreases. Firstly, tempering should avoid the first type of temper embrittlement range (250–400°C). Secondly, for low-alloy steels tempered in the 450–600°C range, rapid cooling should be employed to reduce the effect of the second type of temper embrittlement. Some tempered materials exhibit reduced fracture toughness values within specific tempering temperature ranges.

 

2.4 Relationship Between Fracture Toughness and Impact Toughness

• Fracture toughness KIC and notched impact toughness (impact absorption energy KV, KU) are both indicators of a material's toughness. Therefore, many effective measures for improving fracture toughness can also enhance impact toughness. However, there are distinct differences between the two. One involves a crack, while the other involves a notch; their degrees of stress concentration differ. They are two mechanical performance indicators that are both related and distinct.

 

Impact Test for Bolt Specimens

3.1 General Rules for Impact Testing

• According to GB/T 3098.1-2010 "Mechanical properties of fasteners - Bolts, screws and studs," the impact test is used to verify the toughness of fastener materials under specified low-temperature conditions. The test shall only be conducted if required by the product standard or agreed upon between the supplier and purchaser.

 

Test instruments and equipment shall comply with GB/T 229-2007 "Metallic materials - Charpy pendulum impact test method." The impact energy for V-notch and U-notch specimens is denoted as KV and KU, respectively, with a subscript number 2 indicating the pendulum striker radius, e.g., KV2, and the unit is J (Joule). The impact absorption energy value is read directly from the dial of the testing machine. Dividing KV or KU by the cross-sectional area S at the specimen notch yields the material's impact toughness value, represented by the symbol aK = K/S, with units in KJ/m² or J/cm². Compared to K, aK does not have a clear physical meaning; it is merely a mathematical expression. Therefore, nowadays, the impact absorption energy K is mostly used as the criterion for material toughness.

 

3.2 Applicable Scope of Impact Test

• Machined test specimens prepared from bolts, screws, and studs with a diameter d ≥ 16 mm; bolts and screws with a total length (including the head) ≥ 55 mm; studs with a total length, ℓt ≥ 55 mm. Applicable to high-strength bolts of grade 8.8 and above.