I. The Stress Conditions and Performance Requirements of High-Strength Bolts in Actual Operation
Bolts play roles such as connection, fastening, positioning and sealing in various mechanisms. When bolts are installed, they need to be pre-tightened, thus they all have to bear static tensile loads. The greater the pre-tightening force, the greater the connection strength and the tightness and sealing performance.
Besides being subjected to the axial pre-tightening tensile load, they usually also bear additional axial tensile (alternating) loads, transverse shear (alternating) loads or the bending loads formed by the combination of the two during operation, and sometimes they are also subjected to impact loads.
Generally, the additional transverse alternating loads will cause the bolts to loosen, the axial alternating loads will cause the bolts to fatigue and break, and the axial tensile loads under the action of environmental media will cause the bolts to delay fracture.
Therefore, when applying high-strength bolts, higher technical requirements are put forward for material composition, metallurgical quality, bolt structure, manufacturing process, installation and use. Generally speaking, high-strength bolts and the steel used for them should meet the following requirements:
High tensile strength to resist elongation, breakage, slippage and wear.
(2) Higher plasticity and toughness to reduce sensitivity to skew, notch stress concentration and surface quality.
(3) For bolts working in environments such as by the sea, by rivers, or in oil fields where the atmosphere is humid or corrosive, the bolt material is required to have a sufficiently low sensitivity to delayed fracture to ensure the safety and reliability of the bolts during operation.
(4) For bolts subjected to alternating loads and impact loads, it is required that they have high fatigue resistance and multiple impact tensile resistance to resist fatigue and multiple impact fractures.
(5) For bolts used in extremely cold regions or at low temperatures, it is also required that they have a low ductile-brittle transition temperature.
(6) For bolts with medium and small diameters, cold heading to form the bolt head and thread rolling (or threading) are often adopted as the production processes. This requires the material to have excellent cold working properties such as cold heading.
II. Delayed Fracture and Characteristics of High-Strength Bolt Steel
The tempered martensite structure of steel has a good combination of strength and toughness, and its strength can be controlled by adjusting the types, quantities of added elements such as carbon and alloy elements, as well as the heat treatment process.
Therefore, it has been widely used in alloy steel. However, tempered martensite steel is prone to delayed fracture in natural environments, and the sensitivity to delayed fracture increases with the increase in strength. At the same time, high-strength bolts are notch parts with high notch sensitivity, and are prone to delayed fracture at concentrated notch areas such as the transition between the shank and the head or the root of the thread.
Therefore, delayed fracture of high-strength bolt steel is a very typical case, and the accidents caused by it occur frequently, with considerable economic losses.
Delayed fracture, also known as lagged fracture, is a phenomenon where materials subjected to static stress suddenly undergo brittle failure after a certain period of time. The delayed fracture phenomenon is a form of environmental embrittlement resulting from the interaction of material, environment, and stress, and it is a type of hydrogen-induced material deterioration (hydrogen damage or hydrogen embrittlement).
For hydrogen-induced delayed fracture, the so-called delay refers to the fact that under constant load (or constant displacement) conditions, atomic hydrogen needs a certain period of time to diffuse and accumulate to a critical value through stress-induced diffusion.
Therefore, after loading, it takes a certain period of time for hydrogen-induced cracks to nucleate and propagate. If atomic hydrogen is removed, delayed fracture will not occur, so hydrogen-induced delayed fracture belongs to reversible hydrogen embrittlement.
Delayed fracture is a major factor hindering the high-strength development of steel used in mechanical manufacturing. It can be roughly classified into the following two categories:
Delayed fracture mainly caused by hydrogen (external hydrogen) from the external environment, such as bolts used in bridges, may occur after long-term exposure to humid air, rainwater and other environments.
(2) Delayed fracture caused by hydrogen (internal hydrogen) that invades steel during manufacturing processes such as pickling and electroplating, for instance, electroplated bolts may experience delayed fracture after loading within a relatively short period of several hours or days.
Delayed fracture in actual steel use under natural conditions mainly occurs in tempered martensitic steel, which generally has the following characteristics:
When the tensile strength is greater than 1200 MPa and the hardness is ≥ 38, the sensitivity to delayed fracture increases significantly.
Delayed fracture usually occurs near room temperature, but from room temperature to around 100℃, the sensitivity to delayed fracture increases with the rise in temperature.
(3) Macroscopically, delayed fracture is not accompanied by significant plastic deformation.
(4) Occurs under static load;
(5) It occurs at a stress much lower than the yield strength.
(6) After tempering at a temperature near 350℃ where low-temperature tempering brittleness occurs, the sensitivity to delayed fracture is the greatest.
(7) Governed by the cracks at the original austenite grain boundaries.
III. Main Influencing Factors of Delayed Fracture of High-Strength Bolt Steel
The main influencing factors of delayed fracture of high-strength bolt steel are:
The influence of strength
A large number of research works have shown that with the increase of tensile strength of high-strength bolts steel, especially when the tensile strength exceeds about 1200 MPa, the delayed fracture strength drops sharply, the crack propagation rate increases or the delayed fracture time decreases.
2. The Influence of Carbon and Common Alloying Elements
Carbon can significantly increase the susceptibility of steel to delayed fracture, especially for high-strength steel. The increase in carbon content brings about changes in the microstructure and composition of steel. It not only raises the solubility of carbon in martensite but also strongly lowers the Ms point of steel, leading to the formation of lath martensite.
At the same time, it increases the amount of carbides at grain boundaries. This is one of the main reasons why carbon increases the susceptibility of steel to delayed fracture. However, an appropriate amount of carbon content is necessary to achieve the required high strength.
It is generally believed that manganese can increase the susceptibility of high-strength steel to delayed fracture. Manganese combines with the impurity element sulfur in steel to form MnS inclusions, and hydrogen-induced cracks often initiate at MnS inclusions, leading to delayed fracture.
The influence of silicon on the resistance to delayed fracture of high-strength steel is rather complex. On the one hand, silicon can inhibit the precipitation of ε carbides during tempering. In the process of stress corrosion, H+ can easily gain electrons on ε carbides to become atomic hydrogen.
Therefore, adding silicon can reduce the amount of hydrogen entering the sample. Moreover, silicon can significantly decrease the apparent diffusion coefficient of hydrogen, thereby reducing the propagation rate of stress corrosion cracks.
Research shows that in 1400 MPa grade steel, the combined addition of silicon and calcium can change the intergranular bonding force and hydrogen diffusion behavior, thereby reducing the sensitivity of steel to delayed fracture. On the other hand, like manganese, silicon can promote the grain boundary segregation of impurity elements, which in turn increases the sensitivity of steel to delayed fracture.
There is still no consistent conclusion on the effect of chromium. Generally, it is believed that chromium can increase the susceptibility to hydrogen-induced delayed fracture, which is more obvious when the chromium content is low. However, some studies have shown that, with the contents of P, S and Mn remaining unchanged, an increase in chromium content can inhibit intergranular fracture, thereby enhancing the steel’s resistance to delayed fracture.
Nickel is an austenite-stabilizing element that can facilitate the formation of a greater amount of retained austenite in steel. Austenite has a large solid solubility for hydrogen, which can trap hydrogen and render it harmless. However, unstable retained austenite with a high local carbon content is prone to transform into martensite under stress, which in turn increases the susceptibility to delayed fracture.
Nickel has no effect on the resistance of high-strength steel to hydrogen sulfide stress corrosion and hydrogen embrittlement. Research indicates that nickel can concentrate on the surface of the sample to inhibit hydrogen penetration, thereby enhancing the steel’s resistance to delayed fracture.
It must be pointed out that the influence of alloying elements on the resistance to delayed fracture of high-strength steels is rather complex. The impact of alloying elements varies among different types of steel, and thus specific analyses should be conducted for different situations.
Within the composition range of low-alloy steels, the tendency of delayed fracture is mainly determined by the microstructure and the environment, and the direct influence of alloying elements is limited.
3. The Influence of Microstructure
Since the strength level of steel is closely related to its microstructure, at a certain strength level, the susceptibility of steel to delayed fracture is always associated with a specific microstructure. Different microstructures have different susceptibilities to delayed fracture.
Generally speaking, the susceptibility of austenite and pearlite to delayed fracture is smaller than that of martensite. In the pearlite structure, the shape of cementite has an important influence on the susceptibility to delayed fracture. Martensite with a higher carbon content is more prone to embrittlement than that with a lower carbon content.
Delayed fracture mainly occurs in tempered martensitic steels under natural conditions. When the alloy composition is fixed, heat treatment is the decisive factor in controlling the microstructure. When quenched and tempered steel is tempered in the temperature range of 300 to 400°C, its resistance to delayed fracture deteriorates sharply, which is the result of the overlap of low-temperature tempering brittleness and hydrogen embrittlement.
Generally speaking, the precipitation of cementite on the grain boundaries, especially in a film-like form, significantly enhances the sensitivity to delayed fracture. Therefore, controlling the nature of the carbides at the original austenite grain boundaries is an important means to improve the resistance to delayed fracture of high-strength steels.
4. The Influence of Process Factors
The phosphating of the surface layer of high-strength bolts is an important factor causing their delayed fracture. This is because the phosphating treatment before the cold heading forming of the bolts forms a thin layer of insoluble metal phosphate (phosphating layer) on the surface of the bolts.
This layer decomposes during the quenching and tempering process, allowing phosphorus to diffuse into the steel and causing phosphorus enrichment in the surface layer of the bolts, which reduces the strength of the grain boundaries and promotes the occurrence of delayed fracture.
It is necessary to remove the surface lubricant after the bolt forming, which increases the cost. Therefore, along with the high-strength development of bolts, the development of non-phosphorus lubricating films is also underway.
Hydrogen that penetrates into steel during pickling and electroplating treatment will accumulate at stress concentration points under the action of stress, which can also cause delayed fracture of high-strength bolts.
Therefore, for high-strength bolts, especially those of grade 10.9 and above, dehydrogenation baking treatment should be carried out within 4 hours after electroplating, or other surface treatment methods with less harm, such as Dacromet, should be adopted.
Improving the shape of the thread flanks can reduce the degree of stress concentration. Besides enhancing the fatigue performance of high-strength bolts, it can also decrease the enrichment and diffusion of hydrogen at the thread flanks, and significantly increase the resistance to delayed fracture of high-strength bolts.
IV. 1Cr17Ni2 and Measures to Improve Its Resistance to Delayed Fracture
1Cr17Ni2 is a martensitic stainless steel, with the new grade being 14Cr17Ni2 (GB/T 1220-2007). After heat treatment, it has high mechanical properties and better corrosion resistance than 12Cr13 (1Cr13) and 10Cr17 (1Cr17). It is widely used in fasteners that require both high mechanical properties and resistance to nitric acid and organic acid corrosion.
The heat treatment system for the steel bar is: annealing, high-temperature tempering at 680-700°C, and air cooling. The heat treatment system for the sample is: 1) quenching at 950-1050°C in oil; 2) tempering at 275-350°C and air cooling. The microstructure is of the martensitic type, belonging to tempered martensitic steel, and it is prone to delayed fracture in natural environments.
High-strength steel often has hydrogen enter the material during smelting, processing and use. The hydrogen that enters the steel is extremely harmful. Even a trace amount of hydrogen can cause delayed fracture through diffusion and enrichment.
For the metal-hydrogen system, the process of delayed fracture requires hydrogen invasion, hydrogen diffusion and enrichment, hydrogen-induced crack initiation and propagation until fracture. The diffusion and enrichment of hydrogen in metals, as a transitional process of the interaction between hydrogen and metals, is the premise and bridge of the delayed fracture process.
The delayed fracture of high-strength steel is caused by the diffusible hydrogen that invades the steel, and its fracture characteristics are mainly intergranular brittle fracture. Therefore, how to improve the effectiveness of hydrogen traps to reduce the concentration of diffusible hydrogen and enhance the strength of grain boundaries, transforming intergranular brittle fracture into transgranular ductile fracture, is the basic starting point for improving the resistance to delayed fracture of high-strength steel.
Through the analysis of delayed fracture of high-strength bolts steel, the main approaches to improving the delayed fracture resistance of 1Cr17Ni2 (14Cr17Ni2) in terms of material metallurgy are as follows:
Reduce grain boundary segregation. Strive to minimize the content of impurity elements such as phosphorus (which weakens grain boundary bonding strength) and sulfur (which promotes hydrogen absorption in corrosive environments), and also reduce the manganese content that promotes the co-segregation of phosphorus and sulfur to prevent grain boundary embrittlement.
(2) Refine grains. Add elements such as Al, Ti, Nb, and V to produce dispersed carbonitrides that refine austenite grains, thereby enhancing strength while also improving toughness.
(3) Adjust the alloying elements (such as adding nickel and reducing the manganese content) to achieve higher notch toughness.
(4) Make the invading hydrogen harmless. By adding an appropriate amount of microalloying elements such as V, Ti, and Nb, fine carbonitrides are formed, which not only refines the austenite grains before quenching but also acts as hydrogen traps, inhibiting hydrogen diffusion and promoting its uniform distribution.
(5) By means of baking the raw and auxiliary materials and alloys used in steelmaking as well as VD degassing, the hydrogen content in steel can be further reduced.
(6) Increase the tempering temperature. Add elements with strong anti-tempering softening ability such as molybdenum and vanadium, so that the tempering temperature can be raised while maintaining the strength unchanged, allowing the carbides to spheroidize and avoiding the tempering temperature range that is prone to cause intergranular embrittlement. This can also make the carbides fine and uniform.
(7) Minimize the amount of hydrogen that penetrates the steel surface as much as possible, that is, add alloying elements that inhibit the formation of corrosion pits, such as molybdenum.
Through the above-mentioned approaches, the resistance to delayed fracture of high-strength bolt steel – 1Cr17Ni2 (14Cr17Ni2) can be effectively enhanced, thereby improving the steel’s resistance to delayed fracture.
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