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Vacuum Heat Treatment of Carbon Boron Steel Fasteners

The greater the hardenability of the steel, the greater the yield ratio after quenching and tempering. The increase in the yield ratio means that the potential of the steel can be better utilized. It is necessary to meet the tensile strength and yield ratio specified for motorcycle products. High-strength fasteners are more practical.

The high-strength fasteners of the engine and chassis parts in motorcycles include 8.8, 9.8 and 10.9 bolts; 8 and 10 nuts, etc. Quenched and tempered alloy steel for high-strength fasteners, compared with carbon steel with the same carbon content, often has higher plasticity and toughness at the same strength, or higher strength at the same plasticity, or strength, It has high plasticity and toughness, and is heat-resistant, and can serve in high-temperature environments below 500°C. Because in the quenched and tempered state, in addition to the solid solution strengthening ferrite, the alloying elements mainly improve the vacuum tempering stability of the steel. After vacuum tempering at the same temperature, the carbides of the alloy steel are more dispersed, so that the steel The strength is higher. If vacuum tempering reaches the same hardness and strength as carbon steel, the vacuum tempering temperature of alloy steel should be higher, so that stress relief is more thorough.

In order to realize the requirements of lightweight motorcycles and increase the market competitiveness of fasteners, while reducing production costs and omitting or simplifying procedures, it is also required to reduce energy consumption. In recent years, low-, medium-carbon, high-strength cold heading boron steels that save alloying elements have been developed on the market. Boron to compensate for the loss of strength and hardenability caused by decarburization. The cold heading boron steel hot rolled wire can be directly drawn and cold heading processed without pre-spheroidizing annealing treatment, which saves the manufacturing cost of fasteners.

At present, strictly speaking, the boron steel used in the fastener industry should be called carbon boron steel, so as not to be confused with the general name “boron steel” of various boron-containing alloy structural steels.

As one of the materials for high-strength fasteners, carbon-boron steel has its characteristics and individuality. It is easy to operate in vacuum heat treatment, and has a small tendency to deformation and cracking in vacuum quenching. This paper mainly introduces the vacuum heat treatment technology of carbon-boron steel fasteners, so as to promote the development of high-quality and low-cost high-strength cold heading steels that meet the resource conditions of our country.

1 Alloy elements of cold heading carbon boron steel

The main function of alloying elements in cold heading steel is to improve hardenability and comprehensive mechanical properties. The reason why alloying elements can improve the comprehensive mechanical properties of steel is that they can solid-solution strengthen ferrite and increase the recrystallization temperature of ferrite. , especially to slow down or prevent the precipitation and aggregation of carbides during vacuum tempering, so that the cold heading steel has a higher resistance to tempering. The reason why alloying elements can improve the hardenability of steel is mainly because they can hinder the transformation of austenite into proeutectoid ferrite, pearlite or upper bainite during vacuum quenching.

2 The main function of boron in cold heading steel

The amount of boron added to cold heading steel as an alloy element is only 0.000 5%~0.0035%. The main function of a trace amount of boron is to increase the hardenability of the steel. The effect of 0.001%~0.003% boron is equivalent to 0.60%Mn and 0.70%Cr respectively. , 0.15% Mo and 1.5% Ni, which can replace the rarer and more expensive alloying elements.

For example, adding a small amount of boron (ML35B) cold heading steel to ML35 steel can make steel with a diameter of φ20 mm harden in oil; adding a small amount of boron (ML35MnB) cold heading steel to ML35Mn steel can replace 40Cr steel It is used in the production of high-strength fasteners of grade 9.8~10.9. The extent to which boron increases hardenability is related to the amount of carbon and boron contained in the steel. The higher the carbon content, the smaller the effect of boron on increasing hardenability; when the carbon content exceeds 0.90%, the effect of boron on increasing hardenability will be completely lost. Therefore, boron has a more obvious effect on increasing the hardenability of low and medium carbon steels, but boron cannot be added to high carbon steels.

The reason why boron can increase the hardenability of steel is that most of the boron dissolved in austenite is adsorbed on the grade boundary, which reduces the energy of the grade boundary, inhibits the formation of ferrite nuclei, and reduces the proeutectoid The rate of formation of ferrite and upper bainite nuclei prolongs the incubation period for austenite decomposition.

3 Main points of vacuum heat treatment of carbon boron steel

3.1 Vacuum quenching heating temperature

Adding a small amount of boron to cold heading steel has no effect on the critical point, Ms point, quenching cracking tendency and retained austenite of the steel. In order to make boron dissolve in austenite and adsorb on the grade boundary, so as to increase the hardenability of steel, the heating temperature for vacuum quenching is generally below 860°C, which exceeds Ac and cannot be too high. If the heating temperature is too high, the boron atoms will diffuse from the grain boundary to the grain, on the one hand, it will weaken or even lose its effect of enhancing hardenability (afterwards, if it is heated at a lower temperature for vacuum quenching, its effect can still be recovered). On the other hand, it is also easy to cause overheating.

3.2 Vacuum tempering

Boron cannot improve the tempering stability of cold heading steel after vacuum quenching, so compared with alloy steels that do not contain boron but have similar hardenability, in order to obtain the same strength, the vacuum tempering temperature of carbon boron steel should be lower , the tempering time should be shorter. Carbon-boron steel also has a tendency to temper brittleness, but the second type of temper brittleness is not as significant as steel containing Mn and Cr.

3.3 Concerns of using carbon-boron steel vacuum heat treatment

Since boron can significantly increase the hardenability of steel, fasteners with larger cross-sections are easy to harden, which can make carbon-boron steel obtain better comprehensive mechanical properties after vacuum tempering. It must be noted that if carbon-boron steel cannot be completely hardened, its mechanical properties such as strength, plasticity, toughness, etc., are lower than other grades of steel with the same hardenability but no boron. This is mainly because boron has the unfavorable effect of promoting the production of acicular ferrite, but there is no other alloying element that can refine and strengthen the structure of steel due to solid solution strengthening ferrite and reducing the pearlite transformation temperature. . Therefore, carbon boron steel is generally not suitable for application without hardening.

4 Vacuum heat treatment of carbon boron steel fasteners

As we all know, boron exists in steel in the form of solid solution to improve the hardenability of steel. The vacuum quenching temperature of carbon boron steel is generally selected at Ac: +40~60°C. If the vacuum quenching temperature is too high, such as greater than 900 ° C, boron atoms diffuse into the grain, boron segregation weakens in the grain boundary area, and the hardenability of carbon-boron steel will decrease. However, if the vacuum quenching is reheated, the hardenability will be improved. function can still be restored.

Boron can also promote the growth of austenite grains and tend to increase the second type of temper brittleness (the effect is weaker than that of Mn and Cr). With the heat treatment process, the mechanical properties of carbon-boron steel motorcycle fasteners can be fully guaranteed.

Cold heading carbon boron steel is an alloy steel formed by adding trace amounts of boron on the basis of carbon steel. After quenching, the hardness can reach 48~55HRC. The vacuum tempering stability of carbon boron steel is poor, so the vacuum tempering temperature should be lowered appropriately, for example, the tempering temperature should be lowered by about 50°C when quenching and tempering 10B33.

The high-strength bolts of motorcycles need to bear the load evenly across the entire section. In order to ensure good mechanical properties, carbon boron steel is best used under full hardening conditions. For bolts with a diameter of <12 mm, it is recommended to use vacuum oil quenching. For bolts with a diameter of ≥12 mm Bolts are recommended to be quenched with vacuum oil. Carbon-boron steel has a large deformation during vacuum quenching, and the quenching temperature for nut parts is reduced by about 20°C. The hydrogen embrittlement tendency is also obvious during pickling, and the electroplated fasteners above grade 10.9 should be treated with hydrogen removal in time.

5 Delayed fracture resistance of carbon boron steel

The stronger the motorcycle fastener, the higher its delayed fracture susceptibility. The structure of low-carbon boron steel after vacuum quenching and vacuum tempering is lath martensite, and the laths are staggered. This structure has good strength and toughness, and has a large resistance to crack growth. At approximately the same strength level, low carbon martensitic steel has lower delayed fracture sensitivity than 40Cr medium carbon alloy steel, which is closely related to the special microstructure and substructure characteristics of low carbon martensite.

Boron in carbon-boron steel has a very fast moving speed. During the cooling process, boron can quickly fill in character defects such as dislocations and atomic voids, thereby reducing the deformation energy of the grain boundary and making the grain boundary more stable. Boron also occupies the gap position of y-iron and α-iron, and the preferential segregation of boron at the grain boundary strengthens the grain boundary to some extent; the addition of trace elements such as boron is beneficial to hinder the initiation and expansion of delayed fracture cracks along the grain boundary.

Tests show that with the increase of tensile strength, the delayed fracture strength ratio of high-strength bolts tends to decrease. In the range of 1100-1200 MPa, the delayed fracture resistance decreases in the order of low-carbon boron steel, medium-carbon boron steel, medium-carbon chromium-molybdenum steel, and medium-carbon chromium steel.

In short, in the manufacture of motorcycle fasteners, replacing precious and rare elements with boron can achieve better economic benefits. As far as cold heading carbon-boron steel is concerned, it has no obvious effect on cold forging, but it is more prominent during vacuum heat treatment. The effect of boron on hardenability has a special rule, so attention must be paid to carbon-boron steel when vacuum heat treatment its characteristic. The development of modern steel technology has driven the innovation of motorcycle fasteners and provided a large number of solutions to these problems.