Bolts
Hydrogen Embrittlement
Hydrogen Embrittlement
    The effect of hydrogen embrittlement on metals has been known to man since around 1930 and has been an ongoing threat to product safety ever since. Hydrogen embrittlement on steel bolts is a common reason for metal fracture under static tension load.Due to the fact that hydrogen embrittlement can’t be tested by formal inspection procedures and because of its delayed and hidden properties, it is much more serious than other fractures and the consequences can be disastrous.
This article will analyze he factors that lead to delayed fractures by hydrogen embrittlement through some typical cases and put forward practical methods to prevent them.

1. Typical Case Analysis

1.1 Case one: 40Cr Steel Bolt fracture

(picture 1.)

The fixing bolt on this car engine base cracked two days after being fitted. Specification: M12x30 Quality: 2.9 Material: 40Cr Surface: Black phosphate

(picture 2.)

This crack occurred in the thread: the edge of the crack is flat, clean,with a relatively rough composition and with no obvious signs of corrosion, plastic deformation or mechanical scratch. The whole fracture is a shiny gray with shiny silver traces.

(chart 1.)

    These samples are taken from the edge of the fracture by a wire-electrode cutting machine and cleaned by ultrasonic waves. An H-3000 Hydrogen determinator checks for hydrogen content.

At microscopic level, the fracture separation has crystallized and the crystal grain is well-defined with claw marks, which are the typical features of hydrogen embrittlement. The hydrogen content test can detect a certain amount of hydrogen around the fracture surface. For general strength bolts, cracks occur when the hydrogen content is over 5x10-6, however, for high strength bolts, it can be as little as 1x10-6.     Due to the effect of strain, the hydrogen atoms located in the space between the lattices will, under diffusion, gather at the defective strain point. The hydrogen atoms can’t move freely because of the interaction of dislocation and the fixed dislocation line, which causes the matrix to be embrittled. Consequently, the hydrogen content of this fractured bolt is sufficient to cause hydrogen cracks.increases hydrogen embrittlement.     Secondly, the bolt fixed to the engine is made of 40Cr’s of high strength metal. Metallographic examination and hardness gradient tests prove that the bolt has a 0.1mm carbon-enriched layer which makes the carbon content on the surface higher than on the inside. Heat treatment increases the surface hardness and, in turn, decreases the surface plasticity, which leads to the phenomenon of hydrogen embrittlement and a higher crack extension rate in high strength bolts. Moreover, the carbon-enriched surface layer further

1.2 Case two: Axle Fixing Bolt fracture, Quality10.9

(picture 3.)

Axle fastening bolt, 200℃ x 4h hydrogen removing treatment after galvanizing. Material: ML35CrMo Specification: M10x1.25x128 Quality:10.9 Installation torque: 80±10N•m Surface: Yellow zinc plating

(picture 4.)

The photograph of this fracture was taken through an electronic microscope: the radioactive stripes have faded in marks A and B and in marks C and D two crescent-shaped ends, with a width of about 1mm, can be seen.

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(Picture 5 & Picture 6.)

These are the micro structures of marks A and B, which belong to intergranular and understanding fractures with few claw marks in the crystal face.

(Picture 7 & Picture 8.)

These are the micro structures of marks C and D, which belong to intergranular and understanding fractures with secondary fractures of grain boundary.

(Picture 9.)

This is the micro feature of mark E, in the form of isometric dimples.

(Picture 10.)

This is the micro feature of mark F, in the form of shear dimples.

This is the micro feature of mark F, in the form of shear dimples.

According to the data in the Chart2, there is a certain amount of hydrogen content that infiltrated the bolt during galvanization as a result of incomplete hydrogen exclusion. The surface coating prevents hydrogen diffusion, the thicker the coating,the less diffusion there is.

Even when heated to 200℃, and stay warm for 4 hours, there is still hydrogen remaining within a mass fraction of 4.35x10-6. As we all know, the reliability of the bolt is affected not only by its quality but also by related techniques. Suppose we take two undamaged bolts of the same batch:one sent by an installer and one sent by a manufacturer and then measure the screw fastening axial force and torque analyzer to analogy installment and friction coefficient. The results show that when the torque reaches 80 N•m, the axial pretightening force of the bolt sent by the installer reached 53.4KN but 31.1KN when sent by the manufacture. The relative friction coefficient is revealed in Chart 3.

The big difference in friction coefficient of the same specification, model and surface bolt can be attributed to preservation time, transport or oil pollution. Though no plasticizing deformation can be found, analysis from the surface fracture shows dimpled areas occupying 80% of the whole area which proves axial stress was ever present in the bolt. The conclusion from the fracture coefficient test shows that the total fracture coefficient (0.11, with thread 0.07) of the bolt sent by the installer is less than the 0.02 sent by the manufacturer. When employing torque control installation of the bolt, minor friction coefficients will result in a greater axial pretightening force. The test result of pretightening force---torque measurement reveals that when installing torque of 80 N•m, the pretightening force of the bolt sent by the installer is 53.4 KN. This surpasses the standard 50.8 KN for a bolt of this quality and specification and even comes close to surpassing the yield strength of 57.5KN. In addition, the bolt can bend and over-screw when there is a torque installation error. Even after going through a hydrogen exclusion process, there is still a risk of hydrogen content in galvanized bolts. The harder it is, the greater the axle pretightening force and the greater the chance of hydrogen embrittlement. According to GB/I 5267.1-2002 The Galvanized Coating Of Fasteners, there is a risk of hydrogen embrittlement when the hardness of galvanized fasteners is over 320 HV and galvanizing treatment should be avoided when the hardness is over 365 HV and the surface hardness of the fractured bolt has reached 365HV. Although the results of hydrogen content tests show that the hydrogen mass fraction of the fractured bolt is below 5x10-6, it can still lead to hydrogen induced micro cracks throughout the thread due to unequal distribution, stress bearing and too much pretightening force. The top of the micro crack, which hydrogen atoms move to, is the gathering area of stress; the bolt fractures after an accumulation of extending cracks and so decreases the bearing area.

1.3 Case Three : Hex Socket Head Cap Screw fracture

transit circular arc between bolt and head

GB/T70.1-2000 M20x95 fractured in the process of installing with torque 490N.m. The picture of the fractured bolt can be seen in picture 11 with quality 12.9, material 40Cr,black phosphate and no contact with corrosive medium.At macro level the crack is located in the level the crack is located in the Rangle

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fracture. While extension and end areas are intergranular and dimple fracture.

It is smooth and without visible traces and plasticizing deformation.(Picture 12)In micro level, the fraction face is clean, no obvious corrosion products or oxidation but visible cracks source, extension and end areas. Observe in high magnification , cracks source areas is in the outer layer of the sample. It belongs to intergranular cracking with some claw marks on its crystal grain, which showed the feature of hydrogen induced delayed .

(Picture 13) From the energy spectrum analysis on the fractured bolt surface, the fillet in the picture has mottled phosphate layer whose phosphorus content has reached 10%.

(Chart 4) The surface hardness average of the three samples is in chart 4. The surface hardness surpassed the maximum 435HV0.3 of quality 12.9.

(Chart 5) The hydrogen content of the samples taken from fracture surface is 4.47ppm. To the reasons of the failure of the screw, the incomplete experience phosphate layer on the screw proves that the bolt experience phosphate treatment before heat treatment. Phosphorizing liquid is acidic, and when improper technology happened, these will lead to hydrogen infiltrating. The hydrogen content of the screw is 4.47, which is very high for high tensile quality 12.9 bolt,and have the risk of delayed hydrogen induced

fracture.For the fracture incentives, firstly , it can be attribute to the incomplete exclusion of hydrogen in heat treatment. For delayed hydrogen induced fracture, the main reasons is that the bolt exists moderate tempering brittleness, Martensite bit tempered structure and insufficient tempering. Second, the average surface hardness of the fracture bolt is 443.7HV0.3,which has surpassed the standard 435HV0.3 of quality 12.9 and has reached about 1445Mpa when converted into material strength Rm, all these made the matrix brittled. Third, the uneven division of hydrogen in the material which tend to gather in the place of stress will lead to local high density. Due to the fact that the fillet under the bolt head and the sub-surface in the end of the first tread meet the conditions of stress and hydrogen enrichment, so the hydrogen induced micro-cracks origin and grow from there and finally the bolt fractured.

2. Analysis and Discussion

    GB/T70.1-2000 M20x95 fractured in the process of installing with torque 490N.m. The picture of the fractured bolt can be seen in picture 11 with quality 12.9, material 40Cr,black phosphate and no contact with corrosive medium.At macro level the crack is located in the level the crack is located in the Rangle

    The factors that lead to hydrogen induced fracture of high tensile bolt include : ①strength; ② hydrogen induced; ③stress; ④strain rate, among which the most important factors are strength and hydrogen content.

    Firstly, the higher the strength, the bigger the chance of delayed hydrogen induced fracture. There are too many gathering point of bit error, grain boundary and precipitation equal hydrogen in the metal structure, the bolt has strong hydrogen absorption capacity in the process of acid washing, galvanizing and the inner-stress of the matrix is relatively high. Hydrogen induced delayed fracture of high tensile steel bolt is result from too much emphasis on the strength and hardness scale. Some material of quality 12.9 cannot avoid tempering brittleness, the temperate of 430℃ or below 425℃ contributes to the delayed hydrogen induced fracture.When making quality 10.9 or 12.9 bolt, it will be better to use ML35CrMo (SCM435) or 42CrMoA(SCM440) Steel.

    Secondly hydrogen induced fracture of steel bolt is related to improper heat treatment. The strength and micro structure of steel are closely related. Therefore, under certain strength, hydrogen induced delayed fracture is related to certain structure. Under different structures, the sensitivity of the hydrogen induced delayed fracture order from big to small are as follow: Martensite, upper bainite, lower bainite, Sorbite, pearlyte and austenite. So, improving the technology of heat treatment to control the tensile strength hardness and tempered structure within 2 levels are very necessary.

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    Third , in principle, the hydrogen needs to be removed soon after electroplating, because the hydrogen in the plating and in the metal coating on the surface will diffuse to the inside of the steel, and the number of hydrogen increases with time. It is generally believed that hydrogen needs to be removed within 1 to 3 hours after plating. In addition, the hydrogen removing process is the key to avoid hydrogen embrittlement. At present, there is a common international standards for hydrogen removing process of steel fasteners-------Heat to 190-230℃, and keep warm for 24 hours, then air cooling. During the hydrogen removing process, most domestic steel bolts are heated to 180-220℃, keep warm in 2-4 hours. Although this process can reduce the hydrogen in the bolt, but it is not applicable to all types of steel, nor guarantee the reliability of the product. Therefore, hydrogen removing process should be regulated respectively, according to the varieties, intensity and working stress.

    In case 2, although the hydrogen removing process lasted for 4h at 200℃ after galvanizing, hydrogen embrittlement fracture failure still happened. Measurement results show that there is still residual hydrogen mass fraction of 4.35×10-6, and this shows that the 200℃×4h hydrogen removing process did not achieve very good effect. From the references, for example the 40Cr-10.9 bolt fractured while the mass fraction of hydrogen is only 0.0003%. But according to practice experience, hydrogen induced delayed fracture happens only when the SCM435/SWRCH45K bolt over mass fraction of hydrogen 0.0010%~0.0015%. Study shows that the hydrogen content in the steel remains the same after more than 12h hydrogen removing process. So, for 40Cr steel,200℃×4h hydrogen displacement process is obviously below the requirement, the time should be 8h or more.

    Fourth, in case 3, mottled phosphate layer exists in screw fillet corners, and the hosphorus content is 10%. According to the GB/T 3098.1-2010 standard, white phosphating layer is not allowed to exist at the surface, phosphorus must be removed before the heat treatment.

3. Conclusion

    Based on the analysis of the three typical steel bolt hydrogen induced delayed fracture failure cases, this paper discusses the main incentives that affect the sensitivity of hydrogen embrittlement of steel bolts. From the analysis result, it is better to choose quality10.9 or12.9 bolt which is made of ML35CrMo(SCM435) or 42CrMoA(SCM440) steel. Improve the heat treatment process to control the bolt tensile strength and hardness, and the tempering structure should be within level 2. The hydrogen removing process should be controlled strictly, and last for more than 8h.

    Hydrogen embrittlement delayed fracture of fasteners is a complex physical and chemical changes process, and there are still many problems that people need to explore and research. However, as long as we achieved a deep knowledge of the principle and law of bolt hydrogen embrittlement delayed fracture, and take the necessary precautions measures, we will be able to prevent hydrogen embrittlement delayed fracture ,eliminating hidden quality problems.

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