Hydrogen embrittlement of titanium alloy fasteners (HY-industry technical centre)

After hydrogen penetrates into the metal material, it will cause metal damage and cause brittle fracture failure of metal parts under the stress below the yield limit of the material. This phenomenon is called “hydrogen embrittlement”.

Metal hydrogen embrittlement has two main manifestations: one is delayed fracture; the other is material performance deterioration and brittleness. The main manifestation of hydrogen embrittlement of alloy steel is the former, and the main manifestation of hydrogen embrittlement of titanium alloy is the latter.

01 Principle of hydrogen embrittlement of titanium alloy

Mechanism of hydrogen embrittlement fracture of alloy steel is: when a certain amount of hydrogen penetrates into the alloy steel material, it will be released in the material in the form of free hydrogen atoms, hydrogen ions, etc., and gather from the low stress area to the high stress area to the pores in the material Defects such as, inclusions, microcracks, etc. gather and combine with each other to form hydrogen molecules, thereby increasing the pressure of hydrogen. When the pressure reaches a certain level, the microcracks of the material will expand and extend to release the pressure, and hydrogen molecules escape the material in the form of hydrogen. Under the action of tensile stress, free hydrogen continues to accumulate and pressurize the newly expanded microcracks, causing them to continue to expand, and finally evolve into larger cracks. Repeated aggregation, cracks repeatedly expand, enlarge, and extend, eventually leading to material fracture. The whole process takes a certain amount of time, which forms the so-called “delayed fracture”.

Mechanism and characteristics of hydrogen embrittlement of titanium alloys are different from those of alloy steels. The hydrogen in titanium alloys cannot penetrate into the titanium matrix in molecular form, but after contacting the surface of titanium alloys, physical adsorption and chemical adsorption (active adsorption) occur on the surface first. The molecules dissociate into hydrogen atoms, and the hydrogen atoms diffuse into the titanium alloy matrix at a very fast speed. When the amount of hydrogen absorbed exceeds its maximum solubility, the hydrogen atoms diffused into the titanium will exist in a solid solution state or in the form of a hydride. The distribution of hydrogen atoms after diffusion in the titanium alloy is not uniform, but a certain degree of “segregation”. Similar to alloy steel, material defects (such as dislocations, grain boundaries, precipitated phases or the interface between inclusions and matrix phases, pores, microcracks, etc.) are places where hydrogen likes to accumulate and are often the source of hydrogen embrittlement fractures.

02 Classification of hydrogen embrittlement of titanium alloy

• The typical form of the first type of hydrogen embrittlement is hydride hydrogen embrittlement. When the hydrogen-containing alpha titanium is cooled or the hydrogen-containing beta titanium eutectoid decomposition, a new compound titanium hydride (TiH) will be precipitated. Titanium hydride is a stable and brittle substance. The bonding force between it and the matrix crystal grains is relatively weak. The elasticity and plasticity of the two are quite different. The strain after stress is not coordinated. The matrix crystal grains and the titanium hydride crystal grains The interface between the micro-cracks will produce micro-cracks, this kind of cracks generally rapidly expand and expand along the intergranular, and eventually lead to the fracture of the material.

• Second type of hydrogen embrittlement is related to the deformation speed, and the sensitivity to hydrogen embrittlement decreases with the increase of the deformation speed. The crack sources of hydrogen embrittlement are gradually formed under the interaction of stress and hydrogen. Some of them are reversible and some are irreversible. Reversible hydrogen embrittlement exists in (α+β) titanium alloys, which is a complex hydrogen embrittlement phenomenon. When the amount of dissolved hydrogen of (α+β) titanium alloy does not exceed the limit solubility, the hydrogen is in a solid solution state. In this state, the material undergoes low-speed deformation, then unloads, stands still, and then performs high-speed stretching. The plasticity of the material can be restored . However, if the material is continuously and slowly loaded or static load is continuously applied, after a period of time, the material will suddenly fracture (that is, “delayed fracture”). If the normal performance test is repeated on the fracture, it will be found that the material still has normal plasticity and instantaneous strength. This phenomenon is rare in titanium alloy fasteners.

03 The source of hydrogen for titanium alloy fasteners

1. In the smelting process of titanium alloy, hydrogen, hydrogen-containing compounds, smelting, casting and other processes in sponge titanium will bring hydrogen or hydrogen atmosphere, which is the main channel for hydrogen to enter the titanium alloy.

2. Titanium alloy fasteners come into contact with hydrogen-containing media during the machining process, including water vapor, mold release lubricants, coolants, etc., which will cause hydrogen to adhere to the surface of the part and penetrate into the material.

3. Titanium alloy fasteners are exposed to hydrogen or water-containing atmosphere or quenching medium during heat treatment.

4. Titanium alloy fasteners are exposed to hydrogen-containing media in the process of degreasing, pickling, and anodizing. Among them, pickling has a more significant hydrogen absorption effect for high-strength titanium alloys that are sensitive to hydrogen embrittlement.

Generally speaking, the other three items above except item 1 are collectively referred to as “hydrogen pollution”.

04 Hydrogen embrittlement detection of titanium alloy fasteners

From the analysis of the hydrogen embrittlement mechanism of titanium alloys, it is known that the hydrogen embrittlement of titanium alloys mainly depends on how much hydrogen it absorbs, or its hydrogen content. Therefore, detecting the hydrogen content of the titanium alloy is the main method to determine whether hydrogen embrittlement occurs in the close part of the titanium alloy. At present, the international titanium alloy fastener standards generally control the hydrogen content within 125ppm, which can basically ensure the safety of hydrogen embrittlement of various titanium alloy bolts. As for products such as nuts, washers, ring groove rivets (nail rods), as long as they do not undergo major deformation during installation, the hydrogen content index can refer to the bolt standard, which is less than or equal to 125ppm; for ordinary products that require large deformation during installation For products such as rivets and threaded hollow rivets, stricter control can be considered, and the value should be less than or equal to 85ppm.

There are many methods for detecting the hydrogen content of titanium alloys, but at present they can only use the vacuum melting method or the vacuum extraction method in accordance with the methods specified in the corresponding product standards.

Specific operation is: take a sample at a specified specific location, crush it into powder, put it in a closed crucible container and heat it at high temperature to about 2000 ℃ to make all the hydrogen in the material precipitate, and then compare the hydrogen content with standard materials.

When testing the hydrogen content of fasteners, pay attention to:

In order to ensure the accuracy of the measured hydrogen content, the prepared sample must be kept clean and all foreign objects on the surface of the fastener, including the lubricating layer, must be removed. The lubricating layer generally contains high-molecular organic compounds (such as epoxy resins, phenolic resins, etc.), and the basic components of these substances are hydrocarbons.

These hydrocarbons will not decompose at normal temperature, but in a high temperature environment of 2000 ℃, the hydrocarbons will decompose and precipitate hydrogen, which greatly increases the actual value of hydrogenation, and this actual value does not reflect the fastener material at all. Actual hydrogen content.

One sample for the hydrogen content of titanium alloy fasteners is sufficient. Years of experience has shown that in the same inspection batch, the error of the inspection results of each sample is about 1ppm, which is very consistent.

There are many methods for detecting the hydrogen content of titanium alloys, but at present they can only use the vacuum melting method or the vacuum extraction method in accordance with the methods specified in the corresponding product standards.

The specific operation is: take a sample at a specified specific location, crush it into powder, put it in a closed crucible container and heat it at high temperature to about 2000 ℃ to make all the hydrogen in the material precipitate, and then compare the hydrogen content with standard materials.

When testing the hydrogen content of fasteners, pay attention to:

In order to ensure the accuracy of the measured hydrogen content, the prepared sample must be kept clean and all foreign objects on the surface of the fastener, including the lubricating layer, must be removed. The lubricating layer generally contains high-molecular organic compounds (such as epoxy resins, phenolic resins, etc.), and the basic components of these substances are hydrocarbons.

These hydrocarbons will not decompose at normal temperature, but in a high temperature environment of 2000 ℃, the hydrocarbons will decompose and precipitate hydrogen, which greatly increases the actual value of hydrogenation, and this actual value does not reflect the fastener material at all. Actual hydrogen content.

One sample for the hydrogen content of titanium alloy fasteners is sufficient. Years of experience has shown that in the same inspection batch, the error of the inspection results of each sample is about 1ppm, which is very consistent.

05 Hydrogen embrittlement prevention of titanium alloy fasteners

According to the hydrogen embrittlement mechanism of titanium alloy fasteners, the technical measure to prevent hydrogen embrittlement fracture of titanium alloy fasteners should be to strictly control the hydrogen content in the fastener material. Specifically, it is controlled from the following aspects:

Strictly control the hydrogen content of raw materials: The hydrogen in fasteners mainly comes from the smelting of raw materials, so the hydrogen content of raw materials must be strictly controlled. When the material is re-inspected in the factory, not only the mechanical performance inspection, but also the hydrogen content re-inspection must be carried out to ensure that the hydrogen content of the titanium alloy material used for production meets the requirements of the corresponding material standards, and the fasteners produced have May meet the product’s hydrogen content requirements.

Control the hydrogen pollution during the fastener manufacturing process and in the use state: The fastener manufacturing process often requires drawing, extrusion, upsetting, thread forming and other pressure processing processes, and requires heat treatment, cleaning, surface anodizing treatment, etc. , Surface processing. In these processes, semi-finished parts will inevitably come into contact with acidic substances such as lubricants and mold release agents, causing hydrogen contamination on the surface of the parts and hydrogen infiltration. In order to prevent hydrogen embrittlement of fasteners, effective measures should be taken in these processes to reduce hydrogen pollution. For example, try to avoid or reduce the direct contact between metal materials and air. The blanks after pressure processing should be cleaned as soon as possible. When removing dirt on the surface of fasteners, pickling process should not be used as much as possible, especially concentrated pickling should not be used. In order to avoid hydrogen pollution of titanium alloy fasteners in the storage, transportation and working environment, it is generally necessary to perform anodizing treatment to form an oxide film on the surface of the part to physically isolate it from the air. The oxide layer is blue and golden. , Taupe and many other types.

Titanium alloy fasteners must be tested for hydrogen content before they leave the factory. It is particularly important for titanium alloy fasteners with higher strength after solid solution and aging heat treatment.

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