[Electroplating] Analysis and Prevention of Hydrogen-induced Fracture in Auto Parts

The mechanism of hydrogen embrittlement, the fracture morphology of hydrogen embrittlement, the apparent form of hydrogen embrittlement, the factors affecting hydrogen embrittlement, the prevention of hydrogen embrittlement and the inspection of hydrogen embrittlement were introduced respectively, and the typical hydrogen induced fracture in automobile production was introduced. The analysis and research of the case put forward the basis for the determination of hydrogen embrittlement fracture: brittle fracture, intergranular fracture, microscopic pore, secondary crack and hairline pattern. According to the mechanism of hydrogen embrittlement and the main influencing factors, the importance of timely and effective dehydrogenation treatment of high-strength parts is emphasized.

Key words: auto parts; hydrogen embrittlement; fracture characteristics; hydrogen embrittlement prevention

In the 21st century, the automobile industry technology is developing at a high speed. Among the components of the automobile safety and regulation items, high-strength parts are widely used, such as high-strength bolts and spring washers and washers. Since these parts are often used in harsh and demanding parts, the hydrogen-induced fracture of parts is attracting more and more attention in the industry in common product quality accidents.

When a high-strength part is under load, if there is hydrogen-induced damage inside the metal, the phenomenon that the part breaks after a period of time is called hydrogen-induced fracture (also called hydrogen embrittlement), which is a kind of delayed fracture failure. Since there is no warning of hydrogen embrittlement, once a sudden break occurs, it will bring great harm to the performance and driving safety of the whole vehicle.

Unless hydrogen embrittlement is particularly severe, hydrogen-induced fractures require a certain period of incubation. For the auto OEM, once the parts appear on the whole vehicle outside the main engine, hydrogen embrittlement means that this is a systematic and batch quality accident, involving a wide range, and the cost of recall and rework is also higher. high. For suppliers, they face large-volume replacement of parts and claims from OEMs. Therefore, it is of great significance to analyze the hydrogen embrittlement of auto parts and propose practical preventive measures. It should be emphasized that this article does not analyze the hydrogen-induced damage defects in raw material smelting.

1 Hydrogen embrittlement mechanism

1.1 Hydrogen permeable pathway
There are two main ways of hydrogen permeation.
(1) Hydrogen permeation caused by the pickling process in the surface pretreatment, which mainly introduces hydrogen through two steps. First, the following chemical reaction occurs between the steel and the pickling solution:

Among them, a part of hydrogen enters the inside of the steel in the form of hydrogen atoms, and the other part escapes as a gas of hydrogen molecules. Second, the pickling solution undergoes the following ionization reaction:

H+ enters the interior of the metal in a free ion state and then diffuses into the crystal lattice of the metal.
(2) Hydrogen permeation caused by the electroplating process. When electroplating automotive parts, the cathode of the plated part has a reaction of hydrogen ion reduction to hydrogen atoms:

This reaction causes hydrogen evolution, and some of the hydrogen will infiltrate into the interior of the metal in an atomic state.

1.2 Formation mechanism [1]
Dislocations in the metal, grain boundaries, phase interfaces between the inclusions and the matrix, and defects such as pores are places where hydrogen atoms are easily segregated. In addition, in the region where the local stress concentration is high at the root of the notch and the tip of the microcrack, once the part starts to load, hydrogen atoms diffuse and concentrate to these regions, and local hydrogen concentration enrichment and segregation occur to form a new hydrogen group.
When the strain rate of material deformation is low, the hydrogen cluster moves with dislocation motion, the dislocations lag behind the hydrogen cluster, and the hydrogen clusters “pinning effect” on the dislocations, so that the dislocations cannot move freely, causing localized materials. hardening. Under the continuous action of external force, the strain rate of material deformation is accelerated, and new dislocations are continuously generated. These new dislocations form a new hydrogen cluster. When the dislocations in motion and the hydrogen cluster encounter obstacles, dislocations are generated. Superposition, plugging and accumulation of hydrogen gas. When the stress is greater than the critical value of the material strength, a microcrack tip is formed at the end of the local hardening zone and the dislocation end of the dislocation, thereby generating new stress concentration, new hydrogen enrichment, and new position. Wrong and hydrogen clusters, new dislocations are pinned, and the cycle is repeated, resulting in the continuous formation and expansion of cracks until the parts are brittle.

It should be specially pointed out that when the strain rate of the material deformation is higher than a certain critical value, the hydrogen group motion can not follow the dislocation motion, the hydrogen group lags behind the dislocation, and the dislocations in turn cause a “pinning effect” on the hydrogen cluster. At this time, the parts will not be hydrogen embrittled, and only general mechanical fracture will occur. In other words, the hydrogen embrittlement burst has a stress interval, and hydrogen embrittlement does not occur when it exceeds its upper or lower stress [1].

2 Hydrogen embrittlement fracture characteristics
2. 1 macroscopic appearance
The main features of the macroscopic morphology of hydrogen embrittlement fractures are as follows: (1) the fracture has brittle fracture characteristics, no macroscopic plastic deformation, the fracture is flush, sometimes accompanied by radial patterns; 2 the section is clean and fresh, but relatively rough, and the hydrogen-brittle fracture zone is crystalline. Shape, brighter color, non-hydrogen embrittlement fracture zone is dark gray fiber, accompanied by inconspicuous shear lip; 3 oblique fracture and slowly rotate the light, the sparkling reflective facet on the fracture. These facets are caused by the reflection of the crystals facing the light after the disordered polycrystals are split along some specific crystallographic planes inside the grains.

2.2 Microscopic morphology
Since there is no crack source before the force is applied, the initiation of the crack source is related to the continuous stress. Typically, the crack source is not generated at the surface of the part, but originates, nucleates, and expands at the microcrack tip triaxial stress region of the subsurface. Therefore, the main features of the micro-morphology of hydrogen-brittle fractures are as follows: (1) in the case of crystal brittle fracture, rarely intergranular or a mixture of the two [2], the typical fracture in the fracture zone is intergranular brittle fracture along the grain boundary. The crystal grain boundary is sharp, the fracture surface is flat, there is no attachment, and sometimes white bright and irregular thin strips are visible. This line is the reflection of the final fracture position of the grain boundary, and there is a large amount of chicken claw tear. Cracked edges, non-delayed fracture zones are mostly dilapped tear morphology; 2 microscopic cracks are intermittent and tortuous jagged, cracks are generally not bifurcated; 3 high-power scanning electron microscopy can be seen around the grain boundaries with microscopy Stomata, secondary crack, hairline pattern or chicken claw pattern.

3 hydrogen embrittlement expression
The main manifestations of hydrogen embrittlement are: 1 At the moment of fracture, the part is subjected to a continuous static stress. This "static stress" is a low stress, which is much lower than the yield limit stress of the material; 2 is delayed in time. Breaking, it takes a period of gestation period, the parts will be expressed in the form of static brittle fracture; 3 in the number of parts, the number of batches, batch breaks, is not an individual, accidental phenomenon; 4 room temperature conditions The lowest hydrogen embrittlement sensitivity [3].

4 factors affecting hydrogen embrittlement

The main factors affecting hydrogen embrittlement are:
1 is related to the material strength level, that is, the higher the yield strength of the material, the more serious its hydrogen embrittlement tendency;
The microstructure of the 2 parts has a great influence on the hydrogen embrittlement sensitivity. The higher the strength and hardness of the material, the higher the hydrogen embrittlement sensitivity, and the higher the hydrogen embrittlement sensitivity of the thermodynamically unstable tissue, such as quenching. Martensite is more sensitive to hydrogen embrittlement than tempered martensite;
3 is related to the degree of hydrogen absorption of the plated metal. For example, the chrome plating layer has a large hydrogen absorption capacity, and the high-strength parts should be prevented from being chrome-plated, and the iron group element of the galvanized layer is secondarily absorbed;
4, related to the surface treatment process of the parts, taking bolts as an example, 8. 8 grade bolts galvanizing generally does not occur hydrogen embrittlement, 10. 9 bolts should not be galvanized, 10. 9 and 12. 9 bolts are mostly used The Dacromet process, the bolts of grades 10.9 and 12.9 must be dehydrogenated after phosphating;
5 The more severe the damage caused by hydrogen, the more severe the hydrogen embrittlement tendency of the part, and the gestation period of the fracture is usually shorter.

5 hydrogen embrittlement prevention

For externally threaded fasteners of class 10.9 and above, surface hardened self-tapping screws, and combination screws with hardened steel washers, dehydrogenation must be performed after electroplating. The specific process is as follows: The plated part is placed in an oven or a tempering furnace, and is kept at 200 to 230 ° C for 3 to 12 hours or longer, and under thermodynamic action, hydrogen is allowed to escape from the crystal lattice for a sufficient period of time. The precautions are: dehydrogenation must be timely, and should be carried out before the passivation process; in the oven or tempering furnace, the parts should be stacked as much as possible, and should be laid flat and layered.

6 hydrogen embrittlement inspection
Taking bolts as an example, the threaded fasteners can be tightened. On the special fixture, when the screw is subjected to the corresponding process assembly torque, it is kept for 48 h, and the threaded fasteners are not broken after being loosened.

7 typical hydrogen embrittlement case

7.1 Hydrogen embrittlement of 8.8 grade bolts

The bolt material is ML35 steel, the process conditions are quenching, tempering and electrogalvanizing surface treatment; the fracture morphology is along the crystal + microscopic pore + hairline pattern, the microstructure is troostite; the hardness requirement is 253~319 HV, measured hardness values ​​of 451, 453 and 457 HV.
It is generally believed that hydrogen embrittlement does not occur after electroplating of grade 8.8 bolts, and dehydrogenation is not necessary. However, since the actual strength value of the bolt is much higher than the upper limit of the strength value of the 8.8 bolt, it causes hydrogen embrittlement fracture after a period of installation and fastening.

7.2 Hydrogen embrittlement of oil scale

The oil gauge material is 65Mn steel, the process conditions are quenching, tempering and electrogalvanizing surface treatment; the fracture morphology is along the crystal + microscopic pore + hairline pattern + hydrogen bubble, the microstructure is tempered troostite; hardness The required values ​​are 40 to 45 HRC and the measured hardness values ​​are 42, 43 and 43 HRC.
Although the part meets the defined technical requirements, hydrogen debris still occurs despite the fact that the dehydrogenation is not complete and the oil scale is bent at a small angle in the oil pipe.

7.3 Hydrogen embrittlement of self-tapping screws

The self-tapping screw material is 10B21 steel, the process conditions are quenching, tempering, carburizing and electro-galvanizing treatment; the surface of the fracture micro-morphology is along the crystal + secondary crack + microscopic stomata + hairline pattern, the heart is the dimple, The microstructure is martensite + sorbite + a small amount of ferrite; the surface hardness is required to be >600 HV, the measured surface hardness values ​​are 690,692 and 695 HV, the core hardness is required to be 280~390 HV, and the measured core hardness value is 284, 286 and 292 HV.
Due to incomplete dehydrogenation and high hydrogen embrittlement sensitivity of the carburized layer, hydrogen induced cracking occurred in the self-tapping screws of the attacking Steel Plate.

7.4 10. Hydrogen embrittlement of 9-stage bolt gasket

The gasket material is 65Mn steel, the process conditions are quenching, tempering and Dacromet surface treatment; the fracture morphology is along the crystal + secondary crack ten microscopic pores + hairline pattern, the microstructure is tempered troostite • The hardness requirement is 40 to 45 HRC; the measured hardness values ​​are 42, 43 and 43 HRC.
Since the manufacturer does not strictly implement the Dacromet production process, pickling is used instead of shot blasting, resulting in high-strength gasket hydrogen-induced fracture.

8 Conclusion
According to whether the fracture morphology has brittle fracture, grain fracture, microporosity, secondary crack and hairline pattern, it can be judged whether hydrogen embrittlement has occurred in the automobile parts. It can be known from the above hydrogen embrittlement mechanism and main influencing factors. Intensity automotive parts must be treated in a timely and efficient manner.

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