Causes of surface quenching crack, torque overrun and hydrogen

embrittlement of fasteners and improvement measures
Fasteners are a kind of mechanical parts used for fastening and connection and are widely used.

Fasteners are widely used in all walks of life. All kinds of fasteners can be seen in all kinds of

machinery, equipment, vehicles, railways and so on. They are one of the most widely used

mechanical basic parts. It is characterized by a wide variety of specifications, different performance

and uses, and the degree of standardization, serialization and generalization is also very high.

Once the fastener fails, it will cause serious impact. Therefore, it is necessary to strengthen the

analysis of the causes of fastener failure and find the corresponding improvement measures.

Based on the knowledge of fasteners, China standard parts network shares the following:

1. Surface quenching crack
Surface quenching crack refers to the crack produced in the process of quenching or placing at

room temperature after quenching, which is also called aging crack. In the process of quenching,

when the stress produced by quenching is greater than the strength of the material itself and

exceeds the plastic deformation limit, it will lead to cracks. Quenching cracks often occur shortly

after the beginning of martensitic transformation. The distribution of cracks does not have a certain

law, but they are generally easy to form at the sharp corners and abrupt changes of section of the

workpiece. Quenching cracks caused by too fast cooling in the martensitic transformation zone are

often transgranular distribution, and the cracks are straight, with no branching small cracks around.

Quenching cracks caused by too high quenching heating temperature are distributed along the grain,

with sharp and fine crack ends and overheating characteristics. Coarse acicular martensite can be

observed in structural steel and eutectic or angular carbides can be observed in tool steel. The high

carbon steel workpiece with decarburized surface is more likely to form network cracks after quenching,

because the volume expansion of the surface decarburized layer during quenching and cooling is

smaller than that of the non decarburized core, and the surface material is pulled and cracked into

a network due to the expansion of the core. Quenching cracks on the surface can cause sudden

fracture of bolts, and the fracture source of such fracture is located on the surface.

2. Torque overrun
Torque alarm often occurs in the bolt assembly process of controlling torque by angle method.
Failure modes and causes of fastener torque overrun include:

(1) After assembly, the final torque of the parts is higher than the upper control limit or lower than the

lower control limit. The reason is that the assembly torque control range of parts is unreasonable,

which is manifested in that the set control range is too small and the control range deviates upward

or downward.

(2) If it is not pre tightened to the preset angle, the torque reaches the upper limit alarm. The reason

is that the friction coefficient of the parts exceeds the upper limit, the matching friction coefficient of

the parts exceeds the upper limit, and the interference between the parts causes a sharp increase

in the assembly torque.

(3) Normal installation, lower torque limit alarm. The reason is that the friction coefficient of the part

itself exceeds the lower limit or the matching friction coefficient of the part exceeds the lower limit.

When the part is screwed in, the fitting torque is greater than the initial torque (that is, the screw in t

orque consumption is too large). It is common to tighten the lock nut.

3. Hydrogen embrittlement
Fasteners are prone to hydrogen embrittlement, which is the main cause of fastener fracture.

Hydrogen embrittlement is a phenomenon when hydrogen atoms enter and diffuse into the whole

material matrix. When hydrogen atoms enter the material matrix, the material matrix produces lattice

distortion, destroys the original equilibrium state, and is easy to crack under external force. When the

external load is applied to the screw, the hydrogen atoms migrate to the high stress concentration area,

resulting in great stress between the crystal boundary edges, resulting in the fracture between the

crystal particles of the fastener. When the fastener contains critical hydrogen before installation, it will

break within 24h. When hydrogen enters the fastener, it is impossible to predict when fracture will occur.

4. Improvement measures
4.1 measures to prevent surface quenching cracks:
(1) Reasonably adjust the gap between the induction quencher and the workpiece, select appropriate

medium frequency power supply parameters and quenching process parameters in strict accordance

with the process requirements, ensure uniform temperature rise of the product circumference, and

prevent local temperature from exceeding the normal quenching temperature.

(2) Improve the quenching inductor structure, change the circular section structure of the upper end

and tail end of the inductor into a rectangular section structure, reduce the heating speed of the end

and tail inductor, and prevent the rapid temperature rise of the end and tail from exceeding the process

control temperature and over burning, resulting in cracks.

(3) Reduce the number of conductive magnets of the quenching inductor in the transition area after

quenching, and appropriately reduce the heat there.

(4) The preheating heating cooling quenching method is adopted to make the heating temperature of

the product uniform.

(5) Appropriately extend the delayed cooling time after medium frequency heating.

(6) Implement self tempering. In strict accordance with the process technical parameters, reasonably

control the quenching coolant pressure, flow, temperature and cooling time. After stopping the liquid

spraying, use the waste heat of the workpiece to recover the temperature of the hardened layer, so

as to carry out self tempering, so as to maintain high surface hardness and good wear resistance,

timely stabilize the quenched structure and reduce the peak tensile stress.

4.2 torque system
Torque control method is to screw the bolt to a small torque, generally 40% ~ 60% of the tightening

torque (formulated after process verification), and then screw a specified angle from this point. This

method is based on a certain angle, which means that the bolt produces a certain axial elongation

and the connector is compressed. The purpose of this is to screw the bolt to the close contact surface,

and overcome some uneven factors of surface concavity and unevenness, while the required axial

clamping force is generated by the rotation angle. After calculating the rotation angle, the influence

of friction resistance on axial clamping force no longer exists, so its accuracy is higher than that of

simple torque control method. The key point of torque control method is to measure the starting point

of rotation angle. Once the rotation angle is determined, quite high tightening accuracy can be obtained.

4.3 prevention measures of hydrogen embrittlement
(1) Normal electroplating and strict hydrogen removal. Using the reversibility of hydrogen in metal to

remove hydrogen from electroplated bolts is an important method to slow down or eliminate hydrogen

embrittlement. During treatment, the plated steel bolts are placed in the oven for heating. The baking

temperature is about 200 ° C, and the baking time varies according to the strength of the steel. The

higher the strength, the longer the baking time. The hydrogen in the bolt material forms hydrogen

overflow at high temperature to achieve the purpose of hydrogen removal.

(2) Low hydrogen embrittlement electroplating. Low hydrogen embrittlement electroplating is a process

developed in the 1960s and 1970s for the study of hydrogen embrittlement of aircraft parts, including

low hydrogen embrittlement cadmium plating, low hydrogen embrittlement cadmium titanium plating,

low hydrogen embrittlement zinc plating, etc. Low hydrogen embrittlement electroplating requires stress

relief tempering before plating. Instead of pickling with strong acid, sand blasting is used to remove

oxide scale and surface dirt, or vacuum heat treatment is used to avoid oxide scale. In the electroplating

process, on the one hand, adjust the bath formula, on the other hand, reduce the voltage, strictly control

the current density and reduce the adsorption of hydrogen particles. The subsequent process also

requires strict baking hydrogen removal, and the hydrogen removal time shall be at least more than 18h.

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