Formation and prevention of turning crack of screw for electronic parking brake system

The cracking of screws made from medium-carbon cold heading steel for the electronic parking brake system was studied through the analysis of the microstructure and the systematic investigation of the whole process, and the corresponding improvement measures were proposed at last. Results showed that the cracks initiated at the root of the threads due to insufficient plasticity generated by the large pressure during thread rolling were the resource of the fracture, which was further extruded during the turning process, while the centering condition was not well. It was necessary to adjust the appropriate parameters of rolling pressure and rolling time to avoid heavy work hardening and also pay attention to the cleanness on the surface of the rolling plate or the wheel.


Introduction
With the fast industrial upgrading and development of automobiles, electrification, and intelligent technology, the Electronic Parking Brake (EPB) as a safety configuration has been widely used in vehicles [1,2] .When the vehicle is parked, the motor is turning forward, the parking torque is increased through the double-stage gear reduction mechanism, and then the rotary motion of the motor is converted into the linear motion of the lead screw through the screw nut mechanism, thus pulling the parking cable [3][4][5] .The structure is shown in Figure 1.The complex manufacturing process of lead screw nuts for electronic parking brake systems is as follows.Hot rolled wire → sandblasting → spheroidizing annealing → cold drawing → thread rolling → turning.The processing deformation is large and the deformation speed is fast, which puts forward higher requirements on the mechanical properties, formability, surface quality, fatigue life, and processing adaptability of raw materials.Medium carbon cold-heading steel is usually used as the raw material of such products to meet the composite mechanical properties and processability.
Material researchers always pay attention to fracture in the manufacturing process of the lead screw nut, and it is necessary to find out the resource to evaluate the usability of the rest batch of materials.The impact factors are complex.On the one hand, the internal defects of raw materials such as inclusions, porosity, shrinkage, carbide net, and other segregation [6][7][8] ; on the other hand, the external factors such as the high rotation speed used in the drawing process, the large compression rate of some passes and the working cone angle, etc. [9][10][11] .
In this paper, a sample of processing crack defects was inspected and analyzed.The direct cause of the cracking was found through the analysis of the microstructure and the systematic investigation of the whole process.The corresponding improvement measures were proposed in combination with the production process.

Fracture morphology and analysis
The chemical composition of cold-heading steel samples is shown in Table 1.The hot rolled wire with a diameter of 10 mm was drawn to the refined wire with a diameter of 9 mm after blasting and spheroidization annealing.The rolling process was normal, but there was a fracture in the turning process.First, a LEICA DVM6 3D stereo microscope was used to observe the macro morphology of the fracture after turning.Secondly, EVO18 Zeiss scanning electron microscope and EDS spectroscopy were used to examine and analyze the crack source.Table 1.Chemical composition of steel (weight percentage %).

The fracture morphology
The macro morphology of fracture on the screw was observed by stereo microscope.The top view of the fracture was shown on the left, and the lateral view was shown on the right.From the observation of radial cracks, it can be seen that the fracture source is at the crack of the tooth base, as shown in Figure 2.

The longitudinal morphology of the fracture
The longitudinal metallographic samples were further prepared and observed.Cracks or peeling were found at the root of the upper R corner on both sides of the threads, which were symmetrically in the same direction with an angle of 15-30° from the surface to the inner, with the depth in the range of 10-125 microns, as shown in Figure 3. Figure 4 shows the morphology of the cracks before and after corrosion at the root of the threads.It was observed that the cracks are symmetrical, with depths of 102 microns and 125 microns, respectively.The microstructure after corrosion was spheroidized, the deformation of the structure at the crack was large, the crack defect on one side was thicker than that on the other side, and abnormal inclusion or structure was not found.After etching with a nitric acid solution, each crack was observed along the longitudinal section under a scanning electron microscope, as shown in Figure 5.No abnormal inclusions and abnormal structures were found near the crack, granular cementites were uniformly distributed, matrix metal rheology was regular, and metal streamline bending could be observed at the crack.

Figure 5 EDS analysis results of large-size inclusions.
The linear distribution analysis of elements was further carried out in the cracked and non-cracked areas.As shown in Figure 6, the cracked area was filled with cold mounting material, so the content of carbon was high, and the distribution of other components was not significantly different from that in the non-cracked area.The matrix was Fe, containing trace amounts of C, and there was a layer of FeO with a height of 10 μm at the edge.

Internal defect analysis
Further analysis of the internal defects of the material was studied and it was shown that the size of inclusions was limited to 10 μm, with the components of CaO-Al 2 O 3 -SiO 2 -MgO, as shown in Figure 7.There was no direct relationship with edge defects.The microstructure and mechanical properties of raw materials were also in the normal range, with a yield strength of 386-387 MPa, a tensile strength of 670-682 MPa, an elongation of 30%, and a section shrinkage of 56%.
Figure 7 EDS analysis results of internal inclusions.

Causes of cracks at the root of the threads
Cracks took place during the cold work such as thread rolling.At first, the cold work hardening of the thread surface was caused by the strength increased with the fibrous organization during the thread rolling.The plastic deformation was obvious, and the surface hardness increased, and then, the cracks were initiated due to insufficient plasticity under the too-large rolling pressure or the long rolling time.The surface folding cracks with large openings were found on the side of some screws, and deformed fibrous structures were found around the surface folding cracks, indicating that the cracks generated during the process of thread rolling were further extruded and deformed to form folding cracks, which may be related to contaminants on the surface of the rolling plate or the rolling wheel.

Mechanism of cracking
After the threads were rolled, the surface hardness of the screw increased, with a large residual stress.The thread at both ends of the screw needed to be removed turning by the tool with strong impact resistance.During the turning process, the poor centering condition leads to the transverse strike between the tool and the sample, which leads to the crack at the root of the threads being further expanded by the external force.

Analysis of improvement measures
In the rolling process, it is necessary to adjust the appropriate rolling pressure and rolling time to avoid heavy work hardening; Given the partial folding crack generated after plastic deformation, attention should be paid to the cleaning on the surface of the rolling plate or the wheel.In the subsequent turning process, the alignment accuracy should be ensured to avoid the transverse impact between the turning tool and the sample, which will lead to the further expansion of the crack at the root of the threads.

Conclusion
The resource of the fracture was the crack at the root of the threads, which was distributed along the circumference, with the depth in the range of 101 μm to 125 μm.There was no decarburization, abnormal inclusion, or abnormal organization at the crack.No cracks were found on the surface of the screw or threads.From the review of the whole production process, and the analysis of mechanical properties and inclusion control, it was concluded that the machining fracture was caused by the widespread surface cracks at the root of the threads: (1) Composition and mechanical properties met the requirements of the raw materials with good surface quality, no circumferential or transverse defects, and the hardness was appropriate and uniform after annealing; (2) In consideration of rolling processing, it was necessary to further adjust the appropriate rolling pressure and rolling time to avoid too strong work hardening; (3) For some folding cracks generated after plastic deformation, attention should be paid to cleaning the surface of the rolling plate or wheel.
(4) In the subsequent turning process, the alignment accuracy should be ensured to avoid the transverse impact between the turning tool and the sample, which will lead to the further expansion of the crack of the tooth base.

Figure 3 .
Figure 3. Distribution of cracks in tooth base of screw.Figure4shows the morphology of the cracks before and after corrosion at the root of the threads.It was observed that the cracks are symmetrical, with depths of 102 microns and 125 microns, respectively.The microstructure after corrosion was spheroidized, the deformation of the structure at the crack was large, the crack defect on one side was thicker than that on the other side, and abnormal inclusion or structure was not found.

Figure 4 .
Figure 4. Microstructure at the crack of tooth base: (a) No corrosion; (b) Corrosion.

Figure 6
Figure 6 The linear distribution of elements at crack and non-crack: (a) Crack; (b) Non-crack.