Connecting rod is an important transmission component in diesel engine. Because of the complex force, it is required to have good structural stiffness and fatigue strength to ensure the reliability of transmission mechanism. Because of its importance, the connecting rod of diesel engine has strict requirements on raw materials, forging process and heat treatment.
During the trial production of a certain type of connecting rod developed by our company, three connecting rods had surface cracks. In this paper, the causes of cracks are investigated one by one through macro-examination, metallographic analysis, chemical composition and hardness gradient analysis, so as to avoid similar cracks recurring.
Macroscopic examination
The cracks of the three connecting rods occur on the parting surface near the large end. The macroscopical morphology of the crack is that the crack is rigid, secondary cracks occur, and the whole crack distributes longitudinally. The whole crack coincides with the fibre streamline, and the tail is thin. According to the cross section of the connecting rod, the crack depth is about 10 mm, which is caused by the propagation of the crack.
Metallographic analysis
Samples were taken at a distance of about 8 mm from the edge of the processing area. Cracks on one side of the processing zone are complete, with a certain angle to the surface and a depth of about 10 mm, which is consistent with the forging deformation streamline, while on the other side, only a small section remains in the subsurface layer, which is the longitudinal tail of the crack.
After polishing, the crack did not penetrate the surface of the connecting rod, which was about 0.2 mm away from the surface. The front of the crack has a large angle with the surface. The crack is rigid and tortuous. The tail of the crack is finer and the residual crack is finer. After corrosion, the crack tip coincides with the forged fiber streamline of the connecting rod and does not penetrate the connecting rod surface. There is no decarbonization on both sides of the crack, the front end is more straight, the middle part has obvious twists and turns, the tail is more sharp and thin, and there are more oxides on both sides. The distribution of residual cracks is consistent with the banded structure of the connecting rod. There is no decarburization and the ends are thin. The matrix structure of the connecting rod is tempered sorbite, while the surface structure is fine and uniform tempered sorbite.
Physical and chemical analysis shows that the sub-surface structure of the connecting rod is basically fine needle-like martensite tempered structure, and the matrix is lath-like martensite tempered structure. Because most of the cracked area on the connecting rod surface has been processed, the sampling position is located at the end of the crack, where the crack does not penetrate the surface, and the front end coincides with the fiber streamline. The middle part of the crack is tortuous and the tail is thin, which is a typical stress crack. There is no decarburization on both sides of the crack, and there are more oxides in the middle and on both sides of the crack.
Chemical Composition Analysis
The connecting rod material is 42CrMoA. From Figure 2, it can be seen that the surface of the connecting rod corroded is different from that of the substrate, and the color of the 4mm area of the surface is obviously darker. The chemical composition of surface and matrix was analyzed by spectrometer. The results showed that there was no significant difference between the two chemical compositions, which met the technical requirements of connecting rod.
Hardness analysis
The hardness gradient test was carried out on the specimen made at the unprocessed part of the connecting rod. The results show that the hardness of the area with dark color on the surface of the connecting rod is higher than the hardness of the matrix in the range of 4.0mm, and the hardness of the surface decreases obviously, as shown in Table 2. The hardness gradient test shows that the hardness of the sub-surface of the connecting rod is higher than that of the matrix, and there is decarbonization on the surface.
Analysis and discussion
Because the material of connecting rod is 42CrMoA and the carbon content is high, the transformation of structure will be more intense during quenching, resulting in greater stress, thus increasing the risk of quenching cracking. Due to the deformation of the parting surface of forgings, serious fibre streamlines will appear, and the transverse properties of the parting surface are poor, which will also increase the risk of quenching cracking.
From metallographic analysis, we draw the following conclusions:
(1) Cracks are caused by stress cracks along the fibre streamline outward and inward, which are produced during quenching.
(2) The crack initiation is located in the machined area. Cracks occur on the splitting surface near the large end, and all occur on the one side of the connecting rod.
Therefore, the causes of cracks basically determine the following two factors:
(1) Cracks during quenching.
The causes of cracks during quenching may be as follows: 1) excessive composition; 2) wrong heat treatment process; 3) wrong operation.
The chemical composition detection at the crack site conforms to the technical conditions of the material and can exclude the component exceeding the standard. Because only a few connecting rods find cracks instead of batch cracks, the possibility of irrational heat treatment process and operation errors is not great. There is randomness in the location of heat treatment cracks, but the crack positions of the three connecting rods are basically the same, and the crack appearance and depth are basically the same. 。
Based on the above analysis, it can be excluded that the crack is caused by quenching.
(2) Defects produced during forging expand during heat treatment.
According to the characteristics of forging products, the parting surface is the high incidence area of forging defects. The crack locations of the three connecting rods all appeared on the parting surface, which coincided with the high incidence area of forging defects. Therefore, it can be preliminarily determined that the crack is a defect produced during forging and formed during heat treatment.
Reduction of Crack Generation Process
Because the connecting rod is formed by 16 t die forging hammer, metal will collapse on the parting surface during the strike process, and the collapse position is shown in Figure 9. With the continuous flow of excess metal in the cavity, the collapsed metal forms a flying edge, which gradually gathers at the position of the forging die bridge until the forging is formed.
However, in production, the inclination of 16t die forging hammer equipment is large and the inclination angle reaches 2(the result of later detection). In the process of striking, it will inevitably lead to the deviation of the upper die of the forging die. In the later period of striking, when the upper die is cooperated with the lower die under the function of the guide lock of the die, it will cause the shaking of the upper die, thus causing the collapsed metal to not flow according to the predetermined position at the forging die bridge, causing local accumulation, and then in the process of stri The hammer head is struck repeatedly to form a cascade. In the production process, if the operator does not set the blank right, the possibility of collapse and the amount of collapse will be increased if the blank is biased to one side. At this time, there is too much metal accumulation at the forging die bridge on one side of the blank to flow out effectively. Under the action of the hammer head, some overlapping metal will be squeezed into the connecting rod body to form defects.
In the forging process of connecting rod, defects are formed in individual connecting rod body, which are not completely cleared before heat treatment, forming quenching crack source. In the quenching process, the strength of individual connecting rod body is reduced due to the defects, and the great stress produced during quenching exceeds the strength of the material itself, thus causing cracks.
The quenching temperature of this batch of connecting rods is 850 C, and the carbon content in the raw materials is high. The transformation of structure will be more intense and the quenching cracking degree will be larger, so the stress will be greater and the probability of quenching cracking will be increased. If there are defects on the shallow surface of the connecting rod, crack propagation will be formed during quenching. Due to the serious fibre streamline on the parting surface of forgings, the transverse property of the parting surface is poor, so the crack will propagate along the fibre streamline during quenching.
Combined with the test of the equipment, it is found that the horizontal inclination of the hammer arm of 16t die forging hammer reaches 2, while the strike stroke of 16t die forging hammer is 1500 mm, and the horizontal displacement of the upper die is about 3 mm. The wrong die in the downward forging process is shown in Fig. 10. When the forging die goes down further and the contact between concave lock and convex lock produces the guiding effect, the forced pulling of the upper die by the forging die lock will offset part of the displacement. Considering the die slope, the trimming surface is about 3 mm away from the body, so the original defect location will be within 6 mm away from the surface. The crack growth depth of heat treatment is related to the stress produced during heat treatment. Combining with the original defect, the crack growth will appear parallel to the parting surface and along the metal streamline direction, and the depth is not necessarily.
From the above analysis, it can be concluded that:
(1) Cracks only appear on the splitting surface of the connecting rod.
(2) The original defect will extend along the metal streamline in the range of 6 mm from the surface under the action of heat treatment stress.
(3) Conditions for producing original defects: both excessive shaking of hammer head and deviation of swing material are available. Although the shaking of hammer head exceeds the standard in production, the deviation of pendulum is accidental (the process requires the pendulum to be corrected and centered). Therefore, the cracked connecting rod will only appear in the batch of individual products.
conclusion
Because the defect is located in the connecting rod body, it is a sub-surface crack, but under the cutting force, some surface defects disappear after being extruded. Therefore, it is not effective to detect defects only by magnetic particle inspection. It can increase the detection of ultrasonic flaw with double crystal probe, but it should be noted that the surface roughness of the flaw detection area should reach MRR Ra6.3 micron.
The precision of forging equipment itself is very important to the folding of die forging products. Forging factories must reasonably control the shaking of forging press itself according to the characteristics of products.
