As shown in Figure 1, cut the metallographic sample close to the crack source, and then conduct rough grinding, fine grinding, polishing and sample preparation for the axial section perpendicular to the journal surface. Firstly, the defects such as graphite morphology, ball diameter and inclusions are detected, and then the quenching layer shape is observed to detect the quenching structure and matrix structure. In order to analyze the microstructure characteristics of the metallographic sample, the test surface of the sample is divided into four areas, in which the crack source and nearby area are area I, the area along the edge of the rolling groove and near the crack source is area II, the edge and nearby area of the rolling groove on the other side away from the crack source is area IV, and the surface area of the connecting rod journal between area II and Area IV is area III, See Figure 2 for details. The red arrow indicates the area.

The section is tested according to GB / t9441-2009 metallographic structure inspection of nodular cast iron. The testing equipment is Olympus gx71 metallographic microscope. Test results of zone I: as can be seen from Fig. 3, most of the graphite is spherical, a few are agglomerated, the roundness is general, the spheroidization level of graphite is grade 2 and the ball diameter is grade 5. There is a coarse angular micro porosity distributed along the grain boundary near the crack source, with a size of about 0.25 mm × 0.17 mm。 Micro porosity is easy to appear at the hot spot of the casting and is distributed at the eutectic grain boundary of the final solidification. Zone I of the crack source and nearby area is located near the transition fillet between the connecting rod crotch and the connecting rod journal of the nodular cast iron crankshaft, which belongs to the hot spot. When the molten iron at the hot spot solidifies slowly and the feeding channel is blocked during casting production, small cavities will be formed between dendrites, resulting in loose defects. The transition fillet between the connecting rod journal and the crank has micro porosity defects, which will destroy the connection strength of the base metal and become a potential fatigue crack source to promote and induce the internal cracking of ductile iron crankshaft.

Test results of zone II: it can be seen from Fig. 4 that molten iron has good inoculation effect, graphite has good roundness, large quantity, uniform distribution and no shrinkage cavity and other defects. The spheroidization level of graphite is grade 2 and the ball diameter is grade 6. Test results of zone III: on the side of zone III close to zone II, the graphite spheroidization grade is grade 2 and the ball diameter is grade 6. The graphite morphology of zone III is shown in Figure 5. The graphite morphology of the most surface part of the journal is flake, and the spherical graphite in the inward and near surface part of the vertical journal surface is deformed along the axis of the journal

Elephant. The depth of the graphite deformation zone from the journal surface to the center is about 0.4 mm, and the width is basically half of the width of the connecting rod journal. The graphite morphology of the journal to the inner center is normal, which is spherical graphite.

Test results in Zone IV: the graphite on the journal surface is in flake shape (Fig. 6), the extension direction of flake graphite is basically parallel to the journal surface, and some graphite near the surface is also deformed. Due to the sharp angle effect formed by the flake graphite, many small cracks have been generated along the flake graphite, some of the cracks are still extending, and some of the metal on the surface of the journal has been completely separated from the journal matrix, resulting in unevenness on the edge of the rolling groove and the surface of the adjacent connecting rod journal. In addition, it can be seen from the microscope that the deformation of zone III graphite is affected by the deformation zone of Zone IV graphite and is the continuation of the deformation zone of Zone IV graphite.

After induction quenching of nodular cast iron crankshaft, there is a quenching transition area between the journal quenching area and the non quenching area. The surface stress of the quenching transition area is tensile stress, which will reduce the fatigue strength. Therefore, during process design and production, the quenching transition area is specially controlled to avoid the fillet between the journal and the crank, so as to keep the residual tensile stress away from the journal fillet, That is, the non quenched area is generally controlled at 4 ~ 7 mm. After the metallographic sample is corroded, first observe the shape of the quenching area of the connecting rod journal. It can be clearly seen from Figure 2 that the outline shape of the induction quenching zone is crescent shape, and there are obviously no non quenching zones on both sides of the quenching zone. The quenching transition zone on the crack source side of zone II is just in the rolling groove and edge, while the rolling groove position of Zone IV is diathermy and quenched.

The length of quenching zone is related to the effective coil width of inductor, and the contour shape of quenching zone is related to the distribution of silicon steel sheet. From the shape of the quenching area of the journal, we can see the problems existing in the induction quenching process: first, the effective coil of the inductor is too wide, resulting in the size of the heating area is too long, and there are no non quenching areas on both sides of the journal; Second, the profile slope on both sides of the induction quenching zone is too large, and the distribution of silicon steel sheets needs to be optimized.

When the fillet is rolled normally, the roller with hardness hrc60-65 rolls on the rough soft surface of the fillet rolling groove, and various metal protrusions on the surface of the rolling groove are extruded and filled with valleys to obtain better roughness and achieve the purpose of mechanical strengthening. When the rolling groove is quenched and then rolled, the rolling wheel will squeeze, slip and even crush the brittle and hard quenching layer without plastic deformation ability. The quenching layer on the surface of the journal connected to the rolling groove will also be squeezed and slip, which will lead to the deformation of the graphite in Zone IV along the edge of the rolling groove and many small cracks around the deformed graphite. In the process of fine grinding journal after rolling, the machining allowance is only about 0.20 mm, which can not eliminate the graphite deformation area and all cracks, resulting in the graphite deformation area and some cracks remaining in the bench test.

According to JB / t9205-2008 metallographic inspection for induction quenching of Pearlite Nodular Cast iron parts, the microstructure of quenching layer is tested. It is found that the structure of the quenching zone is composed of coarse needle quenched martensite + white residual austenite + graphite, which is a typical secondary quenching structure (Fig. 7).

According to the metallographic structure, it can be concluded that: with the connecting rod shaft diameter pulling pad and holding pad, the high temperature generated by friction makes the quenching structure re austenitized. At the same time, a large amount of carbon around the graphite ball is dissolved in the matrix, the carbon content of the matrix increases, and high-carbon martensite is generated under the chilling action of the connecting rod bearing. In addition, due to the cooling speed is too fast, a large amount of austenite remains and becomes residual austenite.