1. Status description
GCr15 steel bearing ring is spheroidized and annealed after hot processing and forging, and the final heat treatment is quenching and tempering, so as to improve the strength and surface wear resistance of the workpiece.
The outer circle and the inner hole are ground and then pressed into two parts on a 63kn breaking machine before use.
Normally processed products should have a relatively flat fracture after pressing.
The upper part of the figure is the bearing ring sample provided by the bearing manufacturer, and the fracture surface is a straight edge.
The lower part of the figure is the bearing ring product processed by the company, and the pressed part is uneven fracture.
The surface layer of the inner wall of the bearing ring at the lower left corner is the starting point of the crack.
The crack source can be seen as a multi-source step, and it extends to the outer surface layer in a brittle radial shape at the upper right corner.
There are inclined fine porcelain shaped shear lips at the outer surface layer (see Fig. 1).
Cut the sample block perpendicular to the section line from the height center of the bearing ring.
The upper part of the insert is the press fracture part of the bearing manufacturer, and the right side is the press fracture.
The fracture is relatively flat. The sample block number is 1 # sample.
The lower part of the inlay is a press fracture piece processed by the company, and the right side is also a press fracture, which is arc-shaped and uneven.
There are obvious white bright layers on the surface of the inner wall of the sample block.
The sample block number is 2 # sample (see Fig. 2).
2. Chemical analysis
Sample blocks with a size of 25mm long × 25mm width × 15mm thick were cut from the bearing rings of 1 # and 2 # samples respectively for chemical composition detection.
The detection equipment was labspark 5000 precision direct reading spark spectrometer.
The inspection results (see attached table) show that the chemical composition meets the requirements of material standards.
Inspection results of chemical composition (mass fraction) of raw materials (%)
| GCr15 | C | Si | Mn | Cr | S | P | Mo | Ni | Cu |
| standard value | 0.95～1.05 | 0.15～0.35 | 0.25～0.45 | 1.40～1.65 | ≤0.025 | ≤0.025 | ≤0.100 | ≤0.300 | ≤0.250 |
| 1 # sample | 0.992 | 0.261 | 0.366 | 1.522 | 0.012 | 0.009 | 0.021 | 0.030 | 0.028 |
| 2 # sample | 0.988 | 0.250 | 0.371 | 1.508 | 0.015 | 0.011 | 0.024 | 0.028 | 0.025 |
3. Metallographic examination
(1) 1 # sample inspection
The surface of the pressed fracture is flat and transgranular with no oxidation decarburization on the surface layer.
The subsurface layer is cryptoacicular martensite + granular carbide + a small amount of residual austenite (see Fig. 3).
The core structure is also cryptoacicular martensite, granular carbide and a small amount of residual austenite.
The black-and-white area is quite obvious.
This is the characteristic structure of bearing steel after heating and quenching at a lower temperature and should belong to the normal quenching and tempering structure of bearing steel (see Fig. 4).
The local area of the section is a fine-grained fracture crack along the circular arc pit, and the intermittent circular arc pit belongs to the shedding pit of non-metallic inclusions.
In the part with more granular inclusions, the crack extends along the edge of the inclusion due to the weak binding force between the inclusions and the matrix structure.
The microstructure of the subsurface layer and the core is still cryptoacicular martensite + granular carbide + a small amount of retained austenite, and there is a small amount of banded carbide in the core (see Fig. 5-fig. 6).
(2) 2 # sample inspection
The white bright layer at the surface layer of the inner wall of the bearing ring is relatively serious. The white bright layer is a ferrite structure with approximately equiaxial distribution, which belongs to the fully decarburized layer formed by high temperature and high oxidation atmosphere.
The measured depth of the fully decarburized layer is 0.15mm, and the strength of this structure is very low (see Fig. 7).
The darker color below the white bright layer is the carbon poor layer, the depth of the carbon poor layer is 0.10mm, and the structure is cryptoacicular martensite and a small amount of retained austenite.
Since the carbon potential of the carbon poor layer is low, white granular carbides are less precipitated (see Fig. 8).
The left side of the inner wall of the bearing ring is the crack initiation part, and the decarburization layer on the surface is obvious.
The full decarburization layer is cleaved at the opening of the crack, and the brittle crack phenomenon of grain shedding occurs at the initial part of the fracture (see Fig. 9).
The fracture propagation shows the characteristic morphology of intergranular cracking.
In some areas, the grains have fallen off, and the intergranular cracking characteristics of the secondary fracture are more obvious (see Fig. 10).
There are a lot of intergranular melt holes near the fracture, which is the characteristic structure of melting of low melting point non-metallic inclusions.
At the same time, there are pores with grain shedding and intergranular secondary cracks with oxide infiltration.
It shows that during the hot working forging process, the heating temperature is high, the grain boundary is weakened, the intergranular bonding force is significantly reduced, and the forging hot crack has been formed locally under the forging stress (see Fig. 11).
There is also obvious intergranular cavity structure in the center of the sample, and the distribution of granular carbides is not uniform (see Fig. 12).
4. Conclusion and analysis
The inclusion element sulfur exists in the matrix of steel materials in the form of compound.
It first combines with manganese to form 1600 ℃ high melting point manganese sulfide, and the remaining sulfur combines with iron to form 1200 ℃ low melting point iron sulfide and 980 ℃ eutectic iron sulfide.
Iron sulfide and eutectic iron sulfide are the low melting point inclusions that form intergranular caves.
Although sulfide inclusions are less harmful than alumina inclusions, a large amount of sulfide also damages the matrix structure, reduces the strength of the material, increases the brittleness of the material, and is very easy to form forging hot cracks.
The existence of banded carbides also reduces the strength and toughness of the material, and the serious banded carbides can cut off the continuity of the matrix.
The fracture surface of sample 1 # is relatively flat, mainly characterized by transgranular fracture, and locally presents circular arc pits cracked along granular inclusions, which is caused by uneven distribution of low melting point sulfide inclusions.
The forging heating temperature of 2 # sample is too high, the grain boundary is widened, the intergranular melt is weakened, and even intergranular melt holes with low melting point inclusions are formed.
The material strength is significantly reduced. Under the forging stress, microcracks with intergranular cracking are generated in local areas.
It is also due to the high-temperature heating of forging that the cracks are filled with high-temperature oxides.
There are intergranular microcracks with hot forging cracking and severe decarburization layer on the surface of the inner wall of the forging, which further reduce the tensile strength of the material, and make the workpiece form multi-step stress concentration crack sources during the compression fracture process, and then form the brittle fracture characteristics of radial expansion.
