Fracture Analysis of Partial Guide Roller of 35CrMo Steel Coiling Furnace

Abstract : Through the macroscopic and microcosmic analysis of the partial fracture of the deflector spoke in the coiling furnace, it is believed that various problems in the deflector guide material are the reasons for the fracture. Keywords : fracture ; macrostructure ; microstructure

The partial guide roller of the coiling furnace broke after only one month of use . The following analysis was carried out to find out the cause .
1 Macro Analysis
The partial fracture morphology of deflector roller in coiling furnace is shown in Fig . 1 . The fracture starts from the fiber zone in the center and expands radially to both sides . There is no shear lip on the fracture , which indicates that the fracture is relatively brittle . The radial stripes on the outer arc side are basically straight , and a radial layer appears in the radiation area ; the radial stripes in some areas on the inner arc side are curved ( circled in the figure ), these characteristics indicate that there is a tissue or structure on the section. difference .

with 50 % hydrochloric acid aqueous solution
sample . The low-magnification structure of the sample shows that due to segregation, 7 layers are separated on the section of the guide roller , and the layers on the outer arc side are uniform and clear ;
On the inner arc side, the boundary between layers is blurred . It shows that the low-magnification structure on the outer arc side is mostly cast ( obvious coarse columnar crystals ). It shows that the dense intergranular pores on the inner arc side are loose , and columnar crystals can also be seen .
2 microscopic analysis
In the case of retaining the fracture , the sample is taken from the part where the inner arc side fracture is separated from the roll base ( pointed by the arrow in Figure 1 ) .
The samples were cold mounted, ground and polished , etched with 4 % nital solution, observed and analyzed on OLMPUS BX51 microscope and sisc 6.0 image analysis system .
Figure 3 shows the microstructure of the part where the inner arc side is separated from the roll base due to fracture . Tempered sorbite with retained martensite orientation + partial network ferrite + a small amount of bainite and martensite . The reticulated ferrite is accompanied by granular and linear inclusions .
3 Analysis and Discussion
35CrMo alloy structural steel deflection guide roller has good hardenability , moderate machinability , high durable strength at high temperature after quenching and tempering , and good low temperature impact toughness .
Through the analysis, it is found that there are many problems in the deflector roller which breaks prematurely and fails . ①Casting : During the centrifugal casting process , 7 segregation layers appeared on the section of the deflector roller , and microscopic analysis found that there were granular and linear sulfides precipitated along the grain boundaries ; The layered weak interface makes the strength along the thickness direction of the steel plate significantly lower than the longitudinal direction, and it is easy to generate cracks parallel to the rolling direction . When the tensile test is carried out , under the action of three-dimensional stress , the crack first cracks at the center of the steel plate, such as tiny flaky inclusions or cracks and other defects. With the increase of the test force , the crack tip produces a large plastic deformation . The section of the crack tip of the sample is necked , and the local deformation of the crack along the thickness direction of the sample is constrained by the surrounding materials , resulting in a larger tensile stress from the surface. The closer to the crack tip , the greater the stress from the surface. surface stress is maximum . When the out-of-plane stress along the thickness direction exceeds the tensile strength of the material's weak interface , delamination cracks are first generated near the center of the sample thickness .
( 2 ) The delamination fracture behavior of the specimen depends on the toughness and internal defects of the material , Ordinary-level ship boards rarely appear , It mainly occurs in high-strength high-strength materials with high toughness ; in addition , The proportion of delamination phenomenon is related to the thickness of the material. Thin samples below 10 mm rarely appear, and the frequency of occurrence is higher in thick samples . As the thickness of the sample material increases , The proportion of delamination increases ; the thickness of the sample affects the degree of delamination . The smaller the thickness of the sample , the smaller the size of the delamination cracks , and there are delamination cracks with larger length and width in the middle of the thickness sample . This indicates that delamination cracks are generated under the action of higher out-of-plane stress

in conclusion
( 1 ) The fracture delamination of the tensile specimen is caused by the action of three-dimensional stress on the defects such as segregation and banded structure inside the steel plate when the tensile specimen is deformed during the test .
( 2 ) The fracture delamination phenomenon of the tensile specimen is related to the material , Ordinary-grade ship plates occur rarely , mainly in high-strength and tough materials ; in addition , the proportion of delamination is related to the thickness of the material, and thin samples below 10 mm rarely occur. , the proportion of delamination phenomenon increases .
( 3 ) There is no significant difference in tensile properties and impact toughness between samples with tensile delamination and samples without delamination . Therefore, it is recommended to further track the UT test results of steel plates with stratified batch numbers , verify the necessity of UT inspection, and optimize the production organization .

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