Effect of Mixed Rare Earth on the Microstructure Properties of Gray Cast Iron

Gray cast iron has good casting many other excellent properties, but its mechanical properties are low. Adding rare earth elements to gray cast iron can improve the mechanical properties of gray cast iron, such as tensile strength, wear resistance, hardness. At the same time, rare earth elements can also reduce the whitish tendency of gray cast iron, purify molten iron remove impurities. In this experiment, the effect of different amounts of mixed rare earths on the graphite morphology matrix structure of gray cast iron was studied, the influence of different amounts of mixed rare earths on the mechanical properties of gray cast iron was studied, so as to find the best amount of rare earths to be added. Provide a basis for industrial production.

1 Test materials methods

It is smelted by an intermediate frequency induction electric furnace. The molten iron is discharged at a temperature of 1380~1420℃, the inoculation pouring temperature is about 1350℃. There are 6 test rods in one group. The same box is molded. Hand sand casting. The molding material is raw sand bentonite (4%). ~5%), water (4%~5%). Use a saw to make the mixed rare earth into small flakes with a particle size of 2~5mm, weigh it with a balance. Sprinkle it into the molten iron in the ladle before pouring, stir while pouring, so that the rare earth elements in the ladle are sufficient Melt distribute evenly.

The tensile strength wear resistance of the material were measured on the WE-30 hydraulic universal testing machine the ML-10 abrasive testing machine respectively; the Rockwell hardness of the sample was measured with the HRS-150 Rockwell hardness testing machine; the XJL-2 vertical The metallographic microscope was used to observe the graphite morphology metallographic structure of gray cast iron.

2 Test results analysis

2.1 Metallographic structure observation

2.1.1 The effect of mixed rare earth on graphite morphology

Graphite structure of gray cast iron with different amounts of mixed rare earths (uncorroded). The graphite in gray cast iron without rare earths is thick flake-like. The main form is type A graphite, there are also very small amounts of chrysanthemum-like dendritic graphite; 0.1% mixed rare earth, graphite is finer, the head becomes blunt, belonging to the fine A type graphite; adding 0.3% mixed rare earth, the graphite becomes finer, the distribution is more uniform, has no directionality, the head is blunt; add 0.5% of the mixture Rare earth graphite are refined evenly distributed, the head becomes rounded blunt.

It can be seen that the addition of mixed rare earths can refine homogenize the graphite structure in gray cast iron. When the amount of rare earth added is 0.1%, the graphite structure is an ideal fine A-type graphite. This is because the rare earth elements react with sulfur oxygen in the molten iron to generate rare earth compounds, thereby removing the surface active elements on the graphite ridges, reducing the interface energy between the graphite base the molten iron; it can promote the graphite to dislocation The method grows along the normal direction of the basal plane, so that the graphite becomes vermicular spherical, with the increase of rare earth addition, the tendency of graphite spheroidization will increase. This effect becomes obvious when the rare earth addition is 0.3%.

2.1.2 The influence of mixed rare earth on the matrix structure

Rare earth elements have good inoculation deterioration effects on cast iron. The matrix structure of gray cast iron with different mixed rare earths added was observed. The matrix structure of gray cast iron without rare earths contains more pearlite is distributed in continuous flakes; when the mixed rare earth is added at 0.1%, the pearlite is refined distributed in flakes granules, but uniform; mixed rare earths When the addition amount is 0.3%, the pearlite is uniformly distributed in the form of flakes dispersed in the form of particles; continue to increase the amount of mixed rare earth, the amount of ferrite increases, the pearlite is refined the dispersion increases.

It can be seen that adding rare earth elements can refine homogenize the microstructure, but adding excessive mixed rare earth elements will increase ferrite. This is because the microstructure of general gray cast iron is basically composed of austenite, graphite different amounts of cementite at high temperature. Rare earth has the effect of refining the austenite structure can increase the amount of The number of austenite dendrites makes the distance between the secondary arms smaller the dendrites become finer [2], so the addition of rare earth elements can refine the matrix structure of gray cast iron. In addition, rare earth is a carbide forming element, which generally increases the amount of pearlite. However, when the addition of rare earth causes D E type graphite to appear in cast iron, the amount of ferrite will increase significantly, the hardness strength of ferrite are lower than that of pearlite. Therefore, the content of ferrite should be controlled. Should exceed 0.5%.

2.2 Mechanical performance test results

Test the mechanical properties of the specimens, make 3 specimens for each group of tensile test, take the average of the test results; make 3 specimens for each group of Rockwell hardness test, measure 4 points for each specimen, take the measured result Average value; the wear test refers to the national standard GB/T12444.1-1990 "Metal Wear Test Method", the wear sample is round (Ф6.3mm×20mm), GCr13 standard steel ball (Ф8mm) is selected as the friction pair, at room temperature Under the conditions, set the working load of 50N, speed v=360r/min, friction time t=20min, 3 samples for each group of tests, the average value of the test results is drawn according to the tensile strength, hardness wear of gray cast iron The relationship curve with the amount of mixed rare earth added.

It can be seen that the tensile strength, hardness wear resistance of gray cast iron without rare earth are lower. The tensile strength hardness of rare earth gray cast iron are significantly improved compared with ordinary gray cast iron, they have similar regularities. . When the amount of misch rare earth added is between 0 0.1%, as the amount of misch rare earth increases, the tensile strength, hardness wear resistance of gray cast iron gradually increase; when the amount of rare earth added is 0.1%, gray cast iron The tensile strength, hardness abrasion resistance all reach a peak at the same time; if you continue to increase the mixed rare earth, the tensile strength, hardness abrasion resistance will all decrease.

It can be seen that when the mixed rare earth is added to gray cast iron, the mechanical properties of the cast iron, including strength, hardness, wear resistance have been significantly improved, which is consistent with the situation reflected by the metallographic structure. In particular, the amount of mixed rare earth added is At 0.1%, the tensile strength, hardness wear resistance of the material reach the peak at the same time, the mechanical properties are good. This is due to the dual nature of the inoculation effect of rare earths. When the amount of mixed rare earth is between 0 0.1%, the graphite is refined, uniformly distributed, the head becomes blunt. The blunt flake graphite can reduce the impact on the substrate, reduce the tendency of microcracks, improve the gray The tensile strength of cast iron. The pearlite in the matrix structure is refined, the finer the crystal grains are, which results in more structure interfaces, more defects, hindering the movement of dislocations, improving tensile strength hardness. As the amount of mixed rare earths continues to increase, the graphite flakes become more slender, the more slender the graphite, the stronger the impact cutting action on the matrix, more microcracks will occur, which will make the gray cast iron more resistant to tension. The intensity is reduced. In addition, the amount of ferrite in the matrix structure increases, the tensile strength hardness of ferrite are smaller than that of pearlite, so the mechanical properties are reduced. The wear resistance of gray cast iron has an important relationship with its microstructure. Tests show that gray cast iron has good wear resistance when it has a uniformly distributed graphite pearlite matrix.

3 conclusion

(1) The effect of rare earth on the structure of gray cast iron is mainly manifested in the refinement homogenization of the overall macrostructure microstructure, at the same time, it will transform the flake graphite into a vermicular spherical shape. Rare earth can promote the fineness of graphite flakes, blunt head uniform distribution, increase ferrite, refine pearlite increase dispersion distribution.

(2) The amount of mixed rare earth added within a certain range can increase the tensile strength hardness of gray cast iron improve its wear resistance. However, if the amount exceeds a certain value, the mechanical properties will decrease instead.

(3) Under the test conditions, when the mixed rare earth is added at 0.1%, the tensile strength, hardness wear resistance reach the peak at the same time, the mechanical properties are good.