Effect of casting microstructure state of high chromium white cast iron on abrasion resistance

Effect of casting microstructure state of high chromium white cast iron on abrasion resistance
Abstract: through the experimental study, cast microstructure of cast high chromium white cast iron as the main: chromium austenite and M7C3 eutectic carbides and chromium Qu's body and M7C3 eutectic carbide; modification with RE, the grain refinement, and effectively improve the mechanical properties and wear resistance.
Keywords: cast high chromium white cast iron; rare earth; wear resistance
High chromium cast iron is a kind of common wear resistant cast iron. Chromium carbide into the large amounts, with higher hardness (1300 ~ 1800HV) of the M7C3 type carbides, thereby improving abrasion resistance. At the same time, M7C3 in the solidification process of carbide forming the isolated rods organization distribution, make HISTEP Chrome White Cast iron toughness a certain degree of improvement. The high chromium white cast iron produced at home and abroad at present mostly after high temperature quenching and tempering process, in order to obtain martensitic matrix, but the matrix as the cement mill grinding ball material in high stress three body abrasion force energy, its toughness is still insufficient. And long production cycle, process the complex, equipment investment, energy consumption and labor intensity are large.
The experiments of containing carbon content in Hypoeutectic region, about 15% of the chromium containing high chromium white cast iron were state analysis of microstructure and properties of as cast. The experimental results show that the cast state high chromium white cast iron main microstructure is chromium austenite and M7C3 eutectic carbides and chromium Qu's body and M7C3 eutectic carbide. After rare earth modification can effectively improve the shape and distribution of carbides and homogeneous microstructure, grain refinement and significantly improve the toughness and strength, to improve the abrasion resistance.
Test methods and results
Test used in alloys in acidic medium frequency non core induction furnace melting, melting temperature 15-30 deg.c, pouring temperature of 13.8 ~ 1450 DEG C, sand casting, chemical composition, mechanical properties and microstructure of see in table L.
The maple engine performance test, impact toughness in jb30a type swing hammer impact testing machine on the determination and sample size of 10 * 10 * 55mm, with no gap, no processing.
Wear performance test in the AMSLERAl35/138 dynamic load wear testing machine on. Diameter of specimen size 32 x 10mm. The center hole diameter 6mm, abrasive with 28 / 75 mesh quartz sand. Test pre grinding LH, three body abrasion with horizontal and vertical direction of the shock, impact load for 50 ~ 100kg. Formal wear between 20h. Test weight loss values in automatic electro-optical analytical balance determination.
Two, metallographic analysis
1 the influence of carbon content on Microstructure
L know la table, matrix structure of 4A are troostite and M7C3 when components with the increasing of carbon content, total number of eutectic M7C3 increased, the morphology is also from the short piece to the thick massive development.M7C3 with high hardness and high abrasive wear resistance, increased the number can improve the abrasion resistance; but carbon content higher than eutectic carbon content, very coarse primary carbides, under the impact of the abrasive fragmentation, thereby increasing the wear weight loss.
2 the influence of the mixed rare earth modification on the microstructure
Fig. 200 microstructure of sample 6B 1 *.
Fig. 200 microstructure of sample 10B 2 *.
Figure L, 2 respectively for the same carbon chromium content in B group. To test the microstructure and treated by rare earth modification. The matrix microstructure is mainly M7C3 eutectic carbide chromium austenite. This shows that the addition of Rare Earths on most of the visual effects of the organization is to change grain carbide morphology distribution of rare earth elements in eutectic solidification selectively segregation in eutectic carbide preferential growth direction of liquid iron in water, to prevent the liquid phase of iron, chromium and phosphorus atoms into the eutectic carbide crystal, thus reducing the growth rate of the eutectic carbide in the preferred direction. At the same time, the eutectic austenite dendrite into cold in the liquid phase of growth, and gradually formed by carbide shell, limit and reduce the carbide growth rate. So, the carbide distribution becomes uniform. On the refinement of the organization is rare because of rare earth elements and carbon, nitrogen, Oxygen reacts with the stability of a large number of high melting point compounds. These compounds can be as small emblem of heterogeneous nuclei, atoms in the molten iron is in these nuclei accumulate up to refine the microstructure.
As seen from the table 1, the strength and impact toughness of the samples treated with rare earth modification were significantly increased, while the hardness changed little.
3 the influence of microstructure on the wear resistance
Figure 3 for cast under flexion bainite and M7C3 organization for abrasive resistant material, the matrix is an important and troostite is pearlite interlamellar spacing is very small, it is a tough matrix and on carbide with inlaid and support capacity. As shown in Table 1, cast state the overall hardness for 50 ~ 56HRC. in wear at the beginning of the matrix itself can withstand wear. Therefore, it can be effectively avoid premature carbide exposure that chipped or broken.
Figure 3 M7C3 T+ 200 x
Figure 4 M7C3 M+ 200 x
Figure 4 the microstructure of the M7C3 eutectic carbides in chromium austenite, the former has strong ability and ability to support the mosaic, but also has high impact toughness. Under impact abrasive wear, metal materials must have certain toughness, otherwise it can not resist or absorb these shocks, the impact can be transformed the material is brittle fracture, crack, material surface will be peeled off, caused a serious damage. From the study indicate that the fracture toughness of high chromium cast iron as cast austenite: the critical stress intensity factor KIC value ratio of martensite is high, is better than that of martensite is resistance to crack propagation, especially with thermal passivation crack, shrinkage crack, slag casting defect in the repeated impact conditions.
In addition. The wear resistance of austenitic chromium is increased with increasing the matrix work hardening degree and a total of eutectic carbide content increased. In containing practical application of impact load, the wear surface will produce a sufficiently deep hardening layer enable the practical application of the original 500 ~ 600hv increase to 900 ~ 1000HV.. Ball mill show that, depth of work hardening of austenitic chromium are up to 5mm.
Based on the above point of view, it can be concluded that, in a high stress force energy repeated impact working conditions. Determine the as cast austenitic, Cr flexion's body as a wear resistant material matrix. Table 2, table 3 are quartz sand abrasive different microstructure of high chromium cast iron body wear group and antiwear properties of test results and 45 steel and high chromium white cast iron relative wear resistance of cotton fabric.
The above test results show that:
(1) in the quartz sand for three body abrasive wear to austenite as the matrix of high chromium white cast iron anti-wear performance is slightly better than to bend bainite antiwear properties of matrix of high chromium white cast iron.
(2) in the two groups of tests, the strength, toughness and slightly lower hardness of the material, its wear rate is low; through modification of the material wear resistance properties.
(3) of the test material wear resistance is 20 times as much as that of 45 steel around, so two kinds of matrix of cast state high chromium white cast iron has good wear resistance.
Three, conclusion
1. Cast state high chromium white cast iron microstructure mainly chromium austenite and M7C3 is eutectic carbide and chromium Qu's body and M7C3 eutectic carbide, in toughness and abrasion resistance are demanding conditions, choose to austenite or troostite for matrix cast state high chromium white cast iron is appropriate.. treatment with mixed rare earth modification cast state high chromium white cast iron microstructure uniform, grain refinement can effectively improve the morphology of carbides distribution, improve the strength, toughness and abrasion resistance.
3 chromium austenite has strong work hardening characteristics under high stress impact condition, which is the basis of strong support for carbide, and it is the basis of cement mill grinding ball material.