Primary analyses of cast microstructure design of high chrome white cast iron grinding media (2)

2.As-cast microstructure design

The organization is dependent only on the chemical composition and cooling rate and is assured by the following major technical measures.

2.1 chemical composition

  • Increasing carbon content can increase the volume of carbides in the microstructure on the one hand. On the other hand, it can avoid the low carbon martensite matrix left because more carbon enters into the chromium carbide. Still, the carbon content is limited by the eutectic composition. If the eutectic carbon equivalent is exceeded, very coarse carbides will form. When CR15% , the equivalent of eutectic carbon is 3.6% ; when CR20% , the equivalent of eutectic carbon is 3.2% ; when CR25% , the equivalent of eutectic carbon is 3.0% ; when CR30% , the equivalent of eutectic carbon is 2.7% . The carbon content usually depends on operating conditions ranging from 2.4% to 3.2%.
  • On the basis of carbon content determination, the CR content should be determined according to whether a single M7C3 type carbide can be formed. For example, when the carbon content is 2.8% ~ 3.2%, the CR content is only 15% . We can get M7C3single tissue.
  • The general white iron is obtained mainly by adjusting the silicon according to the wall thickness. Silicon is also a very important element for alloy white iron. The control of silicon must be very careful. As long as it is white, silicon has little effect on wear resistance; for the basic need of deoxidization, the silicon content should be 0.3% ~ 0.6%. When the silicon content is too high, it is disadvantageous to form high carbon martensite.
  • Some pearlite structures should be avoided while multi-element alloying is considered to realize as-cast microstructure expectation, which usually occurs when multi-element alloying is not appropriate to delay austenite transformation. It has been found in practice and theory that fine lamellar martensite + proper amount of austenite can be obtained stably in as-cast microstructure condition by the proper combination of V, Mo, Ni, Cu, Mn and so on, during the subsequent sub-temperature treatment, part of a residue m was changed, and the matrix obtained secondary hardening, and dispersed fine secondary cementite was distributed in the matrix.
  • The addition of trace elements such as V, TI, W, and NB resulted in the formation of MC carbides with higher hardness and the refinement of eutectic structure at the initial stage of solidification due to the high melting point of MC carbides.
  • The sulfur element is expected to be controlled at 0.06% , while the appropriate amount of phosphorus is found to be beneficial to the improvement of material strength and the reduction of internal stress. It should be generally controlled at 0.1% ~ 0.2% .

2.2 Cast model design

  • Metal type is used to speed up the cooling rate and refine the grains.
  • Ceramic fiber is used as a basic coating, and acetylene fume is used as a working coating to control cooling rate and reduce internal stress

2.3 Control the as-cast microstructure of solidification and phase transformation

(1) the martensite is refined by multi-element alloying with V, Mo, Cu, Mn, and so on.

(2) the mixture of rare earth + bismuth + magnesium + potassium + calcium was used to treat the liquid iron by a two-step method, that is, adding the modifying agent into the furnace and the ladle, respectively, in this way, the effects of austenite dendrite refinement and eutectic crystallization can be successively exerted.

(3) a mixed inoculant consisting of v Slag + Ti + Zn was used to form an effective carbide inoculant particle in the actual liquid structure, and a massive carbide was formed in the subsequent crystallization process.

(4) Si20A150Fe + SiC is used as the comprehensive deoxidizer, CAC2 is used as the desulfurized, and the medium electromagnetic stirring force is adjusted to the medium frequency induction furnace to realize the moderate stirring of alloy elements and treatment agent.

(5) using a centrifugal metal mold casting ball, the grain is further refined.

3.The realization mechanism of as-cast microstructure

(1) to determine the proper carbon content, one can ensure that 20% ~ 40% alloy carbides are obtained while leaving high carbon austenite, and the other can make alloy carbides of a single M7C3 type with the proper amount of chromium.

(2) most alloying elements play the role of delaying austenite transformation in austenite. On the one hand, the influence degree is influenced by the lattice properties of alloy element atoms in-structure On the other hand, it depends on their distribution coefficient between austenite and carbide, i. e. (% in carbide)/(% in austenite). For example, NI is 0.25, and MN is 2, and CR is 4.5. The fine lamellar martensite and proper amount of austenite can be obtained by alloying of V, Mo, Cu, Mn, etc.

(3) Because of the rapid cooling rate and the difficulty of gas precipitation, the metal mold is compact, and the inoculant added increases the uniformity of tissue transformation. Because the cooling rate is fast, the elements are not easy to segregate, so the composition is more uniform. The difference between centrifugal casting and permanent mold casting is mainly due to their different crystallization conditions: the cooling mode of centrifugal casting is similar to that of air cooling, in centrifugal casting, the molten metal is solidified in a dynamic state, and there is a strong flow of liquid to melt, drop and increase the nucleus of the dendrite In centrifugal casting, the liquid metal is crystallized under high pressure, and the oscillation of the casting mold is accompanied by these characteristics and phenomena, which make the grains fine and the Matrix of volume solidification mode change from strong columnar crystal to equiaxed crystal.

(4)The austenite dendrites were refined by “Two-step” re-compound modification, the morphology and distribution of carbides were improved, and the matrix was integrated. The M7C3 type carbide is isolated block-like and plate-like so as to obtain better thermal conductivity and thermal diffusivity, thus reducing the overall internal stress, purifying the molten iron by deoxidation, desulfurization, and degassing, and reducing the amount of non-metallic inclusions, improved its form.

(5) If a proper amount of retained austenite is retained in the matrix because of its small specific volume, the expansion of austenite to martensite can be partly offset, and the internal stress can be reduced. If martensite transformation of retained austenite occurs in strain, the retained austenite is beneficial to resist abrasive wear. Its functions are:  1) to prevent crack propagation; 2)to combine austenite with carbide is better than that of martensite with carbide; 3) can absorb energy and consume external work.

white cast iron grinding balls