Cast iron

Cast iron metallographic preparation

Cast iron is widely used in many applications, especially in cars and engineering industry, and metallurgical is an important part of the production process quality control.Learn how to accurate metallographic analysis of cast iron, and reproducible results are obtained.

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The main characteristics of cast iron

Iron is one of the most diverse metal, for centuries, it has always been an important building material, is used in many buildings, such as railway station, market hall and office and home.

If with carbon and other elements to form alloy, there are many kinds of iron.The word "iron" refers to contain 2.5 1-3% - 4% of carbon and silicon (typical) of iron - carbon - silicon alloy.In the gray cast iron, carbon in the form of graphite;In white cast iron, it exists in the form of iron carbide and alloy carbide.

Cast iron has good castability and workability, mechanical properties of moderate.Due to the economic advantages of cast iron, it is used toThe car,Metal manufacturing and processing industriesA variety of applications.In addition, specific cast iron is sea water pump shell, mill roll, and can choose materials used in geotechnical equipment parts.

Cast iron
Figure 1: austemper ductile cast iron, Beraha color etching (magnification: 500 x, DIC)


Cast iron metallographic quality control process

Because the graphite morphology have important influences on the mechanical property of cast iron, grey cast iron metallographic quality control is an indispensable part of cast iron production process.

Cast iron is the basic metallurgical quality control process is as follows:
  • Use the standard reference comparison chart and/or image analysis, without etching polishing samples on the determination of the shape, size and distribution of graphite.
  • Then according to the specification, etching, the samples to check the structure of the matrix.

Cast iron
Figure 2: cast iron GGG60, not etched (200 x magnification:)
Figure 3: cast iron GGG60, etching with nitric acid 3% ethanol (200 x magnification:)

The difficulties of cast iron metallographic preparation

The main challenge is to keep graphite cast iron sample preparation of the original shape and size, to ensure that the correct characterization of microstructure of cast iron.

Cast iron
Figure 4: contains the flake graphite gray cast iron, inadequate polishing (200 x magnification:)
Figure 5: contains the flake graphite gray cast iron, polishing display correct (200 x magnification:)

Under a microscope, the graphite's image seems to be two-dimensional.But it is three-dimensional.This means that in the process of grinding and polishing, a certain proportion of graphite was cut very shallow, only very weak fixation in the matrix.Therefore, graphite will may not be fully retained, especially very large graphite or accumulate graphite pieces.As a result, the graphite phase are not always well retained or polishing.

In malleable cast iron, the graphite exists in the form of flocculent mass or tempering carbon.This is a kind of fragile graphite, it is difficult to retain in metallographic preparation process.

The preparation of common error is insufficient to eliminate the blur matrix metal after grinding, make the real shape and size of graphite is only a blur.This special in the ferritic and austenitic cast iron, they are easy to deformation and scratches.For these materials, a thorough diamond grinding and the final polishing is very important.

Most of the cast iron standard microscope with magnification of 100 times, this makes the graphite looks black.Carbon to verify whether completely retained, however, requires a higher magnification, because good polishing of graphite is grey.

Cast iron

Figure 6: inadequate polishing graphite particle covered with metal stains, etching with nitric acid 3% ethanol (200 x magnification:)
Figure 7: polishing, right shows the shape and size of graphite for evaluating, using 3% ethanol nitrate etching (200 x magnification:)

Cast iron
Figure 8: polishing good flake graphite (500 x magnification:)

To overcome the time limit

Due to time constraints, using the manual method of cast iron metallographic preparation and microstructure analysis, preparation is often difficult to maintain consistent results.But, as a result of the test pieces of geometry, automatic preparation and often impossible.Solution is to design a specific test pieces, to adapt to the automatic system.

Cutting and set about cast iron sample advice


cutting

  • Alloy white cast iron is very hard (600 HV), it is difficult to cut, especially the big cross section.In spite of this, diamond cutting wheel is not suitable for cutting of white cast iron.On the contrary, it is recommended to use cubic boron nitride cutting wheel.
  • For large cross section, automatic cutting is more effective than manual cutting.
  • If the cut graphite cast iron, it is suggested that choose alumina cutting wheel according to the hardness of cast iron.
With the sample

  • The preparation of quality control samples do not usually set samples.
  • If the sample is failure analysis, it is best to use hot compression set.
  • For soft to medium hardness of cast iron, it is recommended to use phenolic resin (MultiFast).To achieve the best effect of edge protection, the use of LevoFast (containing mineral and glass filler melamine resin).
  • Should be used for a hard cast iron, reinforced resin (DuroFast).
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Tips of cast iron grinding and polishing

Please refer to the below white cast iron and graphite cast iron is a complete preparation method in detail.

  • Suggestions for most of the cast iron used for rigid circular diamond millstone to carry on the fine grinding, because it is very smooth, make the sample and won't leave graphite ups and downs.
  • Hard white cast iron and austemper ductile cast iron can use diamond (MD - 220 Piano) plane grinding, with the diamond (MD - AllegroFine grinding).
  • Soft to medium hardness, contains ferrite, austenite and pearlite matrix of cast iron shall be carried out using silicon carbide foil/sand paper plane grinding, with a diamondMD - LargoTo carry on the fine grinding.
  • For cast iron is easy to corrosion in the process of polishing, recommend the use of anhydrous diamond slurry (DP - A) andYellow lubricant.

Cast iron

Table 1: the preparation method of white cast iron.
Or,DiaPro diamondSuspension can be replaced by respectivelyDP - SuspensionP9,muM, 3,muM and 1muM, andDP - Blue lubricantUsed together.

Cast iron

Table 2: the preparation method of graphite cast iron.

* in graphite very difficult circumstances, you can tryMD - PlanTo carry on the fine grinding polishing cloth.
* * this step is optional

Or,DiaPro diamondSuspension can be replaced by respectivelyDP - Suspension P9,muM, 3,muM and 1muM, andDP - Blue lubricantUsed together

Cast iron
In figure 9: gray cast iron, the preparation of silicon carbide foil/sand paper to carry on the fine grinding, still shows have scratches
In figure 10: gray cast iron, the preparation of rigid disc with diamond MD - Largo to carry on the fine grinding, good display edge protection

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Advice on cast iron cleaning

  • Because a lot of cast iron is easy to corrosion, cleaning must quickly, and should always use cold water to clean.But, can't contact with the water sample.
  • Suggest using ethanol thoroughly clean, with strong warm wind rapid drying.If you still have corrosion, can only wash and rinse with anhydrous ethanol.

Advice on cast iron etching

  • To assess the shape, size and distribution of graphite, and the porosity of castings, cast iron samples in the original microscopy without etching.After the preliminary assessment was carried out on the microstructure, can use the samples of 1-3% nitric acid alcohol solution to etch, in order to detect the micro structure.
  • Graphite cast iron is not suitable for electrolytic polishing, because the graphite can be washed off electrolyte.However, if you just need to quickly determine the microstructure of the matrix, you can use the electrolytic polishing and etching.

The following Beraha reagent can be used for color etching, and can be modified according to the alloy:
1000 ml of water
200 ml of hydrochloric acid
24 g two ammonium fluoride
In 100 ml of this reservoir, add 1 g potassium metabisulfite

Cast iron
Figure 11: austenite tempering nodular cast iron, using 3% ethanol nitrate etching, polarized light (200 x magnification:)

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conclusion

Cast iron containing 2.5 4% carbon black alloy and 1 to 3% silicon.Cast iron metallographic preparation of the main difficulties of the preservation of graphite flake, globular or tempering true shape and size of the form.Especially with soft ferrite matrix of cast iron stain easily, easy to deformation and scratches.

In the process of grinding, the matrix can daub on the graphite, unless a very thorough diamond polishing, graphite won't show up in its true form.Therefore, we suggest that using silicon carbide foil/sand paper plane grinding, and then use the diamond to carry on the fine grinding and polishing.Can choose simple using silica gel as the final polishing.

In-depth understanding of other materials

If you want to learn more about the material and phase property of the metal and other materials, please check our page.

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