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Austenitic cast iron materials

Microstructure and propertiesThe principal characteristic of austeniticgrades of cast iron is that they have astable austenitic basic structure atambient temperature. Most varieties arealso referred to by the trade name Ni-Resist because they contain at least20% nickel, which is the main reason fortheir austenitic microstructure.

Compared to unalloyed and low alloyedgrades of cast iron, austenitic gradesoffer a range of exceptional properties.These include the following:

- Good scaling resistance- High resistance to heat- Impressive thermal expansion

characteristics, which areadjustable within certain limits

- Resistance to corrosion from seawater and alkaline media

- Good running properties- High elongation at fracture- Cold toughness- Resistance to erosion- Non-magnetizable

With this profile of properties, austeniticgrades of cast iron offer an alternative tonon-corrosive, heat-resistant steels andeven, in some cases, to Ni base alloys.

In comparison to these, they offernumerous economic advantagesbecause the process control involved inproduction is simpler. For example, thelower melting and pouring temperaturesreduce the risk of reactions between the

molten mass and the mold material,which in turn means lower costs forcleaning and re-working. In general, thesurface quality is superior to thatachieved with cast steel. With better flowand mold filling characteristics it is notonly possible to achieve greaterdimensional accuracy with the castparts, but thinner walls as well. Inaddition, austenitic cast parts are also

better for mechanical machining thancast steel.

In comparison to unalloyed or lowalloyed cast iron materials, however, thehigher feeding requirement and the highlevel of shrinkage of the austenitic basicstructure mean that a more complexgate and feeding system is required,one which is more similar to the typeused for cast steel. The melting processand the handling of the molten massalso require greater care in order toprevent the absorption of gas and the

degeneration of graphite duringsolidification.

Grades of material and theinfluence of alloying elementsThis class of materials is standardized inDIN EN 13835. In a similar way to white,wear-resistant grades, this standardspecifies not only minimum values forproperties but specifies chemicalcompositions as well. These have aparticularly important influence on theproperties of the materials in this class.

These days, grades with spheroidalgraphite are of greater importance thangrey cast iron grades because theyprovide greater strength and toughnessand because they are less susceptibleto growth at higher temperatures.

The diverse alloying elements havedifferent functions when it comes to theproduction of the desired profile ofproperties. The main purpose of theprimary alloying element, nickel, is tostabilize the austenitic microstructure. Inaddition, nickel also increases thematerials tensile strength and

elongation at fracture, with little effect onhardness and yield strength.Furthermore, nickel also has a majorinfluence on the coefficient of thermalexpansion, which can be adjusted to acertain degree by adjusting the nickel

content. This value can vary band 18.7 m/(mK) dependinnickel content.Although chromium improves to corrosion, strength and temperature properties, the amcan be added is limited becausotherwise lead to excessive focarbide and, as a result, emband excessive hardness. T

cause major problems for macmaterial and it would alsonegative impact on its properties.Copper has a positive impact iresistance to corrosion frowhereas manganese is specifically to cold-tough Manganese keeps the austenlong term at temperatures of a196C.For high temperature apsilicium and molybdenum aused. Silicium increases the

to scaling by forming a protecof SiO2. However, excessivemake the material brittle if it isto high temperatures oveperiods. Molybdenum, in makes the material more reheat.

Corrosion resistanceUnlike corrosion-resistant austenitic cast iron does not passive film because the content is too low. Instead, resistance to corrosion is prthe inherent resistance of tholding matrix and/or the formprotective layer made of products, on which all the otheelements are involved. Uniformsomewhat stronger on austeiron than on corrosion-resistaUnlike these kinds of steel, there is no risk of hole ocorrosion which can be cdamage to the passive film.

100 m

Austenitic microstructure of EN-GJS-

AXNiCr 20-2

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Classification of grades of austenitic cast iron according to DIN EN 13835

Graphite type Material designation Tensile

strength

Rm

N/ mm2

min.

0.2%

Tensile yieldstrength

Rp0,2

N/ mm2

min.

Elong-ation to

fracture

A

%

min.

Impact resistancevalues

Charpy V-notchtest

according to EN10045-1

J

min.

Abbreviation Number

Normal grades

Lamellar EN-GJL-A-XNiCuCr15-6-2 EN-JL3011 170 - - -

EN-GJS-A-XNiCr20-2 EN-JS3011 370 210 7 131)

EN-GJS-A-XNiMn23-4 EN-JS3021 440 210 25 24

EN-GJS-A-XNiCrNb20-2 EN-JS3031 370 210 7 131)

EN-GJS-A-XNi22 EN-JS3041 370 170 20 20

EN-GJS-A-XNi35 EN-JS3051 370 210 20 -

Spheroidal

EN-GJS-A-XNiSiCr35-5-2 EN- JS3061 370 200 10 -

Special grades

Lamellar EN-GJL-A-XNiMn13-7 EN-JL3021 140 - - -EN-GJS-A-XNiMn13-7 EN-JS3071 390 210 15 16

EN-GJS-A-XNiCr30-3 EN-JS3081 370 210 7 -

EN-GJS-A-XNiCr30-5-5 EN-JS3091 390 240 - -

Spheroidal

EN-GJS-A-XNiCr35-3 EN-JS3101 370 210 7 -1)

Optional requirement subject to agreement between the manufacturer and customer