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H

Mechanical Properties of

High-Alloy and White Ironigh-alloy and white irons, or specialirons, are an important part of the

cast iron family. The alloy content of high-alloy iron exceeds 3%, and the metal can-not be produced by ladle additions to oth-erwise standard iron compositions.

High-alloy iron usually is cast in found-ries that are specially equipped to produceits highly alloyed compositions and that maybe equipped with heat-treating furnaces andquenching equipment or cooling facilitiesto provide for the most economical use of

alloys. This iron often is melted in electricarc or induction furnaces, which provide forprecise control of composition and tem-perature. High-alloy iron is sold at premiumprices and is expected to outperform ordi-nary compositions in applications that in-volve severe service conditions. Thus, gradesof this iron, each with their own uniquecharacteristics, are categorized for threekinds of service:Corrosive ServiceThis category in-

cludes nickel-alloyed iron (Ni-Resist)and high-silicon (Si) iron;

Elevated Temperature ServiceThis cat-egory also includes nickel-alloyed iron(Ni-Resist) and high-Si iron, and it ad-ditionally covers aluminum-alloyed ironand high-chromium white iron;

Wear- and Abrasion-ResistantWhite IronThese specialirons include nickel-chromiumwhite iron (Ni-Hard), high-chromium white iron andmoly-chromium white iron.Many of these types of special

iron are covered by standard speci-fications, but some are of a pro-

prietary composition. In addition,some require heat treatment to de-velop their most useful properties.

Corrosion-Resistant IronHigh-alloy cast iron is used to

produce many parts for engi-neering applications that requireresistance to corrosives in the op-erating environment such as sea-water, sour well oils, commercialorganic and inorganic acids, andalkalis. The ability to easily cast

it into complex shapes and theease of machining some types ofthe metal, make high-alloy iron

an attractive material for the productionof parts for chemical processing plants,petroleum refining, food handling andmarine service. Two types dominate high-alloy corrosion-resistant cast ironnickel-alloyed iron and high-Si iron.

Nickel-Alloyed IronNi-Resist hasbeen produced for many years for corro-sion-resistant applications. This versatileiron owes its excellent corrosion resistanceto the presence of nickel (Ni) in concen-trations of 13.5-36%, chromium (Cr)

content of 1.8-6% and, in one type, cop-per (Cu) content of 5.5-7.5%. Ni-Resistiron is used to solve corrosion problemsrelated to pumping, transportation andprocessing of sour well oils, salt water,some acids and alkalis. Most of the Ni-Re-sist compositions can be produced as ei-ther gray or ductile cast iron.

Gray Ni-Resist provides resistance to oxi-dation at elevated temperatures and corro-sion. The high Ni content assures the for-mation of flake graphite during solidifica-tion even when the compositions contain

relatively high Cr contents (up to 6% in type2b). High Ni concentrations also prevent thetransformation of the austenitic matrix.

The mechanical and physical proper-ties of Ni-Resist gray iron reflect the mi-

crostructure of flake graphite in an aus-tenitic matrix. In general, tensile strengthwill be in the range of 25,000-35,000 psi(170-240 MPa) and, although highly al-loyed, gray Ni-Resist should not be con-sidered high-strength iron.

Ductile Ni-Resist iron provides thesame corrosion resistance as the gray Ni-Resist iron, but develops higher tensilestrength and appreciable ductility. Noductile Ni-Resist contains Cu because thismetal interferes with the nodulizing pro-

cess. Cr should be present in concentra-tions of more than 2% for improved re-sistance to grain boundry corrosion wherethis could be a problem.

Seawater applications account for thepurchase of many Ni-Resist castings each

year, particularly for pumps and valves. Insuch applications, it provides substantial ad-vantages over unalloyed or low-alloy iron,particularly with respect to tolerance forhigher fluid velocity and galvanic compat-ibility with dissimilar metals.

Ni-Resist is cathodic to galvanized

steel, aluminum alloys and unalloyed grayiron; it is anodic to some copper-base al-loys, stainless steel and nickel-base alloys.Stainless steel trim is used successfully inNi-Resist valves, and stainless steel impel-

lers and shrouds perform well inNi-Resist pump castings.

High-Si IronIron with ahigh Si content comprises thesecond major group of high-al-loy iron produced for corrosion-resistant applications. It is widelyused by the chemical industryfor processing and for the trans-

port of highly corrosive fluids.This iron depends principally onthe presence of 14.20-14.75% Sifor its outstanding corrosion re-sistance. It is particularly resis-tant to industrial acids, includ-ing sulfuric and nitric acids at alltemperatures, combinations ofoxidizing acids and organic ac-ids at all concentrations andtemperatures, and phosphoricacids at room temperature.

High-Si cast iron is used ex-

tensively in equipment for theproduction of sulfuric and nitricacids; for fertilizer, textile and

Silicon iron, which exhibits oxidation resistance at elevatedtemperatures, may be alloyed with Mo to enhance its tensileand creep properties. Pictured, a high silicon-molybdenumductile iron exhaust manifold with a 4.5-mm runner wall thick-

ness glows cherry red as it endures temperatures greaterthan 1500F (816C) during a rigorous cycling in Chrysler Corp.s2.2L turbo engine.

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Table 1. Summary of High-Alloy Gray and White Iron ASTM Specifications

explosives manufacture; for sewage dis-posal and water treatment; for handlingmineral acids in petroleum refining andin metal cleaning or pickling; in electro-plating; for processing of paper, beverages,and paints and pigments; and as anodesfor the impressed-current, cathodic pro-tection of iron pipes or other ferrous ves-

sels buried in the soil.Specific high-Si iron casting applicationsinclude pump rotors, agitators, kettles,evaporators, separator towers and Rachidrings, tank outlets, crucibles, insoluble an-odes, and pipe and fittings for plumbing inchemical laboratories of hospitals, collegesand industry. The size of castings may varyfrom small pump rotors and laboratory sinkfittings to tower sections 48 in. (1.22 m) indiameter and 48 in. (1.22 m) high.

Elevated-Temperature ServiceThe service requirements for many

iron castings involve high temperature. Tobe suitable for service at elevated tempera-

tures, cast iron must satisfy three majorrequirements: first, it must resist forma-tion and fracture under service loads atthe highest temperature to which it willbe exposed; second, it must resist oxida-tion by the ambient atmosphere at the ser-vice temperature; and third, to be resis-tant to growth, it must be structurally

stable, preferably not subject to phasetransformations within the temperaturerange to which it will be subjected.

Special iron is capable of sustaininggreater stresses than ordinary alloy ironat temperatures above 1100F (600C). Inaddition to its higher load-carrying capac-ity at elevated temperatures, high-alloyiron offers greater resistance to oxidationand stable microstructure. High-alloy ironsfor elevated-temperature applications in-clude gray iron, ductile iron and white iron.

Four basic alloy systems dominate thehigh-temperature category:

Ni-Resist that depends on Ni to de-velop a stable austenitic matrix and Cr

to combine with the Ni to form a tightoxidation-resistant scale. In addition,it may be further alloyed with molyb-denum (Mo) for improved mechani-cal properties at high temperatures;

intermediate Si iron with a stable ferriticmicrostructure that develops excellentresistance to oxidation at elevated tem-

peratures and often is alloyed with Moto enhance its tensile and creep proper-ties at high temperatures;

aluminum-alloyed iron that develops ahigh degree of oxidation resistance andgood high-temperature mechanical prop-erties when further alloyed with Mo;

high-chromium white iron that dependson Cr content for its resistance to oxidation.

Abrasion-Resistant White IronHigh-alloy white cast iron is specially

qualified for abrasion-resistant applications.The predominant carbides in its microstruc-

ture provide the high hardness necessary forcrushing and grinding other materials with-

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out degradation. The supporting matrixstructure may be adjusted by alloy contentand/or heat treatment to develop the mostcost-effective balance between resistance toabrasive wear and toughness required towithstand repeated impact loading. High-alloy white iron is cast easily into shapes re-quired for crushing and grinding or the han-

dling of abrasive materials.Abrasion resistance concerns the con-ditions under which a metal or alloy isused. The ability of a part to resist a weightloss due to abrasion depends upon its mi-crostructure, the actual mechanical opera-tion of the part, and the kind and size ofmaterial being moved, crushed or ground.

In many crushing and grinding appli-cations, the life of a part may be limitedby its capacity to sustain repeated impactloading without fracture. In applicationsfor which impact loading is not severe, thelife of a casting will depend upon its ca-

pacity to withstand the movement of theabrasive medium against the metal sur-face with a minimum loss of metal.

White iron as a class of material offersconsiderable versatility for a variety ofabrasion-resistant applications. Specificcompositions and heat treatments mayserve to develop the carbide distributionand the matrix microstructures to give

maximum service life and cost effective-ness in white iron castings.

Most of the white iron designated forabrasion-resistant applications fallswithin the high-alloy iron category, butunalloyed white iron is common and pro-vides satisfactory service where the abrad-ing material is not fine or where replace-

ment is not frequent or expensive. All al-loyed iron contains Cr to prevent the for-mation of graphite and to ensure the sta-bility of the carbides in the microstruc-ture. Alloy white iron also may contain Ni,Mo, Cu or combinations of these metalsto prevent or minimize the formation ofpearlite in the microstructure.

Unalloyed white iron castings develophardnesses in the range 350-550 Bhn. Theirmicrostructures consist of primary iron car-bides with a microhardness of 900-1200Vhn in a pearlite matrix with amicrohardness of 220-300 Vhn. Alloyed

martensitic white iron, however, developsBrinell hardnesses in the 500-700 range.Carbide hardness remains at 900-1200 Vhn,but martensite (always associated with someretained austenite) exhibits a microhardnessof 600-700 Vhn. For many abrasion-resis-tant applications, the more costly alloyedwhite iron with martensitic matrix struc-tures provide the most economical service.

SpecificationsASTM Specification A532 covers

the composition and hardness ofabrasion-resistant white iron. Manycastings are ordered to these specifica-tions; however, a large portion of abra-sion-resistant white iron castings arepurchased in accordance with modifi-

cations recommended by foundries forspecific applications.

For economical performance, it isdesirable that designers, metallurgistsand foundrymen work together tospecify the optimum composition, heattreatment and foundry practice to de-velop the most suitable abrasion-resis-tant parts for each application.

Martensitic white iron falls into twomajor groups:

the low-Cr group alloyed with 1-4%Cr and 3-5% Ni, with one modifica-tion that contains 7-11% Cr;

the high-chromium iron containing 14-28% Cr with 1-3% of Mo, often alloyedfurther with additions of Ni or Cu.A third but minor category comprises

the straight 25-28%-Cr white iron.

This article has been excerpted from theIron Casting Handbook.

For more information, see Resources for CastingDesigners & Buyers, p. 67.

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