Heat Treatment of Cast Irons Daniel H
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THERMAL PROCESSING Heat Treatment of Cast Irons Daniel H. Herring, The HERRING GROUP, Inc., Elmhurst, Ill. Iron (Fe), derives it name from the Latin word ferrum. In its pure form iron is lustrous, silvery, soft, and ductile. However, pure iron is a poor engineering material, generally not as strong as most plastics. Cast irons are based on the Fe-C system, and the solid-state transformations on which cast iron heat treatments are based are similar to those applied to steels. ron is the fourth most abundant ele- erties. The basic types of cast iron are best bon cast iron (Fig. 3). At a rapid cooling ment on Earth and is one of the most differentiated by their microstructure as rate, dendrites of austenite form as the widely disbursed elements in the opposed to their chemical analysis because alloy cools below the liquidus and grow Earth's crust. In nature it is found in the various types overlap (Table 2). until the eutectic temperature is reached. various compounds with oxygen, sulfur, or The metallurgy of cast iron is more com- At the eutectic, graphite formation is sup- more complicated ores such as carbonates plex than its economics and, in fact, is one pressed, but austenite and cementite pre- and silicates (Table 1). Because iron is so of the more complex metallurgical systems cipitate to form ledeburite, a form of abundant, combines readily with other ele- [Fig 2]. Iron-carbon alloys with less than eutectic that consists of spheres of austen- ments (such as manganese to form steel) 2% carbon are metastable; the true stable ite embedded in cementite. Ledeburite and requires relatively little energy to system being iron-graphite (Fe-C). The forms at the Fe-Fe3C eutectic (solid line extract it from ore, it is one of the most general term cast iron includes pig iron, “nm”). On further cooling, the cementite attractive elements to use for the products gray iron, malleable iron, chilled iron, grows as the austenite decreases in carbon we require in everyday life (Fig. 1). white iron, and nodular or ductile iron. content (along the solid line “no”) At the If an iron alloy exceeds about 2% car- eutectic (point “o”), the remaining Cast Irons bon, the carbon does not have to nucleate austenite transforms to pearlite. At room Cast iron is a generic term used to designate from decomposition of austenite, but temperature, the iron is hard and brittle a family of metals with a wide variety of instead, it can form directly from the melt and is called white iron because the sur- properties. All cast irons contain more than by a eutectic reaction. Note that cementite face of a fractured piece of iron is white 2% carbon and an appreciable amount of (Fe3C) can still nucleate at the eutectic and (somewhat) lustrous. silicon (usually 1-3%). The high carbon more readily than graphite, but on suffi- Upon slow cooling of a 3% carbon cast and silicon content means that they are ciently slow cooling, graphite itself is able iron, austenite forms from the melt, but easily melted, have good fluidity in the liq- to form and grow. eutectic freezing is now slow enough so the uid state and have excellent pouring prop- Consider the solidification of a 3% car- products of the eutectic reaction are Table 1 Common Iron Ores [1] 105 Diamond Mineralogical Chemical Chemical composition Class name formula Platinum 104 Magnetite Fe3O4 72.36% Fe, 27.64% O2 Oxide Palladium Germanium Hematite Fe O 69.94% Fe, 30.06% O Oxide 2 3 2 103 Rhenium Beryllium 36.80% Fe, 31.63% O2, Indium Ilmenite FeTiO3 Oxide 31.57% Ti Hafnium Silver 102 HFeO 62.85% Fe, 27.01% O , Relative price Cobalt Limonite 2 2 Oxide FeO(OH) 10.14% H O Zirconium 2 Vanadium Titanium Pyrite FeS 46.55% Fe, 53.45% SSulfi de Selenium Tin 2 10 Mercury Tungsten Nickel 48.20% Fe, 37.99% CO , Copper 2 Calcium Magnesium Siderite FeCO3 Carbonate Aluminum 13.81% O2 Zinc 1 Lead Iron 1 10 100 103 104 105 106 107 108 Fig 1 (right). Cost of materials versus their relative use [2] Relative production IndustrialHeating.com – December 2004 23 THERMAL PROCESSING austenite and graphite (the reaction takes Graphite forms in cast iron in several Subcritical heating is used for both. Stress place at the dotted line “nm”). The eutec- different shapes including flakes, nodules relief is done at temperatures between 1020 tic graphite tends to form flakes surround- and spheroids (Fig. 4). Because graphite and 1200˚F (550 and 650˚C) without sig- ed by eutectic austenite. As cooling con- has very little (cohesive) strength and nificantly lowering strength and hardness. tinues, the austenite decreases in carbon reduces the effective metallic cross section Heating at temperatures between 1290 and content (along the dotted line “no”), of the casting, both strength and ductility 1400˚F (700 and 760˚C) lowers the hard- while the remaining austentite transforms are affected. ness for improved machinability. to pearlite. Because the fracture surface Nodular iron, also known as ductile iron appears dull gray the material is known as Types of iron or spheroidal graphite iron, is cast iron in gray iron (or pearlitic gray iron). Pig Iron is the term that is generally which the graphite is present as tiny balls, Cooling at an extremely slow rate results applied to a metallic product that contains or spherulites, instead of graphite flakes (as in phase changes similar to those of a slow over 90% iron. Typically it contains in gray iron), or compacted aggregates (as cooled component, except the eutectoid approximately 3% carbon, 1.5% silicon in malleable iron). The nodular irons typi- cooling is sufficiently slow to permit and lesser amounts of manganese, sulfur cally contain from 3.2-4.1% C, 1.8-2.8% Si graphite to precipitate rather than pearlite. and phosphorus. Pig iron along with scarp and up to 0.80% Mn as major constituents. No new graphite flakes will form, but the metal is the base material for both cast iron Several types of matrix structures (includ- ones present will increase in size. The final and cast steel. ing ferritic and pearlitic) can be developed microstructure consists of graphite flakes Gray irons are alloys of iron, carbon and by alloying and heat treatment. The vari- embedded in a ferrite matrix. The resultant silicon, in which more carbon is present ous grades of regular, unalloyed ductile iron material is called ferritic gray iron (cooling than can be retained in solid solution in are designated by their tensile properties of actual castings cooling is seldom slow austenite at the eutectic temperature. The (Table 3). enough to obtain this structure). carbon precipitates as graphite flakes. The Heat treatment of ductile cast iron The cooling rate of a portion of a casting gray irons typically contain from 1.7% - includes stress relief and annealing, as well sometimes may vary, resulting in a struc- 4.5% carbon and 1% - 3% silicon as major as heat treatments used for steels including ture containing patches of both white and constituents. normalize and temper (for higher strength gray iron, called mottled iron [4]. The most common heat treatments and wear-resistance), quench and temper applied to gray cast irons are (for the highest strength), and austemper- stress relief because of ing. Ferritizing (for the most ductile 1500 nonuniform cooling of cast- microstructure) is done by austenitizing at Delta ferrite ings and annealing to 1650˚F (900˚C), followed by holding at 2600 1400 improve machinability. 1290˚F (700˚C) to completely transform Melt 2400 1300 Table 2 Typical Composition of Unalloyed Cast Irons [3] Malleable Austenite + melt Melt + Element Gray iron, % White iron, % Nodular iron, % carbide iron, % 2200 1200 Carbon 2.5-4.0 2.00-2.60 1.8-3.6 3.0-4.0 Austenite Silicon 1.0-3.0 1.10-1.60 0.5-1.9 1.8-2.8 2000 1100 Manganese 0.25-1.0 0.20-1.00 0.25-0.80 0.10-1.00 Austenite + carbide Acm 1000 Sulfur 0.02-0.25 0.04-0.18 0.06-0.20 0.03 max 1800 Temperature, ˚F Temperature, Temperature, ˚C Temperature, Phosphorous 0.05-1.0 0.18 max 0.06-0.18 0.10 max 900 1600 Table 3 Common Grades of Ductile Iron [3] A3 Austenite Type Tensile Yield Typical + Ferrite Ferrite + 800 Hardness, Heat (TS-YS- strength, strength, Elong., % micro- 1400 austenite BHN treatment A1 +carbide %EL) ksi ksi structure Ferrite 700 60-40-18 60 40 18 137-170 Annealed All ferrite Ferrite + carbide 1200 65-45-12 65 45 12 149-229 - Ferritic 600 Ferrite & 80-55-06 85 55 6 179-255 - pearlite 1.0 2.0 3.0 4.0 Normal- 100-70-03 100 70 3 229-302 All pearlite Carbon content, wt% ized Fig 2. Fe-C-Si diagram at 2% silicon. Quench & Tempered 120-90-02 120 90 2 250-350 Silicon strongly promotes graphite formation. Tempered martensite 24 December 2004 – IndustrialHeating.com THERMAL PROCESSING austenite to ferrite and graphite. contain from 2.0-2.65% C, 0.90-1.65% Si cast irons. Malleable irons are relatively soft and and 0.25-0.55% Mn as major constituents. Chilled cast iron is produced by casting can be bent without breaking. They Malleable cast iron can be heat treated the molten metal in such a way as to pro- include ferritic (or standard) malleable to the same microstructures as ductile duce a surface virtually free of graphitic iron and pearlitic malleable iron.