Simplified Selection of Stainless Steels

Three parameters are important to consider when selecting a particular grade of stainless steel for a given application. These parameters are in order of decreasing importance:

Corrosion resistance. This is the most important property to consider when selecting a stainless steel. Actually, the corrosion resistance is always the primary reason when considering Fe-Cr-Ni alloys. For best results, the maximum allowable corrosion rate, usually 50 μm/year (2 mils per year), along with an exact knowledge of the corrosive environment, must be known.

Mechanical strength. The mechanical strength is the second most important parameter, especially for designing structural applications.

Fabrication. The capabilities of the stainless steel to be machined, welded, cold worked, and heat treated are, in combination with the two previous parameters, an important parameter to take into account from a technical and a cost-assesment point of view.

 

Based on an approach developed by the company Carpenter Specialty Alloys8, it is possible  to summarize the selection process graphically (Figure 1), plotting the grade of the  stainless steel as a function of both corrosion resistance and mechanical strength.

 

 

 

 

Stainless Steel Application Guidelines

 

 



 

 

Stainless Steels

In 1821, the French engineer Pierre Berthier observed that a certain amount of chromium added to iron alloys, in addition to enhancing their stiffness, also improved remarkably their corrosion resistance to acids. Almost a century later, in 1909, Leon Guillet and Albert Portevin in France studied independently the microstructure of Fe-Cr and Fe-Cr-Ni alloys. In 1911, the German metallurgist P. Monnartz, following the pioneering activity of his predecessors, explained the passivation mechanism and determined the lowest percentage of chromium required to impart a rustless ability to steels. The new alloy did not corrode or rust when exposed to weather, and the new iron alloy was then simply referred to as rostfrei Stahl in Germany, rustless or rustproof iron in Great Britain, and acier inoxydable in France. However, in the United States and United Kingdom, it was later denoted by the more modern designation still used today, stainless steel. In 1913, the first casting of a stainless steel was performed at Sheffield in the United Kingdom.

 

Stainless steels are a large family of iron-chromium-based alloys (Fe-Cr) that are essentially low-carbon steels containing a high percentage of chromium, at least above 12 wt.% Cr, to impart the same corrosion resistance conferred by pure chromium in chrome plate. This addition of chromium gives the steel its unique corrosion-resistance properties denoted as stainless or rustproof. The chromium content of the steel allows the formation on the steel surface of a passivating layer of chromium oxide. This protective oxide film is impervious, adherent, transparent, and corrosion resistant in many chemical environments. If damaged mechanically or chemically, this film is self-healing when small traces of oxygen are present in the corrosive medium. It is important to note that in order to be corrosion resistant, the Fe-Cr alloy must contain at least 12 wt.% Cr and that when this percentage is decreased, for instance by precipitation of chromium carbide during heating, the protection is lost and the rusting process occurs. Moreover, the corrosion resistance and other useful properties of stainless steels are largely enhanced by increasing the chromium content usually well above 12 wt.% Cr. Hence the chromium content is usually 15 wt.%, 18 wt.%, 20 wt.%, and even up to 27 wt.% Cr in certain grades. In addition, further alloy additions (e.g., Mo, Ti, S, Cu) can be made to tailor the chemical composition in order to meet the needs of different corrosion conditions, operating temperature ranges, and strength requirements or to improve weldability, machinability, and work hardening. Generally, the corrosion resistance of stainless steels is, as a rule, improved by increasing the alloy content. The terminology heat resistant and corrosion resistant is highly subjective and somewhat arbitrary.  The term heat-resistant alloy commonly refers to oxidation-resistant metals and alloys (see Ni-Cr-Fe alloys in the nickel and nickel alloys section), while corrosion resistant is commonly applied only to metals and alloys that are capable of sustained operation when exposed to attack by corrosive media at service temperatures below 315°C. They are normally Fe-Cr or Fe-Cr-Ni ferrous alloys and can normally be classified as stainless steels. There are roughly more than 60 commercial grades of stainless steel available, and the global annual production was roughly 25 million tonnes in 2004.

 

 

Following a classification introduced by Zapffe and later modernized to accommodate new grades, stainless steels can be divided into five distinct classes (see Table 1). Each class is identified by the alloying elements that affect their microstructure and for which each is named. These classes are as follows:

(i) austenitic stainless steels;

(ii) ferritic stainless steels;

(iii) martensitic stainless steels;

(iv) duplex or austenoferritic stainless steels; and

(v) precipitation-hardened (P-H) stainless steels.

In practice, empirical parameters called nickel and chromium equivalents can be utilized to assess the relative stability of austenite and ferrite, respectively. These equivalents are defined as follows:

 

Eq(Ni) = wNi + 30 × wC + 0.5 wMn,

Eq(Cr) = wCr + wMo + 1.5 wSi + 0.5 wNb,

where wi denotes the mass fraction of the chemical element indicated by the subscript. The Ni and Cr equivalents are usually used to assess the phase formation in weldments. Hence modifying the chemistry of the weld metal can ensure a better result by avoiding hot cracking.

 

Low-Alloy Steels

Steels that contain specified amounts of alloying elements other than carbon and the commonly accepted amounts of manganese, copper, silicon, sulfur, and phosphorus are known as alloy steels. Alloying elements are added to change mechanical or physical properties. A steel is considered to be an alloy when the maximum of the range given for the content of alloying elements exceeds one or more of these limits: 1.65 wt.% Mn, 0.60 wt.% Si, or 0.60 wt.% Cu, or when a definite range or minimum amount of any of the following elements is specified or required within the limits recognized for constructional alloy steels: aluminum, chromium (up to 3.99%), cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium, or other element added to obtain an alloying effect. According to the previous definition, strictly speaking, tool and stainless steels are also considered alloy steels. However, the term alloy steel is reserved for those steels that contain a minute amount of alloying elements and that usually depend on thermal treatment to develop specific properties. Subdivisions for most steels in this family include through-hardening grades, which are heat treated by quenching and tempering and are used when maximum hardness and strength must extend deep within a part, while carburizing grades are used where a tough core and relatively shallow, hard surface is needed. After a surface-hardening treatment such as carburizing or nitriding for nitriding alloys, these steels are suitable for parts that must withstand wear as well as high stresses. Cast steels are generally through-hardened, not surface treated. Carbon content and alloying elements influence the overall characteristics of both types of alloy steels. Maximum attainable surface hardness depends primarily on the carbon content. Maximum hardness and strength in small sections increase as carbon content increases, up to about 0.7 wt. % C. However, carbon content greater than 0.3% can increase the possibility of cracking during quenching or welding. Alloying elements primarily influence hardenability. They also influence other mechanical and fabrication properties including toughness and machinability. Lead additions (i.e., 0.15 to 0.35 wt.% Pb) greatly improve the machinability of alloy steels by high-speed-tool steels. For machining with carbide tools, calcium-treated steels are reported to double or triple tool life in addition to improving surface finish. Alloy steels are often specified when high strength is needed in moderate to large sections. Whether tensile or yield strength is the basis of design, thermally treated alloy steels generally offer high strength-to-weight ratios. In general, the wear resistance can be improved by increasing the hardness of an alloy, by specifying an alloy with greater carbon content, or by using nitrided parts, which have better wear resistance than would be expected from the carbon content alone. Fully hardened and tempered, low-carbon (i.e., 0.10 to 0.30 wt.% C) alloy steels have a good combination of strength and toughness, at  both room and low temperatures.

 

 

Carburizing alloyed steels. The properties of carburized and hardened cases depend on  the carbon and alloy content, the structure of the case, and the degree and distribution of residual stresses. The carbon content of the case depends on the carburizing process and on the reactivity of iron and of the alloying elements to carburization. The original carbon content

of the steel has little or no effect on the carbon content produced in the case; hence the last two digits in the AISI-SAE specification numbers are not meaningful as far as the case is concerned. The hardenability of the case, therefore, depends on the alloy content of the steel and the final carbon content produced by carburizing. With complete carbide solution, the effect of alloying elements on the hardenability of the case is about the same as the effect of these elements on the hardenability of the core. As an exception to this statement, any element that inhibits carburizing may reduce the hardenability of the case. Some elements that raise the hardenability of the core may tend to produce more retained austenite and consequently somewhat lower hardness in the case. Alloy steels are frequently used for case hardening  because the required surface hardness can be obtained by moderate quenching speeds. Slower quenching may mean less distortion than would be encountered with water quenching. It is usually desirable to select a steel that will attain a minimum surface hardness of 58 or 60 HRC after carburizing and oil quenching. Where section sizes are large, a high-hardenability  alloy steel may be necessary, whereas for medium and light sections, low-hardenability steels will suffice.

 

 

The case-hardening alloy steels may be divided into two classes: high- and mediumhardenability case steels.

High-hardenability case steels. The five AISI-SAE grades 2500, 3300, 4300, 4800, and 9300 are high-alloy steels; hence both the case and the core possess a high hardenability. They are used particularly for carburized parts with thick sections, such as pinions and heavy gears. Good case properties can be obtained by oil quenching. These steels are likely to have retained austenite in the case after carburizing and quenching, and hence refrigeration may be required.

 

Medium-hardenability case steels. The AISI-SAE grades 1300, 2300, 4000, 4100, 4600, 5100, 8600, and 8700 have medium hardenability, which means that their hardenability is intermediate between that of plain carbon steels and the higher-alloy carburizing steels discussed previously. In general, these steels can be used for average-size case-hardened produced by oil quenching. The core properties of case-hardened steels depend on both the carbon and alloy content of the steel. Each of the general types of alloy case-hardening steel is usually made with two or more carbon contents to permit different hardenability in the core. The most desirable hardness for the core depends on the design and the type of application. Usually, where high compressive loads are encountered, relatively high core hardness is beneficial in supporting the case. Low core hardnesses may be required if great toughness is important.

 

The case-hardening steels may be divided into three general classes, depending on the hardenability of the core:

(i) low-hardenability core such as AISI-SAE 4017, 4023, 4024, 4027, 4028, 4608, 4615, 4617, 8615, and 8617;

(ii) medium-hardenability core such as AISI-SAE 1320, 2317, 2512, 2515, 3115, 3120, 4032, 4119, 4317, 4620, 4621, 4812, 4815, 5115, 5120, 8620, 8622, 8720, and 9420;

(iii) high-hardenability core such as AISI-SAE 2517, 3310, 3316, 4320, 4817, 4820, 9310, 9315, and 9317.

Carbon Steels (C-Mn Steels)

Iron containing more than 0.15 wt.% C chemically combined is normally termed steel. This 0.15 wt.% C is a somewhat arbitrarily chosen borderline, and sometimes the nearly chemically pure “ingot iron” is referred to as mild steel. To make it even more confusing, the term mild steel is often also used as a synonym for low-carbon steels (see below), which are materials with 0.15 to 0.30 wt.% C. Steels that have been worked or wrought while hot are covered with a black scale, also called a mill scale made of magnetite (Fe3O4), and are sometimes called black iron. Cold-rolled steels have a bright surface, accurate cross section, and higher tensile and yield strengths. They are preferred for bar stock to be used for machining rods and for shafts. Carbon steels may be specified by chemical composition, mechanical properties, method of deoxidation, or thermal treatment and the resulting microstructure. However, wrought steels are most often specified by their chemical composition. No single element determines the characteristics of a steel; rather, the combined effects of several elements influence hardness, machinability, corrosion resistance, tensile strength, deoxidation of the solidifying metal, and the microstructure of the solidified metal. Standard wroughtsteel compositions for both carbon and alloy steels are designated by the SAE-AISI four-digit  code, the last two digits of which indicate the nominal carbon content (Table 1).

 

 

Plain Carbon Steels

 

Carbon steels, also called plain carbon steels, are primarily Fe and C, with small amounts of Mn. Specific heat treatments and slight variations in composition will lead to steels with varying mechanical properties. Carbon is the principal hardening and strengthening element in steel. Actually, carbon increases hardness and strength and decreases weldability and ductility. For plain carbon steels, about 0.20 to 0.25 wt.% C provides the best machinability. Above and below this level, machinability is generally lower for hot-rolled steels. Plain carbon steels are usually divided into three groups:

 

(i) Low-carbon steels (e.g., AISI-SAE grades 1005 to 1030), or mild steels, contain up to 0.30 wt.% carbon. They are characterized by a low tensile strength and a high ductility. They are nonhardenable by heat treatment, except by surface hardening processes.

 

(ii) Medium-carbon steels (e.g., AISI-SAE grades 1030 to 1055) have between 0.31 wt.% and 0.55 wt.% C. They provide a good balance between strength and ductility. They are hardenable by heat treatment, but hardenability is limited to thin sections or to the thin outer layer on thick parts.
 

(iii) High-carbon steels (e.g., AISI-SAE grades 1060 to 1095) have between 0.56 wt.% and about 1.0 wt.% C. They are, of course, hardenable and are very suitable for wear-resistant and/or high-strength parts.

 

Low-carbon or mild steels. The lowest carbon group consists of the four AISI-SAE grades 1006, 1008, 1010, and 1015. All these grades consist of very pure iron with less than 0.30 wt.% C having a ferritic structure, and they exhibit the lowest carbon content of the plain carbon group. These steels exhibit a relatively low ultimate tensile strength and are not suitable for mechanically demanding applications. Both tensile strength and hardness rise with increases in carbon content and/or cold work, but such increases in strength are at the expense of ductility or the ability to withstand cold deformation. Hence mild steels are selected when cold formability is required. They are produced both as rimmed and killed steels. Rimmed steels are used for sheet, strip, rod, and wire where excellent surface finish or good drawing qualities are required, such as oil pans and other deep-drawn and formed products. Rimmed steels are also used for cold-heading wire for tacks, and rivets and low-carbon wire products. Aluminum-killed steels (i.e., AK steels) are used for difficult stampings or where nonaging properties are needed. Silicon-killed steels (i.e., SK steels) are preferred to rimmed steels for forging or heat-treating applications. With less than 0.15 wt.% C, the steels are susceptible to serious grain growth, causing brittleness, which may occur as the result of a combination of critical strain from cold work followed by heating to elevated temperatures. Steels in this group due to their ferritic structure do not machine freely and should be avoided for cut screws and operations requiring broaching or smooth finish on turning. The machinability of bar, rod, and wire products is improved by cold drawing. Mild steels are readily welded.

 

 

Carburizing steels. This second group consists of the 13 AISI-SAE grades 1016, 1017, 1018, 1019, 1020, 1021, 1022, 1023, 1024, 1025, 1026, 1027, and 1030. Because of their higher carbon content they exhibit enhanced tensile strength and hardness but at the expense of cold formability. For heat-treating purposes, they are known as carburizing or case-hardening steels. Killed steels are recommended for forgings, while for other uses semikilled or rimmed steel may be suitable. Rimmed steels can ordinarily be supplied with up to 0.25 wt.% C. Higher carbon content provides a greater core hardness with a given quench or permits the use of thicker sections. An increase in manganese improves the hardenability of both the core and the case along with machinability; in carbon steels this is the only change in composition that will increase case hardenability. For carburizing applications, grades AISI 1016, 1018, and 1019 are widely used for thin sections or water-quenched parts. AISI 1022 and 1024 are used for heavier sections or where oil quenching is desired, and AISI 1024 is sometimes used for such parts as transmission and rear axle gears. AISI 1027 is used for parts given a light case to obtain satisfactory core properties without drastic quenching. AISI 1025 and 1030, although not usually regarded as carburizing types, are sometimes used in this manner for larger sections or where greater core hardness is needed. For cold-formed or cold-headed parts, the lowest manganese grades (i.e., AISI 1017, 1020, and 1025) offer the best formability at their carbon level. AISI 1020 is used for fan blades and some frame members, and 1020 and 1025 are widely used for low-strength bolts. The next highest manganese types, i.e., AISI 1018, 1021, and 1026, provide increased strength. All carburizing steels can be readily welded or brazed.

 

 

Medium-carbon steels. This group consists of the 16 AISI-SAE grades 1030, 1033, 1034, 1035, 1036, 1038, 1039, 1040, 1041, 1042, 1043, 1045, 1046, 1049, 1050, and 1052 with a carbon content of between 0.31 and 0.55 wt.% C. They are usually selected for their higher mechanical properties and are frequently further hardened and strengthened by heat treatment or by cold work. They are usually produced as killed steels and are suitable for a wide variety of automotive-type applications. Increases in the mechanical properties required in section thickness or in depth of hardening ordinarily indicate either higher carbon or manganese content or both. The heat-treating practice preferred, particularly the quenching medium, has a great effect on the steel selected. In general, any of the grades over 0.30 wt.% C may be selectively hardened by induction heating or flame methods. The lower-carbon and manganese steels in this group find usage for certain types of cold-formed parts. AISI 1030 is used for shift and brake levers. AISI 1034 and 1035 are used in the form of wire and rod for cold upsetting such as bolts, and AISI 1038 for bolts and studs. The parts cold-formed from these steels are usually heat treated prior to use. Stampings are generally limited to flat parts or simple bends. The higher-carbon AISI 1038, 1040, and 1042 are frequently cold drawn to specified physical properties for use without heat treatment for some applications such as cylinder head studs. Any of this group of steels may be used for forgings, the selection being governed by the section size and the physical properties desired after heat treatment. Thus, AISI 1030 and 1035 are used for shifter forks and many small forgings where moderate properties are desired, but the deeper-hardening AISI 1036 is used for more critical parts where a higher strength level and more uniformity are essential, such as some front-suspension parts. Forgings such as connecting rods, steering arms, truck front axles, axle shafts, and tractor wheels are commonly made from the AISI 1038 to 1045 group. Larger forgings at similar strength levels need more carbon and perhaps more manganese; for instance, crankshafts are made from AISI 1046 and 1052. These steels are also used for small forgings where  high hardness after oil quenching is desired. Suitable heat treatment is necessary on forgings from this group to provide machinability. It is also possible to weld these steels by most commercial methods, but precautions should be taken to avoid cracking from too rapid cooling.

 

 

High-carbon steels. These are the 14 AISI-SAE grades 1055, 1060, 1062, 1064, 1065, 1066, 1070, 1074, 1078, 1080, 1085, 1086, 1090, and 1095. These steels contain more carbon than is required to achieve maximum “as quenched” hardness. They are used for applications requiring improved wear resistance for cutting edges and to make springs. In general, cold forming cannot be used with these steels, and forming is only limited to flat stampings and springs coiled from small-diameter wire. Practically all parts from these steels are heat treated before use. Uses in the spring industry include AISI 1065 for pretempered wire and 1066 for cushion springs of hard-drawn wire; 1064 may be used for small washers and thin stamped parts, 1074 for light flat springs formed from annealed stock, and 1080 and 1085 for thicker flat springs. 1085 is also used for heavier coil springs. Finally, valve spring wire and music wire are special products.

 

Easily machinable carbon steels. The three AISI-SAE grades 1111, 1112, and 1113 are intended for applications where easy machining is the primary requirement. They are characterized by a higher sulfur content than comparable carbon steels, machinability improving within the group as sulfur increases but at the expense of cold-forming, weldability, and forging properties. In general, the uses are similar to those for carbon steels of similar carbon and manganese content. These steels are commonly known as Bessemer screw stock and are considered the best machining steels available. Although of excellent strength in the cold-drawn condition, they have an unfavorable property of cold shortness and are not commonly used for vital parts. These steels may be cyanided or carburized, but when uniform response to heat treating is necessary, open-hearth steels are recommended. The nine AISI-SAE grades 1109, 1114, 1115, 1116, 1117, 1118, 1119, 1120, and 1126 are used where a combination of good machinability and more uniform response to heat treatment is required. The lower-carbon varieties are used for small parts that are to be cyanided or carbonitrided. AISI 1116, 1117, 1118, and 1119 contain more manganese for better hardenability, permitting oil quenching after case-hardening heat treatments. The higher-carbon 1120 and 1126 provide more core hardness when this is needed. Finally, grades AISI-SAE 1132, 1137, 1138, 1140, 1141, 1144, 1145, 1146, and 1151 exhibit a composition similar to that of carbon steels of the same carbon level, except they have a higher sulfur content. They are widely used for parts where large amounts of machining are necessary or where threads, splines, or other contours present special problems with tooling. AISI 1137 is widely used for nuts and  bolts. The higher-manganese grades 1132, 1137, 1141, and 1144 offer greater hardenability,  the higher-carbon types being suitable for oil quenching for many parts. All these steels may be selectively hardened by induction or flame heating.

 

 

 

 

 

Steelmaking

Steel is The Iron that contain the Carbon lower than 1,7%, as figure 1.

Steel is very important for our life, more than 95% of the equipment is made of steel. In line with the increasing demand for steel, the main raw material requirements and support: Alloy, Refractory, Flux, Spare Parts, Energy etc. will also increase.

The material requirements for steelmaking can be seen as figure 2.

Figure 2. The Material Requirements of Steelmaking

 

 

 

The continous process of steelmaking can be seen as figure 3.

 

 



Figure 3. The Process of Steelmaking

 

 

The most important processes in the manufacture of steel is how to maintain the properties of steel produced. So, maintaining the composition by adjusting the impurity content is very important. Impurities are usually controlled, Sulfur, Phosphor, Silicon, as well as gas and other alloy material if used.

  

 

 

Desulfurization

 

In absence of carbon, silicon and aluminium in metal, the desulphurization reaction can be written as:

 

[S] + [Fe] + (CaO) = (CaS) + (FeO)

 

 

Sulphur distribution ratio will be higher if:

 

aCaO is high (higher basicity)

aFeO is low (reducing slag)

k‘ is high (higher temperature)

 

Figure 4. Melt and Slag in The Laddle ( From Dr. Ing. Zulfiadi Zulhan)

 

Figure 5. Bottom stirring of Desulphurization Process

 

 



Desiliconization

Silicon in metal is oxidized to SiO2, the reaction taking place is:

[Si] + 2[O] = (SiO2)

The partitioning between slag and metal will depend upond the activities of silicon and oxygen in metal and on the the activity of silica in slag

[Si] + 2[O] = (SiO2)

 

Dephosphorization

 

Phosphor in metal is oxidized to P2O5, the reaction taking place is:

2[P] + 5[O] = (P2O5)

The partitioning between slag and metal will depend upond the activities of Phosphorus Pentoxide and oxygen in metal and on the the activity of Phospor in slag

2[P] + 5[O] = (P2O5)

 

Desulphurization need Reductive Environment, but the Desiliconization and Dephosphorization need Oxidative Environment. Degassing is done with vacuum condition.

 

 



 





Cast Irons

Cast irons contain much higher carbon and silicon levels than steels, theoretically higher than 1.8 wt.% but typically 3 to 5 wt.% Fe and 1 to 3 wt.% Si. These comprise another category of ferrous materials that are intended to be cast from the liquid state to the final desired shape. Various types of cast irons are widely used in industry, especially for valves, pumps, pipes, filters, and certain mechanical parts. Cast iron can be considered a ternary Fe-Si-C alloy. The carbon concentration is between 1.7 and 4.5%, most of which is present in insoluble form (e.g., graphite flakes or nodules). Such material is, however, normally called unalloyed cast iron and exists in four main types:

(i) white iron, which is brittle and glass hard;

(ii) unalloyed gray iron, which is soft but still brittle and which is the most common form of unalloyed cast iron;
(iii) the more ductile malleable iron;

(iv) nodular or ductile cast iron, the best modern form of cast iron, which has superior mechanical properties and equivalent corrosion resistance.
 

In addition, there are a number of alloy cast irons, many of which have improved corrosion resistance and substantially modified mechanical and physical properties. Generally, cast iron is not a particularly strong or tough structural material. Nevertheless, it is one of the most economical and is widely used in industry. Its annual production is surpassed only by steel. Iron castings are used in many items of equipment in the chemical-process industry, but its main use is in mechanical engineering applications: automobile and machine tools. Some of the best known classes, listed below, include the high-silicon and nickel cast irons. 

• gray cast iron

• white cast iron• chilled iron (duplex) • malleable cast irons
• ductile or nodular cast irons

• alloy cast irons

• high-silicon cast irons
• nickel cast irons

Gray Cast Iron or Graphitic Iron
 

Gray cast irons contain 1.7 to 4.5 wt.% of C and other alloying elements such as Si, Mn, and Fe. Due to the slow cooling rate during casting, the carbon is precipitated as thin flakes of graphite dispersed throughout the metal. Therefore, gray cast irons are relatively brittle. Gray cast iron is the least expensive material, is quite soft, has excellent machinability, and is easy to cast into intricate shapes. Various strengths are produced by varying the size, amount, and distribution of the graphite. For instance, ultimate tensile strength typically ranges from 155 to 400 MPa and the Vickers hardness from 130 to 300 HV. Gray iron has excellent wear resistance and damping properties. However, it is both thermal and mechanical shock sensitive. Gray iron castings can be welded with proper techniques and adequate preheating of the components.
 

 

White Cast Iron

White cast iron is made by controlling the chemical composition (i.e., low Si, high Mn) and rate of solidification of the iron melt. Rapid cooling leads to an alloy that has practically all its carbon retained as dissolved cementite that is hard and devoid of ductility. The resulting cast is hard, brittle, and virtually unmachinable, and finishing must be achieved by grinding. Typically, the Vickers hardness ranges from 400 to 600 HV. Its main use is for wear- or abrasion-resistant applications. In this respect white irons are superior to manganese steel, unless deformation or shock resistance is required. The major applications of cast irons include pump impellers, slurry pumps, and crushing and grinding equipment.



Malleable Cast Irons

Malleable iron exhibits a typical carbon content of 2.5 wt.% C. It is made from white cast iron by prolonged heating of the casting. This causes the carbides to decompose, and graphite aggregates are produced in the form of dispersed compact rosettes (i.e., no flakes). This gives a tough, relatively ductile material. There are two main types of malleable iron, standard and pearlitic. The latter contains both combined carbon and graphite nodules. Standard malleable irons are easily machined. This is less so for pearlitic iron. All malleable cast irons withstand cold working and bending without cracking. 


Ductile (Nodular) Cast Irons 

This is the best modern form of cast iron as it has superior mechanical properties and equivalent corrosion resistance. Ductility is much improved and may approach that of steel. Ductile iron is sometimes also called nodular cast iron or spheroidal graphite cast iron, as the graphite particles are approximately spherical in shape, in contrast to the graphite flakes in gray cast iron. Ductile cast iron exhibits a typical microstructure. This is achieved by the addition of a small amount of nickel-magnesium alloy or by inoculating the molten metal with magnesium or cerium. Furthermore, composition is about the same as gray iron, with some nickel, and with more carbon (3.7 wt.% C) than malleable iron. There are a number of grades of ductile iron. Some have maximum machinability and toughness, others have maximum oxidation resistance. Ductile iron castings can also be produced to have improved low-temperature impact properties. This is achieved by adequate thermal treatment, by control of the phosphorus and silicon content, and by various alloying processes.

 
High-Silicon Cast Irons
Cast irons with a high silicon level (i.e., 13 to 16 wt.% Si), which are called Duriron, exhibit,  for all concentrations of H2SO4, even up to the boiling point, a constant corrosion rate of  130 μm/year (i.e., 5 mpy). For these reasons, it is widely used in sulfuric acid service. Duriron is a cheap material that does not contain any amount of strategic metal. Nevertheless, it is very hard and brittle and thermal shock sensitive, so it is not readily machined or welded.

 

 

 

  

The Iron-Carbon (Fe-C) and Iron-Cementite (Fe-Fe3C) Systems

Because carbon is a ubiquitous element in both iron- and steelmaking processes due to its essential use as a reductant during the extractive process of iron from its ores, carbon has   a predominant role in  siderurgy  (i.e., the metallurgy of iron and its alloys). Although other  alloying elements may be added to produce steels for special purposes, usually the structure of iron and steels is determined first by the content of carbon, secondly by the type of other alloying elements, and finally by the rate of cooling from the molten state. For all the above reasons, a solid grasp of the iron-carbon system is a mandatory step for understanding iron and iron alloys (i.e., steels and cast irons). As for the phase diagram of pure iron, the major phases occuring in the Fe-C phase diagram (Figure 1) can be accurately characterized by means of X-ray diffraction, thermal analysis, and dilatometry techniques. In practice, the iron-carbon phase diagram is a graphical plot of phases existing in thermodynamic equilibrium as a function of temperature versus the mass fraction of total carbon in the iron. The diagram depicted in this book is only a detail of the entire diagram. Actually, the phase diagram extends on the abscissa axis at left from pure iron free of carbon to a content of total carbon reaching 6.70 wt.% C that corresponds to the theoretical composition of iron carbide or cementite (Fe3C), while temperatures range from 200°C to 1600°C, the temperature at  which the system is fully liquid. The binary phase diagram exhibits, in addition to the four  critical points of the allotropes of pure iron, three other important characteristics:

 

(i) a eutectic point at 4.30 wt.% C and 1148°C;

(ii) a eutectoid point at 0.77 wt.% C and 727°C;

(iii) a peritectic transformation occurring at 1495°C.

 

 

 

Moreover, experimentally the following solid phases were identified.

 

Alpha-ferrite (α-ferrite, bcc). Sensu stricto and historically, ferrite consists of a solid solution of carbon inside a body-centered cubic crystal lattice in alpha-iron. The solubility of carbon in alpha-iron is extremely low, ca. 0.01 wt.% C at ambient temperature, and reaches only 0.025 wt.% C at 723oC. Therefore, at room temperature under conditions of equilibrium, any carbon present in excess of that small amount will exsolute in the form of cementite. Due to this low carbon content, some textbooks treat the ferrite phase substantially as pure iron, but this view must be discontinued to avoid confusion.  Usually, the ferrite of an alloyed steel may contain in solid solution appreciable amounts of other elements; ab extenso, any solid solution of which alpha-iron is the solvent is called ferrite (i.e., a solid solution of any element in alpha-iron). Alloying elements that stabilize ferrite are listed in Table 1.
 

 

 

Beta-ferrite (b-ferrite, bcc). Like alpha-ferrite, beta-ferrite consists of a solid solution of  any element in body-centered cubic beta iron.

Delta-ferrite (δ-ferrite, bcc).
Like alpha-ferrite, delta-ferrite consists of a solid solution of  any element in body-centered cubic delta iron. In the case of carbon, its maximum solubility in delta-iron is only 0.1 wt.% at 1487°C.
Gamma-austenite (g-austenite, fcc). Austenite is a solid insertion solution of carbon into the crystal lattice of face-centered cubic gamma-iron. It has been definitively established that the carbon atoms in austenite occupy interstitial positions in the face-centered cubic space lattice causing the lattice parameter to increase progressively with the carbon content. Cementite. Cementite is an iron carbide with the chemical formula Fe3 C. At room temperature,   cementite is a hard, brittle, and ferromagnetic material with a Curie temperature of   210°C. It is formed by chemical reaction between iron and excess carbon. Three distinct origins must, however, be distinguished:

(i) primary cementite resulting from the separation during solidification of liquid iron with carbon content ranging between 4.3 wt.% and 6.69 wt.% C;

(ii) secondary cementite formed after demixion of carbon as a result of a decrease in miscibility during the cooling of ferrite;

(iii) tertiary cementite resulting from demixion during the cooling of austenite. Actually, at

room temperature under conditions of equilibrium, any carbon present in excess of that small amount must exist in a form other than that of a solute in a solid solution.

Perlite. A biphasic eutectoidic constituent that consists of an interlamellar growth of ferrite and cementite. Perlite is formed during the transformation of austenite with a eutectoid composition (i.e., 0.77 wt.% C).

 

Ledeburite. A biphasic eutectic constituent resulting from the solidification of a molten metal having a eutectic composition. Hence it consists of an austenite containing 1.7 wt.% C in solid solution and cementite.

 

In the phase diagrams in Figures 2.1 and 2.2, the transition temperatures or critical points   previously identified for the four iron allotropes must now be replaced by two temperature  limits or points. Actually, due to hysteresis phenomena occurring upon heating and cooling, the equilibrium curves are greatly influenced by the rate of cooling and heating, and they form distinct plots. The various temperatures at which pauses occur in the rise or fall of  temperatures when iron or steel is heated from room temperature or cooled from the molten  state are called arrest points, denoted by uppercase letter A. Due to the previously mentioned hysteresis behavior during heating and cooling, the arrest points obtained on heating   are denoted Ac and those obtained on cooling are denoted Ar,  while arrest points at equilibrium are denoted Ae. Historically, the subscripts c , r, and e were derived from the first letters  of the French words chauffage (heating), refroidissement (cooling), and équilibre (equilibrium),  respectively. These arrest points are described in detail in Table 2.

 

 

From the iron-carbon phase diagram several important characteristics regarding the classification   of iron and iron alloys can be seen. Iron alloys are classified according to their total content of carbon. Steel are particular iron alloys having a carbon content below 2.1 wt.% C.  Above this limit, we have cast irons up to a practical limit of 3.75 wt.% C. A steel containing  0.77 wt.% C is called a eutectoid steel.  Eutectoid steel consists of an intimate mixture of alpha- ferrite and cementite forming an intergrowth of thin plates or lamellae known as perlite.  Therefore, a steel having a carbon content below 0.77 wt.% C is called a hypotectoid steel. Its  structure consists of a small amount of pearlite with an excess of alpha-ferrite, which collectsat the grain boundaries. Hypotectoid steels are hence softer and more ductile than eutectoid steels. On the other hand, a steel with more than 0.77 wt.% C is called a hypertectoid steel. Its   structure consists of pearlite with an excess of cementite. Hypertectoid steels are harder,  more brittle, and les ductile than eutectoid steels. Above 2.11 wt.% C, molten iron solidifies   always below 1350°C and the resulting low liquidus temperature iron alloys are called cast irons due to their ease of melting. The eutectic point in the Fe-C diagram is located at  4.3 wt.% C. At this composition, when the alloy solidifies, it forms a mixture of austenite  with 1.7 wt.% C in solid solution and cementite; this eutectic structure is called ledeburite. In  practice, cast irons exhibit a carbon content ranging between 2.11 and 3.75 wt.%. Upon cooling, cast irons exhibit a mixture of pearlite and cementite.

 

 

 

The iron-carbon phase diagram only applies to alloys that contain only iron and carbon. But because other desired or undesired alloying elements are usually originally present from the ironmaking process (e.g., O, C, Si, P, Mn, V) or are added intentionally (e.g., Ni, Cr, Mo), during steelmaking the iron-carbon phase diagram cannot show accurately the conditions that apply to actual steels. Hence it has to be modified appropriately to take into account the effect of the elements. These additional elements impact both the arrest points and equilibrium lines, and they also determine the existence or not of certain phases. Alloying elements with their related impact on iron-carbon phases are listed in Table 3.