Iron and Steels

Abstract

Whereas the fundamental properties of all metallic elements are covered systematically and comprehensively in Chap. 4, this chapter treats iron as both a base and an alloying element of metallic materials. The properties of metallic materials depend sensitively not only on their chemical composition and on the electronic and crystal structure of the phases formed, but also to a large degree on their microstructure, including the kind and distribution of lattice defects. The phase composition and microstructure of iron is strongly dependent, in turn, on the thermal and mechanical treatments, which are applied under well-controlled conditions to achieve the desired properties.

According to the complexity of the interrelations between the fundamental (intrinsic) and microstructure-dependent (extrinsic) properties of iron, this chapter provides a substantial amount of explanatory text.

Iron is technically the most versatile and economically the most important base metal for a great variety of structural and magnetic materials, most of which are called steels. With increasing temperature, iron undergoes two structural phase transitions,

$$\alpha{\text{ (bcc)}}\leftrightarrow\gamma\text{ (fcc)}\text{ at }{\mathrm{911}}\,{\mathrm{{}^{\circ}\mathrm{C}}}$$

and

$$\gamma\text{ (fcc)}\leftrightarrow\delta\text{ (bcc)}\text{ at }{\mathrm{1392}}\,{\mathrm{{}^{\circ}\mathrm{C}}}\;,$$

and a magnetic phase transition,

$$\alpha_{\text{ferromagnetic}}\leftrightarrow\alpha_{\text{paramagnetic}}\,{\text{at}\,{\mathrm{769}}\,{\mathrm{{}^{\circ}\mathrm{C}}}}\;,$$

at ambient pressure. At elevated pressure levels, Fe forms a third structural phase ε (hcp). These phasetransitions, their variation upon alloying, and the concomitant phase transformations are the thermodynamic, structural and microstructural basis for the unique variety of iron-based alloys and of their properties [9.1, 9.2, 9.3, 9.4, 9.5].

The wide variety of standards for steels which have developed from national standards and efforts of international standardization are compiled in [9.6]. In the present section we are using different standard designations as provided by the sources used. In the following sections, the SAE (Society of Automotive Engineers), AISI (American Iron and Steel Institute) and GlossaryTerm

UNS

(unified numbering system) designations are the dominating ones.

1 Phase Relations and Phase Transformations

1.1 Iron-Carbon Alloys

The most frequent alloying element of iron is carbon. The Fe-C phase diagram (Fig. 9.1) shows the important metastable phase equilibria involving the metastable carbide Fe₃C, called cementite, in solid gray lines, whereas the stable equlibria with graphite C are shown in solid brown lines. The formation of Fe₃C predominates in most carbon and low-alloy steels because the activation energy of its nucleation is considerably lower than that of graphite. At higher carbon contents (2.5 − 4.0 wt% C) and in the presence of Si (1.0 − 3.0 wt% Si), graphite formation is favored. This is the basis of alloying and microstructure of gray cast iron (Sect. 9.7).

Fig. 9.1

Fe-C phase diagram. Metastable equilibria involving cementite Fe₃C are shown as solid gray lines, stable equilibria with graphite C are shown as solid brown lines (after [9.7]) (dashed brown lines – Curie temperature)

The Fe-C phase forms interstitial solid solutions of α- and γ-Fe. The solid solution phase of α-Fe is called ferrite, the solid solution phase based on γ-Fe is called austenite in the binary Fe-C system. These terms for the solid solutions phases of α- and γ-Fe are applied to all other Fe-based alloy systems as well. Since phase transformations induced by cooling from the austenite phase field play a major role to induce particular microstructures and properties, some resulting microstructures have also been given particular terms and form the basis of the nomenclature in steels. Cooling of Fe-C alloys from the austenite phase field can lead to three different phase transformations below the eutectoid temperature of 1009 K (736^∘C). Their kinetics of formation depends on composition and cooling rate. The transformation products are:

• Pearlite , a lamellar product of ferrite and cementite. It is formed by a discontinuous (pearlitic) transformation, i. e., both phases are formed side by side in the reaction front. The lamellar spacing decreases with decreasing temperature of formation. The maximum rate of formation occurs at about 500^∘C.

• Bainite , a plate- or spearhead-shaped product consisting of a ferrite matrix in which carbide particles are dispersed. The bainitic transformation mechanism depends sensitively on alloy composition and the temperature of transformation, yielding essentially two microstructural variants. A somewhat coarser transformation product formed at about 450^∘C is called upper bainite and a finer transformation product formed at about 350^∘C is termed lower bainite.

• Martensite , a plate-shaped product formed by a diffusionless, athermal transformation. Thermodynamically it is a metastable ferrite, designated as α^′ and supersatured in carbon. But by the displacive mechanism of the transformation, the distribution of the C atoms in the martensite lattice is anisotropic such that is has a body-centered tetragonal crystal structure and its c and a parameters vary with the C content accordingly (Fig. 9.2). The temperature below which martensite begins to form upon queching is termed martensite start temperature M_s and depends strongly on the C concentration (Fig. 9.3). M_f designates the temperature at which the transformation is complete. In order to promote the diffusionless martensitic transformation, the diffusion-dependent transformations to pearlite and bainite have to be suppressed by rapid cooling, usually termed quenching.

Since martensite formation is used as a main hardening mechanism in steels, the hardenability is a main concern of alloy design and consequence of alloy composition. The lower the rate of formation of the diffusion-dependent transformations, the higher is the fraction of martensite formed upon cooling from the austenite range, i. e., the hardenability (Sect. 9.2). The rates of pearlite and bainite formation are reduced by alloying with carbon and all substitutional alloying elements except by Co. But the decrease of M_s with increasing alloy content has, also, to be taken into account.

Fig. 9.2

Lattice parameters of Fe-C martensite as a function of composition (after [9.7])

Fig. 9.3

Concentration dependence of the martensite transformation temperatures. M_s – martensite start; M_f – martensite finish, i. e., austenite is transformed completely

Fig. 9.4

Fe-Ni phase diagram. T_C – Curie temperature (after [9.7])

Subsequent heat treatment of the phases formed is termed annealing with regard to ferrite and bainite, and tempering with regard to martensite. These heat treatments play a major role in optimizing the microstructure to obtain specific properties. Upon subsequent heat treatment the transformation products listed above undergo the following reactions:

• Pearlite is coarsened by the transition of the cementite lamellae into spherical particles, thus reducing the interfacial free energy per unit volume. The process is called spheroidization and consequently the resulting microstructural constituent is termed spheroidite .

• Bainite is coarsened as well both by recovery of the ferrite plates and by coarsening of the carbide particles.

• Martensite is essentially transformed into bcc ferrite by the precipitation of carbide particles during tempering. The tempering treatment usually leads to the precipitation of metastable carbides from the martensite phase. Different metastable carbides may be formed depending on alloy composition (including substitutional alloying elements), temperature, and time of annealing. A compilation of all metastable carbides occurring in Fe-C(-X) alloys is given in Table 9.1.

Table 9.1 Metastable and stable carbide phases occurring in the Fe-C(-X) alloy system (after [9.7])

1.2 Heat Treatments

The heat treatments referred to above need to be specified rather succinctly such that they can be correlated with the ensuing microstructures and properties. Furthermore, the specifications of heat treatments require taking the cross section and form of the part to be heat treated into account (at least if the cross sections get larger than, say, 0.5 mm). The finite thermal conductivity and the heat capacity of the material will cause any temperature change applied to the surface to occur at a decreasing rate with increasing depth in the heat-treated part. Thus, not only the time and temperature of an isothermal treatment but also the rate of cooling or the rate of heating are common parameters to be specified. Beyond those referred to above, the following treatments are widely applied to steels.

1.2.1 Austenitizing

Heating to and holding in the range of the austenite phase is commonly the first stage of transformation heat treatments. The higher the austenitizing temperature, the more lattice defects such as dislocations and grain boundaries are annihilated. This lowers the rate of nucleation of subsequent phase transformations.

1.2.2 Soft Annealing

This term is used for heat treatment of hardenable steels containing ≥ 0.4 wt% C at temperatures closely below the eutectoid temperature for a duration of ≤ 100 h. It results in a microstructure of coarse grained, ductile ferrite, and coarsened cementite.

1.2.3 Normalizing

This heat treatment is applied to obtain a uniform, fine-grained microstructure. The first step consists of heating the metal rapidly to, and holding it at a temperature 30 − 50 K above the $(\alpha+\gamma)/\gamma$ phase boundary (also referred to as the A_c3 line) for hypo-eutectoid steels, and heating rapidly to and holding at about 50 K above the eutectoid temperature (also referred to as A_c1 line). This step results in the formation of a fairly fine-grained austenite and ferrite structure in the hypo-eutectoid and in a fine-grained austenite with coagulated grain boundary cementite in the hyper-eutectoid compositions. Upon cooling, the austenite transforms into pearlite and this microstructural state has a favorable combination of strength, ductility, and machinability.

1.3 Substitutional Iron-Based Alloys

For the phase diagrams with substitutional alloying components shown in later sections, a major aspect pertaining to both the binary alloys shown and the steels alloyed with these components is whether the α or the γ phase of Fe is stabilized. i. e., which phase field is expanded or contracted upon alloying.

1.3.1 Fe-Ni

Figure 9.4 shows the Fe-Ni phase equilibria indicating that Ni stabilizes the fcc γ phase. If Fe-rich alloys are quenched from the γ phase field they transform martensitically to bcc α^′ martensite. The transformation temperatures are shown in Fig. 9.5. The Fe-Ni phase diagram is particularly relevant for the controlled thermal expansion and constant-modulus alloys as well as for the soft magnetic Fe-Ni based materials at higher Ni contents. These, in turn, derive their magnetic properties in part from the occurrence of the superlattice phase FeNi₃.

Fig. 9.5

Martensitic transformation temperatures of Fe-rich Fe-Ni alloys . The reverse transformation is characterized by the A_s (austenite start) and A_f (austenite finish) temperatures (after [9.7])

Fig. 9.6

Fe-Mn phase diagram (after [9.7])

Nickel is added to Fe-C alloys to increase the hardenability and to increase the yield strength of ferrite by solid solution hardening.

1.3.2 Fe-Mn

Figure 9.6 shows the Fe-Mn phase equilibria indicating that Mn is stabilizing the fcc γ phase similar to Ni. It should be noted that quenching Fe-rich alloys from the γ-phase field leads to two different martensitic transformations which may result in a bcc structure (α^′ martensite) or an hcp structure (ε^′ martensite). The transformation temperatures are shown in Fig. 9.7 . The martensitic transformation can also be induced by deformation. This property is exploited in the design of wear-resistant steels (Hatfield steel: 12 wt% Mn, 1 wt% C).

Fig. 9.7

Martensitic transformation temperatures of Fe-rich Fe-Mn alloys . The superscripts indicate the transforming phases (after [9.7])

Fig. 9.8

Fe-Cr phase diagram (after [9.7])

Manganese is contained in practically all commercial steels because it is used for deoxidation of the melt. Typical contents are 0.3 − 0.9 wt% Mn. Manganese increases the hardenability of steels and contributes moderately to the yield strength by solid solution hardening.

1.3.3 Fe-Cr

The Fe-Cr phase diagram, Fig. 9.8, is the prototype of the case of an iron-based system with an α-phase stabilizing component. Chromium is the most important alloying element of corrosion resistant, ferritic stainless steels and ferritic heat-resistant steels. If α-Fe-Cr alloys are quenched from above 1105 K and subsequently annealed, they decompose according to a metastable miscibility gap shown in Fig. 9.9. This decomposition reaction can cause severe embrittlement which is called 475^∘C-embrittlement in ferritic chromium steels. Embrittlement can also occur upon formation of the σ phase.

Fig. 9.9

Metastable miscibility gap in the Fe-Cr alloy system (after [9.7])

Fig. 9.10

Fe-Si phase diagram (after [9.7])

In carbon steels, Cr is added to increase corrosion and oxidation resistance because it promotes the formation of stable passivating and protective oxide layers. Moreover, Cr is a strong carbide former which modifies and delays the formation of pearlite and bainite, thus increasing the hardenability. In heat-resistant steels Cr contributes to the high-temperature yield strength.

1.3.4 Fe-Si

The phase diagram Fe-Si, Fig. 9.10, shows that Si is a strong ferrite former. The main application of binary Fe-Si alloys is in the form of steels with ≤ 3.5 wt% Si which have an optimum combination of high magnetic moment, low magnetostriction, and low magnetocrystalline anisotropy such that they are the ideal material for high induction and low magnetic power loss applications such as power transformers. Data are given in Chap. 21.

In Fe-C steelmaking, Si is one of the principal deoxidizers. It may amount to 0.05 − 0.3 wt% Si in the steel depending on the deoxidizing treatment and the amount of other deoxidants used. At these levels of concentration Si contributes only moderately to the strength of ferrite and causes no significant loss of ductility.

2 Carbon and Low-Alloy Steels

The largest group of steels produced both by number of variants and by volume is that of carbon and low-alloy steels. It is characterized by the fact that most of the phase relations and phase transformations may be referred to the binary Fe-C phase diagram or comparatively small deviations from it. These steels are treated extensively in [9.5].

2.1 Compositions and Properties of Carbon Steels

According to the effect of carbon concentration on the phases formed and on their properties, Fig. 9.11 shows the variation of the effective average mechanical properties of as-rolled 25 mm bars of plain carbon steels as an approximate survey of the typical concentration dependence.

Fig. 9.11

Variations in average mechanical properties of as-rolled 25 mm diameter bars of plain carbon steels as a function of carbon content (after [9.5])

Carbon steels are defined as containing up to 1 wt% C and a total of 2 wt% alloying elements. Apart from the deoxidizing alloying elements Mn and Si, two impurity elements are always present in carbon steels: phosphorous and sulfur. Phosphorus increases strength and hardness significantly by solid solution hardening, but severely decreases ductility and toughness. Only in exceptional cases may P be added deliberately to increase machinability and corrosion resistance. Sulfur has essentially no effect on the strength properties since it is practically insoluble in ferrite. However, it decreases the ductility and fracture toughness. But S is added deliberately along with an increased Mn content to promote the formation of MnS. This compound is formed in small particles which are comparatively soft and serve as effective chip breakers in free-cutting steel grades, thus increasing machinability. On the basis of these effects of the most common alloying and impurity elements, carbon steel compositions are specified as listed in Table 9.2 and free-cutting carbon steel compositions are specified as listed in Table 9.3.

Table 9.2 Standard carbon steel compositions applicable to semi-finished products for forging, hot-rolled and cold-finished bars, wire rods, and seamless tubing (after [9.5]). Selected grades

Table 9.3 Standard free-cutting (re-sulfurized) carbon steel compositions applicable to semi-finished products for forging, hot-rolled and cold-finished bars, and seamless tubing (after [9.5])

A survey of the alloying elements used and of the ranges of composition applied in carbon and low-alloy steels may be gained from the SAE-AISI system of designations for carbon and alloy steels listed in Table 9.4. Extensive cross references to other standards may be found in [9.6].

Turning to the mechanical properties, it should be emphasized that the microstructure has a decisive influence on the properties of all steels. Therefore the composition and the prior thermal, mechanical, or thermomechanical treatments which determine the phase transformations and ensuing microstructural state of a steel will always have to be taken into account. Accordingly, tabulated property data will invariably be given with reference to mechanical and thermal treatments applied. The terms used and their specific definitions are outlined in Sect. 9.1.

In plain carbon steels the C content and microstructure are determining the mechanical properties. Manganese is providing moderate solid solution strengthening and increases the hardenability. The properties of plain carbon steels are also affected by the other common residual elements Si, P, and S. Furthermore, the gasses O, N, and H and their reaction products may play a role. Their content depends largely on the melting, deoxidizing and pouring practice. While Fig. 9.11 illustrates the general effect of C content on the mechanical properties if the austenite grain size and transformation microstructure are held essentially constant. Tables 9.5 and 9.6 list the mechanical properties of representative carbon and low alloy steels in specified states as a function of deformation and heat treatment.

2.2 Hardenability

Hardening of steels is the heat treatment consisting of heating to the range of austenite, cooling in water, oil, or air, and subsequent tempering. The term hardenability refers to the suitability of a steel to be transformed partially or completely from austenite to martensite to a specified depth below the free surface of a workpiece when cooled under specified conditions. This definition reflects that the term hardenability does not only refer to the magnitude of hardness which can be attained for a particular steel, but it relates the extent of hardening achievable to the macroscopic or local cooling rate or isothermal holding time in the transformation range and, thus, to the mechanisms, kinetic phase transformations, and their effects on the mechanical properties.

Table 9.4 SAE-AISI system of designations for carbon and low-alloy steels (after [9.5])

Table 9.5 Mechanical properties of selected carbon and low-alloy steels in the hot-rolled, normalized, and annealed conditions (after [9.5])

The amount of martensite formed upon cooling is a function of C content and total steel composition. This behavior is due to two decisive factors of influence:

• The temperature range of transformation of austenite to martensite depends on the C content, as shown in Fig. 9.3 of Sect. 9.1.

• The diffusion-dependent transformations of austenite (to ferrite, pearlite, and bainite) compete with the martensitic transformation such that the volume fraction available for the latter will decrease as the volume transformed by the former increases. This transformation kinetics of the diffusional phase transformations is strongly dependent on alloy composition.

The lower the cooling rate may be while still permitting one to obtain a high fraction of martensite, the higher the hardenability of steel. This can best be tested by varying the cooling rate and analyzing the resulting microstructure and its hardness (and other mechanical properties). The method used most widely is an ingeniously simple testing procedure, the Jominy end-quench test illustrated in Fig. 9.12. The material to be tested is normalized and a test bar 100 mm long and 25 mm in diameter is machined. This specimen is austenitized and transferred to the fixture (shown in Fig. 9.12) holding it vertically 13 mm above the nozzle through which water is directed against the bottom face of the specimen. While the bottom end is quenched by water, the top end is slowly cooled in air and intermediate cooling rates occur at intermediate positions. After the test, hardness readings and microstructural analyses may be taken along the bar. They can be correlated to the pre-determined approximate cooling rate at a given temperature, as shown for a characteristic example in Fig. 9.13.

Fig. 9.12

Jominy end-quench apparatus

Fig. 9.13

Plot of end-quench hardenability data of an AISI 8650 steel (0.49 wt% C, 0.98 wt% Mn, 0.29 wt% Si, 0.59 wt% Ni, 0.47 wt% Cr, 0.19 wt% Mo) (after [9.5])

The hardenability of numerous commonly used steels has been characterised quantitatively by extensive investigations of their transformation behavior as a function of temperature and time. These investigations have been carried out in two modes of heat treatment:

• Quenching from the austenite range to a temperature in the transformation range at which the specimen is held isothermally in a salt or a lead bath for different times, and finally quenched to room temperature to be investigated regarding the transformation products by microstructural and supplementary measurements. The resulting plots of the beginning of formation of the different transformation products (sometimes including the fractional amounts and end of transformation) are termed time-temperature-transformation (TTT) diagram or isothermal transformation (GlossaryTerm

  IT

  ) diagrams. An example is shown as Fig. 9.14.

• Cooling from the austenite range through the transformation range at different cooling rates, and investigating the temperature of onset of transformation of the different products for different cooling rates by microstructural and supplementary measurements, e. g., dilatometry. The resulting plot is called a continuous-cooling-transformation (GlossaryTerm

  CCT

  ) diagram. An example is shown in Fig. 9.15. The critical cooling rate means the lowest rate for which a fully martensitic state can be obtained.

Apart from empirical determinations of these transformation diagrams, methods of prediction based on nucleation theory and phenomenological growth theory using the Johnson–Mehl–Avrami equation have been devised to estimate TTT diagrams [9.8].

The hardenability increases with increasing carbon and metallic alloy element concentration (with the exception of Co). The transformation kinetics and ensuing hardenability properties are documented extensively in compilations of TTT and CCT diagrams such as [9.9].

Table 9.6 Mechanical properties of selected carbon and alloy steels in the quenched-and-tempered condition, heat treated as 25 mm rounds (after [9.5])

Fig. 9.14

Time-temperature-transformation (TTT ) diagram of a 4130 grade low-alloy steel. A – austenite; F – ferrite; C – cementite; M – Martensite (the suffix indicates the amount of martensite formed in vol.%)

Fig. 9.15

Continuous-cooling-transformation (CCT) diagram for a 4130 grade low-alloy steel. A_c3 and A_c1 signify the temperatures of the $\gamma/(\gamma+\alpha)$ and eutectoid reation, respectively. A – austenite, F – ferrite, B – bainite, P – pearlite, M – martensite. The cooling rate is measured at 705^∘C. The calculated critical cooling rate is 143 K s⁻¹ (after [9.5])

3 High-Strength Low-Alloy Steels

High-strength low-alloy (GlossaryTerm

HSLA

) steels are designed to provide higher mechanical property values and/or higher resistance to atmospheric corrosion than conventional low-alloy steels of comparable level of alloy content. Higher yield stress is achieved by adding ≤ 0.1 wt% N, Nb, V, Ti, and/or Zr (micro-alloying) which form carbide or carbonitrite precipitates, and by special, closely-controlled processing which yields mostly fine-grained microstructures.

HSLA steels contain 0.05 − 0.25 wt% C, ≤ 2 wt% Mn and mainly Cr, Ni, Mo, and Cu as further alloying elements. Their yield stress is in the range ≥ 275 MPa. They are primarily hot-rolled into usual wrought product forms and commonly delivered in the as-rolled condition.

The particular processing methods of HSLA steels include [9.5]:

• Controlled rolling of micro-alloyed, precipitation hardening variants to obtain fine equiaxial and/or highly deformed, pancake-shaped austenite grains. During cooling these austenite grains transform into fine ferrite grains, providing an optimum combination of high yield strength and ductility.

• Accelerated cooling of controlled-rolled steels to enhance the formation of fine ferrite grains.

• Quenching of steels containing ≤ 0.08 wt% C such that acicular ferrite or low-carbon bainite is formed. This microstructural state provides an excellent combination of high yield strengths of 275 − 690 MPa, ductility, formability, and weldability.

• Normalizing of V-alloyed steel, thus increasing yield strength and ductility.

• Intercritical annealing , i. e., annealing in the γ + α phase field to obtain a dual-phase microstructure which, after cooling, consists of martensite islands dispersed in a ferrite matrix. This microstructure exhibits a somewhat lower yield strength but a high rate of work-hardening, providing a better combination of tensile strength, ductility, and formability than conventional HSLA steels.

HSLA steels include numerous standard and proprietary grades designed to provide specific desirable combinations of properties such as strength, ductility, weldability, and atmospheric corrosion resistance. Table 9.7 lists characteristic compositions and Table 9.8 lists mechanical properties of these characteristic variants.

In view of the multitude of compositional and processing variants of HSLA steels it is useful to have a summarizing overview as provided in Table 9.9.

Table 9.7 Compositional limits of HSLA steel grades according to ASTM standards [9.5]

Table 9.8 Tensile properties of HSLA steel grades specified in ASTM standards [9.5]

Table 9.9 Summary of characteristics and uses of HSLA steels according to ASTM standards [9.5]

4 Stainless Steels

Stainless steels are treated extensively in [9.4]. Compared to carbon or low-alloy steels, they are characterized by an increased resistance against corrosion in aggressive media. The corrosion resistance is achieved basically by an alloy content of at least 11 − 12 wt% Cr. This content is required to form a dense, pore-free protective surface layer consisting mainly of chromium oxides and hydroxides. The corrosion resistance can be further increased by additional alloying with elements such as Ni, Mo, W, Mn, Si, Cu, Co, Al, or N.

Since chromium has a high affinity to carbon, the formation of chromium carbides may reduce the local concentration of Cr in solution and thus deteriorate the corrosion resistance. This can be avoided by:

• Low carbon content of the steel

• Suitable heat treatment

• Bonding of carbon by other elements with higher carbon affinity, such as Ti and Nb, so-called stabilization.

Similar effects can be attained by the formation of chromium nitrides. Thus, in addition to the chemical composition of the steel, its corrosion properties are strongly influenced by its heat treatment condition.

Depending on the intended field of application, the corrosion resistance of the steel must often be combined with other useful properties such as high strength or hardness, high temperature strength, good formability, low temperature fracture toughness, weldability or machinability. However, since optimization of one property is generally only possible at the expense of others, the property spectrum of a stainless steel is often the result of a compromise. Consequently, a large number of steel grades has been developed to meet different property requirements. Some steels have been developed just for a single application. The following section gives typical representatives of the various types of stainless steel grades.

Depending on alloy composition and cooling conditions from elevated temperature, stainless steels may occur in different types of microstructure:

• Ferritic (bcc)

• Austenitic (fcc)

• Martensitic

• Mixtures of two or all three of these phases.

The bcc structure is promoted by the ferrite forming elements Cr, Mo, W, Ti, V, Nb, Al, and Si, whereas the fcc structure is promoted by the austenite forming elements C, N, Ni, Mn, Cu, and Co. All alloying elements suppress the austenite to martensite transformation by reducing the M_s temperature, so that the steel may remain fcc at and below room temperature at a sufficiently high alloy content. For a rough estimate of the structural components of a stainless steel as a function of alloy composition, the so-called Schaeffler diagram (initially determined by M. Strauss and E. Maurer in 1920) can be used (Fig. 9.16) [9.2]. It relates the equivalent Cr and Ni content to the observed fractions of martensite, austenite, and ferrite. One of the formulas of the Ni and Cr equivalent used most frequently is that of Schneider [9.10] (in wt%)

$$\begin{aligned}\text{Cr}_{\text{equ}}&=\text{Cr}+2\,\text{Si}+1.5\,\text{Mo}+5\,\text{V}\\ &\quad\,+5.5\,\text{Al}+1.5\,\text{Ti}+0.7\,\text{W}\;,\end{aligned}$$

(9.1)

$$\begin{aligned}\text{Ni}_{\text{equ}}&=\text{Ni}+\text{Co}+0.5\,\text{Mn}+0.3\,\text{Cu}\\ &\quad\,+25\,\text{N}+30\,\text{C}\;.\end{aligned}$$

(9.2)

Fig. 9.16

Structure of iron-based alloys as a function of the concentrations of the chromium and nickel equivalent elements. A: austenite (fcc); F: ferrite (bcc); M: martensite (tetragonal-bcc) (after [9.2])

4.1 Ferritic Chromium Steels

High chromium (≥ 18 wt% Cr) and low carbon concentrations result in a fully ferritic structure of the steels at all temperatures, i. e., with a bcc delta ferrite structure and no phase transformations. Therefore these steels cannot be strengthened by quenching and tempering. The possibility to increase strength by cold deformation is limited since it decreases ductility and toughness. Steels containing < 18 wt% Cr form some austenite during heating which can be transformed into martensite by fast cooling, thus strengthening the steel. A tempering treatment just below A_r1 results in a mixture of δ-ferrite, α-ferrite, and carbides formed from tempered martensite.

Fig. 9.17

Notched bar impact energy as a function of test temperature of various types of stainless steels (DVM samples: dimension $10\times 10\times{\mathrm{55}}\,{\mathrm{mm^{3}}}$, notch depth 3 mm, notch root radius 1 mm)

In general the toughness of the conventional stainless ferritic chromium steels is not very high, the impact transition temperature often being at or above room temperature and reaching 100^∘C after welding (see curve Cr-Ferrite in Fig. 9.17 ). This is due to (i) the tendency of the ferritic stainless steels to pronounced grain coarsing on heat treatments, leading to a relatively large grain size (which cannot be refined by phase transformation), and (ii) the precipitation of chromium carbides at the grain boundaries. Both effects render the steel rather brittle after welding. Furthermore, precipitation of chromium carbides at the grain boundaries causes susceptibility to intergranular corrosion due to the formation of zones with local chromium depletion along the grain boundaries. Some improvement can be achieved (i) by quenching or a suitable diffusion heat treatment for equilibration of the chromium distribution, and (ii) by a stabilization of the steels with small amounts of Ti and/or Nb which are added to bind the carbon and nitrogen in more stable compounds. In unstabilized steels the carbon content is generally restricted to ≤ 0.1 wt%. Up to 2 wt% Mo improves the corrosion resistance of ferritic chromium steels, especially in chloride-containing media.

In Table 9.10 the chemical composition of a number of ferritic stainless steels is given. The most common representative is the grade X6Cr17/AISI 430. Table 9.11 presents some information on hot deformation and recommended heat treatment parameters. Tables 9.12 and 9.13 show typical data of the mechanical and physical properties, respectively, and Table 9.14 reviews the weldability of these steels. With respect to their magnetic behavior, in the annealed state the ferritic stainless steels can be considered in general to be soft magnetic materials, suitable e. g., for automotive applications.

Good ductility and toughness at sub-zero temperatures can be achieved when the carbon and nitrogen content in ferritic steels is reduced to very low concentrations (below 100 ppm) (see curve Cr-superferrite in Fig. 9.17). With higher alloy concentrations in the order of 26 − 30 wt% Cr, up to 2 wt% Mo, and up to 4 wt% Ni, these so-called superferritic steels exhibit excellent corrosion properties, i. e., high resistance against transcrystalline stress corrosion cracking and chloride-induced pitting corrosion, as well as intercrystalline and general corrosion. However, in order to maintain the low C and N contents even after welding, an effective inert gas shielding is required during welding as it is known for welding of Ti. Obviously, the high purity requirements which have to be observed during melting, hot forming and welding, and some embrittling effects which occur in high Cr steels after lengthy high temperature exposure (475^∘C embrittlement and σ-phase formation) have precluded as yet a wider application of these steels, in spite of their attractive properties. In addition to their good corrosion resistance these steels possess a higher thermal conductivity than austenitic stainless steels, which is of special interest in heat exchanger applications, and a lower rate of work hardening on cold deformation.

Table 9.10 Chemical composition of ferritic stainless steels

Table 9.11 Heat treatment conditions of ferritic stainless steels

Table 9.12 Mechanical properties of ferritic stainless steels

Table 9.13 Physical properties of ferritic stainless steels

Table 9.14 Weldability of ferritic stainless steels

4.2 Martensitic and Martensitic-Ferritic Chromium Steels

Steels with 11.5 − 18 wt% Cr and up to 1.2 wt% C are austenitic at high temperatures and can be transformed into martensite by fast cooling. Due to the high chromium content, the hardenability of these steels is relatively high, so a fully martensitic transformation can be achieved even with relatively large cross sections and moderate cooling rates. They are normally used for applications requiring a combination of high hardness with good corrosion and wear resistance. With carbon contents below about 0.35 wt% C the steels are hypo-eutectic, and some ferrite will be present after heat treatment. They can be austenitized at $\geq{\mathrm{960}}\,{\mathrm{{}^{\circ}\mathrm{C}}}$. Steels with ≥ 0.40 wt% C and 13 wt% Cr are already hyper-eutectic and will contain some undissolved primary carbides after quenching. To dissolve more carbon, these steels are usually quenched from a higher austenitization temperature around 1050^∘C. The steels cannot be hardened if the carbon concentration is below 0.12 wt% C and the chromium content ≥ 16 wt% Cr.

In some grades a small content of 0.5 − 1 wt% Mo (and sometimes some W, V, or Nb) is used to increase the tempering resistance, i. e., to retain higher hardness on tempering by means of precipitation reactions. Increased S contents or Se additions improve the machinability of the steels. About 1 − 2.5 wt% Ni together with reduced carbon content is applied if hardenabilty of higher cross sections and an improved weldability are required.

The hardness after quenching depends on the carbon content, as illustrated in Fig. 9.18 for a 12 wt% Cr steel [9.2]. In the as-quenched state the steels are hard but very brittle. Thus a tempering treatment is necessary to adjust toughness and strength to the level required for a specific application. For high hardness, annealing temperatures of about $100{-}300\,{\mathrm{{}^{\circ}\mathrm{C}}}$ are applied, whereas increases in ductility and toughness require tempering at or above 650^∘C.

The chemical composition for a number of martensitic Cr steels is given in Table 9.15. Tables 9.16–9.19 review data on recommended heat treatment conditions, mechanical properties, physical properties, and weldability, respectively.

Fig. 9.18

As-quenched hardness as a function of the carbon content of a martensitic 12 wt% Cr steel

It should be noted that a different way to produce stainless steels with high hardness is by precipitation hardening. Such steels have a low carbon content and contain in addition to chromium a few wt% of Ni and Cu. The hardening is caused by Cu precipitates. Others use precipitation hardening by intermetallic phases such as NiTi, TiAl, or NiAl.

Table 9.15 Chemical composition of martensitic and martensitic-ferritic chromium steels

Table 9.16 Heat treatment conditions of martensitic and martensitic-ferritic chromium steels

Table 9.17 Mechanical properties of martensitic and martensitic-ferritic chromium steels

Table 9.18 Physical properties of martensitic and martensitic-ferritic chromium steels

Table 9.19 Weldability of martensitic and martensitic-ferritic chromium steels

4.3 Austenitic Stainless Steels

By adding of austenite forming elements, mainly Ni, the range of stability of the fcc phase is extended down to and below room temperature. The favorable combinations of ductility, toughness, hot and cold formability, weldability, and corrosion resistance have made the austenitic CrNi steels by far the most important and popular stainless steels. The most widespread representatives are the steel grades AISI 304 (X5CrNi18-10, 1.4301) and AISI 316 (X5CrNiMo17-12-2, 1.4401). The austenitic steels are applied in the solution-annealed (at $1000{-}1100\,{\mathrm{{}^{\circ}\mathrm{C}}}$) and fast-cooled state which yields a microstructure that is free of carbide precipitates and has a homogeneous distribution of the alloying elements necessary for good corrosion resistance.

Compared to the bcc ferrite phase the fcc austenite phase is characterized by a higher solubility but a lower diffusivity of almost all alloying elements. The first fact allows the production of single phase fcc alloys with a broad composition spectrum. This permits adjustment of the properties of the steel to specific requirements of corrosion and oxidation resistance, cold and hot strength etc. The low diffusivity makes precipitation processes rather sluggish.

The austenite of the quenched steels can be unstable and can transform into martensite as a result of cold deformation or cooling to sub-zero temperatures, especially if these steels are relatively weakly-alloyed. This transformation leads to increased strength and reduced ductility. Martensite can be detected by magnetization measurements because the austenite is paramagnetic whereas the martensite is ferromagnetic. Since the fcc austenitic steels do not exhibit the ductile-brittle transition characteristic of ferritic alloys, the austenitic steels stay sufficiently ductile even at liquid He temperature (4.2 K) and are thus preferred materials for cryogenic applications. Their yield strength of 200 − 250 MPa is lower than that of the ferritic steels, but due to their pronounced work hardening by cold deformation (making extensive cold-forming operations difficult and often requiring intermediate soft annealing heat treatments), they have a higher tensile strength and fracture elongation. A pronounced solid solution strengthening is possible, especially by higher concentrations of nitrogen (up to about 0.4 wt% N). The nitrogen-alloyed CrNi steels are characterized by particularly favorable combinations of mechanical and corrosion properties.

Table 9.20 Heat treatment conditions of austenitic stainless steels

As mentioned above, after fast cooling from $\geq{\mathrm{1000}}\,{\mathrm{{}^{\circ}\mathrm{C}}}$, the austenitic stainless steels are free of carbide precipitates. But since steels with ≥ 0.05 wt% C are already carbon-supersaturated at temperatures below 900^∘C, holding the material at temperatures between about 400 and 900^∘C will lead to preciptation of chromium carbides (mainly M₂₃C₆ type), preferentially at the grain boundaries. This will result in an increased susceptibility to intergranular corrosion due to local chromium depletion. The means to avoid this undesirable effect are the same as for the ferritic steels: bonding of the carbon (and nitrogen) atoms by an overstoichiometric alloying with Nb, Ta, or Ti (so-called stabilization of the steels), or the reduction of the carbon content to below 0.03 wt% C resulting in the development of the extra low carbon (GlossaryTerm

ELC

) steels. However, in strongly oxidative media (such as concentrated nitric acid) even steels that have been sufficiently stabilized by Ti may exhibit intergranular corrosion due to selective dissolution of the TiC. Under less severe corrosion conditions, the Ti stabilized steels are as stable as the Nb stabilized grades but at somewhat lower costs.

Chemical compositions, heat treatment conditions, physical properties, and hints to weldability of austenitic stainless steels are presented in Tables 9.20–9.24. The austenitic stainless steels are often considered to be nonmagnetic or nonmagnetisable. However, in fact they are paramagnetic with a relative permeability of less than about 1.6 (often less than 1.1). Small volume fractions of residual δ-ferrite or martensite which can be formed on deep cooling or cold deformation or of magnetisable precipitates may lead to some small ferromagnetic behavior. The steels are the less prone to martensite formation on deep cooling or cold deformation the higher the alloy content is.

Table 9.21 Chemical composition of austenitic stainless steels

Table 9.22 Mechanical properties of austenitic stainless steels (hot rolled, solution annealed)

Table 9.23 Physical properties of austenitic stainless steels

Table 9.24 Weldability of austenitic stainless steels (welding methods not in parantheses are to be preferred)

4.4 Duplex Stainless Steels

In recent years, duplex steels are a class of stainless steels that have found increased interest and are becoming more common [9.11] due to lower alloying cost and attractive properties. They have a mixed structure with nearly equal volume fractions of 50% ferrite and 50% austenite (the practical range of the ferrite content is between 30 and 60 vol . %). According to their chemical composition, mechanical and corrosion properties they are often subdivided into lean, standard, super, and hyper duplex steels, respectively Typical examples are presented in Table 9.25. The lean duplex steels contain about 22 wt% Cr and low concentrations of Ni (≈ 2 wt%) and Mo (< 0.5 wt%). The super and hyper duplex steels are characterized by higher concentrations of 25 − 32 wt% Cr, 6 − 9 wt% Ni, 3 − 5 wt% Mo and up to 0.5 wt% N. Higher contents of Mn and N allow to further reduce the Ni content and thus to reduce alloying cost.

Table 9.25 Chemical composition of duplex stainless steels

The duplex steels solidify as δ-ferrite which will partly transform into austenite upon cooling. Thus, the phase fractions of ferrite and austenite depend not only on the concentrations of ferrite and austenite forming elements, but also on the annealing and cooling conditions which have to be controlled within narrow limits. The influence of annealing temperature and cooling conditions on the ferrite content of a steel with 0.05 wt% C, 25 wt% Cr, 8 wt% Ni, 2.5 wt% Mo and 1.5 wt% Cu is illustrated in Table 9.26 [9.3]. During the partial phase transformation of ferrite into austenite below 1350^∘C, a redistribution of the alloying elements occurs with the ferrite-forming elements Cr, Mo, and Ti enriched in the α-phase and the austenite-forming elements C, N, Ni, and Mn enriched in the γ-phase. The reduced C content in the α-phase delays the formation of chromium carbides. The relatively high Mo and N contents ensure good resistance against pitting corrosion.

The advantages of the duplex steels compared to austenitic steels are a substantially higher yield and tensile strength (Table 9.27) and a better resistance against chloride-induced stress corrosion cracking and pitting corrosion. A few data on heat treatment conditions and physical properties are given in Tables 9.28 and 9.29, respectively. In comparison with ferritic stainless steels, the duplex steels have a better weldability, a higher low temperature toughness down to ${\mathrm{-50}}\,{\mathrm{{}^{\circ}\mathrm{C}}}$ or even ${\mathrm{-80}}\,{\mathrm{{}^{\circ}\mathrm{C}}}$, and a lower susceptibility to general and intergranular corrosion. With respect to optimum toughness, the δ-ferrite content should not surpass 60 vol . %. The volume fraction of ferrite determines also the soft magnetic properties of the duplex stainless steels.

Table 9.26 Influence of annealing temperature and cooling conditions on the ferrite content of a steel with 0.05 wt% C, 25 wt% Cr, 8 wt% Ni, 2.5 wt% Mo and 1.5 wt% Cu [9.11]

Table 9.27 Mechanical properties of duplex stainless steels

Table 9.28 Heat treatment conditions of duplex stainless steels

Table 9.29 Physical properties of duplex stainless steels

Brittle Cr-rich phases can deteriorate ductility, toughness, and corrosion resistance of duplex steels due to the formation of σ-phase at slow cooling through the temperature range of $1000{-}500\,{\mathrm{{}^{\circ}\mathrm{C}}}$ and of the α^′ phase when the steel is exposed for a longer time to temperatures between 350 and 525^∘C (so-called 475^∘C embrittlement). Therefore, duplex steels are usually water-quenched from a solution treatment at $1000{-}1150\,{\mathrm{{}^{\circ}\mathrm{C}}}$, and service temperatures are restricted to less than $250{-}300\,{\mathrm{{}^{\circ}\mathrm{C}}}$. These problems are the more pronounced the higher the alloy content is.

The duplex stainless steels can be welded with all common welding processes except oxyacetylene welding. Recommended welding details are given in [9.11]. Preheating and post-weld stress relief heating are not necessary. The susceptibility to hot cracking of the weld seam is very low due to the ferrite component, but the formation of too much ferrite and of embrittling phases in the heat affected zone requires special attention, e. g., with respect to heat input and interpass temperature.

5 Heat-Resistant Steels

Heat-resistant steels are treated extensively in [9.1], creep data are compiled in [9.12]. Steels are considered heat-resistant if they possess – in addition to good mechanical properties at ambient temperature – special resistance against short or long term exposure to hot gases, combustion products and melts of metals or salts at temperatures above about 550^∘C where non- or low-alloyed steels are no longer applicable due to extensive scaling and creep.

Thus, heat-resistant steels are characterized by a combination of good high temperature strength, scaling resistance, a sufficient hot and cold formabilty, and weldability. They are sufficiently stable against embrittling processes at the high application temperatures. The resistance against scaling and hot gas corrosion is affected by the formation of a protective dense, pore-free, and tightly adherent oxide layer at the surface. The main alloying elements leading to such an oxide layer are Cr, Al, and Si. The oxidation and scaling resistance increases with increasing Cr content between about 6 and 25 wt% Cr. Higher Cr concentrations do not lead to further improvement. Additions of up to 2 wt% Al and up to 3 wt% Si enhance the effect of Cr. Small additions of rare earth metals, e. g., of Ce, can improve the adherence and the ductility of the oxide layer. In order to keep the protective oxide layer intact during temperature changes, the steels should exhibit low volume changes and, if possible, no phase transformations during heating and cooling. Consequently, there are two main groups of heat resistant steels: ferritic and austenitic steels, both showing no phase transformations.

The ferritic Cr (or Cr-Al-Si) steels are less expensive but have a lower creep strength above 800^∘C. They may suffer from three embrittling mechanisms:

• The 475^∘C embrittlement due to decomposition in the metastable miscibility gap of the Fe-Cr solid solution, occuring between about 350 and 550^∘C at Cr contents above 15 wt% Cr (Fig. 9.9)

• The formation of the brittle intermetallic FeCr σ-phase at temperatures between about 550 and 900^∘C (Sect. 9.1, Fig. 9.8)

• Grain coarsening at temperatures above about 900^∘C.

However, these embrittling mechanisms will not impair the behavior at high operating temperatures if taken into account properly, but will deteriorate the toughness after cooling to room temperature. Heating to temperatures above the range of occurrence of the embrittling phases followed by sufficiently fast cooling will suppress the embrittling effects.

The Cr and Ni alloyed austenitic steels possess a higher temperature strength and better ductility, toughness, and weldability. The susceptibility to embrittling effects is considerably lower. At Ni contents above 30 wt% Ni, they are outside the stability region of the brittle σ-phase.

The properties of the ferritic-austenitic steels lie between those of the ferritic and austenitic steels. They are characterized by a higher fracture toughness, cold formability, high temperature strength, and weldability than the fully ferritic grades, and by a higher chemical resistance in sulphurous gases than the austenitic grades.

It is obvious that the scaling resistance of the heat-resisting steels will be detrimentally influenced by any other corrosion mechanism which may be destroying the oxide layer, e. g., by chemical reactions with other metal oxides, chlorine, or chlorides. Thus in general, the heat resistance cannot be characterized by a single test method or measuring parameter but will depend on the specific environmental conditions.

Table 9.30 Chemical composition of heat-resisting steels

Table 9.31 Recommended conditions for heat treatment and hot forming of heat-resisting steels

Table 9.32 Mechanical properties of heat-resisting steels at 20^∘C

Table 9.33 Long-term mechanical properties of heat-resistant steels at high temperatures: average values of scatter bands

Table 9.34 Physical properties of heat-resisting steels

Table 9.35 Resistance of heat-resisting steels in various media

In Table 9.30 chemical compositions of the most important grades of heat resistant steels are presented [9.13]. Table 9.31 contains some information about recommended temperature ranges for heat treatment and hot forming. In Tables 9.32 and 9.33 the mechanical properties at room temperature and at high temperatures are listed, respectively. Table 9.34 shows some physical properties. The ferritic and ferritic-austenitic steels are magnetisable while the austenitic grades are nonmagnetic. Qualitative data on the high temperature behavior in special gas atmospheres are given in Table 9.35. In carburizing atmospheres carbon can diffuse into the steel, reacting with the chromium to form chromium carbides which can lead to embrittlement and reduced scaling resistance due to chromium depletion in the matrix. Higher Ni and Si contents reduce the carburization susceptibility. In sulfur-containing atmospheres, which contain the sulfur mostly in the form of SO₂ or H₂S, the formation of sulfides at the surface may inhibit the formation of the protecting oxide layer. Under oxidizing conditions this process will proceed rather slowly, but under reducing conditions the pick-up of sulfur occurs very fast. This is especially true with Ni-alloyed steels due to the formation at about 650^∘C of a low-melting Ni/NiS eutectic. Thus under such conditions the ferritic steels are more stable than the austenitic grades. In sulfur-containing atmospheres the maximum service temperatures will be about $100{-}200\,{\mathrm{{}^{\circ}\mathrm{C}}}$ lower than in air.

The heat-resistant steels are weldable by the usual processes, with arc welding preferred over gas fusion welding. For the ferritic steels, the tendency to grain coarsening in the heat affected zone has to be kept in mind. The application of austenitic filler metals will lead to better mechanical properties of the weld connection than those of the base metal (however, with respect to the scaling resistance, different thermal expansions of the ferritic and austenitic materials may be a problem). Filler materials should be at least as highly alloyed as the base metal. In sulfurizing atmospheres it is advisable to use ferritic electrodes for the cap passes only in order to ensure a tough weld. Post-weld heat treatments are generally not necessary.

6 Tool Steels

Tool steels are the largest group of materials used to make tools for cutting, forming, or otherwise shaping a material into a part or component. An extensive account is given in [9.14]. Other major groups of tool materials are cemented carbides (Sect. 9.6) and ceramics including diamond (Chap. 6).

The most commonly used materials are wrought tool steels, which are either carbon, alloy, or high-speed steels capable of being hardened by quenching and tempering to hardness levels ≤ 70 HRC. High-speed tool steels are so named because of their suitability to machine materials at high cutting speed. Other steels used for metalworking applications include steels produced by powder metallurgy, medium-carbon alloy steels, high-carbon martensitic stainless steels, and maraging steels.

Wrought tool steels are essentially hardenable alloy steels with relatively high contents of the carbide forming elements Cr, Mo, W, and V. If the steels are quenched and tempered, the dependence of their hardness on tempering temperature indicates the level of hardening achieved as well as its temperature stability, (Fig. 9.19). The rate of effective softening at tempering temperatures up to about 300^∘C is mainly due to the competing effects of recovery and the precipitation of iron carbides (Table 9.1). The hardening at higher temperatures is associated with the precipitation of alloy carbides which can form at elevated temperatures only because of their high melting points and transformation kinetics. They give rise to a second maximum on isothermal tempering curves, as curves 3 and 4 in Fig. 9.19 , which is referred to as secondary hardening.

The alloy carbide phases which precipitate and give rise to secondary hardening are listed in Table 9.36. Co is an alloying element which raises the high temperature stability of tool steels by raising their melting temperature. Figure 9.20 shows this effect by a comparison of the hardness versus temperature data for non-Co-based and Co-based high-speed tool steels.

Table 9.36 Alloy carbides occurring in tool steels

Fig. 9.19

Isothermal (1 h) tempering curves of 4 typical tool steels. Curves 1 and 2: softening of AISI grade W (water-hardening) and O (oil-hardening) steels; Curves 3 and 4: softening and secondary hardening of AISI grade A2 (air-hardening medium alloy) and M2 (Mo high-speed) steels (after [9.14])

Fig. 9.20

Comparison of the hot hardness of Co-bearing versus non-Co bearing high-speed tool steels (after [9.14])

Table 9.37 gives composition ranges for the tool steels most commonly used. According to the AISI classification, each group of similar composition and properties is given a capital letter, somewhat related to the major alloying element. Thus, high-speed steels are classified by M for molybdenum and T for tungsten. Within each group individual types are assigned code numbers.

The basic properties of tool steels that determine their performance in service are hardness, wear resistance, ductility and fracture toughness, and in many applications stability against softening at elevated temperatures. Characteristic mechanical properties at room temperature as a function of hardening treatment are listed for group L and group S steels in Table 9.38.

Table 9.37 Composition ranges of principle types of tool steels according to AISI and UNS classifications. All steels except group W contain 0.25 max wt% Cu, 0.03 max wt% P, and 0.03 max wt% S; group W steels contain 0.20 max wt% Cu, 0.025 max wt% P, and 0.025 max wt% S. Where specified, sulfur may be increased to 0.06 to 0.15% to improve machinability of group A, D, H, M and T steels. (after [9.14])

Table 9.38 Mechanical properties of group L and group S tool steels at room temperature as a function of hardening treatment (after [9.14])

7 Cast Irons

The term cast iron pertains to a large family of multi-component Fe-C-Si alloys which solidify according to the eutectic of the Fe-C system (Sect. 9.1; Fig. 9.1). They are treated extensively in [9.15]. Their comparatively high C and Si contents lead to solidification either according to the metastable equilibria involving Fe₃C or according to the stable equilibria involving graphite, depending, also, on the content of further alloying elements, melt treatment, and rate of cooling. Since, in addition, the metallic phases can be alloyed and their microstructures varied by annealing and transformation treatments as in other ferrous alloys, a multitude of microstructural states and associated properties result.

7.1 Classification

The C rich phases determine the basic classification of cast irons. According to the color of their facture surfaces, Fe₃C-containing grades are called white, graphite-containing grades are called gray, and alloys which solidify in mixed states are called mottled. In addition, the shape of the graphite phase particles and the microstructure of the metallic matrix phases are taken into account since they are also characterizing the mechanical properties.

Table 9.39 Classification of cast irons according to commercial designation, microstructure and color of fracture surface (after [9.15])

7.1.1 Shape of Graphite Phase Particles

Lamellar (flake) graphite (GlossaryTerm

FG

) is characteristic of cast irons with near-zero ductility; spheroidal (nodular) graphite (GlossaryTerm

SG

) is characteristic of ductile cast iron; compacted (vermicular) graphite (GlossaryTerm

CG

) is a transition form between flake and nodule shape; temper graphite (TG) results from a tempering treatment and consists of small clusters of branched graphite lamellae.

7.1.2 Microstructure of Metallic Matrix Phases

Ferritic, pearlitic, austenitic, bainitic (austempered). More details are presented in Fig. 9.21 and Table 9.39.

Fig. 9.21

Classification of cast irons (after [9.15])

Table 9.40 Range of composition, microstructural and mechanical characteristics of typical unalloyed cast irons (after [9.15])

Fig. 9.22

Section through the metastable Fe-C-Si phase diagram at 2 wt% Si (after [9.15])

7.2 Iron-Carbon-Silicon Equilibria and Carbon Equivalent

Since C and Si are the alloying elements which dominate the solidification behavior and the resulting microstructures of cast irons, their phase equilibria need to be taken into account. Figure 9.22 shows a section through the metastable ternary Fe-C-Si diagram at 2 wt% Si which approximates the Si content of many cast irons. Compared to the binary Fe-C system, the addition of Si decreases the stability of Fe₃C and increases the stability of ferrite, as indicated by the expansion of the α-phase field. With increasing Si concentration, the C concentrations of the eutectic and the eutectoid equilibria decrease while their temperatures increase.

Fig. 9.23

Approximate C and Si concentrations for the composition ranges of steels and different grades of cast irons (after [9.15])

These relations are the basis for correlating the C and Si concentrations with the ranges of formation of steels and of the main groups of cast irons as shown in Fig. 9.23. The relations are expressed in terms of the carbon equivalent (GlossaryTerm

CE

) $\text{CE}=(\text{wt{\%}\,C})+(1/3)(\text{wt{\%}\,Si})$. The concentration of the eutectic (upper dashed line) is given by CE_e = 4.3. Accordingly, alloys with CE < 4.3 are hypoeutectic and alloys with CE > 4.3 are hypereutectic. In P-containing cast irons the relation is $\text{CE}=(\text{wt{\%}\,C})+(1/3)(\text{wt{\%}\,Si}+\text{wt{\%}\,P})$. In addition, shows the limit of solubility of C in austenite (lower dashed line), which is the upper limit of the range of steels. It is given by $\text{CE}_{\gamma_{\max}}=2.1=(\text{wt{\%}\,C})+(1/6)(\text{wt{\%}\,Si})$.

7.3 Grades of Cast Irons

Table 9.40 lists the composition ranges of typical unalloyed cast irons, indicating that they are classified mainly by the type of carbon-rich phase formed and by the basic mechanical behavior.

7.3.1 Gray Iron

This most common type of cast iron is characterised by flake graphite and requires a high CE to ensure a sufficient graphitization potential which is also increased by Al addition. Gray irons may be moderately alloyed, e. g., by 0.2 − 0.6 wt% Cr, 0.2 − 1 wt% Mo, and 0.1 − 0.2 wt% V which promote the formation of alloy carbides and pearlite. Upon plastic deformation the flake form of graphite promotes early internal crack formation and, thus, causes the low ductility of gray iron.

7.3.2 Ductile Iron

This cast iron is characterized by the spheroidal graphite phase (SG) in its microstructure. Spheroidal graphite is formed during solidification if the melt has been treated by the addition of a component which promotes the particular nucleation and growth behavior of graphite in the form of nodules. The most common alloying component added to nucleate spheroidal graphite is Mg, but Ca, Ce, La, and Y have also been found to favour spheroidal graphite formation. Basically this microstructural modification leads to higher yield strength and higher ductility because the plastic deformation of the metallic matrix phases can be extended to higher strains than in gray iron before fracture sets in.

7.3.3 Malleable Irons

The melt treatment of malleable cast irons involves Mg, Ca, Bi, or Te additions. But malleable irons have an as-cast structure consisting of Fe₃C in a pearlitic matrix. By heat treatment in the range of $800{-}970\,{\mathrm{{}^{\circ}\mathrm{C}}}$ the cementite phase is transformed into graphite (TG). The cooling is controlled in such a way as to promote pearlite formation, ferrite formation, or a mixture of the two.

7.3.4 Alloy Cast Irons

Alloying elements beyond the levels mentioned above are added to cast irons almost exclusively to enhance resistance to abrasive wear or chemical corrosion, or to extend their stability for application at elevated temperature. The function of the alloying elements is essentially the same as in steels. Table 9.41 lists the groups of grades with typical compositions and the microstructural constituents present in the as-cast state.

Table 9.41 Ranges of alloy content of typical alloy cast irons (after [9.15]). Where a single value is given rather than a range, that value is a maximum limit

7.4 Mechanical Properties of Cast Irons

Due to the multitude of as-cast structures as a function of alloy composition, melt treatment, cooling rate (as influenced by the cooling conditions and the cross section of the work piece), and subsequent heat treatment, there is a wide range of mechanical properties which can be achieved according to the requirements of the application. Table 9.42 gives a survey in terms of characteristic examples.

Table 9.42 Mechanical properties of cast irons