DUCTILE IRON DATA FOR DESIGN ENGINEERS

 

 

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SECTION III.  ENGINEERING DATA (part 1)

Introduction

Ductile Iron is not a single material, but a family of versatile cast irons exhibiting a wide range of properties which are obtained through microstructure control. The most important and distinguishing microstructural feature of all Ductile Irons is the presence of graphite nodules which act as "crack-arresters" and give Ductile Iron ductility and toughness superior to all other cast irons, and equal to many cast and forged steels. As shown in Figure 2.8, Section II, the matrix in which the graphite nodules are dispersed plays a significant role in determining mechanical properties.

Matrix control, obtained in conventional Ductile Iron either "as-cast" through a combination of composition and process control, or through heat treatment, gives the designer the option of selecting the grade of Ductile Iron which provides the most suitable combination of properties. Figure 3.1 illustrates the wide range of strength, ductility and hardness offered by conventional Ductile Iron. The high ductility ferritic irons shown on the left provide elongation in the range 18-30 per cent, with tensile strengths equivalent to those found in low carbon steel. Pearlitic Ductile Irons, shown on the right side, have tensile strengths exceeding 120 ksi (825 MPa) but reduced ductility. Austempered Ductile Iron (ADI), discussed in Section IV, offers even greater mechanical properties and wear resistance, with ASTM Grades providing tensile strengths exceeding 230 ksi (1600 MPa). Special Alloy Ductile Irons, described in Section V, can be selected to provide creep and oxidation resistance at high temperatures, resistance to thermal cycling, corrosion resistance, special magnetic properties, or low temperature toughness.

The numerous, successful uses of Ductile Iron in critical components in all sectors of industry highlight its versatility and suggest many additional applications. In order to use Ductile Iron with confidence, the design engineer must have access to engineering data describing the following mechanical properties: elastic behaviour, strength, ductility, hardness, fracture toughness and fatigue properties. Physical properties - thermal expansion, thermal conductivity, heat capacity, density, and magnetic and electrical properties - are also of interest in many applications. This Section describes the mechanical and physical properties of conventional Ductile Irons, relates them to microstructure, and indicates how composition and other production parameters affect properties through their influence on microstructure.

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Tensile Properties

The tensile properties of conventional Ductile Iron, especially the yield and tensile strengths and elongation, have traditionally been the most widely quoted and applied determinants of mechanical behaviour.  Most of the world-wide specifications for Ductile iron summarized in Section XII describe properties of the different grades of Ductile Iron primarily by their respective yield and tensile strengths and elongation.  Hardness values, usually offered as additional information, and impact properties, specified only for certain ferritic grades, compolete most specifications.   Although not specified, the modulus of elasticity and proportional limit are also vital design criteria.  Figure 3.2 illustrates a generalized engineering stress-strain curve describing the tensile properties of ductile engineering materials.

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Modulus of Elasticity

Figure 3.2 shows that, at low tensile stresses, there is a linear or proportional relationship between stress and strain. This relationship is known as Hooke's Law and the slope of the straight line is called the Modulus of Elasticity or Young's Modulus. As shown in Figure 3.3, the initial stress-strain behaviour of Ductile Iron lies between those of mild steel and Gray Iron. Annealed or normalized mild steels exhibit elastic behaviour until the yield point, where plastic deformation occurs suddenly and without any initial increase in flow stress. In Gray Iron, the graphite flakes act as stress-raisers, initiating microplastic deformation at flake tips at very low applied stresses. This plastic deformation causes the slope of the stress-strain curve to decrease continually and as a result Gray Iron does not exhibit true elastic behaviour.

Ductile Iron exhibits a proportional or elastic stress-strain relationship similar to that of steel but which is limited by the gradual onset of plastic deformation. The Modulus of Elasticity for Ductile Iron, measured in tension, varies from 23.5 to 24.5 x 106 psi (162 - 170 GPa). In cantilever, three point beam or torsion testing, values as low as 20.5 x 106 have been reported. The Dynamic Elastic Modulus (DEM), the high frequency limit of the Modulus of Elasticity measured by the resonant frequency test, exhibits a range of 23.5 to 27 x 106 psi (162 - 186 GPa).

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Poisson's Ratio
Poisson's Ratio, the ratio of lateral elastic strain to longitudinal elastic strain produced during a tensile test, shows little variation in Ductile Iron. A commonly accepted value is 0.275.

Proportional Limit

The proportional limit (also called the limit of proportionality) is the maximum stress at which a material exhibits elastic behaviour. When a material is stressed below the limit of proportionality, and the stress is then removed, the stress-strain curve returns to the origin - no permanent change in dimension occurs. When the stress exceeds the proportional limit, plastic strain reduces the slope of the stress-strain curve. Upon removal of the stress, the strain decreases linearly, following a line parallel to the original elastic curve. At zero stress, the strain does not return to zero, exhibiting a permanent plastic strain, or change in dimension of the specimen (see Figure 3.2).

In Ductile Irons, which exhibit a gradual transition from elastic to plastic behaviour, the proportional limit is defined as the stress required to produce a deviation from elastic behaviour of 0.005%. It is measured by the offset method used to measure the yield strength and may also be estimated from the yield strength. The ratio of proportional limit to 0.2% yield strength is typically 0.71 for ferritic grades, decreasing to 0.56 for pearlitic and tempered martensitic grades.

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Yield Strength

The yield strength, or proof stress is the stress at which a material begins to exhibit significant plastic deformation. The sharp transition from elastic to plastic behaviour exhibited by annealed and normalized steels (Figure 3.3) gives a simple and unambiguous definition of yield strength. For Ductile Iron the offset method is used in which the yield strength is measured at a specified deviation from the linear relationship between stress and strain. This deviation, usually 0.2 %, is included in the definition of yield strength or proof stress in international specifications (see Section XII) and is often incorporated in the yield strength terminology, e.g. "0.2 % yield strength". Yield strengths for Ductile Iron typically range from 40,000 psi (275 MPa) for ferritic grades to over 90,000 psi (620 MPa) for martensitic grades.

Tensile Strength

The tensile strength, or ultimate tensile strength (UTS), is the maximum load in tension which a material will withstand prior to fracture. It is calculated by dividing the maximum load applied during the tensile test by the original cross sectional area of the sample. Tensile strengths for conventional Ductile Irons generally range from 60,000 psi (414 MPa) for ferritic grades to over 200,000 psi (1380 MPa) for martensitic grades.

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Elongation

Elongation is defined as the permanent increase in length, expressed as a percentage of a specified gage length marked in a tensile test bar, which is produced when the bar is tested to failure. Elongation is used widely as the primary indication of tensile ductility and is included in many Ductile Iron specifications. Although shown as the uniform elongation in Figure 3.2, elongation also includes the localized deformation that occurs prior to fracture. However, because the localized deformation occurs in a very limited part of the gage length, its contribution to the total elongation of a correctly proportioned bar is very small. Brittle materials such as Gray Iron can fail in tension without any significant elongation, but ferritic Ductile Irons can exhibit elongation of over 25%. Austempered Ductile Irons exhibit the best combination of strength and elongation (See Section IV).

Relationships Between Tensile Properties

The strong influence of graphite morphology and matrix structure on the different tensile properties of Ductile Iron produces significant correlations between these properties. Figure 3.4 illustrates the non-linear least square relationships between tensile and yield strengths and the dynamic elastic modulus.

In 1970 Siefer and Orths, in a statistical study of the mechanical properties of a large number of Ductile Iron samples, identified a relationship between tensile strength and elongation of the form:

(tensile strength ksi)2 x (elongation%) รท1000 = Q
where Q is a constant.

A larger value of Q indicates a combination of higher strength and elongation and, therefore, higher material performance. Crews (1974) defined Q as the Quality Index (QI) for Ductile Iron. Both the QI and the underlying relationship between strength and elongation offer valuable insights into the quality of different Ductile Iron castings and the feasibility of obtaining various combinations of properties. High QI values have been shown to result from high modularity (high percentage of spherical or near-spherical graphite particles), absence of intercellular degenerate graphite, high nodule count, a low volume fraction of carbides, low phosphorus content (<O.03%) and freedom from internal porosity. High quality castings with these characteristics can be produced consistently by a competent, modern Ductile Iron foundry.

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Figure 3.5 illustrates the tensile strength-elongation relationships for different QI levels of Ductile Iron. Each curve is an "iso-quality" line along which strength-elongation values can be displaced by an annealing or normalizing heat treatment which changes the matrix ferrite: pearlite ratio in the matrix.  For example, using the highest iso-quality line Q99.5, three test bars with tensile strength/elongation values of 70 ksi/24.5%, 100 ksi/12%, and 120 ksi/8.3% are of equivalent quality, in spite of a three-fold difference in elongation.    Quench-and-temper heat treatments produce similar curves, but with a slight displacement to higher quality when compared to annealed or normalized samples of the same iron

This iso-quality concept can assist in the arbitration of irons which are of sufficient quality but are off-grade by virtue of their position in Figure 3.5 relative to the ASTM grade limits. For example, 3 different irons, all with a QI of 70, could have strength-elongation values of 64 ksi/17.1%, 70 ksi/14.3% and 78 ksi/11.5%. Although only the 70 ksi iron meets the 65-45-12 grade requirement, the other two irons, on the basis of identical QI, might be judged equally fit for the intended purpose.

The following comparison of QI values reveals determined by Siefer and Orths with those of recently produced commercial Ductile Irons (see Figure 3.9) reveals the impact of 20 years of progress in Ductile Iron technology.

Siefer and Orths (1970) Venugopalan & Alagarsamy (1990)
Q99.5 (metric) = 60,000 Qmax, (metric) = 64,500
Q99.5 (Imperial) = 120,000 Qmax, (Imperial) = 129,000
Q50 (metric) = 30,000 Q50 (metric) = 45-,OOO
Q50 (Imperial) = 60,000 Q50 (Imperial) = 90,000

where: tensile strength (metric) is expressed in kp/MM2,
tensile strength (Imperial) is expressed in ksi,
Q99.5 and Q50 indicate the quality levels exceeded by 0.5% and
50% respectively of the samples tested, and
Qmax is the maximum quality exhibited by a batch of 34 samples

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As might be expected from two decades of progress in Ductile Iron production technology and process control, the maximum QI increased by 7.5% but the median QI increased by 50%, indicating a significant improvement in consistency of properties. The application of the Quality Index concept to Austempered Ductile Iron highlights the superior combination of strength and elongations offered by this material, with ASTM A897-90 Grades 125/80/10 and 150/100/7 having minimum Quality Indices of 156 and 158 respectively.

The inverse relationship between tensile strength and elongation is followed by all Ductile Iron specifications (see Section XII), as shown in Figure 3.6 for ASTM specification A536-80. The various grade specifications shown in Figure 3.6 and their minimum property boundaries are superimposed on the Siefer and Orths diagram (Figure 3.5) in order to indicate the relative qualities of irons required to meet the different grades. Examination of Figure 3.5 reveals several relationships between the ASTM grades and Ductile Iron Quality Indices.

  • The Grade 60-40-18 has the highest QI, 64.8, with the QI decreasing to a value of 29 for Grade 120-90-02.
  • The properties corresponding to a grade designation e.g. 65-45-12, define the minimum QI - 50.7 - required to meet that grade. The property boundaries for that grade define QI levels which increase until the boundary of the next grade is reached - 76.8 for the boundary with 80-55-06 and 76 for the boundary with 60-40-18.
  • The mean Quality Index Q50 for the data of Venugopalan and Alagarsamy - 90 - is substantially higher than the Quality Indices required to meet all requirements for ASTM A536-80.

slagpot.JPG (61932 bytes)

Machined Ductile Iron slag pot half (subsequently austempered acier machining).

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Hardness

The hardness of Ductile Iron is usually and best measured by the Brinell test, in which a 10 mm diameter hardened steel or tungsten carbide ball is pressed into a flat surface of the workpiece. Hardness is expressed as a Brinell Indentation Diameter (BID) or a Brinell Hardness Number (BHN). Hardness may also be described as BHN/3000 to indicate the force applied to the ball is 3ooo kg, the normal value for ferrous materials. The size of the Brinell indentation, and its related volume of plastic deformation, are large relative to the scale of the microstructure and as a result an average hardness is obtained which exhibits good reproducibility for similar microstructures.

Brinell Hardness is included  in many Ductile Iron specifications.   Brinell Hardness should be used for production control and as an auxiliary property test, for example to control machinability. Microhardness testing, using either the Knoop or Vickers indenters, can be used to measure the hardness of the individual components of the Ductile Iron matrix.

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Tensile Properties vs. Hardness

Figure 3.7 and Figure 3.8 illustrate the relationships between Brinell Hardness, tensile strength and elongation respectively. Figure 3.7 indicates that 90% of all castings with a hardness of 150 BHN will have tensile strengths between 40 and 50 kp/MM2 (57-71 ksi), while the equivalent range of strength corresponding to a hardness of 250 BHN would be 66-87 kp/MM2 (94-124 ksi). Figure 3.8 reveals a more complex relationship between BHN and elongation. For a hardness of 150 BHN, 90% of the castings would have elongation in the range 13-24%. At 250 BHN the equivalent range is 2.5 to 8.5%. Because of the magnitude of these variations, Brinell Hardness alone should not be used to determine tensile properties, especially elongation.

Microhardness data for the individual microstructural components can be used to predict the tensile properties of as-cast, annealed, and normalized commercial Ductile Iron. Figure 3.9, from Venugopalan and Alagarsamy, compares strength and elongation data with the following linear progression curves:

tensile strength (ksi) = 0.10 + 0.36 x CMMH

yield strength (ksi) = 12 + 0.18 x CMMH

elongation (%) = 37.85 - 0.093 x CMMH

CMMH is composite matrix microhardness, and is defined as:

CMMH = ((HF x%F) + (HP x%P))/100,

where HF and%F, and HP and%P are the respective hardnesses and volume fractions of ferrite and pearlite.

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Effect of Graphite Shape

As would be expected from the dramatic differences in mechanical properties between Gray and Ductile Irons, that modularity plays a significant role in determining properties within the Ductile Iron family. Figure 3.10 illustrates the correlation between modularity and Dynamic Elastic Modulus. This relationship not only emphasizes the strong influence of modularity on DEM, but also indicates that DEM values obtained by sonic testing can be used to measure modularity (graphite volume and nodule count should be relatively constant).

Nodularity, and the morphology of the non-spherical particles produced as modularity decreases, exert a strong influence on the yield and tensile strengths of Ductile Iron. Figure 3.11 shows the relationships between strength and nodularity for ferritic irons in which modularity has been changed by two methods: through magnesium control, or through lead control. When nodularity is decreased by reducing the amount of residual magnesium (the most common spheroidizing agent used in commercial Ductile Iron) the nodules become elongated, but do not become sharp or "spiky". The result is a 10% decrease in yield strength and a 15% decrease in tensile strength when modularity is reduced to 30%. Small additions of lead reduce modularity by producing intergranular networks of "spiky" or plate-like graphite which result in dramatic reductions in tensile properties.

The effect of nodularity on pearlitic Ductile Irons can be determined in Figure 3.12 and Figure 3.13 by comparing the tensile properties, at constant carbide levels, of irons with nodularities of 90, 70 and 40%. These Figures reveal two important features. First, compared to the Mg-controlled loss of nodularity for the ferritic iron in Figure 3.11, the pearlitic iron is much more sensitive to reduced nodularity. Second, at low carbide levels typical of good quality Ductile Iron, there is relatively little loss of strength as the nodularity decreases to 70% but as nodularity deteriorates further, strength decreases more rapidly.

Although not shown in Figures 3.11, 3.12, 3.13, the effect of nodularity on elongation can be inferred by considering the influence of nodularity on the difference between the yield and tensile strengths, which is proportional to elongation. Both Mg- and Pb-controlled losses in nodularity reduce the difference between the yield and tensile stresses, indicating that loss of nodularity results in reduced elongation. The dramatic decrease in tensile strength produced by lead control indicates that the formation of spiky, intercellular graphite can severely embrittle Ductile Iron.

Designers can virtually eliminate the effect of nodularity on tensile properties by specifying that the nodularity should exceed 80-85% and that there should be no intercellular flake graphite. These criteria can be met easily by good production practices which ensure good nodularity through Mg control and prevent flake or spiky graphite by a combination of controlling flake-producing elements and eliminating their effects through the use of small additions of cerium.

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Effect of Nodule Count

Nodule Count, expressed as the number of graphite nodules/MM2, also influences the mechanical properties of Ductile Iron, although not as strongly and directly as graphite shape. Generally, high nodule count indicates good metallurgical quality, but there is an optimum range of nodule count for each section size of casting, and nodule counts in excess of this range may result in a degradation of properties. Nodule count per se does not strongly affect tensile properties, but it has the following effects on microstructure, which can significantly influence properties,

  • Nodule count influences the pearlite content of as-cast Ductile Iron. Increasing the nodule count decreases the pearlite content, decreasing strength and increasing elongation.
  • Nodule count affects carbide content. Increasing the nodule count improves tensile strength, ductility and machinability by reducing the volume fractions of chill carbides, segregation carbides, and carbides associated with "inverse chill".
  • Matrix homogeneity is influenced by nodule count. Increasing the nodule count produces a finer and more homogeneous microstructure. This refinement of the matrix structure reduces the segregation of harmful elements which might produce intercellular carbides, pearlite or degenerate graphite
  • Nodule count affects graphite size and shape. Increasing nodule count results in a decrease in nodule size which improves tensile, fatigue and fracture properties. Inoculation practices used to improve nodule count often make the nodules more spherical. Thus, high nodule count is generally associated with improved nodularity.

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Effect of Graphite Volume

The volume fraction of graphite in Ductile Iron can also influence certain tensile properties. Figure 3.14 illustrates the effects of carbon content (at constant silicon level) and casting diameter on the Dynamic Elastic Modulus (DEM) of a Ductile Iron casting with a fully pearlitic matrix. Increasing the carbon content, which increases the volume fraction of graphite, decreases the DEM for a constant section size. Casting section size can influence both the volume fraction and size of graphite nodules. Increased section size reduces the cooling rate of the casting, causing more carbon to precipitate in the stable graphite phase, instead of the carbide phase favoured by higher cooling rates. The lower cooling rates of the larger diameter bars also affect graphite nucleating conditions, resulting in reduced nodule count but increased nodule size. The increase in nodule size with section size is the primary cause of the reduced DEM, but an increase in the formation of graphitic carbon during solidification could also be a contributing factor.

Graphite flotation can produce variations in graphite volume within larger castings which can be harmful to mechanical properties. Graphite flotation occurs when low cooling rates and high "carbon equivalent" (carbon equivalent = % carbon + 1/3 (% silicon)) combine to produce large nodules that rise during solidification. The result is a depletion of the larger nodules in the lower part of the casting and an accumulation at the upper surface. The increasingly pronounced curvature, with increasing bar diameter, of the curves in Figure 3.14 is probably an indication of graphite flotation. In these larger bars, graphite flotation at higher carbon levels may have reduced the graphite volume in the center of the bars from which the 1/4 inch (6 mm) diameter test bars were machined. The resultant reduced rate of increase of graphite volume with increased carbon would be reflected in flatter curves at higher carbon levels.

Graphite flotation can cause a serious degradation of properties near the upper (cope) surface of large Ductile Iron castings. However, this phenomenon is readily avoided by reducing the carbon equivalent as the casting section size increases.

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Effect of Carbide Content

Carbide content has both direct and indirect effects on the properties of Ductile Iron castings. Figure 3.12 and Figure 3.13 show that increasing the volume per cent of hard, brittle carbide increases the yield strength, but reduces the tensile strength of Ductile Iron castings. As discussed earlier, this convergence of yield and tensile strengths produces a decrease in elongation with increasing carbide content. The presence of carbides in a Ductile Iron matrix also increases the dynamic elastic modulus and significantly reduces machinability. The formation of eutectic carbide during solidification affects the volume fraction of graphite produced because carbide and graphite compete for the carbon contained in the liquid iron. Fifteen volume per cent of carbide would require 1 per cent carbon, reducing the carbon available for graphite by approximately one third. The formation of carbide thus increases the likelihood of internal casting porosity by reducing the expansion effects produced by the formation of graphite during solidification.

To minimize the detrimental effects on properties and machinability, maximum carbide levels of less than 5% are normally specified. These levels can usually be achieved as-cast by reducing the levels of carbide forming elements through the use of high purity pig iron in the furnace charge and by increasing the nodule count through the application of good inoculation practices. When required, heat treatment can be used to eliminate carbides.

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Effect of Matrix

In Ductile Irons with consistent modularity and nodule count and low porosity and carbide content, mechanical properties are determined primarily by the matrix constituents and their hardness. For the most common grades of Ductile Iron, the matrix consists of ferrite and/or pearlite. Ferrite is the purest iron phase in Ductile Iron. It has low strength and hardness, but high ductility and toughness and good machinability. Pearlite is an intimate mixture of lamellar cementite in a matrix of ferrite. Compared to ferrite, pearlite provides a combination of higher strength and hardness and lower ductility. The mechanical properties of ferritic/pearlitic Ductile Irons are, therefore, determined by the ratio of ferrite to pearlite in the matrix. This ratio is controlled in the as-cast condition by controlling the composition of the iron, taking into account the cooling rate of the casting. It can also be controlled by an annealing heat treatment to produce a fully ferritic casting, or by normalizing to maximize the pearlite content. Annealing, normalizing and other Ductile Iron heat treatments are discussed in Section VII.

Figure 3.15 shows the correlation between tensile properties, hardness and pearlite content in as-cast 1 inch (25 mm) keel blocks. The pearlite content was varied from 15 to 100 per cent by the use of different copper-manganese and tin-manganese combinations. Alloy levels beyond those required to produce a fully pearlitic matrix were also tested to determine their effects on properties. The apparent variation in properties at the 100% pearlite level is therefore not due to scatter in the data but an indication of the effects of higher alloy contents. Figure 3.15 reveals the remarkable consistency in the relationships between mechanical properties and pearlite content for all pearlite levels below 100 per cent, regardless of whether they were produced by Cu or Sn additions.

The effects of Cu and Sn diverge, however, for alloy levels approaching and exceeding those required to produce a fully pearlitic matrix. Additions of copper to a fully pearlitic matrix in the Cu-Mn alloy resulted in further increases in both yield and tensile strengths, probably due to solid solution strengthening. Additions of tin to the fully pearlitic Sn-Mn alloy did not affect the yield strength, but resulted in a decrease in tensile strength that has been related to the formation of intercellular degenerate graphite.

Figure 3.16, Figure 3.17 and Figure 3.18 provide further evidence of the relationships between tensile properties and pearlite and ferrite contents in Ductile Iron castings in the as-cast, fully annealed and normalized conditions respectively. These data, obtained from testing 1 inch (25 mm) keel blocks made from irons with average compositions of 3.75% C, 2.50% Si and 0.23% Mn, also show the influence of varying levels of Cu and Sn on tensile properties. As-cast properties (Figure 3.16) vary mainly through the influence of Cu and Sn levels on the pearlite content of the matrix. Yield and tensile strengths increase, and elongation decreases, until the matrix becomes fully pearlitic at 0.5% Cu for the Cu-hardened alloy and at 0.06% Sn for the Sn-pearlitized alloy. In agreement with Figure 3.15, additions of Cu and Sn beyond these levels have opposite effects on the tensile properties of the two alloys, with the Sn alloy becoming weaker and less ductile.

Figure 3.17 shows that the tensile properties of an annealed, fully ferritic casting are relatively constant, and independent of the quantities of either Cu or Sn. The UTS and BHN data for the Cu alloyed material suggest a slight solution hardening that is not produced by Sn. Ferritization of the fully pearlitic samples containing more than 0.06% Sn has eliminated the embrittling effect seen in the as-cast condition. (These Sn levels are of academic interest only, as the Sn content in commercial Ductile Iron is usually limited to less than 0.05%.)

Both hardness and strength of the normalized keel blocks increase with increasing Cu and Sn contents (Figure 3.18). In the Cu alloyed material, the increase is due to solid solution strengthening, while the initial increase produced by Sn is caused by the elimination of ferrite rings around the graphite particles, indicating that for the Sn series, the base composition provided insufficient hardenability for complete pearlitization.

The exceptional as-cast properties of the fully ferritic base material - 66 ksi UTS, 45 ksi YS and 26% elongation for a Quality Index of 113: - are noteworthy. The Quality Indices of the heat treated samples, which were taken from different keel blocks, ranged from 90 to 113.

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Low Temperature Tensile Properties

Ductile Irons are structurally stable at very low temperatures, but when designing for low temperature applications, the designer must take into consideration the significant effect of temperature on strength and elongation. Ferritic grades of Ductile Iron are generally preferred for low temperature applications because their ductility at low temperatures is superior to that of pearlitic grades. Figure 3.19 illustrates the effect of decreasing temperature on the tensile properties of an annealed ferritic Ductile Iron. As the temperature decreases, both the yield and tensile strengths increase, although the yield strength, which more accurately reflects the effect of temperature on flow stress, rises more rapidly. The room temperature elongation of 25 % is maintained to very low temperatures, - 200oF (- 130 oC), but as the yield and tensile stresses converge, the elongation decreases rapidly to less than 2% at - 330 oF (- 200 oC).

Pearlitic grades of Ductile Iron exhibit a significantly different response to decreasing temperature. Figure 3.20 shows that as the test temperature decreases, the yield strength increases, but the tensile strength and elongation decrease continuously. As a result of the steady deterioration in tensile strength and elongation below room temperature, pearlitic Ductile Irons should be used with caution at low temperatures.

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High Temperature Tensile Properties

Ductile Irons exhibit several properties which enable them to perform successfully in numerous elevated temperature applications. Unalloyed grades retain their strength to moderate temperatures and exhibit significantly better resistance to growth and oxidation than unalloyed Gray Iron. Alloy Ductile Irons (see Section V) provide outstanding resistance to deformation, growth and oxidation at high temperatures. The only high temperature applications in which Ductile Irons, with the exception of Type D-5 Ductile Ni-Resist, do not perform well are those involving severe thermal cycling. In these applications the low thermal conductivity of Ductile Iron, combined with a high modulus of elasticity, can result in internal stresses high enough to produce cracking and warpage. However, the successful use of Ductile Iron in millions of exhaust manifolds and turbocharger casings confirms that in specific thermal cycling applications Ductile Iron provides superior performance.

Figure 3.21 and Figure 3.22 show that the short-term, elevated temperature tensile strengths of unalloyed ferritic and pearlitic Ductile Irons initially decrease slowly, losing only about one-third of their values between room temperature and 425 oC (800 oF). Above this temperature the tensile strengths of both grades decrease rapidly with further increases in temperature. The pearlitic grade exhibits superior strength at all temperatures, due to a combination of higher ambient temperature strength and reduced effect of temperature on strength. Figure 3.21 and Figure 3.22 also describe both stress-rupture and creep behaviour above 425 oC (800 oF). The stress-rupture curves define the stress required to produce rupture failures after 10, 100 and 1000 hours. The creep curves define the stress required at a given temperature to produce a minimum creep rate of 0.0001%/h for both grades. As with the tensile properties, the short-term stress-rupture strength of the pearlitic grade is approximately twice that of the ferritic grade. However, the longer term rupture strength and creep strength of both materials are almost identical. The relatively poor longer term rupture and creep properties of the pearlitic iron, compared to its shorter term properties, are partly due to growth from graphitization and ferritization of the pearlite matrix.

Figure 3.23 is a Larson Miller Diagram which relates the high temperature creep and stress-rupture properties of unalloyed ferritic Ductile Iron to a combination of time and temperature. For example, a sample subjected to a stress of 4 ksi would be expected to have lives of 10, 100 and 1000 hours when tested at temperatures of 675, 625 and 595 oC (1245, 1160, and 1100 oF). Figure 3.23 also shows that the creep and stressrupture properties of Ductile Iron can be improved substantially by increasing the silicon content and adding molybdenum and aluminium. The effect of alloying elements on the high temperature properties of Ductile Iron will be presented in greater detail in Section V.

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Effect of Temperature on Design Stresses

When determining design stresses for a Ductile Iron component, the designer must be aware of both the temperature range in which the component will be operated and the effect of temperature on tensile properties. The increase in yield strength with decreasing temperature for both ferritic and pearlitic Ductile Irons suggests that higher design stresses may be used at low temperatures. Because most low temperature applications also involve performance at room temperatures, the room temperature yield strength must be used in the calculation of design stresses. However, the use of a yield strength-related design stress is acceptable for low temperature applications only when the applied stress state can be simulated by a quasi-static (low strain rate) test. In such cases, both ferritic and pearlitic grades may meet the design criteria. If the application involves impact loading, or if good notch toughness is specified, selection should be limited to ferritic grades. For special low temperature applications requiring maximum elongation and toughness, annealed ferritic grades should be used.

For temperatures up to 575o F (300o C), static design stresses can be based on the room temperature yield strength, as described earlier in this section. For temperatures above 650oF (350 oC), design stresses should be related to creep data for applications in which dimensional accuracy is critical or stress rupture data when deformation can be tolerated but time-to-failure is critical.

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Growth and Oxidation

The microstructural stability of unalloyed Ductile Irons at elevated temperatures depends primarily upon the matrix structure and the temperature. Ferritic Ductile Irons are stable up to a critical temperature of about 1350oF (730oC), while pearlitic grades exhibit growth through graphitization of the carbide component of the pearlite at temperatures above 1000oF (540oC). Above 1500oF (815oC) both ferritic and pearlitic grades of unalloyed Ductile Iron exhibit significant growth, with pearlitic grades growing more rapidly due to graphitization. Growth decreases with increasing section size and can be retarded by increasing the silicon content and alloying with chromium and molybdenum. Gray Iron, which grows by both graphitization and oxidation, exhibits higher growth rates than Ductile Iron. Table 3.1 compares the oxidation of different Ductile Irons and Gray Iron. Unalloyed Ductile Iron exhibits one-half the weight gain shown by Gray Iron. Increases in silicon content and additions of aluminium and molybdenum significantly decrease the oxidation of ferritic Ductile Iron to levels shown by the higher alloy, austenitic grades.

Table 3.1

  Weight
Gain*
 
  Analysis, Percent Oxide Depth
Type of Iron Silicon Alloys mg/cm2 mils mm
Ferritic Ductile 2.8   119.9 18.6 0.47
Ferritic Ductile 4.0 0.8 Al 6.3 3.5 0.09
Ferritic Ductile 4.2 1.9 Mo 0.6 Al 22.8 5.8 0.15
Ferritic Ductile 3.8 2.0 Mo 1.0 Al 15.2 3.7 0.09
Ferritic Ductile 4.0 2.0 Mo 0.9 Al 6.2 2.7 0.07
Austenitic Ductile 2.5 22.5 Ni 0.4 Cr 81.6 24.1 0.61
Austenitic Ductile 5.5 30.0 Ni 5.0 Cr 7.2 1.5 0.04
Austenitic Ductile 2.2 35.0 Ni 2.5 Cr 30.0 9.3 0.24
    1.0 Mo      
Gray Iron 2.0 0.14 Cr 217.2 35.3 0.09

*Net gain, oxidation minus decarburization.

Table 3.1. Oxidation behavious of ferritic and austenitic Ductile Irons in
flowing air at 1500
oF (815oC) for 500 hours.

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Effect of Environment on Tensile Properties

Like some steels, the ambient temperature tensile properties of certain grades of Ductile Iron can be reduced significantly by prolonged exposure to certain environments. Figure 3.24 summarizes the effects of exposure for 30 days to air-saturated, distilled water on the tensile properties of Ductile Iron samples with different hardness levels. Yield strength was not affected by exposure until hardness exceeded 275 BHN, above which it decreased rapidly, attaining a loss of over 40% at a hardness of 430 BHN. Tensile strength and elongation followed similar trends, but the loss of strength and ductility began at lower hardness levels, 175 BHN, and increased more slowly, attaining the same level of reduction (40%) at 430 BHN. Figure 3.24 indicates that exposure to water for 30 days has no significant effect on the tensile properties of ferritic Ductile Irons, but those quenched and tempered to produce hardness levels above 250 BHN are embrittled to a degree which increases with hardness. Embrittlement may be due to a hydrogen-related phenomenon similar to that occurring in high strength steels.

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Fatigue Strength

A fatigue failure occurs in a metal component by the initiation and propagation of a crack under cyclic loading conditions. Fatigue failures play a significant role in machine design and materials selection for the following reasons.

  • Fatigue is probably the primary cause of 80% of the service failures occurring in machines.
  • Fatigue failures can occur at stress amplitudes considerably below the yield strength.
  • Stress concentrations such as material flaws or abrupt changes in component cross-section are much more harmful to material performance under fatigue conditions than under monotonic tensile loading.
  • Fatigue cracks can grow slowly and without an easily detectable change in component dimension or performance. Upon reaching a critical size, catastrophic failure occurs.
  • Design stresses based on fatigue criteria will be lower than those determined using monotonic tensile design values and will be reduced further by stress concentrations caused by material flaws or component design.

The fatigue behaviour of a material is defined by its Fatigue Life - the number of stress or strain cycles at which failure occurs. The fatigue data for a material are normally plotted on a semi-logarithmic graph of stress amplitude versus the log of the number of cycles to failure. The resultant S-N curve defines the relationship between the stress amplitude (S) and the number of cycles to failure (N) when the mean stress is zero. Fatigue data are also plotted on Goodman Diagrams to define fatigue behaviour for non-zero mean stresses.

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Fatigue Limit

The fatigue strength of a material is normally defined by quoting its fatigue limit, also called the endurance limit. The fatigue limit is the magnitude of the cyclic stress at which the fatigue life exceeds a specified number of cycles, usually 106 or 107. The fatigue strength of a material is related to its tensile strength by the endurance ratio - the ratio of fatigue limit to tensile strength. The effect of stress-raisers on the fatigue limit is defined by the notch sensitivity ratio, also known as the fatigue strength reduction factor. The notch sensitivity ratio is the ratio of unnotched fatigue limit to notched fatigue limit. The fatigue limit of a Ductile Iron component is influenced by the following factors: tensile strength, the size, shape and distribution of graphite nodules, the volume fractions of inclusions, carbides and dross, the quantity and location of porosity, the presence of stress-raisers, and the condition of the component surface.

Figure 3.25 illustrates S-N curves for notched and unnotched annealed ferritic Ductile Iron with a tensile strength of 65.8 ksi (454 MPa). With notched and unnotched fatigue limits of 17 ksi (117 MPa) and 28 ksi (193 MPa) respectively, this material has notch sensitivity factor of 1.65 and an endurance ratio of .43. The endurance ratio of Ductile Iron depends upon the tensile strength and matrix. Figure 3.26 shows that the endurance ratios of ferritic and pearlitic grades are similar, decreasing from 0. 5 to 0. 4 with increasing strength within each grade. For tempered martensite matrices, the endurance ratio decreases from 0. 5 at a tensile strength of 60 ksi (415 MPa) to 0.3 at a UTS of 150 ksi (1035 MPa).

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Effect of Nodule Shape and Size

Figure 3.27 shows the influence of nodularity on the notched and unnotched fatigue limits of pearlitic Ductile Iron. The notched fatigue limit varies very little over a wide range of nodularity, while the unnotched fatigue limit increases rapidly with nodularity, especially at very high nodularities. These results indicate that non-spherical graphite initiates fatigue failure in unnotched Ductile Iron, while in v-notched specimens, the crack initiates prematurely in the notch, over-riding any effect of nodularity.

The net result of the different effects of modularity on notched and unnotched specimens is the variation of fatigue strength reduction factor (notch sensitivity ratio) with nodularity shown in Figure 3.28, in which notch sensitivity increases with increasing nodularity. Figure 3.29 illustrates the effect of nodule size on the fatigue limits of Ductile Irons with different matrix hardness. At all levels of hardness, fatigue strength increases as nodule size decreases, but the effect of nodule size is most pronounced as hardness increases.

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Effect of Metal Cleanliness

Under bending and torsional fatigue conditions in which the cyclic stresses reach a maximum at the component surface, fatigue strength is reduced by the presence of inclusions, dross, and other surface defects which act as crack initiation sites. Figure 3.30 shows that increasing the volume fraction of non-metallic inclusions significantly decreases fatigue strength. The influence of non-metallic inclusions on fatigue strength increases as matrix hardness increases. The increasing use of Ductile Iron components with as-cast surfaces places an increased importance on the elimination of surface defects for applications requiring optimum fatigue strength.

The reduction of dross-related surface defects through the use of filters in the mold filling system can result in a 25 per cent increase in fatigue life, as shown in Figure 3.31. The use of good foundry practices, including minimizing residual Mg content, careful deslagging of ladles, good gating and pouring practices, the use of filters in the gating system and the reduction of the effects of flake-forming elements in both the metal and molding materials, can result in fatigue strengths for ascast surfaces that are within 5 per cent of those obtained on components with machined surfaces.

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