Ductile Ni-Resist Irons D-2

  TABLE OF CONTENTS

  PREFACE

  I. FORWARD
  II. INTRODUCTION
  III. ENGINEERING DATA
  A. Introduction (p. 1)
  B. Tensile Properties (p.1&2)
  C. Other Mechanical (p.2)
  D. Physical Properties (p.2)
  E. References (p.2)
  IV. AUSTEMPERED DUCTILE   IRON
  V. ALLOY DUCTILE IRONS
  A. Introduction
  B. Silicon-Molybdenum Ductile Irons
  C. Austenitic Ductile Irons
  D. References

  VI. MACHINABILITY

  VII HEAT TREATMENT
  VIII WELDING, BRAZING AND BONDING 
  A.  Welding
  B.  Brazing
  C.  References
  IX SURFACE TREATMENTS
  X DESIGNING WITH DUCTILE IRON
  XI ORDERING CASTINGS
  XII SPECIFICATIONS
  XIII SEARCH (Index)

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SECTION V.  ALLOY DUCTILE IRONS

INTRODUCTION

Three families of alloy Ductile Irons -   austenitic (high nickel - Ni - Resist), bainitic and ferritic (high silicon-molybdenus) - have been developed either to provide special   properties or to meet the demands of service conditions that are too severe for conventional or austempered Ductile Irons. While conventional and austempered Ductile Irons contain limited percentages of alloying elements primarily to provide the desired microstructure, alloy Ductile Irons contain substantially higher levels of alloy in order to provide improved or special properties. The high silicon levels, combined with molybdenum, give the ferritic Ductile Irons superior mechanical properties at high temperatures and improved resistance to high temperature oxidation. The high nickel   content of the austenitic Ductile Irons, in conjunction with chromium in certain grades, provides improved corrosion resistance, superior mechanical  properties at both elevated and low temperatures and controlled expansion,   magnetic and electrical properties.   Bainitic irons are used where high  strength and good wear resistance are obtainable in either the as cast state or heat treated using from 1 - 3% alloy (Ni and Mo).  The bainitic irons   are not as widely used as the austenitic or Si-Mo Ductile Irons, so they will not be covered in this chapter.  The reader is encouraged to contact us for more information or consult other publications such as the "Iron Castings Handbook" available through the American Foundrymen's Society.

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SILICON-   MOLYBDENUM DUCTILE IRONS

Alloy Ductile Irons containing 4-6%   silicon, either alone or combined with up to 2 % molybdenum, were developed   to meet the increasing demands for high strength Ductile Irons capable of   operating at high temperatures in applications such as exhaust manifolds or turbocharger casings. The primary properties required for such applications are oxidation resistance, structural stability, strength, and resistance to thermal cycling.

These unalloyed grades retain their strength to moderate temperatures (Figures 3.21, 3.22, 3.23), perform well under low to moderate severity thermal cycling (Figure 3.37) and exhibit resistance to growth and oxidation that is superior to that of unalloyed Gray Iron (Table 3. 1).  Ferritic Ductile Irons exhibit less growth at high temperatures due to the   stability of the microstructure. Alloying with silicon and molybdenum  significantly improves the high temperature performance of ferritic Ductile Irons while maintaining many of the production and cost advantages of conventional Ductile Irons.

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Effect of Silicon

Silicon enhances the performance of Ductile Iron at elevated temperatures by stabilizing the ferritic matrix and forming a silicon-rich surface layer which inhibits oxidation. Stabilization of the ferrite phase reduces high temperature growth in two ways. First, silicon raises the critical temperature at which ferrite transforms to austenite (Figure 5.1). The critical temperature is considered to be the upper limit of the useful temperature range for ferritic Ductile Irons. Above this temperature the  expansion and contraction associated with the transformation of ferrite to austenite can cause distortion of the casting and cracking of the surface   oxide layer, reducing oxidation resistance. Second, the strong ferritizing tendency of silicon stabilizes the matrix against the formation of carbides and pearlite, thus reducing the growth associated with the decomposition of these phases at high temperature.

The oxidation protection offered by silicon   increases with increasing silicon content (Figure 5.2).  Silicon levels above 4% are sufficient to prevent any significant weight gain   after the formation of an initial oxide layer.

Table 5.1 Effect of silicon and molybdenum on the high temperature tensile and   creep rupture strengths of ferritic Ductile Iron.

Gray Iron: Unalloyed, stress-relieved. Ductile   Irons: Sub-Critically annealed at 1450^oF (788^oC).

Silicon influences the room temperature mechanical properties of Ductile Iron through solid solution hardening of the ferrite matrix. Figure 5.3 shows that increasing the silicon content increases the yield and tensile strengths and reduces elongation. For silicon levels above 6%, the material may become too brittle for engineering applications requiring any degree of toughness. Thus, the best combination of heat resistance and mechanical properties are   provided by silicon contents in the range 4-6%. The solid solution strengthening effect of silicon persists to temperatures as high as 1000^oF (540^oC) but above that temperature the tensile strength of high-silicon alloys is reduced as well  (Table 5.1). Figures 5.4 and 5.5 illustrate the high temperature creep and stress-rupture strengths obtained in ferritic  Ductile Irons containing 4% silicon.

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Effect of Molybdenum

Molybdenum, whose beneficial effect on the creep and stress-rupture properties of steels is well known, also has a similar influence on Ductile Irons. Figures 5.6 and 5.7 show that the addition of 0. 5 % molybdenum to ferritic Ductile Iron produces significant increases in creep and stress rupture strengths, resulting in   high temperature properties that are comparable to those of a cast steel  containing 0. 2 % carbon and 0. 6 % manganese.

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High Silicon with Molybdenum

The addition of up to 2% molybdenum to 4%   silicon Ductile Irons produces significant increases in high temperature tensile strength (Table 5. 1), stress-rupture strength (Tables 5.1 and 5.2 and Figure 5.5)   and creep strength (Figure 5.4).   Molybdenum additions in the range 0-1% to high-silicon Ductile Irons have been found to be very effective in increasing resistance to thermal fatigue (Table 5.3 and Figure 3.37).

Table 5.2 Effect of silicon and molybdenum on stress-rupture strength of ferritic   Ductile Irons.

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Table 5.3 Influence of silicon and molybdenum on the thermal cycling behaviour of ferritic Ductile Iron.

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Applications

High silicon-molybdenum Ductile Irons offer the designer and end user a combination of low cost, good high temperature strength, superior resistance to oxidation and growth, and good performance under thermal cycling conditions. As a result these materials have been very cost-effective in applications with service temperatures in the range 1200-1500^oF (650-820^oC) and where low to moderate severity thermal cycling may occur. Ductile Irons with 4% silicon and 0.6-0.8% molybdenum are presently specified for numerous automotive manifolds and turbocharger casings. High silicon irons containing 1% molybdenum are   used for special high temperature exhaust manifolds and heat treating racks.

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Production   Requirements

High silicon-molybdenum Ductile Irons can be produced successfully by any competent Ductile Iron foundry that has good process control, provided that the following precautions are taken.

Carbon levels should be kept in the range 2.5-3.4%. Carbon content should be reduced as the   silicon level and section size increase.

Silicon may   vary from 3.7 to 6% according to the application. Increasing the silicon content improves oxidation resistance and increases strength at low to intermediate temperatures but reduces toughness and machinability.

Molybdenum contents up to 2% may be used. Increasing the molybdenum level enhances high  temperature strength and improves machinability but reduces toughness and may segregate to form grain boundary carbides. Other pearlite and carbide stabilizing elements should be kept as low as possible to ensure a carbide-free ferritic matrix.

Normal nodularizing and inoculation   practices should be used but pouring temperatures should be higher than for   normal Ductile Iron. Increased dross levels require good gating and pouring practices, and increased shrinkage necessitates larger risers. Castings must be shaken out and handled carefully to avoid breakage, and all castings   should be heat treated to improve toughness.  Castings are commonly given a subcritical anneal - 4h at 1450^oF (790^oC) and furnace cooled to 400^oF (200^oC) - but a full anneal is required if the matrix contains significant quantities of carbides and pearlite. Machinability is similar to normal pearlitic /ferritic Ductile Irons with hardness values in the range 200-230 BHN.

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AUSTENITIC DUCTILE IRONS

A family of austenitic, high alloy Ductile Irons identified by the trade name "Ductile Ni-Resist" have been produced for many years to meet a wide range of applications requiring special chemical, mechanical and physical properties combined with the economy and ease of production of Ductile Iron. Ductile Ni-Resist irons containing 18-36% nickel and up to 6% chromium combine tensile strengths of 55-80 ksi (380-550 MPa) and elongations of 4-40% with the following special   properties:

● corrosion, erosion and wear resistance,

● good strength, ductility and oxidation resistance at high temperatures,

● toughness and low temperature stability,

● controlled thermal expansion,

● controlled magnetic and electrical properties and

● good castability and machinability.

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Specifications and Recommendations

Table 5.4 summarizes the ASTM and ASME specifications for Ductile Ni-Resist Irons and lists typical applications for each grade. Section XII contains further information on international specifications for these materials. The applications listed for each grade take advantage of the following general  characteristics.

Type D-2, the most commonly used grade, is recommended for service requiring resistance to   corrosion, erosion and frictional wear up to temperatures of 1400^oF (760^oC).