Heat Treating of Nodular Irons: Part Two

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Introduction to Austenitizing Ductile Cast Iron

Austenitizing serves as the fundamental first step in most heat treatment processes for ductile cast iron. The primary objective is to produce an austenitic matrix with uniform carbon content prior to further thermal processing. For typical hypereutectic ductile cast iron, the austenitizing temperature must exceed the upper critical temperature to reach the two-phase region where austenite and graphite coexist. This critical temperature varies depending on the alloy composition.

The "equilibrium" austenite carbon content increases proportionally with higher austenitizing temperatures. This relationship allows manufacturers to selectively control (within limits) the matrix austenite carbon content, making austenitizing temperature control particularly crucial in processes that rely on matrix carbon to drive subsequent reactions. This is especially important in structures intended for austempering, where hardenability (or austemperability) depends significantly on the matrix carbon content.

Generally, the time required for complete austenitizing is determined by three key factors: alloy content, the original microstructure, and the section size of the casting. The following sections on specific heat treatments will discuss austenitizing considerations in greater detail where relevant.

Annealing Processes for Ductile Cast Iron

When applications require maximum ductility and superior machinability rather than high strength, ductile iron castings typically undergo a full ferritizing anneal. This treatment converts the microstructure to ferrite while excess carbon deposits on existing graphite nodules. The result is ASTM grade 60-40-18 iron with excellent formability. For optimal machinability, elements that retard the annealing process—such as manganese, phosphorus, chromium, and molybdenum—should be minimized.

Recommended annealing practices vary based on alloy content and the presence of eutectic carbides:

Full anneal for unalloyed 2-3% Si iron without eutectic carbides:

• Heat to 870-900°C (1600-1650°F) and hold for 1 hour per inch of section
• Furnace cool at 55°C/h (100°F/h) to 345°C (650°F)
• Air cool to room temperature

Full anneal for castings containing carbides:

• Heat to 900-925°C (1650-1700°F) and hold for minimum 2 hours (longer for heavier sections)
• Furnace cool at 110°C/h (200°F/h) to 700°C (1300°F)
• Hold at 700°C (1300°F) for 2 hours
• Furnace cool at 55°C/h (100°F/h) to 345°C (650°F)
• Air cool to room temperature

Subcritical anneal to convert pearlite to ferrite:

• Heat to 705-720°C (1300-1330°F) and hold for 1 hour per inch of section
• Furnace cool at 55°C/h (100°F/h) to 345°C (650°F)
• Air cool to room temperature

For alloy-containing irons, controlled cooling times through the critical temperature range down to 400°C (750°F) must be reduced below 55°C/h (100°F/h).

It's important to note that certain carbide-forming elements, particularly chromium, form primary carbides that resist decomposition. For instance, just 0.25% Cr creates intercellular carbides that cannot be broken down even with 2-20 hour heat treatments at 925°C (1700°F). The resulting matrix consists of carbides in ferrite with only 5% elongation. Other problematic carbide stabilizers include molybdenum exceeding 0.3%, and vanadium or tungsten exceeding 0.05%.

Understanding Hardenability in Ductile Cast Iron

Hardenability serves as a critical parameter that determines how ductile cast iron responds to normalizing, quenching and tempering, or austempering processes. It essentially measures the ability of the iron to form hardened structures throughout its cross-section during cooling.

The standard measurement method is the Jominy test, where a standardized bar (1 inch diameter by 4 inches in length) is austenitized and then water quenched from one end. The variation in cooling rate along the bar results in microstructural differences, producing hardness gradients that are measured and recorded as the Jominy curve.

Higher austenitizing temperatures increase the matrix carbon content, which enhances hardenability. This shifts the Jominy curve to greater distances from the quenched end and produces higher maximum hardness values.

Alloy elements are added to ductile cast irons specifically to increase hardenability. Manganese and molybdenum demonstrate much higher effectiveness per weight percent added compared to nickel or copper. However, similar to steels, combinations of elements produce synergistic effects that exceed their individual contributions. Common effective combinations include:

• Nickel and molybdenum
• Copper and molybdenum
• Copper, nickel, and manganese

Heavy-section castings requiring through-hardening or austempering typically contain combinations of these elements to ensure adequate hardenability throughout the entire cross-section. Silicon, while having significant effects on other properties, has minimal direct impact on hardenability apart from its influence on matrix carbon content.

Normalizing Treatment for Enhanced Strength

Normalizing—the process of air cooling following austenitizing—can significantly improve tensile strength and is commonly used to produce ASTM type 100-70-03 ductile iron. The resulting microstructure depends primarily on two factors: the casting's composition (which determines hardenability) and the cooling rate during normalization.

The composition influences the time-temperature-transformation (CCT) diagram, positioning the various transformation fields. The cooling rate is determined not only by the casting's mass but also by the temperature and movement of the surrounding air during the cooling process.

Normalizing typically produces a homogeneous structure of fine pearlite in irons with moderate silicon content and at least 0.3-0.5% manganese. Heavier castings requiring normalized structures usually contain alloying elements such as nickel, molybdenum, and additional manganese to ensure sufficient hardenability for developing a fully pearlitic structure after normalizing. Lighter castings made from alloyed iron may develop martensitic or acicular structures after normalizing due to their faster cooling rates.

The standard normalizing temperature range is 870-940°C (1600-1725°F). Holding time at temperature typically follows the rule of 1 hour per inch of section thickness, with a minimum of 1 hour for thin sections. Longer times may be necessary for alloys containing elements that impede carbon diffusion in austenite. For example, tin and antimony segregate to graphite nodules, effectively blocking carbon solution from these sites.

Normalizing is frequently followed by tempering to:

• Achieve desired hardness levels
• Relieve residual stresses that develop during air cooling (especially in complex castings with varying section sizes)
• Enhance toughness and impact resistance

The effect of tempering on hardness and tensile properties depends on the iron's composition and the hardness achieved during normalizing. Standard tempering involves reheating to 425-650°C (800-1200°F) and holding at the selected temperature for 1 hour per inch of cross-section. These temperatures are adjusted within the specified range to meet required specifications.

Quenching and Tempering for Maximum Hardness

The quench and temper process begins with austenitizing at 845-925°C (1550-1700°F) for commercial castings. Oil is the preferred quenching medium to minimize stresses and prevent quench cracking, though water or brine may be suitable for simple shapes. Complicated castings often require oil quenching at elevated temperatures of 80-100°C (180-210°F) to avoid cracking.

Studies of water-quenched ductile iron cubes show that the highest hardness range (55-57 HRC) is achieved with austenitizing temperatures between 845-870°C (1550-1600°F). Temperatures above 870°C result in higher matrix carbon content, which increases retained austenite and consequently reduces hardness.

Castings should be tempered immediately after quenching to relieve quenching stresses. The resulting tempered hardness depends on:

• As-quenched hardness level
• Alloy content
• Tempering temperature
• Tempering time

Tempering in the range of 425-600°C (800-1100°F) reduces hardness, with the magnitude dependent on the factors listed above. Vickers hardness measurements of quenched ductile iron alloys show predictable changes with tempering temperature and time.

The tempering of ductile iron proceeds through two distinct stages:

1. Initial carbide precipitation (similar to steel tempering)
2. Secondary graphitization — the nucleation and growth of small, secondary graphite nodules that form at the expense of carbides

The hardness reduction accompanying secondary graphitization also produces corresponding decreases in tensile and fatigue strength. Since alloy content affects the rate of secondary graphitization, each alloy composition has a unique range of effective tempering temperatures.

Austempering for Optimized Strength-Ductility Balance

Austempering offers the opportunity to produce a unique austenite-ferrite matrix structure that provides a significantly better tensile strength-to-ductility ratio than any other grade of ductile cast iron. Achieving these desirable properties requires careful consideration of section size and precise control of time-temperature parameters during both austenitizing and austempering.

Section Size and Alloying Considerations

As section size increases, the rate of temperature change between austenitizing and austempering temperatures decreases. Various quenching and austempering techniques are available:

• Hot-oil quench (limited to 240°C/460°F)
• Nitrate/nitrite salt baths
• Fluidized-bed methods (suitable only for thin, small parts)
• Lead baths (for tool-type applications)

To prevent formation of undesirable high-temperature reaction products (such as pearlite) in larger sections, salt bath quench severity can be enhanced through:

• Water additions to the bath
• Alloying elements that improve pearlite hardenability (copper, nickel, manganese, or molybdenum)

It's crucial to understand that these alloying elements tend to segregate during solidification, creating a non-uniform distribution throughout the matrix. This can adversely affect the austempering reaction and resulting mechanical properties, with ductility and impact toughness being most severely affected.

Manganese and molybdenum provide the most powerful effect on pearlite hardenability but tend to segregate into intercellular regions where they promote formation of iron or alloy carbides. Nickel and copper have less impact on hardenability but segregate to graphite nodule sites without forming detrimental carbides. Combinations of these elements, with their opposite segregation patterns, are often selected for their synergistic effect on hardenability.

Austenitizing Temperature and Time Effects

Phase diagram relationships show that matrix carbon content increases with higher austenitizing temperatures. The actual carbon content depends on a complex interaction of alloying elements, their concentrations, and their distribution within the matrix.

Silicon content has the greatest influence on matrix carbon in ductile irons—as silicon increases at a given austenitizing temperature, matrix carbon content decreases. Typical austenitizing temperatures range from 845-925°C (1550-1700°F), with times of approximately 2 hours generally sufficient to fully recarburize the matrix.

Austenitizing temperature significantly affects hardenability through its impact on matrix carbon content. Higher temperatures increase carbon content, which enhances hardenability and slows the rate of isothermal austenite transformation.

Austempering Temperature and Time Parameters

The austempering temperature primarily determines the final microstructure and therefore the hardness, strength, and toughness of the austempered product. As austempering temperature increases, both strength and impact toughness change in predictable patterns.

Achieving maximum ductility at any given austempering temperature requires precise time control. The initial increase in elongation occurs as the stage I reaction progresses to completion, at which point the austenite fraction reaches its maximum. Extending austempering time beyond this optimal point reduces ductility as the stage II reaction causes decomposition to the equilibrium bainite product. Typical austempering times range from 1 to 4 hours, depending on composition and desired properties.