Heat Treating of Gray Irons: Part One

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Introduction to Gray Irons

Gray irons form a distinct category of cast irons characterized by their flake graphite formation during solidification, differentiating them from ductile irons which develop spheroidal graphite morphology. The microstructure of gray irons consists of graphite flakes dispersed throughout a matrix that is predominantly pearlitic in its standard form. Both the initial composition and subsequent heat treatment determine the final matrix microstructure.

Typical gray iron compositions contain 2.5 to 4% carbon, 1 to 3% silicon, and varying manganese levels ranging from as low as 0.1% in ferritic gray irons to as high as 1.2% in pearlitic varieties. Additional alloying elements commonly include nickel, copper, molybdenum, and chromium, each contributing specific properties to the final material.

Heat treatments can substantially transform the matrix microstructure of gray irons while having minimal effect on the graphite flakes formed during casting. Through appropriate heat treatment, the matrix can range from ferrite-pearlite combinations to tempered martensite. Despite the possibility of hardening gray iron through quenching, this approach is rarely used commercially to increase overall strength, as adjusting the chemical composition during casting offers a more cost-effective alternative for strength enhancement.

Chemical Composition Effects on Heat Treatment

The chemical composition of gray cast irons significantly influences their response to heat treatment processes. Silicon, a key element in gray irons, produces several important effects:

• Decreases carbon solubility
• Increases carbon diffusion rate in austenite
• Accelerates reactions during heat treating
• Raises the austenitizing temperature
• Reduces combined carbon content (cementite volume)

In contrast, manganese creates different effects:

• Lowers the austenitizing temperature
• Increases hardenability
• Enhances carbon solubility
• Slows carbon diffusion in austenite
• Increases combined carbon content
• Stabilizes pearlitic carbide, increasing pearlite content

These compositional factors must be carefully considered when designing heat treatment processes for gray irons.

Annealing Processes for Gray Iron

Annealing represents one of the most frequently applied heat treatments for gray iron, second only perhaps to stress relieving. The primary purpose of annealing is to soften the iron and minimize or eliminate massive eutectic carbides, thereby improving machinability. However, this process substantially reduces mechanical properties, typically lowering the grade level to approximately the next lower grade (e.g., from class 40 to class 30). The specific property reduction depends on the annealing temperature, holding time, and alloy composition.

Ferritizing Annealing

When improved machinability is the primary goal and only conversion of pearlitic carbide to ferrite and graphite is desired, a ferritizing anneal between 700 and 760°C (1300 and 1400°F) is recommended. For unalloyed or low-alloy cast iron with normal composition, heating above the transformation range is generally unnecessary. Below approximately 595°C (1100°F), short exposure times have insignificant effects on the structure of gray iron.

Medium (Full) Annealing

Medium or full annealing is performed at temperatures between 790 and 900°C (1450 and 1650°F). This treatment is particularly effective when ferritizing annealing would be inadequate due to high alloy content. However, it's advisable to test the effectiveness of lower temperatures before adopting higher annealing temperatures as standard procedure.

Holding times for medium annealing are typically similar to those used in ferritizing annealing. The critical difference is that when using the higher temperatures of medium annealing, the casting must be cooled slowly through the transformation range, approximately from 790 to 675°C (1450 to 1250°F).

Graphitizing Annealing

When gray iron microstructure contains massive carbide particles, higher annealing temperatures become necessary. Graphitizing annealing may serve to convert massive carbide to pearlite and graphite, though in some applications, a subsequent ferritizing annealing treatment may be desirable to maximize machinability.

The presence of free carbide requiring removal through annealing is typically an unintended result of inadequate inoculation or excess carbide formers that inhibit normal graphitization. Therefore, annealing is not considered part of the normal production cycle except for pipe and permanent mold castings.

For effective breakdown of massive carbide within reasonable time frames, temperatures of at least 870°C (1600°F) are required. The rate of carbide decomposition doubles with each additional 55°C (100°F) increment in holding temperature. Consequently, standard practice typically employs holding temperatures between 900 and 955°C (1650 and 1750°F).

Normalizing of Gray Iron

Normalizing gray iron involves heating it to a temperature above the transformation range, maintaining this temperature for approximately 1 hour per inch of maximum section thickness, and then cooling it in still air to room temperature. This process may be employed to enhance mechanical properties such as hardness and tensile strength or to restore as-cast properties that have been altered by other heating processes, including graphitizing or repair welding procedures.

The typical temperature range for normalizing gray iron is approximately 885 to 925°C (1625 to 1700°F). The austenitizing temperature significantly affects both the resulting microstructure and mechanical properties.

The tensile strength and hardness of normalized gray iron depend primarily on three key parameters:

• Combined carbon content
• Pearlite spacing (distance between cementite plates)
• Graphite morphology

Since graphite morphology remains largely unchanged during normalization, its effect on hardness and tensile strength is less significant in discussions of the normalizing process.

The combined carbon content is determined by both the normalizing temperature and the chemical composition. Higher normalizing temperatures increase carbon solubility in austenite, which increases the cementite volume in the resulting pearlite. This higher cementite volume contributes to increased hardness and tensile strength. The alloy composition also influences carbon solubility in austenite, with different elements either increasing, decreasing, or having no effect on this property.

Pearlite spacing, the other critical parameter affecting mechanical properties, is determined by the cooling rate after austenitization and the alloy composition. Faster cooling produces smaller pearlite spacing, resulting in higher hardness and tensile strength. However, excessive cooling rates may cause partial or complete martensitic transformation. Alloying elements can significantly influence the hardness and tensile strength achieved through the normalizing process.