Heat Treating of Gray Irons: Part Two

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Heat Treating of Gray Irons: Understanding Matrix Transformation Methods

Gray irons comprise a family of cast irons characterized by flake graphite formation during solidification, distinguishing them from ductile irons with their spheroidal graphite morphology. The flake graphite in these materials is distributed within a matrix whose microstructure is determined by both composition and subsequent heat treatment processes.

Heat treatment significantly alters the matrix microstructure while maintaining the original size and shape of the graphite formed during casting. Through appropriate thermal processing, the matrix can be transformed across a spectrum of microstructures ranging from ferrite-pearlite combinations to tempered martensite. Despite the ability to harden gray iron through elevated temperature quenching, heat treatment is rarely employed commercially for overall strength enhancement. This is primarily because increasing the as-cast strength can be achieved more economically by either reducing silicon and total carbon contents or introducing alloying elements.

Hardening and Tempering Processes for Enhanced Wear Resistance

Gray irons undergo hardening and tempering primarily to improve mechanical properties, particularly strength and wear resistance. After proper hardening and tempering treatments, these irons typically exhibit wear resistance approximately five times greater than their pearlitic counterparts.

Furnace or salt bath hardening offers greater versatility across gray iron compositions compared to flame or induction hardening techniques. The latter methods require relatively high combined carbon content due to the extremely brief period available for carbon dissolution in austenite. In contrast, furnace or salt bath hardening allows castings to be maintained at temperatures above the transformation range for extended periods, enabling even irons with minimal initial combined carbon to be effectively hardened.

Austenitizing Parameters and Transformation Considerations

Unalloyed gray iron with low combined carbon content requires extended austenitizing time to adequately saturate austenite with carbon. As austenitizing duration increases, more carbon dissolves into the austenite phase, resulting in higher post-quench hardness values. Despite containing the same carbon content (0.60%), unalloyed gray iron demonstrates superior hardenability compared to carbon steel due to its higher silicon content. However, this advantage comes with a trade-off—the silicon reduces carbon solubility in austenite, necessitating higher austenitizing temperatures to achieve equivalent hardness levels in silicon-rich compositions.

Manganese serves as a powerful hardenability enhancer in gray iron. Research has demonstrated that approximately 1.50% manganese content is sufficient to achieve through-hardening in 38mm sections using oil quenching or 64mm sections with water quenching. While manganese, nickel, copper, and molybdenum are well-established elements for improving gray iron hardenability, chromium plays a different role. Although chromium alone does not directly influence hardenability, its contribution to carbide stabilization proves especially valuable during flame hardening processes.

The hardening process for gray iron involves heating the casting to a temperature sufficient for austenite formation, maintaining this temperature until the desired carbon dissolution occurs, and then quenching at an appropriate rate. The specific austenitizing temperature depends on the transformation range of the particular gray iron composition. This transformation range can extend more than 55°C above the A (transformation-start) temperature. For unalloyed gray iron, the approximate A transformation temperature can be calculated using:

A (°C) = 730 + 28.0 (% Si) - 25.0 (% Mn)

Different alloying elements affect the transformation temperature in distinct ways. Chromium elevates the transformation range, with each percentage point raising the temperature by approximately 10-15°C in high-nickel, high-silicon irons. Conversely, nickel lowers the critical range, with gray irons containing 4-5% nickel exhibiting an upper transformation range limit of approximately 710°C.

Quenching Media Selection and Tempering Requirements

Careful temperature management during the heating cycle is essential to prevent cracking. Castings should be heated slowly through the lower temperature range. Once above the stress-relieving range (595-650°C), heating can proceed as rapidly as desired. For production efficiency, a two-furnace approach may be employed—slowly heating castings to about 650°C in one furnace before transferring them to a second furnace for rapid heating to the final austenitizing temperature.

The selection of quenching media significantly impacts the success of gray iron heat treatment. Molten salt and oil are the most frequently utilized quenching media. Water generally proves unsatisfactory for furnace-heated gray iron due to its excessive cooling rate, which frequently leads to distortion and cracking in all but the smallest, simplest components. Recently developed water-soluble polymer quenchants offer a practical alternative, providing water's convenience with reduced cooling rates that minimize thermal shock.

Air quenching represents the least severe cooling method but typically provides insufficient cooling rates for unalloyed or low-alloy gray iron castings to form martensite. However, for highly alloyed gray irons, forced-air quenching often becomes the preferred cooling approach.

After quenching, gray iron castings typically undergo tempering at temperatures well below the transformation range. The tempering duration generally follows the guideline of approximately one hour per inch of the thickest section. This tempering process produces a controlled decrease in hardness while typically improving both strength and toughness of the quenched iron.

Austempering: Achieving Bainitic Microstructures in Gray Iron

Austempering produces a distinctive microstructural transformation in the gray iron matrix. When performed below the pearlite formation temperature range but above the martensite formation range, this process generates an acicular or bainitic ferrite structure, accompanied by varying amounts of retained austenite depending on the transformation temperature. The process involves quenching the iron from above its transformation range into a hot quenching bath and maintaining it at a constant temperature until the austempering transformation completes.

The austenitizing parameters for hot quenching processes, including austempering, mirror those used in conventional hardening. Castings are typically heated to temperatures between 840 and 900°C (1550 and 1650°F), with holding times determined by both the casting size and chemical composition. For the austempering process specifically, gray iron is usually quenched in salt, oil, or lead baths maintained at temperatures between 230 and 425°C. When maximizing hardness and wear resistance is the primary objective, processors typically select quench bath temperatures between 230 and 290°C.

The composition of the iron significantly influences the required holding time during austempering. Alloying additions such as nickel, chromium, and molybdenum increase the transformation time, requiring longer processing cycles to achieve complete microstructural development.

Martempering: Controlling Stress Development During Martensite Formation

Martempering offers a specialized approach to producing martensite without the high internal stresses typically associated with its formation. While similar to conventional hardening, martempering significantly reduces distortion. However, it's important to note that the characteristic brittleness of martensite remains in the gray iron casting after martempering, necessitating subsequent tempering in nearly all applications.

The martempering process involves quenching the casting from above its transformation range into a salt, oil, or lead bath maintained at a temperature slightly above the martensite formation range—typically 200 to 260°C (400 to 500°F) for unalloyed irons. The casting remains in the bath only until it reaches thermal equilibrium with the bath, after which it is cooled to room temperature.

When a fully martensitic structure is the desired outcome, precise timing becomes critical. The casting must remain in the hot quench bath only long enough to reach the bath temperature, making the size and shape of the casting the primary determinants of the appropriate martempering duration.

Flame Hardening: Surface Treatment for Enhanced Wear Resistance

Flame hardening represents the most commonly applied surface hardening method for gray iron. This process creates a composite structure consisting of a hard, wear-resistant outer layer of martensite surrounding a core of softer gray iron that remains below the A₁ transformation temperature during treatment.

Both unalloyed and alloyed gray irons can undergo successful flame hardening, though certain compositions yield superior results. The combined carbon content plays a particularly crucial role, with the optimal range typically falling between 0.50 and 0.70%. While irons with combined carbon levels as low as 0.40% can be flame hardened, those exceeding 0.80% combined carbon (such as mottled or white irons) generally present a high risk of cracking during surface hardening, making flame hardening inadvisable for these compositions.

Effects of Alloying Elements on Flame Hardening Performance

Alloyed gray irons generally demonstrate superior flame hardening characteristics compared to their unalloyed counterparts, primarily due to their enhanced hardenability. Alloying additions can also significantly increase the final hardness achieved through flame hardening. An unalloyed gray iron containing approximately 3% total carbon, 1.7% silicon, and 0.60 to 0.80% manganese typically attains a surface hardness of 400 to 500 HB after flame hardening. It's important to recognize that this Brinell hardness value represents an average between the hardness of the matrix and the relatively soft graphite flakes. The actual matrix hardness—which directly influences wear resistance—approaches 600 HB.

Strategic alloying can elevate these hardness values considerably. For instance, incorporating 2.5% nickel and 0.5% chromium enables an average surface hardness of 550 HB. Comparable results have been achieved using alternative combinations of 1.0 to 1.5% nickel with 0.25% molybdenum. These improvements in hardness translate directly to enhanced wear performance in service.

Following flame hardening, stress relieving becomes an important consideration. Whenever practical or economically feasible, flame-hardened castings should undergo stress relief treatment at temperatures between 150 and 200°C to minimize distortion and prevent potential in-service failures.

Induction Hardening: High-Volume Surface Treatment Technology

Induction hardening offers an alternative surface hardening approach for gray iron castings, particularly advantageous when production volumes are sufficient to justify the relatively high equipment costs and the requirement for specialized induction coils. This process delivers rapid, localized heating that can be precisely controlled for consistent results in high-volume production environments.

Due to the short heating cycles characteristic of induction hardening, combined carbon content becomes a critical factor in achieving satisfactory results. Significant variation in hardness may occur due to fluctuations in combined carbon levels across different castings or even within the same casting. A minimum combined carbon content of 0.40 to 0.50% is generally recommended for cast iron subjected to induction hardening.

When processing castings with lower combined carbon content, operators might attempt to dissolve some free graphite by employing higher hardening temperatures for extended periods. However, this approach carries significant risks, particularly grain coarsening, which can compromise mechanical properties. Therefore, maintaining adequate combined carbon content through proper casting practices remains the preferred approach for ensuring successful induction hardening.

Selecting the Optimal Heat Treatment Process for Gray Iron Applications

The selection of an appropriate heat treatment process for gray iron components depends on multiple factors including the desired property profile, component geometry, production volume, and economic considerations. For applications requiring improved wear resistance throughout the entire component, conventional hardening and tempering or austempering may be preferred. When only surface properties need enhancement while maintaining a tough core, flame or induction hardening typically offers more cost-effective solutions.

The composition of the gray iron must be matched to the intended heat treatment process. Unalloyed irons with adequate combined carbon content can undergo most treatments, while specialized applications may require specific alloying additions to achieve optimal response to the selected heat treatment. When designing components for heat treatment, engineers should consider not only the initial processing requirements but also the dimensional stability and service performance of the treated components.

By properly matching the gray iron composition, component design, and heat treatment process, manufacturers can achieve substantial improvements in performance characteristics while maintaining economic viability in their production processes.