The Tempering of Martensite: Part One

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Introduction to Martensite and Tempering

Martensite is a very strong but typically brittle phase in steel, necessitating modification of its mechanical properties through heat treatment in the range of 150-700°C. This process, known as tempering, represents one of the oldest heat treatments applied to steels, though detailed understanding of the underlying phenomena has only been achieved in recent decades.

Fundamentally, martensite is a highly supersaturated solid solution of carbon in iron which, during tempering, rejects carbon to form finely divided carbide phases. The end result is a fine dispersion of carbides in an α-iron matrix, often bearing little structural similarity to the original as-quenched martensite.

It's important to note that in many steels, the martensite reaction does not fully complete during quenching, resulting in varying amounts of retained austenite which undergoes transformation during subsequent tempering.

The Four Stages of Tempering in Plain Carbon Steels

The tempering of plain carbon steels occurs in four distinct but overlapping stages. Before examining these stages, it should be recognized that the as-quenched martensite may already possess a complex structure due to auto-tempering—a phenomenon that occurs in the first-formed martensite near the martensite start temperature (Ms). This auto-tempering is more prevalent in steels with a high Ms temperature.

Stage 1: Initial Carbide Precipitation (Up to 250°C)

Martensite formed in medium and high carbon steels (0.3-1.5% C) is inherently unstable even at room temperature because interstitial carbon atoms can diffuse within the tetragonal martensite lattice. This instability increases between room temperature and 250°C, leading to the precipitation of α-iron carbide and a partial loss of tetragonality in the martensite structure.

Stage 2: Decomposition of Retained Austenite (230-300°C)

During the second stage, austenite retained during quenching decomposes, typically in the temperature range of 230-300°C. This stage was initially detected through X-ray diffraction, dilatometric, and specific volume measurements by Cohen and colleagues. Although direct observation of retained austenite in the microstructure has historically been challenging, particularly at low concentrations, available evidence suggests that retained austenite decomposes to bainite, ferrite, and cementite in this temperature range.

Stage 3: Formation of Cementite (100-300°C)

During the third stage of tempering, cementite first appears in the microstructure as a Widmanstatten distribution of rods, exhibiting a well-defined orientation relationship with the matrix. At this point, the matrix loses its tetragonality completely and transforms into ferrite.

This reaction begins at temperatures as low as 100°C and is fully developed by 300°C, with particles reaching up to 200 nm in length and approximately 15 nm in diameter. Similar structures are often observed in lower carbon steels in the as-quenched condition, resulting from the formation of Fe₃C during quenching.

The third stage marks the complete replacement of transition carbides and low-temperature martensite by cementite and ferrite. The matrix, having lost its tetragonality, is essentially ferrite no longer supersaturated with carbon. Subsequent morphological changes in cementite particles occur through an Ostwald ripening process, where smaller particles dissolve in the matrix, providing carbon for the selective growth of larger particles.

Stage 4: Coarsening and Spheroidization (300-700°C)

The fourth stage of tempering involves the coarsening and spheroidization of cementite particles, with the eventual loss of their crystallographic morphology. Coarsening begins between 300 and 400°C, while spheroidization continues progressively up to 700°C. At the higher end of this temperature range, martensite lath boundaries are replaced by more equiaxed ferrite grain boundaries through recrystallization.

The final structure consists of an equiaxed array of ferrite grains with coarse, spheroidized Fe₃C particles located partly, but not exclusively, at grain boundaries. Spheroidization of Fe₃C rods is driven by the resulting decrease in surface energy. Particles that preferentially grow and spheroidize are mainly located at interlath boundaries and prior austenite boundaries, though some remain within the matrix. These boundary sites are preferred due to greater ease of diffusion in these regions.

The original martensite lath boundaries remain stable up to approximately 600°C. However, between 350 and 600°C, considerable rearrangement of dislocations occurs within the laths and at lath boundaries that are essentially low-angle boundaries.

Influence of Carbon Content on Tempering Behavior

Carbon significantly influences the behavior of steels during tempering. The hardness of as-quenched martensite is largely determined by carbon content, as is the morphology of martensite laths, which exhibit a {111} habit plane up to 0.3% C, changing to {225} at higher carbon contents.

The martensite start temperature (Ms) decreases as carbon content increases, reducing the likelihood of auto-tempering. During rapid quenching in alloys with less than 0.2% C, up to 90% of the carbon segregates to dislocations and lath boundaries, while slower quenching may result in some cementite precipitation.

When tempering low carbon steels up to 200°C, further carbon segregation occurs, though no precipitation has been observed. Under normal circumstances, detecting tetragonality in martensite is difficult in steels with less than 0.2% C, which can be attributed to the rapid segregation of carbon during quenching.

Mechanical Properties and Applications of Tempered Plain Carbon Steels

Measuring the intrinsic mechanical properties of tempered plain carbon martensitic steels presents several challenges. The low hardenability of these steels limits the formation of fully martensitic structures to thin sections. Additionally, lower carbon steels have high Ms temperatures, making auto-tempering likely, while higher carbon steels often contain retained austenite that influences results. Quench cracking, particularly in steels with carbon content above 0.5%, further complicates reliable testing.

When properly processed, excellent mechanical properties—notably high proof and tensile strengths—can be achieved by tempering in the 100-300°C range. However, elongation is frequently low, and impact toughness is typically poor.

Plain carbon steels with less than 0.25% C are not typically quenched and tempered. Those with 0.25-0.55% C are often heat-treated to enhance mechanical properties, with tempering typically performed between 300 and 700°C. This treatment produces tensile strengths ranging from 800 to 1700 MPa, with toughness increasing as tensile strength decreases. This versatile group of steels finds applications in crankshafts, general machine parts, and hand tools such as screwdrivers and pliers.

High carbon steels (0.5-1.0% C) are more challenging to fabricate and are therefore primarily used in applications requiring high hardness and wear resistance, such as axes, knives, hammers, and cutting tools. Another important application is for springs, where the required mechanical properties are often achieved simply through heavy cold work, as in hard-drawn spring wire. However, carbon steels in the 0.5-0.75% C range are sometimes quenched and then tempered to achieve the desired yield stress.