Dis-moi, quel est bien la limite de solubilité maximale du carbone dans l'austénite à 1148°C ?
Properties

The density of steel varies based on the alloying constituents but usually ranges between 7,750 and 8,050 kg/m (484 and 503 lb/cu ft), or 7.75 and 8.05 g/cm (4.48 and 4.65 oz/cu in).

Even in a narrow range of concentrations of mixtures of carbon and iron that make steel, several different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of pure iron is the body-centred cubic (BCC) structure called alpha iron or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at 0 °C (32 °F) and 0.021 wt% at 723 °C (1,333 °F). The inclusion of carbon in alpha iron is called ferrite. At 910 °C, pure iron transforms into a face-centred cubic (FCC) structure, called gamma iron or γ-iron. The inclusion of carbon in gamma iron is called austenite. The more open FCC structure of austenite can dissolve considerably more carbon, as much as 2.1%, (38 times that of ferrite) carbon at 1,148 °C (2,098 °F), which reflects the upper carbon content of steel, beyond which is cast iron. When carbon moves out of solution with iron, it forms a very hard, but brittle material called cementite (Fe C).

When steels with exactly 0.8% carbon (known as a eutectoid steel), are cooled, the austenitic phase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC austenite structure, resulting in an excess of carbon. One way for carbon to leave the austenite is for it to precipitate out of solution as cementite, leaving behind a surrounding phase of BCC iron called ferrite with a small percentage of carbon in solution. The two, cementite and ferrite, precipitate simultaneously producing a layered structure called pearlite, named for its resemblance to mother of pearl. In a hypereutectoid composition (greater than 0.8% carbon), the carbon will first precipitate out as large inclusions of cementite at the austenite grain boundaries until the percentage of carbon in the grains has decreased to the eutectoid composition (0.8% carbon), at which point the pearlite structure forms. For steels that have less than 0.8% carbon (hypoeutectoid), ferrite will first form within the grains until the remaining composition rises to 0.8% of carbon, at which point the pearlite structure will form. No large inclusions of cementite will form at the boundaries in hypoeutectoid steel. The above assumes that the cooling process is very slow, allowing enough time for the carbon to migrate.

As the rate of cooling is increased the carbon will have less time to migrate to form carbide at the grain boundaries but will have increasingly large amounts of pearlite of a finer and finer structure within the grains; hence the carbide is more widely dispersed and acts to prevent slip of defects within those grains, resulting in hardening of the steel. At the very high cooling rates produced by quenching, the carbon has no time to migrate but is locked within the face-centred austenite and forms martensite. Martensite is a highly strained and stressed, supersaturated form of carbon and iron and is exceedingly hard but brittle. Depending on the carbon content, the martensitic phase takes different forms. Below 0.2% carbon, it takes on a ferrite BCC crystal form, but at higher carbon content it takes a body-centred tetragonal (BCT) structure. There is no thermal activation energy for the transformation from austenite to martensite. There is no compositional change so the atoms generally retain their same neighbours.

Martensite has a lower density (it expands during the cooling) than does austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when steel is water quenched, although they may not always be visible.
Facies d'éclogite

La lawsonite reste stable jusqu'à 1080 °C et 9,4 GPa. La décomposition de la lawsonite libère d'énormes quantités d'H₂O dans le manteau, ce qui peut déclencher une fusion partielle de la plaque et du manteau sus-jacent. La réaction de décomposition de la lawsonite est indiquée ci-dessous.
### Query ###
Dis-moi, quel est bien la limite de solubilité maximale du carbone dans l'austénite à 1148°C ?

### Main Text ###
Properties

The density of steel varies based on the alloying constituents but usually ranges between 7,750 and 8,050 kg/m (484 and 503 lb/cu ft), or 7.75 and 8.05 g/cm (4.48 and 4.65 oz/cu in).

Even in a narrow range of concentrations of mixtures of carbon and iron that make steel, several different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of pure iron is the body-centred cubic (BCC) structure called alpha iron or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at 0 °C (32 °F) and 0.021 wt% at 723 °C (1,333 °F). The inclusion of carbon in alpha iron is called ferrite. At 910 °C, pure iron transforms into a face-centred cubic (FCC) structure, called gamma iron or γ-iron. The inclusion of carbon in gamma iron is called austenite. The more open FCC structure of austenite can dissolve considerably more carbon, as much as 2.1%, (38 times that of ferrite) carbon at 1,148 °C (2,098 °F), which reflects the upper carbon content of steel, beyond which is cast iron. When carbon moves out of solution with iron, it forms a very hard, but brittle material called cementite (Fe C).

When steels with exactly 0.8% carbon (known as a eutectoid steel), are cooled, the austenitic phase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC austenite structure, resulting in an excess of carbon. One way for carbon to leave the austenite is for it to precipitate out of solution as cementite, leaving behind a surrounding phase of BCC iron called ferrite with a small percentage of carbon in solution. The two, cementite and ferrite, precipitate simultaneously producing a layered structure called pearlite, named for its resemblance to mother of pearl. In a hypereutectoid composition (greater than 0.8% carbon), the carbon will first precipitate out as large inclusions of cementite at the austenite grain boundaries until the percentage of carbon in the grains has decreased to the eutectoid composition (0.8% carbon), at which point the pearlite structure forms. For steels that have less than 0.8% carbon (hypoeutectoid), ferrite will first form within the grains until the remaining composition rises to 0.8% of carbon, at which point the pearlite structure will form. No large inclusions of cementite will form at the boundaries in hypoeutectoid steel. The above assumes that the cooling process is very slow, allowing enough time for the carbon to migrate.

As the rate of cooling is increased the carbon will have less time to migrate to form carbide at the grain boundaries but will have increasingly large amounts of pearlite of a finer and finer structure within the grains; hence the carbide is more widely dispersed and acts to prevent slip of defects within those grains, resulting in hardening of the steel. At the very high cooling rates produced by quenching, the carbon has no time to migrate but is locked within the face-centred austenite and forms martensite. Martensite is a highly strained and stressed, supersaturated form of carbon and iron and is exceedingly hard but brittle. Depending on the carbon content, the martensitic phase takes different forms. Below 0.2% carbon, it takes on a ferrite BCC crystal form, but at higher carbon content it takes a body-centred tetragonal (BCT) structure. There is no thermal activation energy for the transformation from austenite to martensite. There is no compositional change so the atoms generally retain their same neighbours.

Martensite has a lower density (it expands during the cooling) than does austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when steel is water quenched, although they may not always be visible.

### Supplementary Text ###
Facies d'éclogite

La lawsonite reste stable jusqu'à 1080 °C et 9,4 GPa. La décomposition de la lawsonite libère d'énormes quantités d'H₂O dans le manteau, ce qui peut déclencher une fusion partielle de la plaque et du manteau sus-jacent. La réaction de décomposition de la lawsonite est indiquée ci-dessous.

### Draft ###

Query: "Dis-moi, quel est bien la limite de solubilité maximale du carbone dans l'austénite à 1148°C ?"

Translation check: "Tell me, what exactly is the maximum solubility limit of carbon in austenite at 1148°C?"

Key terms:
- *limite de solubilité* = solubility limit
- *austénite* = austenite  
- *1148°C* = specific temperature (critical?)

→ Information retrieval task. Metallurgy domain.

**Context mapping:**
● Iron-carbon phase diagram knowledge required
● Austenite solubility characteristics  
● Temperature-composition relationships

**Physical constraints check:**
- Carbon solubility in austenite increases with temperature ✓
- Maximum solubility exists at specific temperature ✓  
- 1148°C is major phase transition point ?

**Memory recall:**
Iron-carbon system → austenite (FCC) has higher carbon solubility than ferrite (BCC).
Critical temperatures: A1 (723°C), Acm (1148°C).
Austenite solubility increases → maximum at Acm temperature.

**Quantitative reasoning:**
Need solubility value at 1148°C. 

From phase diagram memory:
- Room temperature: ~0.021% C in austenite
- 1148°C: ~2.1% C maximum

※ This seems high - verify plausibility.

**Physical verification:**
Steel classification boundary: <2.1% C = steel, >2.1% C = cast iron.
∴ 2.1% represents practical upper limit.

**Confidence assessment:**
● 1148°C as critical temperature: high confidence
● 2.1% carbon content: high confidence  
◐ Exact solubility number: partial - rounding uncertainty

**Potential issues:**
- Unit specification (wt%)
- Temperature precision
- Regional terminology differences

**Final synthesis:**
Query asks for exact maximum carbon solubility in austenite at 1148°C.
Answer: 2.1% carbon (approximately).
Context: eutectoid composition boundary, phase diagram critical point.
Significance: defines steel/cast iron boundary.

**Response structure:**
Direct answer → explanation → contextual significance.

✓ Information complete for response generation.

### Answer###
La limite de solubilité maximale du carbone dans l'austénite à 1148°C est d'environ 2,1% en poids de carbone. Cette valeur représente le contenu en carbone le plus élevé que peut contenir l'acier avant de former de la fonte. À cette température, l'austénite (phase FCC) peut théoriquement dissoudre jusqu'à 2,1% de carbone, ce qui est 38 fois plus que ce que peut dissoudre la ferrite à température ambiante. Cette composition correspond à la limite eutectoïde et explique pourquoi les aciers avec exactement 0,8% de carbone forment une structure perlitique lors du refroidissement lent, le carbone se décomposant en carbure de fer et ferrite.