Two identical bars of 1080 steel are austenitized and then quenched: one in cold water, one in warm oil. The water-quenched bar is substantially harder. What is the correct metallurgical explanation?
AWater is chemically reactive with steel, causing surface hardening reactions during the quench
BWater quenching cools the steel faster, allowing the cooling curve to miss the pearlite nose on the TTT diagram and form more martensite
COil introduces carbon into the steel surface during quenching, reducing the carbon available for hardening
DWater quenching increases dislocation density directly, independent of the phase transformation
The quench medium does not directly harden the steel — it controls the cooling rate. A faster cooling rate (water) produces a steeper cooling curve on the TTT diagram that passes to the left of the pearlite C-curve nose, suppressing diffusion and trapping carbon in a BCT martensite structure. Oil cools more slowly; if the cooling curve clips the nose, some austenite transforms to softer pearlite or bainite instead. The hardness comes from martensite formation, not from the medium itself.
Question 2 Multiple Choice
A machinist needs maximum hardness in a tool steel component. After achieving a fully martensitic microstructure by quenching, she considers tempering at 600°C. Her supervisor cautions against this for a cutting tool application. What is the correct reason?
ATempering above 500°C reverses the martensitic transformation, returning the steel to austenite
BHigh-temperature tempering allows carbon to diffuse out of the BCT lattice and form coarse carbide precipitates, substantially reducing hardness while improving toughness
C600°C tempering introduces residual tensile stresses at the surface, causing delayed cracking
DTempering at any temperature weakens the steel and should be avoided for all tool applications
Tempering allows the supersaturated carbon in as-quenched martensite to diffuse out and precipitate as fine (then coarser at higher temperatures) carbides. This reduces the lattice strain that gives martensite its hardness. High-temperature tempering (500–650°C) maximizes toughness but significantly reduces hardness — the right choice for structural parts, wrong for cutting tools. Low-temperature tempering (150–250°C) retains high hardness with modest toughness improvement, appropriate for cutting and wear applications.
Question 3 True / False
A low-carbon steel (0.15 wt% C) cannot be significantly hardened by quenching, regardless of how rapid the cooling rate is.
TTrue
FFalse
Answer: True
Martensite hardness depends critically on carbon content. Carbon dissolved in the iron lattice creates the BCT distortion that makes martensite hard and brittle. With only 0.15 wt% C, there is insufficient carbon to create significant lattice strain even if the cooling is fast enough to suppress pearlite formation. Plain low-carbon steels form soft martensite ('lath martensite') that is only marginally harder than ferrite. For effective hardening, steels typically require at least 0.3–0.4 wt% C.
Question 4 True / False
Tempering is best understood as a corrective step — a way to partially undo an overly aggressive quench that left the steel too brittle.
TTrue
FFalse
Answer: False
Tempering is not corrective; it is a deliberately planned second step that follows a successful quench. As-quenched martensite is intentionally formed to maximize hardness, then intentionally tempered to improve toughness. The two steps together — quenching to martensite, then tempering to calibrate the hardness-toughness balance — constitute the quench-and-temper process. The temper is engineered for the application's requirements, not applied to fix a mistake.
Question 5 Short Answer
Explain why the nose of the pearlite C-curve on a TTT diagram occurs at an intermediate temperature rather than at the highest or lowest temperatures, referencing the two competing factors that produce it.
Think about your answer, then reveal below.
Model answer: The nose represents the temperature where pearlite forms fastest. Two factors compete: (1) thermodynamic driving force — the free energy difference between austenite and pearlite, which increases as temperature drops below the eutectoid temperature, providing more driving force at lower temperatures; (2) atomic diffusivity — carbon and iron atoms must diffuse to form the layered pearlite structure, and diffusion slows dramatically as temperature decreases. At high temperatures near the eutectoid, diffusion is fast but driving force is small; at very low temperatures, driving force is large but diffusion is nearly frozen. The nose occurs at the intermediate temperature that best balances both factors — maximizing the transformation rate. This matters for quench selection: any cooling curve that passes through the nose region will produce some pearlite, so the quench medium must be fast enough to miss it entirely.
This is the fundamental reason TTT diagrams have C-shaped curves rather than straight lines. The nose temperature (typically around 550°C for plain carbon steels) sets the critical cooling rate. Alloying elements like Mn, Cr, and Mo shift this nose to longer times by slowing diffusion and stabilizing austenite, reducing the required cooling rate and improving hardenability.