Why is as-quenched martensite so hard compared to slowly-cooled pearlite?
AMartensite is an FCC phase, which is inherently harder than the BCC ferrite in pearlite
BSupersaturated carbon trapped in the distorted body-centered tetragonal lattice blocks dislocation motion
CThe rapid quench introduces a very high dislocation density, and dislocation tangling is the primary strengthening mechanism
DMartensite contains more carbon by weight than austenite, and carbon itself is hard
Martensite is hard because of lattice distortion, not simply because it is a different phase. Carbon atoms, unable to diffuse out during rapid quenching, are trapped in interstitial sites of the iron lattice, stretching it into a body-centered tetragonal structure. This distortion — combined with high internal stress — blocks dislocation motion, giving martensite extreme hardness. Option C is partly true (dislocation density does rise) but is not the primary mechanism. Option A is wrong: FCC austenite is actually less hard than BCC-based structures.
Question 2 Multiple Choice
A steel part requires high surface hardness for wear resistance but enough toughness to resist fracture in service. Which heat treatment sequence best achieves this?
AAnneal at high temperature, then slow-cool to produce fully pearlitic microstructure
BQuench rapidly to form martensite, then temper at a moderate temperature to restore toughness
CQuench to martensite and leave it untempered — maximum hardness means maximum performance
DHeat to just below the eutectoid temperature and air-cool to produce bainite
Quench-and-temper is the correct sequence. Quenching produces hard martensite; tempering at an intermediate temperature allows carbon to partially diffuse out as fine carbide precipitates, substantially recovering toughness while retaining much of the hardness. Untempered martensite (option C) is catastrophically brittle and prone to shattering under impact — hardness without toughness is rarely useful in structural applications. Annealing (option A) produces a soft, tough pearlite unsuitable for wear-resistant applications.
Question 3 True / False
Tempering a quenched steel always reduces its hardness compared to the as-quenched state.
TTrue
FFalse
Answer: True
This is correct and represents a fundamental trade-off of heat treatment. Tempering allows carbon to diffuse out of the distorted martensite lattice, relieving both the lattice distortion and internal stresses that produce hardness. Toughness recovers, but hardness necessarily drops. The engineer chooses the tempering temperature to optimize the hardness–toughness balance for the application, accepting this trade-off rather than eliminating it.
Question 4 True / False
Adding alloying elements such as chromium and manganese to steel makes it easier to form martensite because they push the TTT nose to the left, accelerating the austenite-to-pearlite transformation.
TTrue
FFalse
Answer: False
This is backwards. Alloying elements push the TTT nose to the RIGHT — meaning more time must elapse before the austenite-to-pearlite (or bainite) transformation begins. This is called hardenability: the steel can be cooled more slowly and still miss the nose, arriving at martensite. The practical value is that thicker sections can be quenched all the way through without the center transforming to softer products before the cooling front arrives.
Question 5 Short Answer
Why is tempering necessary after quenching steel to martensite, and what happens at the atomic level during the tempering process?
Think about your answer, then reveal below.
Model answer: As-quenched martensite is extremely brittle because the lattice is severely distorted by trapped carbon and carries high internal stress from the rapid quench. Tempering reheats the steel to an intermediate temperature (typically 150–650°C), giving carbon atoms enough thermal energy to slowly diffuse out of the lattice and precipitate as fine carbide particles. This relieves lattice distortion and internal stress, recovering substantial toughness at the cost of moderate hardness.
The key is that martensite's hardness and its brittleness have the same cause: supersaturated carbon in a distorted lattice under high internal stress. Tempering selectively relaxes those stresses by allowing limited, controlled diffusion — enough to reduce brittleness, but not so much (at lower tempering temperatures) that hardness is severely compromised. Higher tempering temperatures allow more diffusion, producing a tougher but softer steel; lower temperatures preserve more hardness at the cost of less toughness recovery.