Case Hardening / Surface Hardening: Correct Option Identification
Case hardening (a type of surface hardening), aims to create a hard wear-resistant surface without sacrificing the toughness of the core. This is typically achieved by modifying the surface composition via diffusion of carbon and/or nitrogen into the steel, followed by a hardening step (commonly quenching).
Among the statements, the one that matches this defining principle is:
✅ (ii) Modify the surface composition by diffusion of carbon and/or nitrogen while keeping a tougher core.
This aligns with the core purpose of processes such as carburizing (carbon diffusion) and nitriding (nitrogen diffusion), where the surface becomes harder due to transformed microstructures (and/or carbide/nitride formation), while the bulk remains relatively tougher.
Footnotes
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Case Hardening (Principle of diffusion/thermochemical surface treatment) - https://en.wikipedia.org/wiki/Case-hardening ↩
-
Carburizing (Carbon diffusion to create hardened case) - https://en.wikipedia.org/wiki/Carburizing ↩
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Nitriding (Nitrogen diffusion to create hardened surface/case) - https://en.wikipedia.org/wiki/Nitriding ↩
Surface Hardening (Carburizing/Nitriding) — Overview
How to Identify the Correct Option (Reasoning Checklist)
- 1Step 1
Surface hardening by thermochemical treatment typically alters the surface using diffusion of C and/or N.
- 2Step 2
The correct statement must mention a tougher core (i.e., hard case with less-hardened interior).
- 3Step 3
Reject statements that claim: through-hardening only, no compositional change, or restriction to only one heat source.
- 4Step 4
Option (ii) matches both: diffusion of C/N and preservation of a tougher core.
Evaluate the four statements
(i) “Harden only high-carbon steels by through hardening”
Through hardening (through hardening), in general, hardens the whole section rather than creating a hard case. Case hardening is primarily a surface-focused strategy, often applied so that even lower-carbon steels can be given a hard surface by diffusion. So (i) is not the correct general description of case hardening. 2
(ii) “Modify the surface composition by diffusion of carbon and/or nitrogen while keeping a tougher core”
This directly matches the hallmark of case hardening / surface hardening: thermochemical diffusion of carbon/nitrogen into the surface, followed by hardening so that the surface becomes hard while the core remains tougher. ✅ Correct. 2
(iii) “Rely only in rapid quenching with no change in composition”
Rapid quenching alone is a quenching step used in ordinary hardening; it does not inherently create a hard case via carbon/nitrogen diffusion. Since case hardening fundamentally involves compositional change at the surface via diffusion, (iii) is incorrect. 3
(iv) “Use only flame or induction as heat sources”
Case hardening is not limited to flame or induction. The key distinction is the surface treatment mechanism (e.g., carburizing/nitriding), not a specific heating method. Different furnace/atmosphere-based methods are also used. Hence (iv) is too restrictive to be correct. 2
Footnotes
-
Carburizing (Carbon diffusion to create hardened case) - https://en.wikipedia.org/wiki/Carburizing ↩ ↩2 ↩3 ↩4
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Nitriding (Nitrogen diffusion to create hardened surface/case) - https://en.wikipedia.org/wiki/Nitriding ↩ ↩2 ↩3
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Case Hardening (Principle of diffusion/thermochemical surface treatment) - https://en.wikipedia.org/wiki/Case-hardening ↩ ↩2
type="tip" title="Exam Strategy Tip" content="When a question says “case/surface hardening,” look for language about diffusion, surface composition change, and tough core—those are the fastest discriminators among options."
type="warning" title="Common Confusion" content="Do not confuse through hardening (hardening the whole section) with case hardening (hard case + tough core). If the option emphasizes the entire body being hardened, it’s likely wrong."
Option Fit vs. Core Principle (Diffusion + Tough Core)
Higher means better matches defining features of case/surface hardening.
Key Concepts You Should Know
Case Hardening Quick-Recall Deck
Knowledge Check
Which statement correctly describes case hardening / surface hardening?
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- Compared to Waterfall (low risk handling), Prototyping (moderate, focus on requirements), and Incremental (moderate, focus on staged delivery), Spiral offers the strongest formal risk management.
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