For through-hardening of steel sections, which alloying addition most effectively increases the depth of hardenability (i.e., the depth to which martensite forms upon quenching)?

Introduction / Context:
Hardenability (depth of hardness) determines how deeply a steel section transforms to martensite during quenching. Alloy selection controls critical transformation kinetics and is central to heat treatment design for gears, shafts, and tools.

Given Data / Assumptions:

• Plain carbon steel baseline; quenching in a given medium.
• Objective: increase hardening depth, not merely surface hardness.

Concept / Approach:
Alloying elements shift Time-Temperature-Transformation (TTT) and Continuous Cooling Transformation (CCT) curves to longer times, delaying pearlite/bainite formation and enabling martensite formation deeper into sections. Nickel is especially effective in improving hardenability without significantly increasing the risk of grain boundary carbides or excessive air-hardening behavior.

Step-by-Step Solution:
1) Identify elements that increase hardenability: Ni, Cr, Mo, Mn, and B (trace).2) Consider roles: Nickel is powerful for deep hardenability and toughness; chromium promotes hardenability and wear resistance but may form carbides; vanadium refines grain and forms stable carbides (tool steels); tungsten supports hot hardness, not primarily depth; cobalt decreases hardenability.3) Choose the element with the strongest, broad effect on depth: nickel.

Verification / Alternative check:
Jominy end-quench data for alloy steels show significant shift in hardness-depth profiles with nickel additions, confirming its effectiveness for through-hardening.

Why Other Options Are Wrong:
Chromium: helps, but not as uniformly effective as Ni for depth and may favor carbide formation.
Vanadium: mainly grain refinement and secondary hardening; limited effect on deep hardenability alone.
Tungsten: promotes red/hot hardness in high speed steels rather than depth in general alloy steels.
Cobalt: typically reduces hardenability, used for hot hardness.

Common Pitfalls:
Confusing surface hardness (case hardening) with hardenability; assuming wear-resistant elements always increase depth of hardness.

Final Answer:
nickel