Alloying for Impact Toughness — Improving Shock Resistance of Steel Shock (impact) resistance in steels is most effectively increased by adding:

Introduction / Context:
Shock resistance, or impact toughness, reflects the ability of a material to absorb energy without fracturing. Alloying elements influence toughness differently: some increase hardness and wear resistance, while others improve ductility and toughness, particularly at low temperatures.

Given Data / Assumptions:

• Plain carbon steel base composition.
• Comparative effect of common alloying elements on impact toughness.
• Room to low-temperature service is relevant.

Concept / Approach:
Nickel is well known to enhance toughness and reduce ductile-to-brittle transition temperature in steels. It improves impact behavior without excessively increasing hardness or promoting brittle phases. Chromium, while valuable for hardenability and wear/corrosion resistance, tends to raise hardness and, unless balanced, can reduce toughness. Additions like sulphur, lead, and phosphorus primarily improve machinability (S, Pb) or strength at the expense of toughness (P), and therefore do not improve shock resistance.

Step-by-Step Solution:
Identify property target: higher impact toughness, not just hardness.Map alloy influences: Ni → toughness; Cr → hardness/wear; S/Pb/P → machinability or embrittlement.Choose the element with the most direct, consistent positive impact on shock resistance: nickel.

Verification / Alternative check:
Low-temperature structural steels often contain nickel to maintain toughness; standards specify Ni additions for improved Charpy V-notch values.

Why Other Options Are Wrong:
Chromium alone does not primarily enhance impact toughness; it is more associated with hardenability and wear.Nickel and chromium together can be balanced in alloy steels, but “nickel” alone cleanly captures the principal toughening effect.Sulphur, lead, phosphorus degrade toughness or target machinability, not impact resistance.

Common Pitfalls:
Assuming higher hardness automatically means better impact resistance; often the opposite is true.

Final Answer:
nickel