Substitution in high-speed steels: Which alloying element can effectively replace tungsten in certain tungsten-based high-speed steel grades while retaining comparable hot-hardness performance?

Introduction / Context:
Tungsten-based high-speed steels (T-series) are renowned for their red hardness but are heavy and costly due to the high tungsten content. Molybdenum (M-series) grades were developed to achieve similar performance with reduced tungsten dependency.

Given Data / Assumptions:

• Objective: identify a practical, widely used substitute for tungsten in HSS.
• Performance target: maintain hot hardness and cutting capability.
• Alloying context: presence of carbide formers and solid-solution strengtheners.

Concept / Approach:
Molybdenum provides comparable strengthening and carbide formation effects in HSS, enabling the M-series (e.g., M2, M42) to deliver red hardness similar to tungsten-rich T-series (e.g., T1), often with added vanadium and cobalt to fine-tune wear resistance and hot hardness. Nickel and silicon are not direct substitutes for tungsten in HSS; cobalt boosts hot hardness but does not replace tungsten's role in the matrix and carbide balance by itself. Vanadium primarily refines carbides and improves wear, not the bulk replacement of tungsten.

Step-by-Step Solution:
Identify key property to preserve: red hardness at cutting temperatures.Recognize established M-series steels as Mo-based replacements.Eliminate elements that do not replicate tungsten's function on their own (Ni, Si, V, Co alone).Select molybdenum as the correct replacement element.

Verification / Alternative check:
Materials handbooks list M-series HSS as molybdenum-bearing analogs to tungsten-based T-series, with similar application ranges.

Why Other Options Are Wrong:
Nickel improves toughness; silicon aids deoxidation; vanadium and cobalt are important additives but not primary replacements for tungsten.

Common Pitfalls:
Assuming cobalt alone replaces tungsten; in practice, cobalt augments hot hardness but Mo substitutes for W in base composition.

Final Answer:
molybdenum