Surface hardening routes — simultaneous absorption: Name the heat-treatment process in which both carbon and nitrogen are absorbed into the steel surface to increase hardness and wear resistance.

Introduction / Context:
Surface engineering tailors hardness and wear resistance while retaining a tougher core. Different processes introduce different interstitials (carbon and/or nitrogen) or rely on rapid heating/quenching without composition change. Choosing the correct method depends on the desired case chemistry and production rate.

Given Data / Assumptions:

• Base material: low/medium carbon steel components.
• Goal: absorb both carbon and nitrogen.
• Industrial feasibility and standard definitions are assumed.

Concept / Approach:
Cyaniding is a rapid case-hardening process performed in molten cyanide salts. During treatment, both carbon and nitrogen diffuse into the surface, producing a hard, wear-resistant case after quenching. Carburising introduces only carbon, while nitriding introduces only nitrogen at subcritical temperatures (no quench required). Flame and induction hardening heat the surface and quench to form martensite without intentionally altering surface chemistry.

Step-by-Step Solution:
Identify the processes that change chemistry: carburising (C), nitriding (N), cyaniding (C+N).Match requirement “absorb carbon and nitrogen” → cyaniding.Eliminate thermal methods (flame/induction) that rely on phase transformation only.Select cyaniding as the correct answer.

Verification / Alternative check:
Process descriptions confirm cyanide bath chemistry contributes both carbon and nitrogen diffusion, typically producing thin, hard cases in short times.

Why Other Options Are Wrong:
Carburising adds only carbon; nitriding adds only nitrogen; flame/induction hardening alter microstructure but not composition.

Common Pitfalls:
Assuming any case hardening always means carbon only; overlooking that cyaniding entails toxic salts and is being replaced by safer carbo-nitriding gas processes in many shops.

Final Answer:
cyaniding