Cast irons — improving toughness by annealing with hematite When cast iron is heated to red heat in contact with powdered red hematite to increase toughness, it is converted into:

Introduction / Context:Malleabilisation (toughening) of cast iron is a heat-treatment process that transforms an otherwise brittle structure into a tougher product suitable for parts like fittings and brackets. The question refers to annealing with oxidising media (red hematite).

Given Data / Assumptions:

• Heating to red heat for prolonged time in the presence of hematite (an oxidising environment).
• Starting material is typically white cast iron (cementite present).
• Objective is to increase ductility and toughness.

Concept / Approach:Under oxidising/annealing conditions, combined carbon (cementite) decomposes, producing temper carbon nodules within a ferritic matrix. The resulting product is often termed malleable or toughened cast iron (here listed as “toughed cast iron”).

Step-by-Step Solution:

Verification / Alternative check:Microstructural examination shows reduction in carbides and presence of temper carbon; mechanical testing confirms increased elongation and impact resistance.

Why Other Options Are Wrong:

• Grey cast iron (A): contains graphite flakes formed on solidification, not via this post-solidification malleabilising route.
• White cast iron (B): starting point; it is hard and brittle, not the toughened end product.
• Mottled cast iron (C): intermediate mixture of white and grey structures, not the toughened, malleable product.

Common Pitfalls:Confusing grey iron (flake graphite) with malleable iron (temper carbon nodules). The oxidising pack anneal specifically aims at malleabilisation, not graphitisation during casting.

Final Answer: