MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS

**Definition:** This place covers:

General methods or devices for heat treatment, e.g. hardening, annealing, heating, quenching or tempering.

Heat treatment adapted for particular articles, e.g. springs, pipes, drills, rollers or wires; furnaces therefor.

Heat treatment of cast-iron or of ferrous alloys.

Changing the physical properties of ferrous metals by deformation, by deformation combined or followed by heat treatment or by other methods.

Diffusion processes for extraction of non-metals, e.g. decarburising; furnaces therefor.

Process control or regulation for heat treatments.

The term "ferrous alloys" refers to alloys based essentially on iron.

**Limiting references (this place does not cover):** - Chemical descaling -> C23 - Cementation by diffusion processes -> C23C - Surface treatment of metallic material -> C23F17/00

**Glossary:** - Alloy: A composition of plural elements at least one of which is a free metal. It also includes material containing any combination of fibres, filaments, whiskers and particles, e.g. carbides, diamond, oxides, borides, nitrides, silicides, or other metal compounds, e.g. oxynitrides or sulfides embedded in a metallic matrix. - Cast iron: Ferrous alloy which solidifies with a eutectic, with a carbon content of 2.1-4 wt%. - Steel: Ferrous alloy with a carbon content of 0.2-2.1wt%. - Air hardening steel: Steel which does not require quenching from a high temperature to harden but which is hardened by cooling in air from above its critical temperature range. - Sub-critical annealing/Stress relief annealing: Heat treatment for relieving or dissipating stresses in weldments, heavily machined parts, castings, forgings by heating them, uniformly heated through, and air cooled/slow cooled with subsequent finishing or heat treatment. - Oil-hardening: Process of hardening a ferrous alloy by heating within or above the transformation range and quenching in oil. - Decarburization: Subjecting the steel to high temperatures and heat treating in a media containing air, oxygen or hydrogen to remove carbon at the surface. - Recrystallization: After all metal crystals have been dissolved by heating enough to lose its structural strength, the metal temperature then falls, allowing the crystals to re-form. - Spheroidizing: Heating the carbon steel to approximately 700 °C for over 30 hours to form spheroidite, to soften higher carbon steels and allow more formability. - Aging or ageing: A process in which the hardness or strength of a metal alloy having a constituent in supersaturated solid solution is increased over time as the constituent precipitates out as a secondary phase containing the constituent. When occurring at room temperature the process is termed "natural aging", while a process that occurs when subjecting the metal alloy to elevated temperature is termed "artificial aging". Aging for a longer time than that corresponding to maximum strength or hardness at the particular temperature is termed "over-ageing". - Hardening: The increase in resistance to deformation. - Precipitation hardening: As the quenched alloy ages, a new material precipitates out of the metallic crystal lattice, filling in abutting spaces, and increasing hardness. - Normalizing: A process of heating metallic material above its critical temperature and cooling in air thereby establishing a fine uniform grain size and improving the microstructural uniformity. - Quenching: The rapid cooling of metallic material either from elevated temperature to room temperature or cooling of metal to sub-ambient temperature, at a specific rate, with a given medium. - Tempering: Heating of a previously quenched or normalized metallic material to an elevated temperature, and then cooling under suitable conditions to obtain the desired mechanical properties. - Martempering: Heat treatment of steel involving austenitisation of steel followed by quenching in heat extracting medium (e.g. salt), at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the martensite start (Ms) point. - Austempering: Isothermal heat treatment applied to steel and cast iron, involves holding the metallic material at the quenching temperature for an extended period of time in order to produce a lower bainite microstructure for steels and a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite for cast-irons. - Case Hardening: Heat treatment or combination of heat treatments of surface hardening involving a change in the composition of the outer layer of an iron-base alloy in which the surface is made harder by inward diffusion of a gas or liquid followed by appropriate thermal treatment.