MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING

Introduced: September 1968

Classification Context

Section:: CHEMISTRY; METALLURGY
Class:: METALLURGY OF IRON
Subclass:: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING

Description

C21D encompasses heat treatment processes and equipment that alter the physical and mechanical properties of ferrous metals and alloys through controlled thermal cycles, including annealing, tempering, hardening, normalizing, and stress relief. The class also covers general-purpose heat treatment apparatus applicable to both ferrous and non-ferrous metals, as well as specific processes for improving workability such as decarburization and softening treatments. This classification excludes specialized surface treatment methods (covered under C21C) and specific alloy composition modifications, focusing instead on structural transformation achieved via temperature control and cooling regimens.

Scope Notes

Glossary: Cast-iron cast-irons Ferrous alloy which solidifies with an eutectic, with C 2.1 - 4%. Sub-critical annealing/Stress relief annealing Heat treatment for relieving or dissipating stresses in weldaments, 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 Decarburisation 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 appropiate thermal treatment. | Limiting references: Apparatus for mechanical working of metal without essentially removing material; Punching metal Apparatus for mechanical working of metal not otherwise provided for, e.g. turning, boring Apparatus for grinding or polishing of metal Mechanical descaling , Cooling-beds for metal rolling Cold coilers Chemical descaling Cementation by diffusion processes Surface treatment of metallic material involving at least one process provided for in class and at least one process covered by Electrolytic descaling Unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid material | Application references: Apparatus for heat treatment of railway trains on the spot

Related Keywords

treatment of steel ALLOYStreatment of alloys of IRONmodifying physical properties of METAL(S)

9 direct subcodes

Child Classifications

Navigate with arrow keys, Enter to open

C21D 1/00 General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
C21D 10/00 Modifying the physical properties by methods other than heat treatment or deformation
C21D 11/00 Process control or regulation for heat treatments
C21D 3/00 Diffusion processes for extraction of non-metals; Furnaces therefor
C21D 5/00 Heat treatment of cast-iron
C21D 6/00 Heat treatment of ferrous alloys
C21D 7/00 Modifying the physical properties of iron or steel by deformation
C21D 8/00 Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
C21D 9/00 Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor