C21D - Definition 
Definition statement
This subclass covers:
General methods or devices for heat treatment, e.g. hardening, annealing, heating, quenching, tempering.
Heat treatment adapted for particular articles, e.g. springs, pipes, drills, rollers, wires; Furnaces therefor.
Heat treatment of cast-iron, of ferrous alloys.
Changing the physical properties of ferrous metals by deformation, by deformation combined or followed by heat treatment, by other methods.
Diffusion processes for extraction of non-metals, e.g. decarburising; Furnaces therefor.
Process control or regulation for heat treatments.
Relationship between large subject matter areas
C22B covers general metallurgical or chemical processes for producing or recovering metals from metal compounds, ores, metalliferous waste or scrap metal and for refining metal.
Changing the physical structure of non-ferrous metal and non-ferrous alloys is covered by C22F.
C23F 17/00 covers surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered in C21D or C22F or C25.
Furnaces in general and details thereof are classified in F27B and F27D, respectively.
References relevant to classification in this subclass
This subclass does not cover:
Apparatus for mechanical working of metal without essentially removing material; Punching metal | B21 |
Apparatus for mechanical working of metal not otherwise provided for, e.g. turning, boring | B23 |
Apparatus for grinding or polishing of metal | B24 |
Mechanical descaling | B21, B23 |
Cooling-beds for metal rolling | B21B 43/00 |
Cold coilers | B21C 47/00 |
Chemical descaling | C23 |
Cementation by diffusion processes | C23C |
Surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by C21D | C23F 17/00 |
Electrolytic descaling | C25F 1/00 |
Unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid material | C30B |
Apparatus for heat treatment of railway trains on the spot | E01B 31/18 |
Informative references
Attention is drawn to the following places, which may be of interest for search:
Removing fumes in general | B08B 15/00 |
Working metallic powder | B22F |
Soldering; welding; cladding or plating by soldering or welding | B23K |
Production of gases | C01, C10 |
General processes of refining or remelting metals; Apparatus for electroslag or arc remelting of metals | C22B 9/00 |
Making cast-iron alloys | C22C 33/08 |
Cast-iron alloys compositions | C22C 37/00 |
Ferrous alloy compositions | C22C 38/00 |
Electrolytic production or refining of metals | C25C |
Furnaces, in general | F27 |
Heat exchange apparatus in which the heat exchange media do not come into direct contact | F28D |
Investigating or analysing materials by determining their chemical or physical properties in general | G01N |
Controlling or regulating in general | G05 |
Electric heating | H05B |
Special rules of classification
When classifying in group C21D 6/00, any aspect of the method for the heat treatment of ferrous alloys which is considered to represent information of interest for search may also be classified in groups C21D 1/02-C21D 1/84. This can, for example, be the case when it is considered of interest to enable searching of heat treatment methods of ferrous alloys using a combination of classification symbols. Such non-obligatory classification should be given as "additional information".
When classifying in group C21D 6/00, any alloying constituent which is considered to represent information of interest for search may also be classified in groups C22C 38/02-C22C 38/60. This can, for example, be the case when it is considered of interest to enable searching of heat treatment of specific ferrous alloys using a combination of classification symbols. Such non-obligatory classification should be given as "additional information".
Glossary of terms
In this subclass, the following terms or expressions are used with the meaning indicated:
| Cast-iron | 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. |
Synonyms and Keywords
In patent documents the following terms "hardening combined with annealing" and "vergüten" are often used as synonyms. Also, these terms can be used as keywords.