Transformation point: indicates a crystal transformation when heating or cooling. This transformation consumes or delivers energy in form of heat, keeping the temperature constant while transforming. Hence these points are also called breaking-point (Arret). They show a hysteresis, therefore they are additionally annotated by:
c (chauffage=heating) and
Transformation from ferrite to austenite at 1333°F
Curie-temperature for hypoeutectoidic steel (1414°F)
Boundary temperature of the gamma-crystal area of hypoeutectoidic steel.
Temperature to dissolve the secondary cementite
Gamma-crystal is characterized by a face-centered, cubic, elementary cell and exists stable above A1. Austenite is non-magnetic.
Austenite can dissolve more carbon than ferrite.
Bainite is a micro-structure resulting from isothermal transformation in the cooling process above the martensite-formation temperature and below the perlitic region (See TTT-diagram). Bainite can form in non-isothermal transformation as well, requiring specific cooling processes.
The cementite crystal (Fe3C), also called iron carbide, can segregate as primary or secundary cementite. Cementite has a hardness greater than that of martensite. Alloying elements other than iron form carbides as well.
Eutectoidic steel contains 0.83% of carbon. The gamma-area expands at this carbon content until A1. Slow cooling will result in perlite only.
Alpha chrystal, characterized by a body-centered, cubic elementary cell, present at hypoeutectoidic steels below A3. Ferrite dissolves less carbon than austenite.
Two or more pieces of steel are heated to welding temperature. Flux is used to prevent the formation of iron scale. By hammer blows or in a forging press the pieces of steel are brought into tight contact; the flux is pressed out of the welding zone at the same time. The elevated temperature and pressure lead to a diffusion weld.
Fast cooling from above Ac1 prevents the segregation of cementite. The primary cells are distorted by the trapped carbon, leading to an increase of hardness. The result–depending on the cooling range–are finest lamellar perlite (sorbite, troostite), bainite or martensite.
heat treatment is used to influence the size and type of the grain structure or the carbides. The variety and effect of heat treatments makes steel so universal.
Steel containing more than 0.83% of carbon. After slow cooling, these steels consist of perlite and secondary cementite.
Steel containing less than 0.83% of carbon. After slow cooling, these steels are a composite of ferrite and perlite.
Interstitial Alloying Elements
Interstitial Alloying Elements occupy the interstices of the primary crystal, increasing the hardness by causing internal tensions. Carbon is the most common interstice atom, followed by nitrogen. At higher temperatures, diffusion processes cause a fast equalization in concentration.
Iron-Carbon-iron Equilibrium Diagram
The iron-carbon equilibrium diagram displays the transformation temperatures and crystal types present. Alloying elements can significantly alter the iron-carbon equilibrium diagram.
Above 1.7% of carbon, ledeburite forms while cooling at 1909°F. Ledeburite is a subassembly of austenite and cementite. Below Ar1, the austenite transforms into perlite.
Wrought iron, smelted in an ingot.
Hardening structure, resulting from fast cooling to below the martensitic temperature. (See TTT-diagram)
Martensite Formation Temperature
Temperature below which austenite transforms into martensite. The martensite formation temperature depends on the substitution elements, interstitial atoms do not influence this temperature. With plain carbon steels the martensite formation temperature is 360°F (See TTT-diagram).
Composite material of diffusion-welded nonferrous and/or precious metals.
Grain-refining heat treatment based on austenite transformation. This "soft" hardening influences the carbide distribution positively (small carbides distributed evenly).
Subassembly of ferrite and secondary cementite
Heating to and holding steel at just below Ac1 followed by slow cooling to soften the steel.
Hypereutectoidic steels are soft annealed by oscillating around Ac1 and slow-cooling thereafter. This leads to a globular microstructure, improving the workability.
Substitutional elements occupy lattice position of the primary cell. They can provoke hardening by causing segregations. Often these substitutional elements act as carbon binders.
Tempering transfers tetragonally deformed martensite into cubic martensite. At the same time, macroscopic stresses are relieved, allowing residual austenite to transform.
© 2005 G.v.Tardy