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What are the limitation of iron iron-carbide diagram?
1) The nonequilibrium martensite does not appear on the diagram; and 2) The diagram provides no indication as to the time-temperature relationships for the formation of pearlite, bainite, and spheroidite, all of which are composed of the equilibrium ferrite and cementite phases.
Why martensite is so hard and brittle?
For two reasons: 1. martensite is bct structure which prevent the movement of dislocations. 2. martensite has higher carbon concentraton.
What structure forms in hypoeutectoid steel after full hardening?
Martensite
What is trip steel?
TRIP steel is Transformation Induced Plasticity steel. It is a composite steel that consists of ferrite, bainite, martensite precipitants and restrained austenite. The austenite will transform into martensite when strained, thus increasing the strength of the steel. To stabilize the austenite you need to introduce alloy elements, usually Manganese.
Is steel a composite material?
Steel is not usually considered a composite, as it is macroscopically homogeneous.However, some steel types, including "classical" iron-carbon steel, can be considered as metal-matrix composites, as they contain a second phase... sometimes.For simple iron-carbon steel, cooling after high-temperature forging or heat treatment will precipitate out iron carbide (cementite, Fe3C) particles and leave a carbon-depleted iron matrix. If cooling is slow, coarse bands of iron / cementite will form, a microstructure called pearlite, which is not very hard.If the cooling speed is increased, the pearlite will become finer (finer bands), until another composite microstructure, with more acicular patterns forms, called bainite. This is also heterogeneous, i.e. a composite of carbon-poor iron and cementite.Going to very fast cooling (quenching) will result in a single-phase (not composite) material called martensite. Here the carbon doesn't have time to "exit the iron", and this martensite phase is very hard, but also normally too brittle. Hence, it is normally re-heated to 200-400°C, a process called "tempering", where again some cementite precipitates out: it becomes a composite again, yielding a somewhat softer, but much tougher material.Alloyed steels (i.e. with other elements than just iron and carbon) strongly vary in behaviour:Normal non-magnetic "austenitic" stainless steels are single-phase, not composites.Tool steels (high carbon + carbide-forming alloying elements) are definitely composites. They form a lot of hard particles, such as chromium carbides, that impart good resistance against wear.Magnetic Fe-Cr stainless steels may be essentially single-phase (very low carbon, better corrosion resistance) or also contain carbides (higher carbon, better strength)So-called "maraging" (martensite aging) steels may achieve both high strength and good corrosion resistance. Here, strength is imparted by an "aging" treatment around 500°C, to precipitate out so-called "intermetallic" particles, making it again a "composite material".These considerations also apply to many other metallic alloys, based on metals such as aluminium, titanium or nickel. In most cases, the strongest variants are engineered to be "microcomposites" or "nanocomposites", i.e. they precipitate out intermetallic particles during heat treatment.The reason behind such engineering is that the particles block dislocations, which are responsible for plastic deformation of metals. For each alloy, there is an optimum heat treatment to achieve the best "blocking ability" for dislocations, and thus the highest strength.
