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Martensite
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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.
Why pearlite makes steel harder?
Because Martensite transformation is almost instantaneous, the Martensite has the identical composition of the parent phase. Formation of Martensite involves a transformation from a body-centered cubic structure to body-centered tetragonal structure. The large increase in volume that results creates a highly stressed structure.
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.
What are some uses for martensite?
hardness of martensite is greater than bionite and fine pearlite.
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 is the difference between pearlite and cementite?
Well, basically, pearlite is the eutectic composition of steel, with an overall composition of 0.8% carbon. It is known to consist of two phases, namely: Ferrite (Fe), the room temperature of iron and Cementite(Fe3C). Therefore, the difference between pearlite and cementite is that pearlite is a composition of steel, and cementite is a composition of Pearlite. So cementite is part of pearlite.
What is austenite-cementite?
ledeburite
What is the percentage of carbon in cementite?
It's 6.67%.
Is martensite formed when a steel is quenched?
Martensite transformation begins when austenite is cooled below a certain critical temperature, called the matrensite start temperature. As we go below the tmartensite start temperature, more and more martensite forms and complete transformation occurs only at a temperature called martensire finish temp. Formation of martensite require that the austenite phase must be cooled rapidly.
What is cementite?
Cementite is a compound that is made of iron and carbon, and is sometimes referred to as iron carbide. Because it is a hard compound, it is often used to mix with steel and harden the steel.
What are the types of cementite?
The are three types of cementite which form in different ways. There's the primary that forms from crystalization from the molten iron above 4.3%C and below 6.7%C (line CD in Fe-Fe3C diagram), secondary cementite which forms from precipitation from austenite at the right side of the eutectoid point. And there's the tertiary cementite which forms as precipitation from ferrite alpha because of the falling solubility of carbon in ferrite as temperature goes down.
What structure forms in hypoeutectoid steel after full hardening?
Martensite
What metals that conduct electricity are brittle?
Carbon Steel - Martensite
