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May22-14, 09:40 AM
I have been doing some internet learning about precipitation hardening and martensitic transformation, and have some questions. Firstly I'll describe my current understanding:
Precipitation hardening (specific to non ferrous alloys)
An alloy undergoes solution treatment (basically solid solution strengthening, but in a greater concentration allowed by the solubility equilibrium). It is then quenched, to trap the 'excess' atoms and prevent them precipitating out of the solid solution. It is then aged to obtain a controlled precipitation and even distribution of second phase particles (intermetallic compounds, or carbides). These form on the grain boundaries and strengthen the material by preventing the movement of dislocations.
Martensitic transformation (specific to ferrous alloys)
The alloy is heated to austenitic state (extra carbon can be diffused into solution if desired). It is quenched to prevent the formation of ferrite/pearlite (which would form if it was cooled slowly). Due to the supersaturation of carbon, the BCC structure distorts into BCT. As a result the material is very hard but brittle, it must be tempered to be 'usable'. Tempering enables some of the carbon to precipitate out of the BCT, increasing ductility but reducing hardness.
Please let me know if my understanding is incorrect.
So, my questions:
1. If the material is cooled slowly instead of quenched, the 'excess' solute atoms will phase separate, creating a heterogeneous grain structure - is this correct? will one type of grain be a lattice of the base metal with the max amount of solute atoms, and the other type of grain be a pure metal lattice of just solute atoms?
2. When it is quenched, the 'excess' solute atoms are trapped in the base metal lattice.The grain structure is still homogeneous, but the lattice is supersaturated - is this correct? If so, isn't this still solid solution strengthening - or does the alloy lose strength because it is supersaturated?
3. In precipitation hardening, the precipitation particles contribute to the enhancement of yield strength and hardness, however in martensitic transformation the precipitation particles seem to do the opposite - why is this? do they allow for elastic deformation by reducing the brittleness of the BCT yet prevent plastic deformation by limiting movement of dislocations?
4. Pearlite is a grain with a lamellar structute of ferrite and cementite - right? cementite is basically iron carbide - right? so in annealed steel with around 1% carbon you get pearlite grains with iron carbides that have precipitated to the grain boundary - still right? This seems to me like it would be pretty strong and hard, yet i have seen it described as being soft and ductile. Is this just a comparative description as to how strong and hard it would be if it had actually been quenched to form martensite?
I never studied this stuff at university and am trying to figure it out using wikipediea, scientific journals and everything in between...
Thanks for any feedback!
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May23-14, 12:08 PM
All good questions.
I plan to respond in some detail, but at the moment one should recognize that precipitation hardening can occur in steels as well as non-ferrous alloys.
There are a variety of steels according to morphology, e.g., austenitic, ferritic, martensitic, perlitic, bainitic, and then duplex steels such as austentic-ferritic, ferritic-matensitic, . . . , which are formed by composition and heat treatment for a particular service environment.
Strength and hardness are sometimes used synonymously.
Meanwhile, there are some useful articles here:
See The Basics of Ferrous Metallurgy at
The Iron-Carbon Equilibrium Diagram
The Effects of Alloying Elements on Iron-Carbon Alloys
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