14-09-2013, 02:04 PM
Metals
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• Steel generally contains: 0.05-2.0 wt. % C
• Cast irons: 2.0-4.5 wt. % C
• Total non-carbon addition > 5 wt. % →
high-alloy steel, otherwise low alloy steel
Classification of “Carbon Steels”
There are four main groups of carbon steels
1. Low carbon steels (> 80% of the steel production)
< 0.20 %C Used structural purposes.
2. Medium Carbon Steels
0.25 – 0.55 %C Used for machine parts.
3. Eutectoids Steels
0.6 – 0.8 %C Used for hard wearing rails.
4. Tools Steels
0.9 – 2.2 %C Used for tools and dies.
Steels
Plain-carbon steels are relatively low strength ( as steels go) but have very
high ductility and relatively low cost – this is why the automotive industry
likes it for auto body parts such as the panels for doors, roofs, hoods, etc.
•It cannot be strengthened above ~690 Mpa (150,000 psi) without substantial
loss in ductility and impact strength.
•It has low resistance to corrosion and oxidation, so coatings such as Zn, Sn
and paints are used for protection.
•It has poor resistance to impact at low temperatures, having a BCC
structure.
•There’s difficulty in evenly heat treating large pieces, which creates a
variability in properties.
Alloy Steels
So, there was a need to create better types of steels, which are alloy steels, ie., additions
of alloys (low amounts of Mn, Ni, Cr, Ti, Si, etc) to improve its properties.
•Alloying increases the cost of the steel, but the enhanced properties are essential in
many applications.
•Alloying improves corrosion resistance (particularly with the addition of Chromium),
producing the stainless steels.
•Alloying improves the hardenability, which is needed for tool steels
•Hardenability means the ability of the steel to form martensite.
•Alloying increases the solid solution strengthening of the ferrite phase
•Alloying can produce a less brittle steel by causing the formation of carbides (WC,
Cast Irons
• The ferrous alloys with greater than 2 wt % carbon;
• relatively low melting temperature, and liquid phase viscosities,
• do not form undesirable surface films when poured, and undergo moderate shrinkage during the
solidification.
• Inferior mechanical properties compared with those of wrought alloys results from a less uniform
microstructure including some porosity.
• Four types cast irons:
• White iron: has a characteristic of white, crystalline fracture surface: Large amounts of cementite
are formed during casting, giving a hard brittle material.
• Gray iron: has a gray fracture surface. Silicon content of (2-3 wt %) promotes graphite
precipitation rather than cementite.
• Ductile iron: addition of small amount of Mg (0.05 wt %) to the molten metal of gray-iron
composition produces spheroidal graphite rather than graphite flakes. Ductility is improved by a
factor of 20. and strength is doubled.
Requirements for Age Hardening
Not all alloys are age hardenable. Four conditions must be satisfied for an alloy to have
an age-hardened response during heat treatment.
1)The alloy system must display a decreasing solid solubility with decreasing
temperature. In other words, the alloy must form a single phase on heating above the
solvus line, then enter a two-phase region on cooling.
2) The matrix should be relatively soft and ductile, and the precipitate should be hard
and brittle. In most age hardenable alloys, the precipitate is a hard, brittle
intermetallic compound.
3) The alloy must be quenchable. Some alloys cannot be cooled rapidly enough to
suppress the formation of the precipitate. Quenching may, however, introduce residual
stresses that cause distortion of the part. To minimize residual stresses aluminum
alloys are quenched in hot water, at about 80 oC, i.e., a hot quench.