Page 214 - Materials Chemistry, Second Edition

201
3.2. Metallic Structures and Properties

A major difference between stainless and plated steels is the former will actually
self-repair itself when scratched. Since the chromium is homogeneously dispersed
throughout stainless steel, a scratch will serve to expose additional Cr sites forming
additional layers of the protective oxide. By contrast, the application of a protective
coating over steel will only be an effective barrier as long as it remains intact. When
this coating is penetrated by a scratch/crack, the bare steel is exposed to the
surrounding environment allowing the possibility for corrosion. Often, aluminum
and silicon are also added to steel that also form native oxides that are effective in
preventing surface corrosion of the underlying metal.
When some stainless steels are overheated (ca. 400–800 C) for a prolonged

period, there exists the possibility for chromium carbide formation. Most often this
results from an attempt to weld steels that are not suitable for such high-temperature
treatment. If such a precipitous reaction causes the bulk Cr concentration to fall
below 10.5 wt.%, corrosion protection is drastically reduced. To make the situation
worse, the carbide usually forms at grain boundaries, leading to intergranular corro-
sion and stress cracking. Amazingly, this process is reversible, by reheating the steel

to temperatures in excess of 1,000 C for a period long enough to redissolve the
chromium carbide particles and form a homogeneous solid solution. Rapid cooling
must then be introduced to suppress the reformation of carbide. Hence, if one wishes
to use a stainless steel at high temperatures, either low C compositions must be used,
or doping with carbide-forming metals such as V, Ti, or Ta that are more easily
oxidized than Cr. As a general rule of thumb, more chromium must be added as the
concentration of carbon is increased to ensure effective corrosion resistance.
There are currently over 200 commercially available types of stainless steels.
Hence, there is an exact composition of stainless steel for virtually any application.
As we have already seen, a tremendous number of substitutional and interstitial
dopants may be alloyed with iron, resulting in significant changes in their physical
properties. In addition, varying the heat treatment of the bulk or surface of steels
will change these properties even further. It is truly mind-boggling to think of all the
combinations of dopant composition/postprocessing that are possible! Fortunately,
all of these combinations fall under the umbrella of four general types of stainless
steels, classified according to their microstructural phases/compositions (Table 3.3).
The industrial applications for austenitic stainless steels far outweigh the other
types due to their facile work hardenability and high corrosion resistance. As we have

seen, the fcc austenite phase is not stable at temperatures below 723 C; however,
austenite-stabilizers such as Ni, Mn, Cu, C, or N may be added to extend the stability
of this phase down to room temperature. Figure 3.28 shows the stable phases that
exist at room temperature, as a function of the Cr and Ni concentrations. An easy way
to think about the effect upon Ni alloying is the replacement of an increasing number
of iron atoms in the lattice with Ni (stable fcc lattice at room temperature), results in
the solid solution being “fooled” into crystallizing in an fcc array rather than bcc
ferrite. Due to high concentrations of easily oxidizable elements such as Cr, Ni, and
Mn, the corrosion resistance is the greatest for austenitic stainless steels. However,
their Achilles’ heel is their reaction with chloride ions. Due to the large concentration