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information
Chapter 1: What is Stainless Steel?
Martensitic Stainless Steel
Normally contains 12-16 % Cr, low Ni, rarely Mo and a relatively high content of carbon (C, 0.12-1.2
%). Due to the carbon content, the martensitic steels can be hardened to more than 1000 HV by
rapid cooling, and they are extremely adapt for manufacturing cutting tools, surgical instruments
and high-quality kitchen knives.
Surgical instruments, both made of martensitic stainless steel.
Their mechanical strength is impressive, whereas the corrosion
resistance is low.
After the hardening, the martensitic steel
types can be neither welded nor cold deformed.
Along with any other heat treatment,
welding will cause the steel to lose
its hardness. The martensitic steel types
are strongly magnetic and due to the low
chromium content, the corrosion resistance
is generally poor.
This is the reason why expensive kitchen
knives frequently suffer from superficial pitting
corrosion after a turn in the dish washing
machine.
Ferritic Stainless Steel
Typically, this type contains 12-18 % Cr, low Ni, low carbon (C ≤ 0.12) and sometimes molybdenum
(Mo). The ferritic stainless steel types possess the same metallurgical structure as mild steel;
however, due to the low carbon content, the ferrites are not hardenable. Consequently, the ferrites
are relatively soft, but due to a shorter elongation, they are less adapted to cold deformation than
the soft austenites. All ferritic stainless steel types are strongly magnetic, and the stabilized types
(such as 4512, 4509 and 4521) are weldable.
The lowest alloyed types (i.e. 4003) possess
a relatively poor corrosion resistance,
in particular in strong acids, whereas the
higher alloyed types, such as 4509 and 4521,
equal the well-known 4301 and 4401 with
respect to pitting corrosion and, to a certain
extent, crevice corrosion. Further¬more, the
resistance to wards stress corrosion cracking
is much better than the parallel austenites
(Chapter 4 + 5).
Ferritic stainless steels are very useful for manufacturing thin-walled
specimens without too much welding or bending. This Syrian made
kitchen utensil has been made of 4016 (AISI 430), a very common
steel within catering and a perfect example that it is actually possible
to perform deep-drawing in ferritic stainless steel.
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01.2013