Cutlery alloys

It is also possible to protect iron against corrosion by forming an alloy with a different metal. Stainless steel is an alloy of iron that contains at least 10% chromium, by mass, in addition to small quantities of carbon and occasionally metals such as nickel. Stainless steel is much more resistant to corrosion than pure iron. Therefore, stainless steel is often used for cutlery, taps, and various other applications where rust-resistance is important. However, chromium is much more expensive than iron. As a result, stainless steel is too expensive for use in large-scale applications, such as building bridges. 

There are two types of alloys. In a substitutional alloy, some of the metal atoms in a crystal lattice are replaced by other atoms (usually of comparable size). Examples are brass, in which approximately one third of the atoms in a copper crystal are replaced by zinc atoms, and pewter, an alloy of tin that contains 7% copper, 6% bismuth, and 2% antimony. In an interstitial alloy, atoms of one or more additional elements enter the interstitial sites of the host metal lattice. An example is steel, in which carbon atoms occupy interstitial sites of an iron crystal, making the material stronger and harder than pure iron. Mild steel contains less than 0.2% C and is used for nails, whereas high-carbon steels can contain up to 1.5% C and are used in specialty applications such as tools and springs. Alloy steels are both substitutional and interstitial atoms from metals such as chromium and vanadium substitute for iron atoms, with carbon remaining in interstitial sites. Alloy steels have a variety of specialized purposes, ranging from cutlery to bicycle frames. 

In a substitutional alloy, atoms of the solute occupy sites in the lattice of the solvent metal (Figure 5.8). To maintain the original lattice structure of the host metal, atoms of both components should be of a similar size. The solute atoms must also tolerate the same coordination environment as atoms in the host lattice. An example of a substitutional alloy is sterling silver (used for silver cutlery and jewellery) which contains 92.5% Ag and 7.5% Cu elemental Ag and Cu both adopt ccp lattices and rn,etai(A-g) rjnet3i(Cu) (Table 5.2). 

Stainless steels are examples of alloy steels, i.e. ones that contain a J-block metal in addition to carbon. Stainless steels have a significant content of the alloy metal and are of high commercial value because of their high resistance to corrosion. All contain a minimum of 10.5% (by mass) of chromium and the resistance to corrosion arises from the formation of a thin layer of Cr203 ( 13 000 pm thick) over the surface of the steel. The oxide layer passivates (see Section 9.4) the steel and is self-repairing, i.e. if some of the oxide coating is scratched off, further oxidation of the chromium in the steel necessarily repairs the wound . A further property that makes stainless steels commercially important is that they can be polished to satin or mirror finishes and this is easily appreciated in the ranges of stainless steel cutlery available to the consumer. 

German silver (nickel silver) An alloy of copper, zinc, and nickel, often in the proportions 5 2 2. It resembles silver in appearance and is used in cheap jewellery and cutlery and as abase for silver-plated wire. See also... 

Silver. Silver is ductile and very malleable, conducts heat, and has the highest electrical conductivity of all metals. It is used to make cutlery, jewelry, coins, long-life batteries, photographical films, and electronic components (circuits, contacts), and in dentistry. Its alloy, sterling silver (silver plus copper) is also used to make jewelry and tableware. German silver (sdver plus nickel) is another alloy used for sU-verware. 

S21 420S29 (750° C temper) (750° C temper) 420S37 (cutlery temper, 200°C) Alloyed to 12% Cr steel 2 (650°C temper)... 

Stainless steel, which is hard and resists corrosion, is made of iron and chromium (12-30%). The properties of stainless steel make it a suitable alloy for making cutlery and utensils. 

Tempering at SOOT (425 C) does not reduce the hardness of the part and in this condition these alloys show an exceptional resistance to fruit and vegetable acids, lye, ammonia, and other corrodents to which cutlery may be subjected. 

---