Fig. 10.1 The iron–carbon phase diagram (up to 2% carbon), slightly simplified for clarity.

Fig. 10.2 Diagram illustrating the hardening and tempering cycle of heat treatments which can be used on steel.

steel, called martensite, which has a distorted body-centered cubic lattice, is formed. Martensite is too brittle for most applications but its brittleness can be reduced by using a low temperature heat treatment, called tempering. The alloy is heated to a temperature in the range 200–400ºC, at which martensite partially converts to ferrite and cementite. The degree of conversion depends on the tempering temperature and the tempering time. Thus, the hardness and brittleness can be controlled quite accurately by choosing suitable heat treatment conditions. The process of heat treatment hardening and tempering is illustrated in Fig. 10.2.

The ability to be hardened by heat treatments is one of the major advantages of steel. Probably its main disadvantage is its susceptibility to corrosion.

10.3 Stainless steel

In addition to iron and carbon the stainless steels contain chromium which improves corrosion resistance. This is achieved by the passivating effect in which the chromium exposed at the surface of the alloy is readily oxidized to form a tenacious surface film of chromic oxide. This film resists further attack from aqueous media thus preventing corrosion. Nickel is also present in many stainless steels. It contributes towards corrosion resistance and helps to strengthen the alloy.

The addition of chromium and nickel to steel causes the critical temperature (Tc) (see Fig. 10.1) to be lowered. If sufficient quantities of these two metals are incorporated, the austenitic structure remains even at room temperature. One of the most commonly used stainless steels contains 18% chromium and 8% nickel (termed 18/8 stainless steel). This alloy has a critical temperature below the point at which atomic movements are possible and is therefore sometimes referred to as austenitic stainless steel. It is not possible to harden these stainless steels by heat treatment because the solid–solid transitions occur below the temperature at which atomic diffusions are possible. Hence, 18/8 stainless steels are used in applications where heat hardening is not necessary, for example, for noncutting instruments, wires and occasionally as denture bases. These applications involve a degree of cold working since the alloy is shaped by either bending, drawing or swaging, all of which result in the formation of a wrought structure. Alloys having wrought structures tend to have much higher values of proportional limit than equivalent alloys which have not been cold worked. It is this property coupled with a relatively high modulus of elasticity which makes wrought stainless steel wires suitable for orthodontics and wrought stainless steel sheets suitable for denture bases. Whilst these alloys cannot be hardened by heat treatments they can be softened if inadvertently overheated and recrystallized. This destroys the fibrous grain structure which results in a marked reduction in yield stress and proportional limit.

When smaller quantities of chromium and nickel are incorporated into steel it is possible to produce an alloy which has adequate corrosion resistance but which can be hardened by heat treatment. An alloy with these characteristics may,

typically, contain about 12% chromium and little or no nickel. This alloy is capable of forming a martensitic structure and is therefore sometimes referred to as martensitic stainless steel. This type of alloy is commonly used to construct cutting instruments and probes which can be hardened by heat treatment, using a technique similar to that previously described for steel.

10.4 Stainless steel denture bases

Stainless steel denture bases are formed from very thin pressed/rolled sheets of wrought stainless steel. The method used for forming a stainless steel denture base deserves special mention. A thin sheet of 18/8 stainless steel (approximately 0.2 mm thick) is pressed between an alloy or epoxy resin die and counter die. The method of applying the pressure required for swaging may vary. Traditionally, a hydraulic press was used but modern techniques involve the use of a sudden pressure wave which adapts the sheet of alloy to the die very quickly. The pressure wave may be generated by using a controlled explosion or a sudden, controlled release of hydraulic pressure. These techniques are known as explosion forming and hydraulic forming respectively.

The wrought stainless steel sheets have high values of modulus of elasticity and proportional limit. This enables sufficient rigidity to be achieved with a very thin sheet of material. The weight of the denture can thus be kept to a minimum. Care must be taken not to overheat the wrought appliance since this may cause re-crystallization and a marked reduction in proportional limit.

A further advantage of stainless steel denture bases is that they conduct heat rapidly through the thin metallic sheet, thus ensuring that the patient retains a normal reflex reaction to hot and cold stimuli. The main disadvantages are the lack of surface detail on the swaged plate and, perhaps more significantly, the involved technique required for swaging, attaching retention tags by welding and processing of the acrylic parts of the denture.

10.5 Wires

Wires are commonly used for the construction of orthodontic appliances and occasionally as wrought clasps and rests on partial dentures. Orthodontic wires are designed to function such

that they apply forces to malaligned teeth in order to change their positions/arrangements and to approximate more closely an ideal dental arch. The main variables involved in wire selection are related to the extent of movement required and the speed at which movement should occur. The aim of the orthodontist is to maximize the rate of tooth movement whilst minimizing the potential for pathological change. Wires are normally produced by drawing an ingot of alloy through dies of gradually decreasing cross-section to produce a circular, ovoid or square section wire in which the grain structure is highly fibrous in nature (see p. 55).

Requirements

The requirements of wires relate to their springiness, stiffness, ability to be bent without fracturing, corrosion resistance and an ability to be simply joined by soldering or welding. The springiness of a wire is a function of its fibrous grain structure which is incorporated during drawing of the wire. The ‘springback’ ability of a wire is a measure of its ability to undergo large deflections without permanent deformation. In terms of mechanical properties this is given by the ratio of yield stress to modulus of elasticity as follows:

A more approximate form of this equation uses proportional limit in place of yield stress.

A wire should have a value of stiffness, as indicated by its modulus of elasticity, which enables it to apply a suitable force for tooth movement during orthodontic treatment. This requirement varies considerably, since it is sometimes necessary to carry out rapid movements using stiff wires, whilst on other occasions it is necessary to apply small forces with flexible wires in order to bring about slow movements. Most wires are produced in circular cross-section and the stiffness of such wires is markedly dependent on thickness, being a function of the fourth power of the radius. Thus increasing the thickness of a wire from 0.6 mm to 0.7 mm increases stiffness by a factor of 1.86. Treatment strategy may involve the initial use of a relatively stiff wire capable of applying a large force in order to move teeth rapidly. Such wires may need to be replaced regularly if they have limited springback ability. A different strat-

egy may be required if the teeth are very badly aligned. In this case it may be difficult to adapt a stiff wire to the teeth so a more flexible wire is used and the resulting tooth movement will be slower. When flexibility is required one approach is to use wires of smaller radius, although these can cause problems relating to tilting in the bracket slots so a better approach may be to choose a wire which is inherently more flexible.

Orthodontic wires are generally shaped by bending and the wire should possess sufficient ductility to resist fracture during this bending procedure. The amount of residual ductility remaining in a wire depends in part on the ductility used up in its manufacture. Hence, manufacturers can supply wires with varying ductility depending upon the extent to which they have been work hardened and/or recrystallized during production.

Wires often remain in the oral cavity for several months, whether they be part of a fixed or a removable orthodontic appliance. The wire should therefore have good corrosion resistance in order that it can withstand attack from oral fluids.

Finally, it is sometimes necessary to join two parts of an appliance together and, ideally, wires should be capable of being easily joined either by soldering or by welding, without impairing the mechanical properties of the wire or reducing the corrosion resistance. The importance of this property is reduced in alloys which can be easily bent into loops as a means of joining.

Available materials

Table 10.1 lists the commonly used wires and summarizes their main properties.

Stainless steel: Stainless steel wires are constructed from the 18/8 austenitic type of stainless steel – containing less than 0.15% carbon. The wires most commonly used are designated as types 302

and 304 by the American Iron and Steel Institute. They have a high value of modulus compared with some other alloys used to construct wires and are therefore used to apply relatively high forces. Lower forces can be achieved by using a wire of smaller diameter. 18/8 stainless steel has a relatively high value of yield stress and the springback properties are thus adequate for most applications.

Stainless steel wires have sufficient ductility to allow bending without fracture. They can be obtained in three grades, often referred to as soft, half-hard and hard, ranging from very ductile (soft) to less ductile (hard). The type of wire is chosen according to the amount of bending which must be carried out. Following bending, a stress relief anneal can be carried out in order to relieve internal stresses. This involves heating the wire to 450ºC for about 10 minutes. The annealing procedure should be carried out only with ‘stabilized’ stainless steel wires which contain small quantities of titanium. Unstabilized wires become brittle during annealing due to reaction between chromium and carbon.

Joining of stainless steel wires can be accomplished either by soldering or by welding. Silver solders are normally used for soldering. They contain silver and copper with small quantities of other elements to lower the fusion temperature. Care must be taken during soldering not to overheat the wires since this may cause recrystallization of the grain structure with a subsequent lack of springiness. In addition, overheating may cause the chromium to react with carbon, forming carbides, a phenomenon referred to as weld decay. This results in a loss of corrosion resistance around the soldered joint and the introduction of a degree of brittleness. The solder itself, of course, is a source of potential corrosion being of a eutectictype composition.

Welding is accomplished by pressing two pieces of wire together between two electrodes then

passing an electric current, sufficient to melt the wires at the point of contact joining them together. The temperature rise ( T) at the joint when a current passes is given by the function

TαI2Rt

where I is the current passing, R is the electrical resistance at the junction of the two wires and t is the time for which the current passes.

High temperatures, sufficient for welding, can only be achieved with alloys giving relatively high values of electrical resistance at the junction. Thus, welding is not a suitable technique for joining gold wires. The electrical current and the time for which the current flows must be controlled such that adequate welding is achieved without overheating the rest of the wire. This would lead to weld decay and a degree of recrystallization, causing loss of the fibrous grain structure.

Gold alloys: Traditionally, two types of wrought gold alloy wires have been used in the past – classified as high gold and low gold. High gold materials contain greater than 75% gold and platinum group metals. Low gold alloys contain smaller quantities of noble metals. A typical material contains 60% gold, 15% silver, 15% copper and about 10% platinum or palladium. The high platinum or palladium content raises the melting point and recrystallization temperature of the wires making them more amenable to soldering operations. Gold alloy wires have a lower value of modulus of elasticity than the stainless steel variety and therefore apply lower forces. An advantage of the gold alloy wires is that they are easily soldered using normal gold solders. Gold alloy wires are rarely used nowadays due to their cost.

Cobalt-chromium alloys (elgiloy): These alloys contain cobalt, chromium, nickel, iron and molybdenum in approximate proportions 40 : 20 : 15 : 16 : 7. They have the unique characteristic of being supplied in a softened state which has excel-

lent ductility. Following bending, the wires can be hardened by heat treating at 480ºC. The precipitation hardening that occurs introduces the required springback properties into the wire.

The modulus of elasticity of the wire is similar to that for stainless steel, indicating a similar performance in terms of tooth movement. These wires are difficult to join by soldering.

Nickel-titanium alloys (nitinol): These alloys contain almost equal amounts of nickel and titanium with small quantities of other metals. They are flexible wires with low modulus values and are used to apply relatively low forces. The low modulus coupled with high yield stress indicates excellent springback properties and they are particularly useful for carrying out large tooth movements using low forces over a long period of time. Nitinol wires have limited ductility and are not easy to bend without fracturing. They are not amenable to joining operations such as soldering or welding.

Nitinol wires possess a rather unique shapememory property which enables a plastically deformed wire to return to its original shape following an appropriate heat treatment. This behaviour has not yet been used to good effect in any dental applications of the wires.

b-Titanium: These alloys consist mainly of titanium with some molybdenum. They are ductile, allowing good formability, and have springback characteristics similar to those of stainless steel. They have a lower modulus value than stainless steel and therefore apply lower forces, and they can be joined by welding.

10.6 Suggested further reading

Thomson, S.A. (2000) An overview of nickel-titanium alloys used in dentistry. Int. Endodont. J. 33, 279.

Kusy, R.P. (1997) A review of contemporary archwires: their properties and characteristics. Angle. Orthod. 67, 197.