Metallurgy of Stainless Steel and Cobalt – Chromium alloys

STRESS RELIEF OF STAINLESS STEEL

Work hardening or cold working is the result of forced interlocking of grains and atoms of the metal. Many of these grains are locked in situations in which the material is under stress. These microscopic regions are under pressure or tension. When a wire with such internal stress is bent to store energy for an orthodontic application the previously stressed area cannot do their full share. Force applied to these stressed regions will bring them to their load limit. If the new load is in the same direction as the internal stress the two loads actually augment one another. The wire is weakened by the internal stress.

 

Stress relief eliminates such prestressed areas within the wire and puts it into condition to work more effectively. As internal stresses are relieved there may also be some change in the wire. A wire that is bent to form an arch is full of residual stresses that tend to slowly return it towards its original form. This goes on gradually at ordinary temp.

The effects associated with cold working such as strain hardening, low ductility and distorted grains can be reversed by simply heating the metal. This process is called annealing. The more severe the cold working the more rapidly the effects can be reversed by annealing.

Stress relieving effects depend on both time and temperature and they can be controlled by the adjustment of either of these factors. In general low temp treatment over a long period of time is most desirable. But the arch formed for a patient in the chair cannot be treated for hours or even for too many minutes.

 

This stress relieving treatment enhances the elastic properties of the wire.

 

The recommended temp time schedule for stress relieving stainless steel is 750oF (399oc) for 11 minutes. ‘Funk’ recommends the use of a colour index to determine when adequate heat treatment is achieved. He suggested that a straw coloured wire indicates that optimum heat treatment has been attained.

 

The stress relief treatment increases the modulus of elasticity, yield strength, and an increase in the modulus of resilience. This also reduces failure caused by corrosion which may occur in areas of high localized stress. If the wire is bent into tight loops of 180 degree or a spiral of small radius the measured increase in elastic strength may be as great as 50%.

 

The austenitic stainless steels are nonmagnetic but may develop ferromagnetic qualities as a result of cold-working during fabrication, which forms some ferrite or martensite, both of which are strongly magnetic. This magnetism can be eliminated by a full anneal to restore the metal to its full austenitic form. The temperature of this transformation is termed the Curie temperature, which is around 700° C.

 

 

 

 

 

There are 3 stages in annealing

RECOVERY

The stage at which the cold work properties begin to disappear before any significant visible changes are observed. There is a slight decrease in tensile strength.

RECRYSTALLISATION

This involves a radical change in the microstructure. The old grains disappear completely and are replaced by a new set of strains free grains. Recrystallization occurs in the most severely cold worked regions in the metal. Usually at grain boundaries or where the lattice was most severely deformed. On completion of recrystallization the material essentially attains its original soft and ductile condition.

GRAIN GROWTH

The recrystallized structure has a certain average grain size depending on the number of nuclei. Thus the grain size for the completely recrystallized material can range from rather fine to fairly coarse.

 

If the fine grain structure is further annealed the grains begin to grow. Excessive annealing can lead to larger grains. A large grain structure is generally detrimental to the strength properties of metal. With small grains a greater increase in strength, hardness, and proportional limit occurs during cold work than with large grains.

 

SENSITIZATION

 

Carbon is an undesirable impurity in stainless steel. Carbon does not enter into the physical structure of these steels. But at temperature between 800 – 1200oF carbon reacts with chromium to form chromium carbide. This is harmless in itself. But chromium tied up as the carbide cannot contribute to the corrosion resistance of the metal. This is called sensitization. The carbon inactivates the chromium at the grain boundaries opening them to corrosion. The grain boundaries are the weaker points in the structure, so corrosive attack on these surfaces can actually cause the metal to disintegrate.

 

Two means of prevention are available:

 

1)      Controlling the sensitizing temp. range (800 – 1200oF) or (425 – 650oc)

The chromium carbide reaction takes time. Speed in handling the metal at the sensitizing temperature range can be a very effective means of minimizing sensitization. Stainless steel should always be quenched immediately after soldering to bring it down to safe temperature as rapidly as possible. Both low temperature silver solder and high temperature solder (gold solders) can be used to advantage to control intergranular corrosion.

 

2)      Stabilization

This is the process by which carbon is made unavailable for the sensitizing reaction. This is done at the time the alloy is manufactured by either keeping the carbon content exceptionally low or by adding other metals that tie it up in different compounds. E. C. Bain, R. H. Aborn and J. J. B. Rutherford have pointed out the advantage of using suitable amounts of titanium, columbium and molybdenum, which combine with the carbon more easily than does the chromium, and adding them to low carbon stainless steel. Steel that has been treated in any of the foregoing ways to reduce the available carbon is called stabilized steel.

WIRE CHARACTERISTICS OF CLINICAL RELEVANCE

Several characteristics of orthodontic wires are considered desirable for optimum performance during treatment. They are:-

 

  1. 1.      Spring back

This is also referred to as maximum elastic deflection, maximum flexibility, range of activation, range of deflection, or working range. Spring back is related to the ratio of yield strength to the modulus of elasticity of the material. Higher values provide the ability to apply large activations with a resultant increase in working time of the appliance. This in turn implies that fewer arch wire changes or adjustments will be required. Spring back is also a measure of how far a wire can be deflected without causing permanent deformation or exceeding the limits of the material.

 

 

  1. 2.      Stiffness or load deflection rate

This is the force magnitude delivered by an appliance and is proportional to the modulus of elasticity how values provide.

 

(a)    ability to apply lower forces

(b)   A more constant force over time as the appliance experience deactivation.

(c)    Greater ease and accuracy in applying a given force.

 

  1. 3.      Formability

High formability provides the ability to bend a wire into desired configurations such as loops coils and stops without fracturing the wire.

 

  1. 4.      Modulus of resilience or stored energy

This properly represents the work available to move the teeth. It is reflected by the area under the line describing elastic deformation of the wire.

 

Stainless steel

 

 

 

 

Bending                                                                            nitinol

Moment

 

 

 

Deflection

 

From this graph its clear that for the same force applied the deflection is more for nitinol. Hence the stored energy is also more for nitinol wire when compared to stainless steel wire.

 

  1. 5.      Biocompatibility

            Includes resistance to corrosion and tissue tolerance to elements in the wire.

 

  1. 6.      Environmental stability

Maintenance of desirable properties of the wire for extended periods of time after manufacture. This ensures a predictable behavior of the wire when in use.

  1. 7.      Joinability

This is the ability to attach auxillaries to orthodontic wires by welding or soldering provides an additional advantage when incorporating modifications to the appliance.

  1. 8.      Friction

Space closure and canine retraction in continuous arch wire technique involves a relative motion of bracket over wire, excessive amount of wire friction may result in loss of anchorage or binding accompanied by little or no tooth movement. The preferred wire material for moving a tooth relative to the wire would be one that produces the least amount of friction at the bracket wire interface.

MECHANICAL PROPERTIES ACCORDING TO ADA SPN NO 32

Alloy Modulus of Elasticity Yield strength Ultimate tensile strength No. of 90o Bends without Fracture
S.S. 179 Gpa 1579 Mpa 2117 5

PROPERTIES OF S. S WIRES IN TENSION, BENDING, AND TORSION

Property 18 – 8 Stainless Steel

TENSION

0.1% yield strength, Mpa

Elastic modulus, GPa

Spring back (YS/E), 10-2

1200

134

0.89

BENDING2.9-degree offset yield strength, MPa

Elastic modulus, GPa

Spring rate, mm-N/degree

1590

122

0.80

TORSION

Spring rate, mm-N/degree

0.078

 

The Mechanical properties of 3 sizes of stainless steel, Nickel Titanium and Titanium Molybdenum wires were studied in Tension Bending and Torsion. It is found that in Tension stainless steel showed least maximum elastic strain or spring back and TMA had the most Spring Rates in Bending and Torsion were highest for stainless steel and were lowest for Nickel Titanium.

CLASSIFICATION OF STAINLESS STEEL

 

Classified according to the American Iron and Steel Institute (AISI) system. This classification parallels the unified number system (UNS) and the German standards (DIN).

 

It is classified into:

 

(a)  Austenitic stainless steels (300 series)

These alloys are the most corrosion resistant of the stainless steels. AISI 302 is the basic type containing 18% chromium, 8% Nickel and 0.15% carbon. Type 304 has similar composition but carbon content is (0.08%). Both 302 and 304 may be designated as 18-8 stainless steel. Type 316 L (0.03% C) is used for implants. ‘L’ signifies low carbon content. Structurally these steels are solid solutions which offer better corrosion resistance. If austenizing elements (Ni, Mn and N) are added the highly corrosion resistant solid solution phase can be preserved even at room temperature.

These austenitic stainless steels are most commonly used by the orthodontist in the form of bands and wires.

 

(b)  Martensitic steels (400 series)

Starting in the 1970’s, in addition to carbon other elements were added to stainless steel to stress their microstructure and thereby increase their tensile strength. Strong but less corrosion resistant alloys were the result. Such steels could only be used for short contacts with the oral environment. Because of their stressed martensitic structure these steels are used primarily for instruments that require sharp or wear resistant edges such as surgical and cutting instruments.

The corrosion resistance is less than that of the other types and is reduced further following a hardening heat treatment.

 

(c)  Ferritic stainless steel (400 series)

The name derives from the fact that the microstructure of these steels is the same as that of iron at room temperature. The difference is that chromium is substituted for some of the iron atoms in the unit cells. The degree of substitution can go as high as 30% in the presence of small amounts of other elements like C, N, and Ni. They have good corrosion resistance but cannot be work hardened. Because temperature change induces no phase change in the solid state; the alloy is not hardenable by heat treatment.

 

The modern super ferrites which belong to this category contain 19-30% chromium and are used in several nickel free brackets. Highly resistant to chlorides these alloys contain small amounts of aluminium and molybdenum and very little carbon.

PERCENTAGE COMPOSITION OF THREE   BASIC TYPES OF STAINLESS   STEEL

 

Type Cr. Ni. Carbon
Ferritic [BCC] 11.5 – 27 0 0.2 max.
Austenitic [FCC] 16 – 26 7 – 22 0.25 max.
Martensitic [BCT] 11.5 – 17 0-2.5 0.15 -1.20

(d)  Duplex steels [SAF 2205]

Duplex steels consist of an assembly of both austenitic and ferritic grains. Besides iron these steels contain molybdenum and chromium and they have lower nickel content. [Cr-22%, Ni-5.5%, Mn – 2%, Mo-3%, C-0.03%, P-0.03%, Si-1%, S-0.02% and others]

 

Their duplex structures results in improved toughness and ductility compared to ferritic steels. Where as their yield strength is more than twice that of austenitic stainless steel. They are highly corrosion resistant. Combining lower nickel content with superior mechanical properties duplex steel has been used for the manufacture of one piece brackets. [eg: Bioline low nickel by CEOSA]

 

(e)  Types [500 series]

501 and 502 are low chromium [4-6%] steel not used for orthodontic appliances.

 

(f)  Precipitation Hardenable steels [pH steels] [600 series] [630/17-4] [631/17-7]

Unlike most stainless steels pH steels can be hardened by heat treatment. The process actually is an aging treatment. Which promotes the precipitation of some elements purposely added. Because of its high tensile strength 17 – 4 PH stainless steel is widely used for “mini brackets”. The manufacturer ORMCO has used steel from the same class PH 17 – 7 to make its edge lock brackets. The added metals lower their corrosion resistance.

(g) Cobalt containing alloys

Used both for wires and brackets. Elgiloy and flexiloy contain a large proportion of nickel.

(h) Manganese containing steels

Known as austenizing element, manganese acts by interstitially solubilizing the really austenizing element nitrogen thus replacing nickel.

A.J. WILCOCK ARCHWIRES

This is an area which has seen vast changes in material manufacturing technology resulting in the availability of three more grades of wire becoming available as several new sizes giving a new meaning to the use of light force in Begg appliance usage.

 

Wilcock archwire have been the main stay of Begg’s tech. It is history now that it was the development of the Australian wire by the late Mr. Arthur J. Wilcock senior of Whittlesea, Victoria Australia that enabled Dr. Begg to develop his light wire tech until recently the grade of wire routinely used was special plus and for those cases resistant to bite opening extra special plus was used. The new grades and sizes of wires are now available. They are available in spools and straight lengths.

 

Premium                      –           0.020”

Premium plus              –           0.010”, 0.011”, 0.012”, 0.014”, 0.016”, 0.018”

Supreme                      –           0.008”, 0.009”, 0.010”, 0.011”, 0.012”

Special plus                 –           0.014”, 0.016”, 0.018”

 

MANUFACTURING

Spinner straightening is a mechanical process of straightening resistant materials usually in the cold drawn condition. The wire is pulled through rotating bronze rollers which torsionally twist the wire into straight condition.

 

The disadvantage of this process is

1)            Resultant deformation

2)      Decreases yield strength value as it becomes strain softened.

 

The wires are available in different grades.

(a)          Regular

(b)         Regular plus

(c)          Special

(d)         Special plus

(e)          Premium

(f)          Premium plus

(g)         Supreme

 

Regular and Regular plus are easily formed and are excellent wires for general use and utility wires. The special, special plus and premium wires are ideal for bite opening and where high resistance is required. The finer wires such as premium plus and supreme are ideal for auxillaries.

 

In pulse straightening the wire is pulsed in a special machine which permits high tensile wires to be straightened and also lower diameters than possible earlier with spinner straightening. The material yield strength is not altered and the surface has a smoother finish and therefore causes lower friction.

 

Spools are available in four grades.

 

Special plus                    0.014”, 0.016”, 0.018”

Premium                        0.020”

Premium plus                0.010”,   0.011”,   0.012”,   0.014”,   0.016”,   0.018”

Supreme                        0.008”,   0.009”,   0.010”,   0.011”,   0.012”

 

Straight lengths are available in all the grades

Regular                                    0.012”                         –           0.024”

Regular plus                            0.012”             –           0.020”

Special                                     0.012”             –           0.020”

Special plus                             0.012”             –           0.024”

Premium                                  0.012”             –           0.020”

Premium plus                          0.010”             –           0.018”

Supreme                                  0.008”             –           0.011”

 

THE PROPERTIES OF THESE WIRES ARE

 

1)            The ultimate tensile strength for P.S wire were 8-12% higher than S.S wires indicating greater resistance to fracture in the oral cavity.

2)            The load deflection rate was higher by 10% for 0.016 P.S wire and 23.5% for 0.020 premium wires indicating that when used for intrusion they deliver significantly higher loads.

3)            The pulse straightened wires have a significantly higher working range and show good recovery patterns.

4)            Frictional resistance of the P.S wires was lesser by a factor of 50% than S.S. wires.

5)            There was no significant difference in stress relaxation properties.

 

USES

The supreme grade wire of sizes 0.008 – 0.011 are used for

 

a)            Unraveling of crowded anterior teeth when used for this purpose these wires are pinned into the malpositional teeth and alongside the main archwire in normally aligned teeth. They have resistance and field diameter very close to NITI wires. Cost wise they are much more economic than NiTi wire.

b)            When used as MAA the lighter forces produced do not tax the anchorage.

 

c)            Mini uprighting springs. When used as M.U.S. they can be slipped behind the main archwire without removing the pins.

 

They also produce very light forces thereby decreasing anchor strain. The 0.011 wire can be used for aligning second molar towards the end of stage III. The wire fits snugly into the 0.036 tube having a finishing wire 0.014. Premium plus wires have been recommended by Mollenhauer for use in high angle cases to prevent undue molar extrusion and due to the low diameter do not produce much force and interior intrusion which is clinically favourable in such cases.