Stainless Steal

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Stainless Steel

Background and Application

Stainless steels are iron-based alloys that consist of about 10.5-12% Cr (Chromium) at

minimum (Newton, 2002).The chromium content is needed to prevent the formation of rust in

unpolluted atmospheres (this is why it is called stainless steel). Stainless steels get there

stainless characteristics form the formation of an invisible chromium rich oxide film. This

oxide forms and heals itself in the presence of oxygen. In many stainless steels other

elements are added to improve particular characteristics including nickel, manganese,

molybdenum, copper, and titanium among others. Carbon is normally present in amountsranging from less than 0.03% to over 1.0% in certain grades. (Davis, 2000)

History of stainless steel

The corrosion resistance of iron-chromium alloys was first recognized in 1821 by

French metallurgist Pierre Berthier, who noticed their resistance to some acids and

suggested their use in cutlery. Metallurgists of the 19th century were unable to produce the

combination of low carbon and high chromium found in most modern stainless steels, and

the high-chromium alloys they could produce were too brittle to be practical.

In the late 1890s Hans Goldschmidt developed an aluminothermic (thermite) process for

producing carbon-free chromium. Between 1904 and 1911 several researchers prepared

alloys that would today be considered stainless steel.

Friedrich Krupp Germaniawerft built the 366-ton sailing yacht Germaniafeaturing a chrome-

nickel steel hull in Germany in 1908. In 1911, Philip Monnartz reported on the relationship

between chromium content and corrosion resistance. On October 17, 1912, Krupp engineers

Benno Strauss and Eduard Maurer patented austenitic stainless steel as ThyssenKrupp

Nirosta.

Similar developments were taking place contemporaneously in the United States, where

Christian Dantsizen and Frederick Becket were industrialising ferritic stainless steel. In 1912,

Elwood Haynes applied for a US patent on a martensitic stainless steel alloy, which was not

granted until 1919.

Also in 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England,

while seeking a corrosion-resistant alloy for gun barrels, discovered and subsequently


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industrialized a martensitic stainless steel alloy. The discovery was announced two years

later in a January 1915 newspaper article in The New York Times. The metal was later

marketed under the 'Staybrite' brand by Firth Vickers in England and was used for the new

entrance canopy for the Savoy Hotel in London in 1929.

Brearley applied for a US patent during 1915 only to find that Haynes had already registered

a patent. Brearley and Haynes pooled their funding and with a group of investors formed the

American Stainless Steel Corporation, with headquarters in Pittsburgh, Pennsylvania. In the

beginning stainless steel was sold in the US under different brand names like 'Allegheny

metal' and 'Nirosta steel'. In 1929 before the Great Depression hit, over 25,000 tons of

stainless steel were manufactured and sold in the US.

(Peckner and Bernstein, 1977)

Production of stainless steel

With specific restrictions in certain types, the stainless steels can be shaped and fabricated

in conventional ways. They are produced in cast, powder metallurgy (P/M), and wrought

forms. Available wrought product forms include plate, sheet, strip, foil, bar, wire, semi-

finished products (blooms, billets, and slabs), and pipe and tubing. Cold rolled flat products

(sheet, strip, and plate) account for more than 60% of stainless steel product forms. Figure 2

illustrates the most commonly employed mill processes for making various wrought stainless

steel products. Production of stainless steels is a two-stage process involving the melting of

scrap and ferroalloys in an electric-arc furnace (EAF) followed by refining by argon oxygen

decarburization (AOD) to adjust the carbon content and remove impurities. Alternative,

melting and refining steps include vacuum Induction melting, vacuum arc re-melting, electro

slag re-melting, and electron beam melting. Melting and refining of stainless steels is,

however, most frequently accomplished by the EAF/AOD processing route. In fact, about

90% of all stainless steel produced in the United States is processed by EAF melting

followed by AOD.During the final stages of producing basic mill formssheet, strip, plate and barand

bringing these forms to specific size and tolerances, the materials are subjected to hot

reduction with or without subsequent cold rolling operations, annealing, and cleaning. Further

steps are required to produce other mill forms, such as wire and tube.

Application

Approximately one third of the stainless steel market lies in chemical and power engineering

industries. The application of stainless steel in these industries include the components


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used in chemical processing equipment and energy harvesting equipment, such as in

boilers, nuclear reactors and heat vessels. The food and drink industry, transport industry

and architecture industry are responsible for 18 %, 9% and 5 % of the market respectfully.

32 % of the application of stainless steel falls in the consumer goods section with the use of

stainless steel components in electronic devices and other domestic utensils. (Davis, 2000)

Less than 1% of the stainless steel market is held in the medical device industry.

Commercial grade stainless steels are adequate for the production of nonimplant devices

such as dental and surgical instruments. Stainless steel which is used as parts of implants

must be suitable for prolonged periods of time in the biological conditions, such as warm

temperatures and saline solutions (Davis, 2003). Austenitic stainless steels are a common

choice for implant applications.

Grades of stainless steel and classification

Stainless steels are commonly characterised by their metallurgical phases, which are

evident in their microscopic structures. The classification groups include the following;

martensitic, ferritic, austenitic, duplex (ferritic-austenitic) and precipitation-hardening.

In order to improve the properties of steel, different alloying elements are added in the

manufacturing process, such as chromium, nickel, molybdenum, copper, titanium,

aluminium, silicon, niobium, nitrogen, sulphur, and selenium(Newton, 2002). Chromium

forms an oxide layer in the surface of the steel, making it resistant to corrosion and

increases scaling resistance at high temperatures. In austenitic structures, the addition of

nickel increases the structural stability and ductility. The makes the alloy easier to form. The

nickel increases the alloys strength and prevents chemical corrosion. Molybdenum again

increases the corrosion resistance and strength at high temperature. It also increases the

creep resistance and the passivity range, and counteracts the alloys tendency to pit when

exposed to chlorine. Copper prevents cracking due to stress corrosion and causes age-

hardening effects. Titanium is added with carbon to reduce intergranular corrosion and

promotes the ferrite formation. Aluminium lowers hardenability and improves its scaling

resistance. Silicon also improves the scaling resistance in variable temperatures. Silicon is

added to all grades in some amount, for deoxidising purposes. Manganese is added to

increase the stability of austenite when near room temperature and improves its properties

at high temperatures (Aksteel, 2012).

Depending the chemical composition of these elements within the alloy, there are different

grades within the classification groups. This grading system represents element compositionand properties of the alloy. The 200 series have an austenitic structure and are non-


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magnetic. The basic grade contains 17% chromium, 7% manganese and 4% nickel.

Although this series can be less susceptible to corrosion then the 300 series, when corrosion

occurs, it spreads rapidly. The 300 series, whose basic composition contains 18% chromium

and 8% nickel, are non-magnetic and have an austenitic structure. This series is known to

perform well in low chlorine conditions. The 400 series are magnetic and have a martensitic

structure. Their basic alloy contains 1% manganese and 11% chromium. These steels are

often avoided in situations where corrosion can have a big effect. The 600 series, commonly

called Precipitation Hardening stainless steels. They are subject to corrosion when

exposed to chlorine(Defense, 2005).

Chemical, Physical and Mechanical properties for each grade

Table 1 shows the percentage of elements in different grades of stainless steel alloys. Thecomposition of these elements are often analysed and calculated using the energydispersive X-ray spectrometer, as part of a SEM or TEM(Huang et al., 2004).

Table 1. Chemical composition of stainless steal

The Mechanical properties of the different grades of stainless steel have been summarisedin table 2.


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Table 2. A table showing the mechanical properties of the different grades of stainless steel.

Physical properties vary depending on the group of stainless steel being considered. As

shown in the bellow tables (Inox, 2007),the properties of the different groups of stainless

steel depend on the chemical composition. A big difference evident between the groups is

their magnetic property. It is because of this testing for austenitic stainless steel cannot be

done without destructive means, but the others can be tested using magnetic methods (S. H.

Salleh, 2009). The thermal conductivity decreases as nickel and copper are added. In

general steel has a very high electrical resistivity, and the addition of alloys increases itfurther. This is why ferritic and martensitic stainless steels electrical resistivity is lower than

the austenitic, duplex, and Precipitation hardening steel, but still higher than other steel

alloys without chromium.


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Table 3 (a). The physical properties of austenitic stainless steel

Table 3 (b). The physical properties of duplex stainless steel


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Table 3 (c). The physical properties of ferritic stainless steel

Table 3 (d). The physical properties of martensitic and Precipitation hardening

stainless steel.


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Phase diagram

Shaeffel published the phase diagram shown in figure 1 below. This diagram illustrates the

effect of the chromium and nickel composition on the microstructure of the steel. The

diagram shoes the factor of strength that these elements have in the formation. Using this

phase diagram and knowing the composition of the austenitic steel allows the proportionality

of the phases to be found.

Figure 1. The Shaeffler diagramwhere: A-austenite; M - martensite; F - ferrite

Martensitic

Physical properties vary depending on the group of stainless steel being considered.

Martensitic stainless steel has a body-centred tetragonal structure. Martensitic stainless

steel such as 440C, are defined as being high carbon martensitic stainless when the carbon

content is between 0.15 to 1.0 %. Precipitation of iron carbides left from carbon during the

tempering process cause the alloy to lose its strength. A secondary hardening must be

carried out by using strong carbide forming elements in order to regain the alloys strength.

These elements include molybdenum, as mentioned previously. (Bhadeshia and

Honeycombe, 2006). As the tempering time of the martensitic steel increases, the average

hardness decreases, due to the growth of carbides. (P. Shanthraj and M.A. Zikry, 2012).

Martensitic structures are formed by the quenching of austenitic steel to room temperature.Martensitic steel differs to ferritic steel due to the deformation of the austenitic lattice.


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Martensitic formation occurs without the atoms diffusing out of the lattice, leaving a higher

volume then before. This structure is highly crystallographic. Due to this deformation process

martensitic structures form in laths in order to the amount reduce strain energy on the

system. The formation process is described by the following equation(Bhadeshia and

Honeycombe, 2006);

{( )}

Where is the fraction of martensitic steel formed, is a constant of -0.011, is the

transformation temperature and is the temperature below. Since there is no diffusion of

atoms in this process, martensitic steel has a very rapid growth rate and therefore can be

estimated as being time independent. This shows that the percentage of martensitic steel

produced depends on the number of nucleation sites triggered. There will always be someretained austenite due to being room temperature, so when equals 95%, the lattice of

austenite is defined as being completely transformed. The chemical composition of the

martensite is identical to the parent lattice due of the lack of diffusion. Because of this

martensite and austenite are intimately related and their orientations have a reproducible

relationship. The [011] plane of the BCC martensite is the most closely packed plane, and

will be roughly parallel to the [111] close packed plane of the FCC austenite.

A low martensite start temperature, or high carbon concentration will change the crystal

structure from BCC to body centred tetragonal. The tetragonality of the ordered martensite is

measured by the ratio between the axis of c and a, where;

So if the carbon percentage is 0, the entire structure will be BCC. This equation shows that it

is the carbon that is responsible for the distortion of the crystal. Carbon affects martensite

differently to austenite because it doesnt bend BCC structures symmetrically like FCC. This

is because of the interstitial sites of the carbons 6 neighbouring atoms are regularly placedin FCC but not in BCC, and since there is no diffusion of atoms taking place, the cubic

structure distorts.

The microstructure of the martensitic steel depends on the process used to form it. Figure 2,

shows images retrieved from the SEM of the microstructure of 440C steel. Figure 2 (a)

shows a quenched sample. This spheroidal carbide precipitates are evident within this the

structure. Figure 2 (b) shows a tempered sample. Both spheroidal and elliptical carbides are

present here, demonstrating how different treatments alter the way the martensiticmicrostructures form. The differences in the microstructure affect the hardness of the steel


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produced(S. H. Salleh, 2009).

Figure 2. The Scanning Electron Micrograph of (a) As-quenched and (b) As-tempered Sample

The application of martensitic steal include dental and surgical instruments (Newton, 2002),

precision mechanical parts and plastic moulds(Huang et al., 2004)due to its high

hardenability, good mechanical properties, and corrosion resistance.

Ferritic

The diagram in figure 6, also shows the phase change of ferrite. Ferrite is stable from low

temperatures up until 909in pure iron. The addition of carbon to this lattice changes this

value, due to additional strain placed on the system. Ferrite in also known as -iron, and has

a BCC structure as shown in figure 3. with a lattice perimeter of 0.286 nm (Krauss, 2005).

BCC crystals when closely packed crystals, have 12 neighbouring atoms, and so gives a

less dense structure then other formations such as that of austenite.

.

Figure 3 The body centred cubic structure of ferritic steel.

The diagram in figure 4 shows the gaps within BCC structure. The largest gap being the

tetrahedral cavity, which is between the two edges and the two central atoms. The secondlargest cavity is the octahedral holes which is between the centre of the faces and the [001]


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edge (Bhadeshia and Honeycombe, 2006). These will allow through structures of a radius in

iron less than 0.37 and 0.19 respectively.

Figure 4 Octahedral and tetrahedral interstitial gaps in BCC structure.

Microstructures of ferrite include proeutectoid ferrite, which is a phase that forms on cooling

before the eutectoid austenite decomposes. They start off as grains on the austenite grain

boundary or as carbon rejecting Widmanstaten side plates. The rejection of carbon causes a

build-up, which is then sufficient to start perlite formation(Krauss, 2005). Other morphologies

of ferrite will be formed depending on the temperatures, chemical composition etc. The

chaotically structured needle shaped, acicular ferrite microstructures, are desired trait in

ferrite, as it improves the strength. This is shown in figure 5 (Bhadeshia and Honeycombe,2006).

Figure 5 An image taken optically of acicular ferrite (AF) microstructures.

Applications of ferrite are numerous, and economical because they are cheap, since they

have no nickel content. Some examples include washing-machine drums and exhaust

systems. They are easier to work with then austenite as they can be formed into more


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complex shapes and joined using most the same joining methods, such as simple welding.

Ferritic grades have been developed with the aim to replace the 304 austenitic grades. They

have a chromium content of in the 20-22% and they are free of nickel and molybdenum

which again brings down the price.

Austenitic

Figure 6. The Fe-C equilibrium diagram. Solid lines equal Fe-Fe3C. Dashed lines equal Fe-graphite diagram.

In its pure form, iron will take on two different crystal structures, FCC and BCC, the latter has

already been discussed. The austenite (or gamma phase iron (-Fe)) holds a FCC structure

and as shown in figure 6, is stable from 910C to 1390C when no carbon is present. On

phase change to BCC, the crystals volume will change by 1%. The geometry of the unit cells

controls the solubility and diffusivity of other elements in to the structure. Despite being

tighter packed, the FCC structure of austenite has bigger gaps then the BCC structure of

ferrite. These octahedral holes (surrounded by 6 atoms and so form an octagon shape) lie,at the centre of the edge of the unit cell. The second gap is a tetrahedral hole surrounded by


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four atoms. These are shown in figure 7. It is these gaps in FCC and BCC that determine the

diffusivity atoms passing though. Austenite will allow atoms of radius in iron of 0.51and

0.28 into its octahedral and tetrahedral holes respectively (Bhadeshia and Honeycombe,

2006). Carbon is relatively small in comparison to the iron atoms (0.77 ) in the austenite

and so are allowed to enter the lattice interstitially if some distortion takes place. An addition

of 0.1-0.2 wt% of carbon is enough to increase the strength of the austenite structure. Bigger

atoms, such as other metallic atoms can only join the structure in-between unit cells. The

solubility of these elements is extremely low at room temperature, and so may precipitate out

during heat treatments. There is a critical temperature at which the internal friction is too high

for migration of elements like carbon to migrate.

Figure 7. Octahedral and tetrahedral interstitial voids in a FCC structure

The microstructure of austenite is a matrix solid solution that have a high work hardening

capability and low stacking fault energy (Plaut et al., 2007). These microstructures change

when exposed to high temperatures. The microstructure can have a very large number of

phases, including carbides such as M23C6 and orthorhombic M7C3 carbide where the

carbon content is high and intermetallic phases. The intermetallic precipitations have a

strong effect on the corrosive properties of the austenite. The quality of the steel is

dependent on the stability of these microstructures. Figure 8 shows the grain boundarymicrostructure taken with a SEM, where (Ti,Mo)C carbides are shown at the grain

boundaries(Terada et al., 2006).


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Figure 8. SEM image of the grain boundary microstructure in austenite.

The applications of austenite include being used as the materials for fusion and fission

nuclear reactors. This is because of their excellent creep resistance at high temperatures

and corrosion and oxidation resistance in comparison to ferritic steels (Xu et al., 2011). It is a

common choice in the use of surgical steels (Newton, 2002)for things like dental impression

trays, steam sterilisers and thoracic retractors.Duplex

Duplex stainless steels are a product of austenite and ferrite microstructures. Between the

phase of austenite and ferrite, there exists a field where duplex can be formed. The

microstructures of duplex are formed by getting the balance between the Mo, Ti, Nb, Si and

Al concentrations and the Ni, Mn, C and N concentrations (depending on the heating

treatment) and increasing the Cr content to above 20 wt%.(Bhadeshia and Honeycombe,

2006). Strengthening the microstructures of duplex steel can be done by increasing the Nicontent. Super-duplex contains 7 wt% of Ni for example. Another microstructure of duplex

steel contains up to 30 wt% of Cr. These structures are cheaper since they have less nickel,

have excellent corrosion resistance and corrosion stress free. However these

microstructures are more brittle than others. Applications include chemical processing

equipment, components in oil and gas offshore rigs, marine equipment and chemical storage

devices (Gunn, 1997).


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Precipitation hardening

There are three main microstructures of PH stainless steel. Martensite. 17-4 PH have an

austenitic structure at the high process temperatures of over 1000, but once cooled to

room temperature the structure changes to martensite. Semi-austenitic microstructures, 17-

7 PH, have a soft structure, which makes them easy to work with at room temperature.

Austenitic microstructure A286, are formed after annealing and hardening and is caused by

precipitation during temperature changes. PH stainless steels are ideal for use in the

automotive and aerospace industry for components such as gears, shafts and valves. They

have been used in turbine blades and to contain nuclear waste due to their corrosion

resistance.

Factors in the selection of Stainless Steel grade for Medical devices

(Heubner, 2009)


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References

AKSTEEL 2012. Stainless Steel Comparator.

BHADESHIA, H. K. D. H. & HONEYCOMBE, R. W. K. 2006. Steels: Microstructure And

Properties, Butterworth-Heinemann.

DAVIS, J. R. 2000.Alloy Digest Sourcebook: Stainless Steels, Asm International.

DAVIS, J. R. 2003. Handbook of materials for medical devices.

DEFENSE, D. O. 2005. Index of Specifications and Standards Federal Supply Class Listing

(FSC) Part III September 2005, DIANE Publishing.

GUNN, R. N. 1997. Duplex Stainless Steels: Microstructure, Properties and Applications,

Woodhead Publishing.

HEUBNER, D. W. A. U. 2009. Stainless Steel- When Health Comes First. Environment and

Human Health Series,2.

HUANG, C. A., HSU, F. Y. & YAO, S. J. 2004. Microstructure analysis of the martensitic

stainless steel surface fine-cut by the wire electrode discharge machining (WEDM).Materials Science and Engineering: A,371, 119-126.

INOX, E. 2007. Stainless steel: Tables of Tecnical Properties. Materials and Applications

Series,5.

KRAUSS, G. 2005. Steels: Processing, Structure, And Performance, Asm International.

NEWTON, T. 2002. Stainless SteelA Family of Medical Device Materials. MEDICAL

DEVICE MANUFACTURING & TECHNOLOGY.

P. SHANTHRAJ AND M.A. ZIKRY 2012. Optimal microstructures for martensitic steels.

Journal of Materials Research,27.

PECKNER, D. & BERNSTEIN, I. M. 1977. Handbook of stainless steels, McGraw-Hill.

PLAUT, R. L., HERRERA, C., ESCRIBA, D. M., RIOS, P. R. & PADILHA, A. F. 2007. A

Short review on wrought austenitic stainless steels at high temperatures: processing,

microstructure, properties and performance. Materials Research,10, 453-460.


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S. H. SALLEH, M. Z. O., J. SYARIF, M. J. GHAZALI, S. ABDULLAH AND Z. SAJURI 2009.

INVESTIGATION OF MICROSTRUCTURES AND PROPERTIES OF

440C MARTENSITIC STAINLESS STEEL International Journal of Mechanical and Materials

Engineering,4, 123-126.

TERADA, M., SAIKI, M., COSTA, I. & PADILHA, A. F. 2006. Microstructure and intergranular

corrosion of the austenitic stainless steel 1.4970. Journal of Nuclear Materials,358, 40-46.

XU, Y., ZHOU, Z., LI, M. & HE, P. 2011. Fabrication and characterization of ODS austenitic

steels. Journal of Nuclear Materials,417, 283-285.

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