Stainless Steels

T. Sourmail and H. K. D. H. Bhadeshia

Introduction

Steels are said to be stainless when they resist corrosion; the is achieved by dissolving sufficient chromium in the iron to produce a coherent, adherent, insulating and regenerating chromium oxide protective film on the surface. It is not surprising therefore that they are used in the harsh environments of the chemical, oil production and power generation industries, and in utility goods such as furniture, automotive trims and cutlery, where both aesthetic appearance and corrosion resistance are important design criteria.

The stainlesscharacter occurs when the concentration of chromium exceeds about 12 wt%. However, even this is not adequate to resist corrosion in acids such as HCl or H 2SO 4; higher chromium concentrations and the judicious use of other solutes such as molybdenum, nickel and nitrogen is then needed to ensure a robust material.

There are requirements other than corrosion which have to be considered in engineering design. For this reason, there is a huge variety of alloys available, but they can be classified into four main categories:

Specialist grades include the precipitation hardened or oxide dispersion strengthened alloys.

Corrosion Resistance

Iron does not occur in its native state because it combines readily with oxygen and other elements. It is extracted from its ore and given the opportunity, tends to revert to a compound by reacting with the environment. Rusting is an example of this reversion process. The process can be retarded by adding chromium, which at sufficiently large concentrations forms a protective oxide film at the surface. The nature of this oxide film depends on the chromium concentration, but when the latter exceeds about 12 wt%, the a passive film of chromium oxide only about 1-2 nm thick covers the steel, which becomes stainless as long as the chromium is in solid solution in the steel.

Corrosion can nevertheless occur if the passive film breaks down, locally or uniformly:

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Stainless steel in boat
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The ice-breaker in the background has a stainless steel hull.
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The exposed railings are all stainless steels.



Phase Equilibria

Main Phases

The designation stainless steelimplies little more than a 12% Cr content. Most of the stainless steels are based on the Fe-Cr-C and Fe-Cr-Ni-C systems, but other alloying elements are also important.

Iron and its alloys can exist in several crystallographic forms , of which the most common are the body-centred cubic (b.c.c.) and face-centred cubic (f.c.c.). In pure iron, the f.c.c. structure persists between 910 and 1400°C, the b.c.c. structure below and above this interval (up to the melting temperature of 1539°C).

The importance of this phase-transformation in the metallurgy of steels cannot be overestimated. This transformation allows a wide range of microstructures to be achieved by controlled heat-treatment. Mechanical properties are essentially related to microstructure, and can therefore be obtained in an extraordinarily large range of strength, toughness, etc.. Stainless steels are routinely produced with strengths from 100 MPa to more than 1 GPa.

Knowledge of the relative stabilities of the b.c.c. and f.c.c. structures of iron alloys is therefore of prime concern. The history of stainless steels started with a martensitic steel (12Cr-0.1C wt%) in Sheffield, UK and the austenitic 18Cr-8Ni wt% in Germany. For this reason, and also because they are most important alloying elements in stainless steels, Cr and Ni form the reference relative to which the influence of other solutes is classified: have long been used as reference to quantify the influence of alloying elements on the b.c.c.<->f.c.c. phase transition: those elements which like Cr promote ferrite are called ferrite stabilisersand those which like Ni promote austenite are called austenite stabilisers. The equations below give a rough guide of the potency of individual elements to act as ferrite or austenite stabilisers when compared with the corresponding effects of Cr and Ni respectively (concentrations in wt%):

Cr equivalent = (Cr) + 2(Si) + 1.5(Mo) + 5(V) + 5.5(Al) + 1.75(Nb) + 1.5(Ti) + 0.75(W)

Ni equivalent = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)

Figure 4 shows that an excessive amount of chromium can eliminate austenite at all temperatures, making it impossible to achive a γ to α transition. This is the domain of the ferritic stainless steels discussed below.

Figure 6: Vertical section of Fe-Cr-C diagram for 0.1C wt%.
FeCrC isopleth

Without carbon, the limit beyond which austenite no longer forms is about 13.5 wt% chromium. However, additions of carbon help stabilise the austenite and therefore increase this limit (Fig. 6).

Chromium and nickel equivalents are also used in the welding industry to plot the microstructures obtained when a weld solidifies and cools to ambient temperature (Fig. 7). Although these diagrams are popular, it should be understood that they are not phase diagrams but rather represent the microstructures obtained under specific cooling conditions.

Figure 7: Schaeffler diagram for weld metals.
Schaeffler diagram

Precipitate Phases

These include carbides, nitrides or intermetallic compounds. Since most stainless steels serve at ambient temperature, the intermetallic compounts are of little relevance as they are extremely slow to precipitate because even though they may be thermodynamically stable phases, they are difficult to nucleate.

It is evident from Figure 6 (Fe-Cr-C phase diagram) that typical martensitic steels should exhibit ferrite and M 23C 6in equilibrium at for example, 600°C. In practice, this carbide is only found after relatively long ageing. because it is preceded by Intermediate phases in the sequence cementite, M 2X and M 7C 3, leading finally to M 23C 6.

These precipitation sequences become more complex in heavily alloyed ferritic or austenitic stainless steels, such as those destined for the power generation industry. Considerable effort is being devoted to understanding and estimating the precipitation sequences in such alloys because the are intended to serve safely for 30 or more years, i.e.,for time periods far in excess of what can be reasonably achieved in the alloy development exercise (Robson and Bhadeshia, 1997; Fujita and Bhadeshia, 2002; Sourmail, 2001; Sourmail and Bhadeshia, 2003).

Martensite formation

Most stainless steels have a high hardenability, meaning that the reconstructive transformation of austenite to (ferrite + carbides) is unlikely to happen unless the steel is cooled particularly slowly.

The most important features of these martensitic alloys are therefore the martensite start (M S) and finish temperatures (M F). For martensitic steels, the range [M S-M F] should be above the room temperature to ensure fully martensitic structure. On the contrary, the [M S-M F] range of austenitic stainless steels is often well below 0°C, which is why they can be used in cryogenic applications; austenite does not have the classical ductile-brittle transition associated with body-centred cubic iron (martensite, ferrite). Cold deformation can induce martensitic transformation to α and ε martensite, the extent depending on the strain and on the chemical composition. Heavily alloyed austenitic steels with up to 20Cr and 25Ni wt% are fully stable.

Categories

On the basis of their main microstructural features, there exist the following key categories of stainless steels:

Martensitic Stainless Steels

The composition is such that the austenite in these steels is able to transform into martensite. This allows a degree of control on the mechanical properties by exploiting the phase change. Typical heat-treatments consist of austenitisation at a temperature high enough to dissolve carbides followed by quenching to obtain martensite. Given the high hardenability inherent in such alloys, the quench rate required to achieve martensite is not high; oil and water quenching are used only when dealing with thick sections.

Typical compositions cover 12 to 18 Cr and 0.1 to 1.2 C wt%. As with other martensitic steels, a balance must be sought between hardness and toughness. An untempered martensitic structure typically is strong but lacks toughness and ductility to an extent which depends on the carbon concentration. As a conseqnece, the martensite is in many cases tempered between 600 and 750°C to optimise the mechanical properties.

In applications such as cutlery, surgical instruments etc., high strength is desirable and toughness/ductility are of little concern. A lower temperature tempering is then used to retain most of the strength. Type 420 steel (0.15-0.4C, 1.0Mn, 1.0Si, 0.04P, 0.03S, 12-14Cr wt%) is a typical composition for such applications. Its proof strength in the quenched and tempered condition can be in excess of 1.2 GPa. Type 440C tempered at 300°C has a proof strength of about 2 GPa.

Table 1 shows the compositions and typical uses of AISI standard martensitic grades:

Table 1: Martensitic Stainless Steels
Grade C Mn Si Cr Ni Mo P S Comments/Applications
410 0.15 1.0 0.5 11.5-13.0 - - 0.04 0.03 The basic composition. Used for cutlery, steam and gas turbine blades and buckets, bushings...
416 0.15 1.25 1.0 12.0-14.0 - 0.60 0.04 0.15 Addition of sulphur for machinability, used for screws, gears etc.416 Se replaces suplhur by selenium.
420 0.15-0.40 1.0 1.0 12.0-14.0 - - 0.04 0.03 Dental and surgical instruments, cutlery....
431 0.20 1.0 1.0 15.0-17.0 - 1.25-2.0 0.04 0.03 Enhanced corrosion resistance, high strength.
440A 0.60-0.75 1.0 1.0 16.0-18.0 - 0.75 0.04 0.03 Ball bearings and races, gauge blocks, molds and dies, cutlery.
440B 0.75-0.95 1.0 1.0 16.0-18.0 - 0.75 0.04 0.03 As 440A, higher hardness
440C 0.95-1.20 1.0 1.0 16.0-18.0 - 0.75 0.04 0.03 As 440B, higher hardness

In addition to the standard grades, a large number of alloyed martensitic stainless steels have been developed for moderately high temperature applications. Most common additions include Mo, V and Nb. These lead to a complex precipitation sequence. A small amount (up to 2 wt%) of Ni is added to improve the toughness.

The 12Cr-Mo-V-Nb steels are used in the power generation industry, for steam turbine blades operating at temperatures around 600°C. Current research focuses on achieving service temperatures of 630-650°C under a stress of 30 MPa.

Ferritic stainless steels

Ferritic stainless steels: typically contain more chromium and/or less carbon than the martensitic grades. Both changes act to stabilise ferrite, so much so that it is the stable phase at all temperatures. Therefore, unlike the martensitic grades, ferritic stainless steels cannot be hardened by heat-treatment. They exhibit lower strength but higher ductility/toughness. Typical applications may include appliances, automotive and architectural trim ( i.e.,decorative purposes), as the cheapest stainless steels are found in this family (type 409).

Table 2: Ferritic Stainless Steels
Grade C Mn Si Cr Mo P S Comments/Applications
405 0.08 1.0 1.0 11.5-14.5 - 0.04 0.03 0.1-0.3 Al
409 0.08 1.0 1.0 10.5-11.75 - 0.045 0.045 (6xC) Ti min
429 0.12 1.0 1.0 14.0-16.0 - 0.04 0.03
430 0.12 1.0 1.0 16.0-18.0 - 0.04 0.03
446 0.20 1.5 1.0 23.0-27.0 - 0.04 0.03 0.25 N

Iron-chromium body-centred cubic solutions are such that there is a tendency under appropriate conditions for like atoms to cluster; at temperatures below a critical value, the solution tends to undergo spinodal decomposition into chromium-rich and iron-rich regions. High chromium ferritic stainless steels such as type 446 thus become susceptible to the so-called '475°C embrittlement', which is caused by this clustering process. At and below 475°C, in steels containing more than 25 wt% of chromium, the spinodal typically exhibits a wavelength of about 10 nm. There is a continuous increase of hardness as the composition wave develops, for example, the hardness of an Fe-28Cr wt% steel can increase by more than 300 HV over an exposure 10,000 h at 450°C (Ishikawa et al.,1995). This results in a severe drop of impact toughness and ductility.

The addition of nicikel appears to accelerate the spinodal and raises the maximum temperature at which it is observed. When post-weld heat-treatment is not possible, the welding of ferritic stainless steels is usually done with a metal filler containing Ni, and there is therefore the possibility of weld embrittlement on prolonged exposure at elevated temperatures.

Austenitic stainless steels

These steels are often in a metastable austenitic state at room temperature or below. Most grades have a martensite-start temperature well below 0°C. However, plastic deformation can induce martensite at temperatures higher than M S(the sample then is attracted by a magnet since the α-martensite is ferromagnetic whereas austenite in such alloys is not). The M Dtemperature is that at which martensite cannot be induced no matter how much the austenite is deformed.

The presence of nickel improves considerably the corrosion resistance when compared to the martensitic and ferritic grades.

Type 304 is the basic 18Cr8Ni (18/8) austenitic stainless steel, so widely used that it accounts for about 50% of all stainless steel production. Other standard grades have different preferred applications; for example, type 316 which contains up to 3 wt% Mo, offers an improved general and pitting corrosion resistance, making it the material of choice marine applications and coastal environments. In severe conditions however, even type 316 cannot cope and molybdenum-enriched alloys such as 254SMO are used.

Table 3: Austenitic Stainless Steels
AISI grade C max. Si max. Mn max. Cr Ni Mo Ti Nb Al V
301 0.15 1.00 2.00 16-18 6-8
302 0.15 1.00 2.00 17-19 8-10
304 0.08 1.00 2.00 17.5-20 8-10.5
310 0.25 1.50 2.00 24-26 19-22
316 0.08 1.00 2.00 16-18 10-14 2.0-3.0
321 0.08 1.00 2.00 17-19 9-12 5 x %C min.
347 0.08 1.00 2.00 17-19 9-13 10 x %C min.
E 1250 0.1 0.5 6.0 15.0 10.0 0.25
20/25-Nb 0.05 1.0 1.0 20.0 25.0 0.7
A 286 0.05 1.0 1.0 15.0 26.0 1.2 ~1.9 ~0.18 ~0.25
254SMO 0.02 0.8 1.0 18.5-20.5 17.5-18.5 6-6.5 ~1.9 ~0.18 ~0.25
AL-6XN 0.03 1.0 2.0 20-22 23.5-25.5 6-7


Figure 8. Micrographs of austentiic stainless steels
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Grain structure of an austenitic stainless steel NF709 (25Cr20Ni) . Many of the grains contain annealing twins. NF709 is a creep-resistant austenitic stainless steel used in the construction of highly sophisticated power generation units.
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Type 302 austenitic stainless steel, cold-rolled and then annealed at 704°C for one hour. Partially recrystallised microstructure. Recrystallised grains are clean whereas the deformed regions show a large concentration of defects. There are annealing twins in the recrystallised region (Hopkin).

Specialist austenitic stainless steels are made with up to 0.4 wt% nitrogen when prepared at ambient pressure, and up to 1 wt% nitrogen using high-pressure melting techniques (Simmons, 1996). The prime reason for adding nitrogen is that it is a very effective solid-solution strengthener. Not only do the misfitting nitrogen atoms interfere statically with moving dislocations, but there is also a drag due to nitrogen atoms being carried along with the dislocations as they move through the lattice (Rawers and Grujicic, 1966). The strength of such alloys makes them suitable for niche applications such as power generator retaining rings, high-strength bolts and superconducting magnet housings.

The solubility of nitrogen in austenite is reduced by nickel but increased by chromium and manganese. Excessive nitrogen concentrations can lead to precipitation, particularly of chromium nitrides.

Nitrogen also increases the resistance to localised corrosion (pitting and crevice) in acid-chloride solutions (see pitting index equation above).

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Stainless steel, thermally insulated, cups and saucers
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Austenitic stainless steel

Duplex stainless steels

Duplex stainless steels typically contain 50% austenite and 50% ferrite (Figure 9a-c). The two-phase mixture also leads to a marked refinement in the grain size of both the austenite and ferrite. This, together with the presence of ferrite, makes the material about twice as strong as common austenitic steels. They contain only about half the nickel concentration of typical austenitic stainless steels; they are therefore less expensive and less sensitive to the price of nickel. With their high chromium concentration, they have excellent pitting and crevice corrosion resistance, and to chloride stress corrosion. The two phase mixture also reduces the risk of intergranular attack; for the same reason, they are not prone to solidifcation cracking during welding.

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Figure 9: Optical microstructures of duplex and superduplex stainless steels (courtesy S. Sharafi). The colour etch is achieved using Beraha's reagent, 10 ml HCl, 100 ml H 2O and 0.5-1.0 g K 2S 2O 5(take appropriate safety precautions when using chemicals).
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(a) Duplex stainless steel, IC378, hot rolled in the direction indicated. The darker etching phase is ferrite and the remainder is austenite
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(b) Duplex stainless steel IC381 (dark phase is ferrite).
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(c) Duplex stainless steel IC381 (dark phase is ferrite).
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(d) Superduplex stainless steel A219 after heat treatment at 1150°C for 2.5 h. The austenite is yellow and ferrite is dark brown, with the sigma phase white.

The archetypal duplex alloys contain 22-23Cr, 4.5-6.5Ni and 3-3.5Mo wt%, representing some 80% of all duplex stainless steel use. Detailed compositions are given in Table 4. A significant application is in the production of tubing in corrosive oil and gas wells; such tubes have been installed particularly in the North Sea industries.

Table 4: Duplex stainless steels (wt%). A219 is a superduplex alloy.
Designation Cr Ni C Mn Si P S Other UTS / MPa Elongation / %
Type 329 28.0 6.0 0.10 2.0 1.0 0.04 0.03 1.5 Mo 724 25
Type 326 26.0 6.5 0.05 1.0 0.6 0.01 0.01 0.25 Ti 689 35
2RE60 18.5 4.5 0.02 1.5 1.6 0.01 0.01 2.5 Mo 717 48
IC378 21.8 5.5 0.03 1.38 0.40 0.03 0.01 3.0 Mo 0.18 Cu 0.07 V 0.14 N
IC381 22.1 5.8 0.02 1.92 0.48 0.03 0.01 3.2 Mo 0.07 Cu 0.13 V 0.14 N
A219 25.6 9.4 0.03 0.70 0.60 0.02 0.01 4.1 Mo 0.27 N

In general, the toughness of stainless steels increases in the order ferritic, duplex and austenitic stainless steels. Duplex stainless steels, because of their high Cr concentration, are prone to the 475°C embrittlement described earlier so their application is frequently confined to temperatures below about 300°C.

The superduplex stainless steelshave a higher chromium and molybdenum concentration to enhance pitting corrosion resistance; these ferrite stabilising elements are balanced using a higher nickel and nitrogen concentrations (austenite stabilisers) in order to maintain about equal amounts of ferrite and austenite (Table 4, Fig. 8d). One definition is that a superduplex stainless steel must have a pitting index which is greater than 40.

duplex

Stainless Steel Seamless Tube and Wire Rod

Stainless Steel Thread

Powder Metallurgically Produced Stainless Steel

Transformation Texture in Stainless Steel

Other Topics

Theses on Stainless Steels

Transformations in Supermartensitic Stainless Steels

Sensitisation of Austenitic Stainless Steels

Tensile Properties of Stainless Steel

Anisothermal Recrystallisation in Austenitic Stainless Steels

Simultaneous Precipitation Reactions in Creep-Resistant Austenitic Stainless Steels

Microstructure of Super-Duplex Stainless Steels

REX734

References

Island Constructed from Stainless Steel, Graz, Austria

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The stainless steel island in the river Mur, Graz, Austria.
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The stainless steel island (a cafe) in the river Mur.
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The stainless steel island in the river Mur.
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The stainless steel island in the river Mur.
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The stainless steel island in the river Mur.

Stainless Steel in Subway, Taiwan

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In the subway, Taipei
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In the subway, Taipei

Stainless Steel in Chinheng Memorial, Tarako National Park, Taiwan

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Chinheng, the engineer who supervised the construction of the Trans East-West Road. He was killed by a rock fall following an earthquake.
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Memorial to Chingheng
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The memorial is signed by the son of Chiang, Kai Shek, who was the President of Taiwan
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Elegant use of Stainless Steel in Pusan, Korea

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Stainless steel in action.
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Stainless steel in action.
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Stainless steel in action.
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Stainless steel in action.
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Stainless steel in action.
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Stainless steel in action.
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Art
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Art

Stainless Steel in Switzerland

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Stainless steel bin.
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Stainless steel bin.
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Rust on a stainless steel bin?
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Stainless steel display
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Stainless steel at Lucern railway station.
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Stainless steel bin at Zurich airport.
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Stainless steel bins at Zurich airport.

Science Museum, London

This is an example of a glass and stainless steel bridge, taken from the Science Museum in London. The toughned-glass slats are structurally supported by stainless steel ropes. The ropes are usually made from pearlitic steel, but here cost is not a major issue since this is a museum exhibit.

Moving Pictures


Glass bridge supported by stainless steel ropes

At the Science Museum, London

Still Pictures

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Stainless steel island, Mur, Graz

Movies



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Rolling of stainless steel into thin sheet. Photograph courtesy of Jan-Olof Nilsson.
Extrusion1
Hot-extrusion. Photograph courtesy of Jan-Olof Nilsson.
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Hot-extrusion of stainless steel. Photograph courtesy of Jan-Olof Nilsson.
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The extruded, seamless stainless steel tubes. Photograph courtesy of Jan-Olof Nilsson.
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Stepwise reduction of diameter. Photograph courtesy of Jan-Olof Nilsson.

Stainless Steel in Dusseldorf, Germany

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Stainless steel plant frame

Stainless Steel in Baosteel, China



Stainless steel barriers in Shanghai


Large stainless steel rings













409L Ferritic stainless steel: low cost, low thermal expansion and high corrosion resistance




Movie inside an elevator faced with stainless steel, in Mumbai, India

Sheffield steel made in China




Laser welding of medical devices

Solidification of stainless steel

Laser welding of endoscope

Stainless steel jewellery

Acknowledgment

The creation of this document was partly supported by the Higher Education Funding Council for England, via the U.K. Centre for Materials Education.

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