[ Wiki ]How to Evaluate Stainless Steel Sinks

THE KITCHEN SINK TAKES A LOT OF ABUSE. POTS AND PANS, DISHWARE AND SILVERWARE, ALONG WITH FOOD PREPARATION, ALL TAKE THEIR TOLL ON THE KITCHEN ESSENTIAL. AS THE CENTRAL FIXTURE IN THE KITCHEN, YOU WANT A SINK THAT CAN TAKE THE ABUSE AND STILL LOOK GOOD. ONE DURABLE CHOICE IS A STAINLESS STEEL SINK. BUT NOT ALL STAINLESS SINKS ARE CREATED EQUAL. TO EVALUATE A STAINLESS STEEL SINK, YOU NEED TO CONSIDER A FEW FACTORS IN ITS MAKEUP.

STEEL GAUGE

To ascertain the strength of the stainless steel, evaluate it based on its gauge. Stainless steel is a metal alloy used in a variety of applications, such as the construction of the Chrysler Building in New York. The thickness of stainless steel equates to its gauge, the number of layers that it takes to make 1 inch. For instance, 16-gauge stainless steel takes 16 layers of steel to make it an inch thick. The lower the gauge number, the thicker the steel, and the greater the sink can resist dents and scratches.

OXIDATION RESISTANCE

Oxidation leads to rust, but the chromium-to-nickel ratio in steel helps prevent it. So prior to purchase, check this ratio, as nickel gives the stainless steel strength and hardness, and chromium offers durability and shine. A typical chromium-to-nickel ratio uses 18 percent chromium and 10 percent nickel. It typically reads 18/10 on the sink's label.

FINISH TYPE

Stainless steel sinks come satin, polished, mirror or matte finishes. The type of finish you choose can be an asset or a liability in the kitchen. If you don't want to be constantly polishing the kitchen sink, avoid choosing a sink with a mirror or polish finish. These finishes show water spots and scratches. Instead, choose a sink with a matte or brushed finish that also provides resistance against scratches.

INSULATION

Some stainless steel sinks come without insulation. Without insulation, water heat escapes quickly, and the sounds are louder. Insulation helps to deaden the sound of water running in the kitchen. Check the type of insulation offered on the stainless steel sink. Foam insulation is of better quality and works more efficiently than sprayed-on insulation.

MOUNT TYPE AND SHAPE

Most kitchen sinks are available in differing mounting configurations. Some mount atop the counter and others underneath counters. Undermount units are best suited for solid surfaces such as stone or granite; they won't work with laminate materials. Choose a sink shape that matches your current sink or one that works well when remodeling.

BOWL DEPTH

Stainless steel sinks also come in multiple bowl depths. Some are shallow, while others offer a much deeper sink. Before buying your new stainless steel sink, verify it is deep enough to work with your cookware.

[ Wiki ]Metallography of Stainless Steels

INTRODUCTION

Stainless steels are referred to corrosion-resistant steels that consist of up to 11% chromium. This set of high alloy steels are further divided into four categories; austenitic, martensitic, ferritic, and austenitic-ferritic (duplex) stainless steels (Figure 1). These categories describe the microstructure of an alloy at room temperature, which is considerably affected by the composition of the alloy.

Figure 1. Duplex steel etched electrolytically with 150x40% aqueous sodium hydroxide solution, showing blue austenite and yellow ferrite.

Corrosion resistance is the main property of stainless steels, and this feature can be further improved by adding certain alloying elements. Such elements impart positive effect on other properties, like oxidation resistance and toughness.

Titanium and niobium, for example, boost resistance against inter-granular corrosion as they take in the carbon element to produce carbides; nitrogen to increase strength, and sulfur to increase machinability because it form tiny manganese sulfides that lead to short machining chips. Stainless steels have excellent surface finishes and corrosion resistance, so they play a major role in the medical, aircraft, food and chemical industries, in architecture, professional kitchens, and jewelry.

Metallography of stainless steels is an important part of the overall quality control of the production process. The important metallographic tests are as follows:

• Detection of delta ferrite and sigma phase
• Measurement of grain size
• Assessment and distribution of carbides

In addition to this, metallography is also utilized in failure analysis of oxidation and corrosionmechanisms.

DIFFICULTIES DURING METALLOGRAPHIC PREPARATION

Grinding and Polishing

This involves the deformation and scratching of austenitic and ferritic stainless steels (Figure 2); inclusions and carbides are retained.

Figure 2. Austenitic steel, color etched (Beraha II).

Solution

Alumina or colloidal silica can be used for systematic diamond polishing and final polishing.

PRODUCTION AND APPLICATION OF STAINLESS STEEL

High alloy steels are produced by melting and remelting processes, which are highly advanced procedures. In an electric arc furnace, a combination of well sorted scrap and iron is initially melted and continuously cast into billet or bloom, or cast into ingot form. These main products can be additionally processed into rod, bar, or plate shapes in a large number of applications. For higher quality steels, the main product can be utilized as feedstock for a secondary process of steelmaking. This process can be remelted twice or even three times by vacuum induction melting and electroslag remelting or vacuum arc remelting, which can be carried out under protective and pressure gases.

The secondary process is usually done to reduce impurities like silicates, sulphides, and oxides, so that with repeated remelts the level of cleanliness increases, producing uniform ingots with excellent physical and mechanical properties.

Application

Stainless steels’ high corrosion resistance depends on the creation of a passive surface oxide layer that spontaneously rebuilds itself when it is damaged mechanically, and is also based on alloying iron with chromium. Stress, pitting, intercrystalline, and vibrational corrosion are different types of corrosion that can occur. If alloying elements other than chromium are added, better resistance against certain forms of attack can be obtained. Molybdenum, for example, enhances resistance against pitting corrosion. Here the primary alloys, properties, and the associated applications of four forms of stainless steels are elucidated.

Ferritic stainless steels have a low carbon content, with 11 to 17% of chromium, and they are non heat treatable alloys. Properties of ferritic stainless steels include moderate strength and toughness, magnetic property, and resistance to atmospheric corrosion. Applications include car trim, razor blades, and magnetic valves.

Martensitic stainless steels have a medium carbon content, with up to 12 to 18% of chromium and 2 to 4% of nickel. They are heat treatable alloys. Properties include high creep resistance, high temperature resistance, and high corrosion resistance. Applications include knives, scalpels, tweezers and hooks in medical applications, high performance parts and drive systems for aircraft.

Austenitic stainless steels have 0.03 to 0.05% of carbon and their main alloying elements are molybdenum (2-4%); nickel (8-25%), and chromium (17-24 %). Niobium and titanium are added for carbide forming. Austenitic stainless steels are not heat treatable. Properties include high corrosion resistance, high ductility, good cold forming properties, resistant to oxidizing acids and alkalis, and easy to work and machine. Applications include implants, bolts, and screws; low temperature applications comprise pipes and vessels and pipes in the food, chemical, and pharmaceutical industries, and kitchen utensils.

Austenitic-ferritic steels (Duplex) have lower nickel content (4-6%) and higher chromium (21-24%). They have 2 to 3% of molybdenum and exhibit a low carbon content. Properties include excellent resistance from stress corrosion, and fatigue resistance in corrosive media. Applications include architecture, equipment for environmental, chemical, and offshore industries.

Difficulties in the Preparation of Stainless Steels

Austenitic steels are ductile and ferritic stainless steels are soft, and both are inclined to mechanical deformation. These steels become highly reflective when subjected to final polishing, but if they are not fully pre-polished, deformation will reappear following etching (Figure 3). Martensitic steels have excellent hardness, and can be easily polished. However, carbides should be preserved properly.

Figure 3. Austenitic steel insufficiently polished 500x showing deformation after etching (Beraha II).

RECOMMENDATIONS FOR THE PREPARATION OF STAINLESS STEELS

High pressures and highly coarse grinding papers or foil should not be used for soft and ductile stainless steels, as this can lead to deep deformation. Generally, the finest possible grit, which is uniform with the surface roughness and sample area, must be utilized for plane grinding. Diamond on a rigid disc, such as MD-Largo, is used to perform fine grinding, or on a MD-Plan cloth as an alternative to certain types of stainless steels. After fine grinding, a complete diamond polish is done on a medium soft cloth. This is followed by a final polish using alumina (OP-A) or colloidal silica (OP-S) for scratch removal. This particular step should be done meticulously and will take several minutes. A better contrast can be obtained through a good final polish. Fine grinding and even final polishing will not remove deformations occurring from the initial grinding step. Such deformations will leave some traces.

A preparation method for stainless steel samples is shown in Table 1, and a preparation method for 6 stainless steel samples is illustrated in Table 2.

Table 1. Preparation method for stainless steel samples, 30 mm diameter mounted, on the semi-automatic Tegramin, 300 mm diameter

Table 2. Preparation method for stainless steel samples, 65x30 mm, cold mounted or unmounted using Struers MAPS or AbraPlan/AbraPol, 350 mm diameter

ELECTROLYTIC POLISHING

When it comes to rapid general structure check and research analysis, electrolytical polishingand etching provides an alternative option to mechanical polishing of stainless steels, because this process does not leave mechanical deformations. However, while electrolytical polishing does provide good result for investigating the microstructure (Figure 4), it is not suitable for detecting carbides, which appear either enlarged or washed out.

Figure 4. Stainless steel weld, polished and etched electrolytically, DIC

The samples need to be ground to 1000# on silicon carbide paper or foil prior to electrolytical polishing. If the initial surface is finer, better electrolytical polish can be obtained. Preparation method is as follows:

• Electrolyte: A3
• Flowrate: 13
• Area: 1 cm²
• Voltage: 35 V
• Time: 25 seconds

External etching with stainless steel etching dish

• Voltage: 15 V
• 10% aqueous oxalic acid
• Time: 60 seconds

ETCHING

Some amount of expertise and patience is required to etch stainless steels. Extensive literature is available for etchants, and it is suggested to test out various etchants to set up a separate stock of solutions that are suitable for a certain material prepared in the lab on a regular basis.

Stainless steels have excellent resistance against corrosion, so very strong acids are needed to expose their structure. When handling these etchants, standard safety precautions should be followed. In most labs, the etchants specified in the literature will be altered based on personal preference or the material that is being etched. Adequate final oxide polishing is required to obtain good etching results. Some etchants are effective in routine applications, and they are as follows:

Chemical etching

• For martensitic steels - 25g picric acid, 925 ml ethanol, 50 ml hydrochloric acid.
• For austenitic steels - Swab etch: 500 ml distilled water, 300 ml hydrochloric acid, 200 ml nitric acid, 50 ml of a saturated iron-III-chloride solution, 2.5g copper-II-chloride, 300 ml hydrochloric acid, 100 ml water, 15 ml hydrogen peroxide (30%); V2A etchant: 100 ml hydrochloric acid, 100 ml water, 10 ml nitric acid, etch at room temperature or up to 50°C temperature.
• Color etchant Beraha II: Stock solution, 800 ml distilled water, 400 ml hydrochloric acid, 48g ammonium biflouride; to 100 ml of this stock solution 1 to 2g of potassium metabisulfite should be added for etching.

Electrolytic etching

• All stainless steels: 10% aqueous oxalic acid
• For austenitic-ferritic steels (Duplex) - 40% aqueous sodium hydroxide solution

The proposed safety precautions should be followed when handling chemical reagents.

STRUCTURE INTERPRETATION

Heat treatment has no effect on ferritic stainless steels, but the properties of these steels can be affected by cold working. At room temperature, ferritic stainless steels are magnetic. In annealed condition, the microstructure includes ferrite grains, wherefine carbides are integrated. Ferritic steels employed for machining purposes include a considerable amount of manganese sulfides to enable free cutting, as shown in Figure 5.

Figure 5. Ferritic stainless steel with manganese 200x sulfides and strings of small carbides, etched electrolytically with 10% oxalic acid.

Heat treatment has a major effect on martensitic stainless steels, which are formed via instant cooling. Tempering treatment can be used to optimize their properties. The alloys of martensitic stainless steels are magnetic in nature. The microstructure can range from pure martensitic structure, through to fine tempered martensite based on the thermal treatment. Complicated heat treatment temperatures are required for different alloys and different sizes of semi-finished products. An often unwanted phase is delta ferrite (Figure 6), because extended annealing times of steels with high chromium content at 700 to 950°C can alter the delta ferrite into a brittle and hard iron-chromium intermetallic sigma phase.

Figure 6. Tempered martensitic stainless 75xsteel with delta ferrite, etched with picric acid.

The sigma phase and the embrittlement are removed by heating to a temperature of 1050°C. Thermal treatment has no effect on austenitic stainless steels, but quick cooling leads to the formation of their softest condition. Austenitic stainless steels are non-magnetic in this state, and their properties are affected by cold working. The steels’ microstructure includes austenite grains that may display twinning (Figure 7).

Figure 7. Cold worked austenitic steel showing twinning, etched with V2A etchant.

When these steels are exposed to increased temperatures of 600 to 700°C, complex carbides are formed inside the austenite grains. This results in an insolvency of chromium in the austenite solid solution, increasing the sensitivity to inter-granular oxidation or corrosion. The risk of inter-granular corrosion can be minimized by reducing the carbon content to less than 0.015% and introducing minute quantities of niobium or titanium. This is because these elements form carbides rather than the chrome (Figure 8). Delta ferrite can be a result of the cold working of austenitic steels or heat treatment conditions in martensitic steels (Figure 9).

Figure 8. Austenite with carbides and some 200x titanium carbon nitrides.

Figure 9. Austenitic steel with strings of delta 125xferrite, showing microsegregations. Blue areas: depletion of alloying elements.

Austenite and ferrite are present in austenitic-ferritic stainless steels (Duplex). The structure is revealed through electrolytic etching in a 40% caustic soda solution, and this helps to estimate the right percentage of individual phases (Figure 10). These steels are ductile and are mainly utilized in the paper, food, and petroleum sectors.

Figure 10. Forged duplex steel showing blue ferrite, white austenite and fine needles of sigma phase, etched electrolytically with 40% aqueous sodium hydroxide.

CONCLUSION

Corrosion resistant steels are referred to as stainless steels, which contain high contents of nickel and chromium. Stainless and ferritic steels are soft, ductile, and inclined to scratching and mechanical deformation during the course of metallographic preparation. Moreover, carbides cannot be retained often. To ensure an effective mechanical polish, the following things should be considered.

• Fine grinding and polishing with diamond must be meticulous, and all deformation should be removed from plane grinding.
• Coarse abrasives for plane grinding should not be used
• A final oxide polish with alumina or colloidal silica should be done to provide a surface that is free from deformation.

A four step process carried out on an automatic preparation system provides good and reproducible results. Chemical etching of stainless steels can be difficult, and the proposed etchants are corrosive and have to be handled carefully. Another option is to use electrolytical polishing and etching, which does not retain carbides, but provides a deformation-free surface.

[ News ]China to reinvestigate anti-dumping case into stainless steel tubes from EU, Japan

China's Commerce Ministry said on Monday it would reinvestigate its anti-dumping case into imports of high performance, seamless stainless steel tubes from Japan and the European Union.

China lost an appeal ruling in October at the World Trade Organization in a dispute in which Japan and the European Union had complained about Chinese use of anti-dumping duties on the steel products.

stainless steel fabrication

FOR MORE, FEEL FREE TO VIEW OUR WEBISTE:

WWW.SUNRAYSTEEL.COM

[ Info ] Steel: Higher, Faster, Deeper, Longer!

Innovation in steel makes it possible.

From the deepest depths of the sea to the stars in the universe, steel allows humans to push the boundaries of the possible! Our latest infographic shows how innovation in steel has helped our civilization smash through new barriers, strengthen our lives, and hit new heights.

[ Summary ] Steel Facts

This article is originally published in world steel asscociation website

Integrity is at the heart of the steel industry.

Nothing is more important to us than the well-being of our people and the health of our environment. Wherever we have worked, we have invested for the future and strived to build a sustainable world. We enable society to be the best it can be. We feel responsible; we always have. We are proud to be steel.

Key facts:

• In 2015, 75 members of worldsteel signed a charter committing them to improve social, economic and environmental performance.
   
• Steel is an integral part of the circular economy promoting zero waste, reuse of resources and recycling, thus helping build a sustainable future.
   
• Steel helps people in times of natural disasters; earthquakes, storms, flooding, and other catastrophes are mitigated by steel products.
   
• Sustainability reporting at a global level is one of the major efforts that the steel industry undertakes to manage its performance, demonstrate its commitment to sustainability and to enhance transparency. We are one of the few industries to have done so since 2004.

A healthy economy needs a healthy steel industry providing employment and driving growth.

Steel is everywhere in our lives for a reason. Steel is the great collaborator, working together with all other materials to advance growth and development. Steel is the foundation of the last 100 years of progress. Steel will be equally fundamental to meeting the challenges of the next 100.

Key facts:

• Average world steel use per capita has steadily increased from 150kg in 2001 to 208 kg in 2015, making the world more prosperous. 

• Steel is used in every important industry; energy, construction, automotive and transportation, infrastructure, packaging and machinery.
   
• The steel industry is the second biggest industry in the world after oil and gas with an estimated global turnover of 900 billion USD. 

• By 2050, steel use is projected to increase to be 1.5 times higher than present levels in order to meet the needs of our growing population.
   
• Skyscrapers are made possible by steel. The housing and construction sector is the largest consumer of steel today, using around 50% of steel produced.

Let's talk about steel

We recognise that, because of its critical role, people are interested in steel and the effect it has on the global economy. We are committed to being open, honest and transparent in all our communications about our industry, its performance and the impact we have.

Key facts:

• The steel industry publishes data on production, demand and trade at national and global levels, which is used for analysing economic performance and making forecasts.
   
• The steel industry presents its sustainability performance with eight indicators on a global level every year.
   
• The steel industry proactively participates in OECD, IEA and UN meetings, providing all the information required on key industry topics which have an impact on our society.
   
• The steel industry shares its safety performance and recognises excellent safety and health programmes every year.
   
• The steel industry collects CO2 emissions data, providing benchmarks for the industry to compare and improve on.

There is always a good reason to choose steel.

 

Steel allows you to make the best material choice regardless of what you want to do. The excellence and variety of its properties mean steel is always the answer.

Key facts:

• Steel is safer to use because its strength is consistent and can be designed to withstand high-impact crashes.
   
• Steel offers the most economic and the highest strength to weight ratio of any building material.
   
•  Steel is the material of choice because of its availability, strength, versatility, ductility, and recyclability.
   
• Steel buildings are designed to be easy to assemble and disassemble, ensuring big environmental savings.

• Steel bridges are four to eight times lighter than those built from concrete.

You can rely on steel. Together we find solutions.

For the steel industry customer care is not just about quality control and products at the right time and price, but also enhanced value through product development and the service we provide. We collaborate with our customers to improve steel types and grades constantly, helping to make the customer manufacturing process more effective and efficient.

Key facts:

•    The steel industry publishes the advanced high-strength steels application guidelines, actively assisting automakers in applying them.
   
• The steel industry provides steel life cycle inventory data of 15 key products which helps customers understand the overall environmental impact of their products.
   
• The steel industry proactively participates in national and regional certification schemes, helping to inform customers and enhance supply chain transparency.
   
• The steel industry invests over €80 million in research projects in the automotive sector alone in order to meet customers’ changing needs.   

Steel enables innovation. Steel is creativity, applied.

 

Steel’s properties make innovation possible, allowing ideas to be achieved, solutions to be found and possibilities to be reality.
Steel makes the art of engineering possible, and beautiful.

Key facts:

• New lightweight steel makes applications lighter and more flexible while retaining the required high strength.
   
• Modern steel products have never been more sophisticated. From smart car designs to high-tech computers, from cutting edge medical equipment to state-of-the-art satellites.
   
• Architects can create any shape or span they desire and steel structures can be designed to suit their innovative designs.
   
• New and better ways of making modern steel are invented every year. In 1937, 83,000 tonnes of steel were needed for the Golden Gate Bridge, today, only half of that amount would be required.
   
• Over 75% of the steels in use today did not exist 20 years ago. 

People are proud to work in steel.

 

Steel provides universally valued employment, training and development. A job in steel places you in the centre of some of the greatest technology challenges of today with an unparalleled opportunity to experience the world. There is no better place to work and no better place for your best and brightest.

Key facts:

• The steel industry employs over 8 million people globally, equivalent to the population of Switzerland.
   
• The steel industry offers employees the opportunity to further their education and develop their skills, providing on average 8 days of training per employee per year.
   
• The steel industry is committed to the goal of an injury-free workplace and organises an industry-wide safety audit on Steel Safety Day every year.
   
• steeluniversity, a web-based industry university delivers education and training to the current and future employees of steel companies and related businesses, offering more than 30 training modules.
   
• The lost-time injury frequency rate has improved by 71% since 2004.

Steel cares for its community.

We care about the health and well-being of both the people who work with us and live around us. Steel is local – we touch people’s lives and make them better. We create jobs, we build a community, we drive a local economy for the long term.

Key facts:

• For 2013, the steel industry reported distributing 876 billion USD to society directly and indirectly, including 100 billion USD in tax revenue.
   
• Many steel companies build roads, transport systems, schools and hospitals in the areas around their sites.
   
• In developing countries, steel companies are often more directly involved in the provision of healthcare services and education for the wider community.
   
• Once established, steel plant sites operate for decades, providing long-term stability in terms of employment, community benefits and economic growth.
   
• Steel companies generate jobs and substantial tax revenues which benefit the local communities in which they operate.

Steel is at the core of a green economy.

The steel industry does not compromise on environmental responsibility. Steel is the world’s most recycled material and 100% recyclable. Steel is timeless. We have improved steel production technology to the point where only the limits of science confine our ability to improve. We need a new approach to push these boundaries. As the world looks for solutions to its environmental challenges, all of these depend on steel.

Key facts:

• Around 90% of water used in the steel industry is cleaned, cooled and returned to source. Most of the loss is due to evaporation. Water returned to rivers and other sources is often cleaner than when extracted.
   
• The energy used to produce a tonne of steel has been reduced by 60% in the last 50 years.
   
• Steel is the most recycled material in the world, with over 650 mega tonnes recycled annually.
   
• The recovery and use of steel industry by-products has reached a worldwide material efficiency rate of 96%.
   
• Steel is the main material used in delivering renewable energy: solar, tidal and wind.

[ Wiki ] Steel in Buildings and infrastructure

Construction is one of the most important steel-using industries, accounting for more than 50% of world steel production. Buildings - from houses to car-parks to schools and skyscrapers - rely on steel for their strength. Steel is also used on roofs and as cladding for exterior walls.

According to the UN's latest forecast dating July 2015, world population will reach 8.5 billion in 2030 and 9.7 billion in 20501..This will be accompanied by rapid urbanisation. As the need for buildings and infrastructure continues to grow worldwide, reducing consumption of natural resources and associated emissions is crucial for future sustainability.

Steelmakers around the world are increasingly providing construction solutions that enable energy-efficient and low-carbon-neutral buildings. These solutions reduce the environmental impact over the structures’ life cycle and help to extend their life span through design for disassembly and reuse.

Steel can provide the solutions to infrastructure and construction needs in developing countries and in climate resilient cities through enabling protective coastal and wind-resistant designs. While buildings currently account for about 20% of global greenhouse gas emissions, they also present many opportunities for reducing emissions and mitigating climate change.2,3

Not only is steel affordable, readily available and safer, its intrinsic properties, such as strength, versatility, durability and 100% recyclability allow for improved environmental performance across the entire life cycle of buildings. 

The advanced high-strength steels used in steel-plate applications also find uses in a number of related industries. Offshore oil rigs, bridges, civil engineering and construction machines, rail carriages, tanks and pressure vessels, nuclear, thermal and hydroelectric plants – all these applications benefit from the attributes of modern steels.

How steel is used in buildings and infrastructure

The possibilities for using steel in buildings and infrastructure are limitless. The most common applications are listed below4.

For buildings

• Structural sections: these provide a strong, stiff frame for the building and make up 25% of the steel use in buildings.
• Reinforcing bars: these add tensile strength and stiffness to concrete and make up 44% of steel use in buildings. Steel is used because it binds well to concrete, has a similar thermal expansion coefficient and is strong and relatively cost-effective. Reinforced concrete is also used to provide deep foundations and basements and is currently the world’s primary building material.
• Sheet products: 31% is in sheet products such as roofing, purlins, internal walls, ceilings, cladding, and insulating panels for exterior walls.
• Non-structural steel: steel is also found in many non-structural applications in buildings, such as heating and cooling equipment and interior ducting.
• Internal fixtures and fittings such as rails, shelving and stairs are also made of steel. 

For infrastructure

 

[ Wiki ]Frequently Asked Questions of Stainless Steel

These are some of the questions that we frequently get asked.

What Is Stainless Steel?

Stainless steel is an alloy of Iron with a minimum of 10.5% Chromium. Chromium produces a thin layer of oxide on the surface of the steel known as the'passive layer'. This prevents any further corrosion of the surface. Increasing the amount of Chromium gives an increased resistance to corrosion.

Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Other elements such as Nickel and Molybdenum may be added to impart other useful properties such as enhanced formability and increased corrosion resistance.

When was stainless steel discovered?
There is a widely held view that stainless steel was discovered in 1913 by Sheffield metallurgist Harry Brearley. He was experimenting with different types of steel for weapons and noticed that a 13% Chromium steel had not corroded after several months. However, the picture is much more complex than this.

What is stainless steel used for?

Stainless steels of various kinds are used in thousands of applications. The following gives a flavour of the full range:

Domestic – cutlery, sinks, saucepans, washing machine drums, microwave oven liners, razor blades

Architectural/Civil Engineering – cladding, handrails, door and window fittings, street furniture, structural sections, reinforcement bar, lighting columns, lintels, masonry supports

Transport – exhaust systems, car trim/grilles, road tankers, ship containers, ships chemical tankers, refuse vehicles

Chemical/Pharmaceutical – pressure vessels, process piping.

Oil and Gas – platform accommodation, cable trays, subsea pipelines.

Medical – Surgical instruments, surgical implants, MRI scanners.

Food and Drink – Catering equipment, brewing, distilling, food processing.

Water – Water and sewage treatment, water tubing, hot water tanks.

General – springs, fasteners (bolts, nuts and washers), wire.

Does stainless steel corrode?

Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is 'stain-less' not 'stain-impossible'. In normal atmospheric or water based environments, stainless steel will not corrode as demonstrated by domestic sink units, cutlery, saucepans and work-surfaces.

In more aggressive conditions, the basic types of stainless steel may corrode and a more highly alloyed stainless steel can be used. 

What forms of corrosion can occur in stainless steels?

The most common forms of corrosion in stainless steel are:

1. Pitting corrosion - The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack. The pitting corrosion resistance can be assessed using the Pitting Resistance Equivalent Number calculated from the alloy content.
2. Crevice corrosion - Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface. In very tight crevices, it is not always possible for the oxygen to gain access to the stainless steel surface thereby causing it to be vulnerable to attack. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.
3. General corrosion - Normally, stainless steel does not corrode uniformly as do ordinary carbon and alloy steels. However, with some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric acid and sulphuric acid at some concentrations are particular aggressive towards stainless steel.
4. Stress corrosion cracking (SCC) - This is a relatively rare form of corrosion which requires a very specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. Typical applications where SCC can occur are hot water tanks and swimming pools. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production.
5. Intergranular corrosion - This is now quite a rare form of corrosion. If the Carbon level in the steel is too high, Chromium can combine with Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. This process is also called sensitisation and typically occurs during welding. The Chromium available to form the passive layer is effectively reduced and corrosion can occur. It is avoided by choosing a low carbon grade the so-called 'L' grades or by using a steel with Titanium or Niobium which preferentially combines with Carbon.
6. Galvanic corrosion - If two dissimilar metals are in contact with each other and with an electrolyte e.g. water or other solution, it is possible for a galvanic cell to be set up. This is rather like a battery and can accelerate corrosion of the less 'noble' metal. It can avoided by separating the metals with a non-metallic insulator such as rubber.

How many types of stainless steel are there?

Stainless steel is usually divided into 5 types:

1. Ferritic – These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These steels have a similar microstructure to carbon and low alloy steels. They are usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. They cannot be hardened by heat treatment. High Chromium steels with additions of Molybdenum can be used in quite aggressive conditions such as sea water. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic.
2. Austenitic - These steels are the most common. Their microstructure is derived from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures. This structure gives these steels their characteristic combination of weldability and formability. Corrosion resistance can be enhanced by adding Chromium, Molybdenum and Nitrogen. They cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. They are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.
3. Martensitic - These steels are similar to ferritic steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic.
4. Duplex - These steels have a microstructure which is approximately 50% ferritic and 50% austenitic. This gives them a higher strength than either ferritic or austenitic steels. They are resistant to stress corrosion cracking. So called “lean duplex” steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. “Superduplex” steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not so much as the ferritic, martensitic and PH grades due to the 50% austenitic phase.
5. Precipitation hardening (PH) - These steels can develop very high strength by adding elements such as Copper, Niobium and Aluminium to the steel. With a suitable “aging” heat treatment, very fine particles form in the matrix of the steel which imparts strength. These steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment. This is in contrast to conventional hardening and tempering in martensitic steels where distortion is more of a problem. Corrosion resistance is comparable to standard austenitic steels like 1.4301 (304). 

Is stainless steel non-magnetic?

It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1. 

It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.

In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

Can I use stainless steel at low temperatures?

Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.

Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.

Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.

In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C. 

Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite.

Can I use stainless steel at high temperatures?

Various types of stainless steel are used across the whole temperature range from ambient to 1100 deg C. The choice of grade depends on several factors:

1. Maximum temperature of operation
2. Time at temperature, cyclic nature of process
3. Type of atmosphere, oxidising , reducing, sulphidising, carburising.
4. Strength requirement

In the European standards, a distinction is made between stainless steels and heat-resisting steels. However, this distinction is often blurred and it is useful to consider them as one range of steels.

Increasing amounts of Chromium and silicon impart greater oxidation resistance. Increasing amounts of Nickel impart greater carburisation resistance.

What is 'multiple certification'?

This is where a batch of steel meets more than one specification or grade. It is a way of allowing melting shops to produce stainless steel more efficiently by restricting the number of different types of steel. The chemical composition and mechanical properties of the steel can meet more than one grade within the same standard or across a number of standards. This also allows stockholders to minimise stock levels.

For example, it is common for 1.4401 and 1.4404 (316 and 316L) to be dual certified - that is the carbon content is less than 0.030%. Steel certified to both European and US standards is also common.