Stainless Steel

September 21, 2023

Stainless steel is a remarkably versatile metal that has become an essential material for modern life. From household appliances to construction beams, stainless steel provides durability, aesthetics, and corrosion resistance.

This in-depth guide will cover everything you need to know about this widely used metal – from its composition and properties to its many applications. Discover what makes stainless steel such a stalwart material.

A Brief History of Stainless Steel

Stainless steel was first developed in 1913 by English metallurgist Harry Brearley. At the time, Brearley sought to make gun barrels more durable. He discovered that adding chromium to steel created an alloy that was resistant to erosion and corrosion.

However, stainless steel did not gain widespread use until the 1920s. Advances in steel production allowed commercializing Brearley’s corrosion-proof steel.

By the 1930s, engineers found ways to enhance stainless steel’s formability, machinability, and weldability. This paved the way for mass adoption. During World War II, demand for stainless steel exploded thanks to defense and aviation applications.

After the war, stainless steel became a construction staple. New alloy variations continued expanding its possibilities. Global stainless steel production exceeded 70 million tons by 2021.

Today, this wonder metal is ubiquitous in everything from kitchenware to medical devices. Stainless steel’s standout properties make it indispensable.

Overview of Stainless Steel Grades

There are over 150 grades of stainless steel designed to optimize different properties. The various alloying metals and percentages result in types categorized into five basic families:

  • Austenitic – Non-magnetic with excellent formability and weldability. Austenitic grades like 304 and 316 are the most common and versatile.
  • Ferritic – Magnetic with excellent ductility but poor weldability. Common in automotive parts and appliances. 409 and 430 are popular ferritic grades.
  • Martensitic – Magnetic like ferritic but with higher carbon for added strength. Used for knives, razors, and some aerospace applications.
  • Duplex – Mixed microstructure with a balance of austenitic and ferritic properties. Duplex 2205 and 2507 boast high strength with stress corrosion resistance.
  • Precipitation Hardening – Extremely strong grades capable of immense cold working without deforming. Ideal for applications requiring elasticity and crash resistance. Common variants include 17-4PH and 15-5PH.

Within each family, the percentages of metals like chromium, nickel, and molybdenum can be tweaked as needed. This flexibility makes stainless steel customizable.

Properties of Stainless Steel

Stainless steel derives its name from one of its defining features – corrosion resistance. But stainless steel offers a combination of beneficial properties that make it so versatile:

Strength – Stainless steel maintains high tensile strength and durability, even in hazardous or pressurized environments. Some martensitic and precipitation hardening grades offer yield strengths exceeding 200,000 PSI.

Workability – Austenitic stainless steel in particular can be formed and machined into complex shapes. It also accepts many finishing techniques like grinding and polishing.

Corrosion Resistance – Chromium creates an invisible oxide barrier that enables stainless steel to resist rusting and pitting, even in wet environments or when exposed to acids.

Hygiene – Low surface roughness and nonporous structure enable thorough cleaning. This makes stainless steel ideal for applications requiring sterility like medical and food production.

Affordability – Stainless steel costs a fraction of what more exotic alloys do. It also maintains intrinsic scrap value for cost savings via recycling.

Aesthetics – Stainless steel finishes range from industrial to contemporary. It takes well to passivation coloring treatments like gunmetal gray. Mirrored and brushed finishes enhance décor.

This blend of virtues makes stainless steel more valuable than the sum of its parts. Small tweaks in composition can optimize for a given application.

How Stainless Steel is Made

Manufacturing stainless steel is a complex, multi-step process. While mills use different methods, here is a general overview:

1. Raw Materials – Steel scrap and alloys like ferronickel, ferrochrome, and ferromolybdenum are melted together in electric furnaces. Rigorous chemical analysis ensures the exact composition.

2. Melting – Electric arc furnaces rapidly melt the raw materials into a liquid state for easier blending at temperatures exceeding 3000°F.

3. Refining – Impurities are removed from the molten steel which is recirculated until homogenous. More alloys are added until the desired chemical balance is achieved.

4. Casting – The molten stainless steel gets poured into molds to form slab ingots or cast directly into semi-finished shapes like billets.

5. Rolling/Forging – Ingots are reheated and passed through rolling mills and presses to form sheets, strips, bars, rods, and tubing.

6. Finishing – Coils of stainless steel undergo further cold rolling to final thicknesses. Other finishing like annealing or pickling may be applied.

7. Fabrication – For end uses, stainless steel is cut, machined, welded, or otherwise fabricated into a finished component or product.

This start-to-finish process allows transforming raw ores into exceptionally functional stainless steel material.

Common Grades of Stainless Steel

With so many alloy possibilities, it helps to be familiar with a few of the most popular grades:

304 Stainless Steel

The workhorse of stainless steel, 304 accounts for over 50% of total stainless steel production. With 18-20% chromium and 8-10% nickel, 304 offers excellent corrosion resistance and durability. It also boasts easy machinability and weldability along with good formability.

Applications range from kitchenware and pipes to chemical containers and building panels. 304, also labeled as UNS S30400, sets the standard for stainless functionality at an affordable cost.

316 Stainless Steel

For harsher environments with exposure to chlorides and acids, 316 is the go-to choice. With 16-18% chromium, 10-14% nickel, and 2-3% molybdenum, 316 resists pitting and stress corrosion cracking. The added molybdenum boosts corrosion protection.

Applications include exhaust manifolds, chemical processing equipment, surgical tools, and marine components. 316, with UNS designation S31600, delivers superior corrosion resistance.

409 Stainless Steel

With 11-13% chromium but no nickel, ferritic 409 steel achieves useful stability without nickel’s cost. While not as strong or shiny as austenitic steels, 409 maintains workability and machinability with decent corrosion resistance.

Applications range from automotive exhaust components to furnaces and storage tanks. The low price point also makes 409, or UNS S40900, a cost-effective alternative for appliances.

430 Stainless Steel

As the second most popular ferritic steel behind 409, 430 also eschews nickel for reduced expense. Its 16-18% chromium imparts useful corrosion resistance and aesthetics. However, the lack of nickel impacts ductility and formability.

Common uses include automotive trim, kitchen appliances, and equipment like tanks in the chemical industry. 430, designated S43000, offers an affordable, functional stainless option.

Duplex 2205

For structural applications requiring the ultimate in strength, 2205 duplex steel delivers. With 22% chromium, 5% nickel, and 3% molybdenum, duplex 2205 balances impressive corrosion resistance with double the strength of austenitic 300 series steels.

2205, product code S32205, frequently gets used for pressure vessels, pipes, fittings, and structural supports across marine, oil/gas, and chemical sectors.

Uses of Stainless Steel

Thanks to its winning properties combination, stainless steel gets used in every modern industry. Some of its top applications include:

Architecture – Stainless steel allows grand, thin designs for buildings and monuments with its strength, longevity, and aesthetics. Prominent examples include the Gateway Arch and Washington Monument.

Automotive/Aerospace – Stainless steel allows reducing vehicle weights along with chrome plating for decorative finishes. Almost every car contains stainless steel. Jet engines and aircraft also rely on specialized stainless alloys.

Medical – The nontoxic, serializable, and corrosion-proof properties of stainless steel make it the global standard for surgical tools, medical devices like stents, and hospital equipment.

Consumer Goods – Stainless steel features prominently in appliances, cookware, cutlery, furniture, and electronics cases thanks to good looks and low maintenance.

Industrial – Factories of all types leverage stainless steel for equipment like piping, tanks, valves, and structural supports resistant to chemicals and moisture.

Marine – The high salinity of ocean environments mandates stainless steel for components like propeller shafts, desalination plants, and marine hardware. 316L or duplex 2205 are common selections.

Stainless steel ideally meets the demands of such diverse use cases. Small alloying tweaks personalize stainless steel to every niche application.

How Stainless Steel is Formed

To transform raw stainless steel into usable products and parts, manufacturers rely on various metalworking processes:

Casting – Molten stainless steel solidifies into a mold cavity. This efficiently produces parts in the final or near final shape. Die casting involving injecting the molten metal under high pressure is common.

Rolling – Passing stainless steel through gradually smaller rollers thins and lengthens it into sheets or strips. Cold rolling can reduce thickness by up to 90% with work hardening.

Forging – Powerful presses and hammers shape stainless steel through compressive force when red hot or cold. This makes sturdy, seamless metal parts.

Extrusion – Forcing stainless bars through a die opening allows producing specialized long metal sections like railroad tracks and truck frames.

Welding – Melting stainless edges together bonds them. Arc welding is most common, but laser and spot resistance welding of stainless is also popular.

Machining – CNC machines cut, drill, tap, mill, and otherwise machine stainless stock into components. Waterjet cutting uses highly pressurized water for precision stainless cutting.

Stamping – Stamping presses punch custom shapes from stainless steel blanks. Progressive stamping repeats the process for high volume production.

These metalworking techniques enable shaping stainless steel into every possible application.

Finishes for Stainless Steel

The finishing processes applied to stainless steel allow customizing its appearance:

Mirror Finish – Highly reflective, chrome-like finish achieved by rolling and polishing. Adds prestigious appearance for applications like architecture and luxury goods.

Brushed Finish – A matte texture with visible linear brushing marks made through abrasion. Provides contemporary style and hides fingerprints.

Embossed Finish – Patterned impressions rolled into the stainless surface for decorative flair. Titanium nitride coatings add unique colored finishes.

Bead Blasting – Propelling glass or metal beads against stainless gives a matte, non-reflective uniform texture ideal for hiding scratches.

Electropolishing – Removes surface roughness through reverse plating to smooth the metal for sterile and aerodynamic surfaces.

Colored Finishes – Passivation or physical vapor deposition infuses stainless steel with durable colored finishes like black oxide (gunmetal), rose, bronze and more.

The chosen surface finish alters stainless steel’s look from utilitarian to ornate as needed.

How to Care for Stainless Steel

While resistant to corrosion, stainless steel still requires proper care and maintenance to maximize longevity:

  • Clean regularly with mild soap and water. Avoid abrasive powders or chloride-containing cleaners which can damage the protective surface layer.
  • Rinse and wipe down after use. Allowing things like food, debris, or water spots to linger increases staining risk.
  • Rub gently in the direction of the grain to prevent scratching during cleaning.
  • Oil surfaces periodically. A light coat of food-grade mineral oil enhances water beading and luster while preventing oxidization.
  • Inspect equipment routinely for cracks or pitting. Quickly repair any damage to prevent deterioration.
  • Passivate after welding. The heat of welding destroys the chromium oxide layer. Passivation restores corrosion protection.

With the proper care regime, stainless steel maintains its innate resilience and aesthetic appeal.

Is Stainless Steel Magnetic?

One of the common stainless steel myths is that it must be magnetic. In reality, only certain grades of stainless steel are magnetic:

  • Austenitic stainless steels like 304 and 316 are non-magnetic as their crystal structure makes them paramagnetic.
  • Ferritic and martensitic stainless steels like 409 and 440 are magnetic due to their body-centered cubic crystal lattice.
  • Duplex stainless steels offer a blend, with somewhat weaker magnetism than ferritic counterparts.
  • Precipitation hardening grades like 17-4PH also exhibit notable magnetism.
  • Work hardening and cold working stainless steel can induce some slight magnetic response depending on how the metal’s crystalline grain structure gets aligned.

So while many stainless steel varieties are magnetic, not all are. Therefore, magnetism or lack of it is not a definitive test to identify grades of stainless steel.

Is Stainless Steel Stronger Than Regular Steel?

Stainless steel is stronger than plain carbon steel in most regards thanks to alloying elements like chromium, manganese, and nickel:

  • The work hardening properties of stainless steel enable achieving double the yield strength of mild steel.
  • Stainless steel maintains its strength and durability as temperatures fluctuate while plain steel weakens when hot.
  • Stainless steel demonstrates a much higher resistance to creep deformation under load over prolonged periods.
  • Austenitic and duplex stainless steels offer ultimate tensile strengths exceeding 100,000 PSI, over triple that of low carbon steel.
  • Stainless steel better retains its elasticity and toughness after cold working compared to ordinary steel.

However, heavily tempered high-carbon steels can match or narrowly exceed certain stainless steel alloys in specific strength parameters. But on a holistic basis, stainless steel possesses superior mechanical properties.

Is Stainless Steel Scratch Resistant?

While very durable, stainless steel is not impervious to scratches. The surface hardness and scratch resistance varies considerably between stainless steel grades:

  • Austenitic stainless steels like 304 are comparatively soft, rating only about 200 on the Vickers hardness scale. They easily scratch at a level below hardened steel.
  • Ferritic and martensitic stainless steels achieve up to 400 Vickers hardness to offer decent scratch resistance akin to carbon steel tools.
  • Highly alloyed grades like 17-4PH or 15-5PH attain Vickers hardness values exceeding 450 thanks to precipitation strengthening. This approaches tool steel scratch resistance.
  • Work hardening via cold rolling or forging also increases stainless steel’s surface hardness significantly.

So while austenitic varieties like 304 scratch fairly easily, higher hardness grades can rival and surpass carbon steel in scratch resistance. Maintaining the protective chromium oxide layer also helps prevent surface marring.

Is Stainless Steel Rust Proof?

Stainless steel is highly rust-resistant but not completely immune when its chromium oxide film gets disrupted. Insufficient oxygen or abrasion allows stainless steel to corrode under certain conditions:

  • Deprived of oxygen, stainless steel can still rust. This limits its application for underwater structures and necessitates coatings.
  • In warm brine or saltwater, even 316 marine-grade steel can pit and crevice corrode, requiring extra molybdenum to bolster corrosion resistance.
  • Mechanical damage from scraping or debris abrading the surface interrupts the passivation layer protecting the steel underneath.
  • Welding destroys the integrity of the chromium oxide layer locally enabling rust around heat affected areas.

However, routine passivation restores stainless steel’s protective layer. Overall, stainless steel provides vastly superior corrosion resistance over plain carbon steel while still not being wholly rust proof.

Advantages of Stainless Steel

Stainless steel has many benefits owing to its unique combination of properties:

  • Strength – Maintains high strength across a range of temperatures and in harsh conditions. Twice as strong as normal steel.
  • Corrosion Resistance – Chromium alloying enables excellent resistance to rusting and chemical attack, even in salty or acidic environments.
  • Longevity – Stainless steel lasts for decades with minimal upkeep while maintaining aesthetic appeal. Recyclable too.
  • Hygiene – Nonporous structure prevents buildup of microbes. Easily sterilized for medical and food applications.
  • Workability – Highly machinable and weldable for ease of fabrication. Forgeable when heated.
  • Affordability – Less expensive than other commercial alloys while retaining higher intrinsic value than plain carbon steel. Abundant too.
  • Fire Resistance – High melting point and noncombustible properties qualify stainless steel for fire safety systems.

Stainless steel brings this wide range of benefits to every industry using it.

Stainless Steel vs. Aluminum

Stainless steel gets compared frequently to aluminum since both metals are ubiquitous across industries. But stainless steel has some advantages over aluminum:

  • Strength – Stainless steel maintains stable tensile strength across moderate temperatures while aluminum weakens at higher temps.
  • Corrosion Resistance – Stainless steel exhibits much higher corrosion resistance, especially for saltwater exposure where aluminum is extremely vulnerable.
  • Appearance – Brushed or mirror-polished stainless steel boasts better aesthetics over unclad aluminum’s dull gray look.
  • Hygiene – Stainless steel rates higher than aluminum for sterile and non-contaminant characteristics.
  • Value Retention – Scrap stainless steel retains decent salvage value unlike aluminum which fetches minimal recycled value.
  • Impact Resistance – Austenitic and ferritic stainless steels take repeated impacts without damage better than aluminum.

However, aluminum compares favorably to stainless steel in some regards:

  • Weight – Aluminum is almost three times less dense than stainless steel, providing substantial weight savings. This makes aluminum ideal for applications where reduced mass is critical.
  • Thermal Conductivity – Aluminum conducts heat much better than stainless steel, making aluminum preferable for heat exchangers and electronics cooling.
  • Non-Magnetic – Aluminum is non-magnetic, unlike certain grades of stainless steel. This allows aluminum for applications that prohibit magnetism.
  • Formability – Softer aluminum alloys generally boast better machining and forming characteristics over stainless steel.
  • Cost – Basic aluminum alloys can cost slightly less than commodity stainless steel grades like 304 despite aluminum’s higher raw material value.

So while stainless steel wins out in properties like strength and corrosion resistance, aluminum bests it in light weight and conductivity. The optimum metal depends on the specific demands of the application. But both metals will continue seeing high demand across industries.

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