Stainless Steel

Type 201 stainless steel is unique because it was developed in response to rising nickel prices on the market. This means it is cheaper, but it also has a much lower nickel content. Without a significant amount of nickel, it is not effective at preventing corrosion. Higher manganese levels help make Type 201 one of the strongest types of stainless steel strip. This grade is favoured by industries seeking greater durability at a lower cost. Manganese and nitrogen are used in greater quantities to compensate for the lack of nickel. Type 201 stainless steel is particularly useful in cold environments as it retains its strength in cold weather. As the cheapest type of stainless steel, Type 201 appears to be the most attractive option. However, it does not demonstrate the same long-term durability in highly corrosive environments.

There are many different types and grades within the stainless steel classification. To date, the most common grade is 304, which accounts for more than 50 per cent of the stainless steel used worldwide. Grade 304 stainless steel is ‘austenitic’—a term relating to its molecular structure and indicating that nickel has been added to the alloy. Austenitic stainless steels are the most common type. Around 70 per cent of commercially produced stainless steel is of the austenitic type.
Grade 304 stainless steel consists primarily of iron, which makes up between 66 per cent and 75 per cent of its composition. The alloy also contains between 18 per cent and 20 per cent chromium and between 8 per cent and 10.5 per cent nickel. With a density measured at approximately eight grams per cubic centimetre, 304 stainless steel may also contain trace amounts of other elements such as silicon, sulphur, phosphorus and manganese. It possesses good corrosion resistance and good formability. Grade 304 is non-magnetic.
Austenitic stainless steels such as 304 have a yield strength that represents a relatively low proportion of their tensile strength, typically between 40 per cent and 45 per cent. The yield strength can be further improved when the material is cold-worked. This is particularly useful when manufacturing items such as spring wire.

Austenitic chromium-nickel stainless steels are frequently used in high-temperature applications. Alloys 309 and 309S are resistant to corrosion thanks to their high chromium and nickel content. They offer greater resistance to oxidation and possess excellent heat resistance whilst delivering good performance at both room and high temperatures. The difference between stainless steels 309 and 309S lies in their carbon content.

Machining these alloys is similar to that of type 304 stainless steel. The chips produced by this alloy are fibrous and will harden very quickly. It is essential to keep the cutting edge sharp at all times and to use chip breakers. Most austenitic stainless steels can be welded rapidly using fusion or resistance welding techniques. Oxy-acetylene welding is not recommended. The working temperature for this alloy is 1177°C, whilst a reheat temperature of 982°C is required.

Rapid quenching is recommended. Full annealing after machining is required to restore maximum corrosion resistance. Although this alloy has a high work-hardening rate, it can be drawn, forged, extruded and stamped. Full annealing is required after cold working to eliminate internal stresses.

Grade 420 stainless steel has a minimum chromium content of 12 per cent. It also contains a high proportion of carbon. As with other grades of stainless steel, this grade may be ideal for heat treatment hardening. In the annealed condition, it offers good ductility and, when polished, ground or hardened, it provides excellent corrosion resistance.

It possesses the highest hardness rating compared to other grades. Its impact resistance is also considerably high. When hardened, 420-grade steel is resistant to fresh water, alkalis, air, foodstuffs and mild acids.

Those with a smooth surface have greater strength. The corrosion resistance properties of the 420 grade tend to decrease when annealed. The corrosion resistance of the 420 grade is lower than that of 430-grade ferritic alloys containing 17 per cent chromium, 410-grade steels and other austenitic grades. This steel grade is used in cutlery such as carving knives, table knives, etc. Grade 420 meets food contact standards; however, continuous exposure of these materials to unwashed food products may cause corrosion.

431, a high-chromium, low-nickel grade with high hardenability, high strength and good corrosion resistance, is generally supplied as martensitic stainless steel in the tensile strength range of 850–1000 MPa (T condition) after hardening and tempering. It has a Brinell hardness range of 248–302. It exhibits high resistance to corrosive agents. It resists general atmospheric conditions and provides high resistance to mild marine and industrial environments.

It is resistant to many organic substances, nitric acid and petroleum products. Furthermore, in the hardened and tempered condition, it possesses high tensile and yield strengths, whilst also exhibiting excellent toughness. Due to its excellent hardenability, 431 can be hardened to as high as RC 44, depending on carbon content and cross-sectional dimensions.

For maximum hardness, small sections can be air-quenched and larger sections can be oil-quenched. Pre-hardened and tempered 431 will readily respond to nitriding, achieving a typical surface hardness in excess of Rc65. However, the nitriding process reduces corrosion resistance and is therefore generally not recommended except in critical applications where the benefits outweigh all other considerations. It is widely used amongst stainless steel grades.

It is widely used for components requiring a combination of high tensile strength, good toughness and good corrosion resistance. It has the potential for use in fasteners and equipment suitable for every sector.

It is a low-cost stainless steel. 430 stainless steel is a type of stainless steel that is hypoallergenic and corrosion-resistant. It is also resistant to harsh environments and chemicals. As this metal can withstand high temperatures and acids, it is an excellent choice for medical devices and food packaging. The benefits of 430 stainless steel are as follows:

430 stainless steel is a corrosion-resistant material used in various applications, including food and drink packaging, industrial manufacturing and healthcare products.
430 stainless steel is also heat-resistant, meaning it can withstand high temperatures without deteriorating. This makes it ideal for use in heating and heating systems.
430 stainless steel is non-toxic and safe to use. This makes it a good choice for products intended for consumption or contact by children or pets.
430 stainless steel has high resistance to wear and tear, making it suitable for use in areas where components are subject to prolonged exposure, such as the construction sector.
430 stainless steel is versatile and can easily be customised to meet specific requirements. This makes it a good choice for products that need to be designed to withstand various conditions and industrial processes.

Super Duplex Steel, which contains a high proportion of chromium, offers exceptional resistance to acids, acid chlorides, caustic solutions and other media in the chemical, petrochemical, pulp and paper industries. It generally replaces 300 series stainless steel, high-nickel super austenitic steels and nickel-duplex steels. The chemical composition of these alloys, based on high chromium, nickel and molybdenum content, enhances intergranular and pitting corrosion resistance.

The addition of nitrogen promotes structural hardening via an intermediate solid solution mechanism, which increases yield strength and ultimate strength values without compromising toughness. Since the launch of the first grade, the popularity of duplex steel has grown steadily.

Recently, the production of high-strength, corrosion-resistant super-duplex coils has been introduced in the maritime and chemical industries, as well as in architectural and mast fittings, wire ropes, lifting and pulley equipment, and well service lines. In fact, advances in wire processing techniques have made it possible to produce steel wires with diameters as small as 1 mm.

The key feature of 904L is its excellent corrosion resistance in dilute sulfuric acid across the entire concentration range up to 35° C. It also offers good resistance to certain other inorganic acids, although its performance may be limited when halide ions are present.

Although the nickel content in traditional stainless steels (such as 316L) makes them highly susceptible to chloride stress corrosion cracking, the higher level of this element in 904L provides good resistance to this form of corrosion.

The nitrogen, molybdenum, and chromium contents in stainless steels have a direct effect on their resistance to pitting and crevice corrosion, and in the case of 904L, these parameters are favorable. However, austenitic grades containing 6% molybdenum or super duplex grades exhibit superior resistance to this type of localized corrosion.

When we look at the list of stainless steel’s weaknesses, it is clear that, due to its relatively low weight, high strength and corrosion resistance, titanium can generally pick up where stainless steel leaves off; so let’s take a closer look at this top-performing material. Unlike many common metals that have been reduced using carbon for thousands of years (consider the iron smelting method used to produce all steel),

titanium has only been commercially available since the 1940s. Although it is now relatively widely available in many grades, the high melting point and the chemical reduction process used to produce titanium are time- and chemical-intensive, resulting in high costs. Whilst stainless steel only acquires its unique properties when alloyed, titanium is an element that can be used—and is frequently used—in its ‘commercially pure’ form. However, titanium is also commonly alloyed, and even within the definition of ‘commercially pure’, there are various grades; this means that, like all other useful metals, its properties can be tailored to specific applications.

Like other corrosion-resistant materials that oxidise to form a barrier when exposed to oxygen in the air, the surface of titanium also begins to oxidise immediately, forming titanium oxide which creates a thin passivation layer that protects the rest of the material from further corrosion. Unlike some other oxide layers, the titanium dioxide layer continues to thicken over time, reaching 25 nm after a few years and giving it a corrosion resistance almost equal to that of platinum.

Titanium is a real non-reactive wonder: it is almost impossible to get it to react with anything at normal temperatures. Combine this with its low density, and you have the recipe for the most common hypoallergenic metal used today in the medical industry for joints, bone repair and other sensitive applications involving contact with the body.

Titanium is a high-cost material and only becomes economically viable when its unique material properties are required. For machining, for example, titanium can have a cost factor 30 times higher than that of stainless steel, as it is relatively difficult to machine in addition to the material cost itself.

Titanium and stainless steel present the machinist with many of the same challenges: rather than the chip breaking cleanly at the cutting edge, the common alloys of both materials fracture easily. Both have a low thermal conductivity coefficient; this means it is easy to cause localised work hardening and premature tool wear. There is an additional challenge in that titanium is more ductile than most machined materials relative to its hardness, so care must be taken to clamp it as securely as possible.