Is a group of iron-based alloys that contain a minimum of approximately 11% chromium, a composition that prevents the iron from rusting and also provides heat-resistant properties. Different types of stainless steel include the elements carbon (from 0.03% to greater than 1.00%), nitrogen, aluminium, silicon, sulfur, titanium, nickel, copper, selenium, niobium, and molybdenum.Specific types of stainless steel are often designated by a three-digit number, e.g., 304 stainless.
Stainless steel’s resistance to rusting results from the presence of chromium in the alloy, which forms a passive film that protects the underlying material from corrosion attack, and can self-heal in the presence of oxygen. Corrosion resistance can be increased further, by:
- increasing the chromium content to levels above 11%;
- addition of 8% or higher amounts of nickel; and
- addition of molybdenum (which also improves resistance to “pitting corrosion”).
The addition of nitrogen also improves resistance to pitting corrosion and increases mechanical strength. Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure.
Resistance to corrosion and staining, low maintenance, and familiar luster make stainless steel an ideal material for many applications where both the strength of steel and corrosion resistance are required. Moreover, stainless steel can be rolled into sheets, plates, bars, wire, and tubing. These can be used in cookware, cutlery, surgical instruments, major appliances, construction material in large buildings, industrial equipment (e.g., in paper mills, chemical plants, water treatment), and storage tanks and tankers for chemicals and food products. The material’s corrosion resistance, the ease with which it can be steam-cleaned and sterilized, and the absence of the need for surface coatings have prompted the use of stainless steel in kitchens and food processing plants.
The invention of stainless steel followed a series of scientific developments, starting in 1798 when chromium was first shown to the French Academy by Louis Vauquelin. In the early 1800s, James Stoddart, Michael Faraday, and Robert Mallet observed the resistance of chromium-iron alloys (“chromium steels”) to oxidizing agents. Robert Bunsen discovered chromium’s resistance to strong acids. The corrosion resistance of iron-chromium alloys may have been first recognized in 1821 by Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery.
In the 1840s, both Sheffield steelmakers and Krupp were producing chromium steel with the latter employing it for cannons in the 1850s. In 1861, Robert Forester Mushet took out a patent on chromium steel.
These events led to the first production of chromium-containing steel by J. Baur of the Chrome Steel Works of Brooklyn for the construction of bridges. A U.S. Patent for the product was issued in 1869. This was followed with recognition of the corrosion resistance of chromium alloys by Englishmen John T. Woods and John Clark, who noted ranges of chromium from 5–30%, with added tungsten and “medium carbon”. They pursued the commercial value of the innovation via a British patent for “Weather-Resistant Alloys”.
In the late 1890s, German chemist Hans Goldschmidt developed an aluminothermic (thermite) process for producing carbon-free chromium. Between 1904 and 1911, several researchers, particularly Leon Guillet of France, prepared alloys that would be considered stainless steel today.
In 1908, Friedrich Krupp Germaniawerft built the 366-ton sailing yacht Germania featuring a chrome-nickel steel hull in Germany. In 1911, Philip Monnartz reported on the relationship between chromium content and corrosion resistance. On 17 October 1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel as Nirosta.
Similar developments were taking place in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless steel. In 1912, Elwood Haynes applied for a US patent on a martensitic stainless steel alloy, which was not granted until 1919.
While seeking a corrosion-resistant alloy for gun barrels in 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in The New York Times.
Was later marketed under the “Staybrite” brand by Firth Vickers in England and was used for the new entrance canopy for the Savoy Hotel in London in 1929. Brearley applied for a US patent during 1915 only to find that Haynes had already registered one. Brearley and Haynes pooled their funding and, with a group of investors, formed the American Stainless-Steel Corporation, with headquarters in Pittsburgh, Pennsylvania.
In the beginning, stainless steel was sold in the US under different brand names like “Allegheny metal” and “Nirosta steel”. Even within the metallurgy industry, the name remained unsettled; in 1921, one trade journal called it “unstainable steel”. In 1929, before the Great Depression, over 25,000 tons of stainless steel were manufactured and sold in the US annually.
THE SPINNING PROCESS
Is fairly simple. A formed block is mounted in the drive section of a lathe. A pre-sized metal disk is then clamped against the block by a pressure pad, which is attached to the tailstock. The block and workpiece are then rotated together at high speeds. A localized force is then applied to the workpiece to cause it to flow over the block. The force is usually applied via various levered tools. Simple workpieces are just removed from the block, but more complex shapes may require a multi-piece block. Extremely complex shapes can be spun over ice forms, which then melt away after spinning. Because the final diameter of the workpiece is always less than the starting diameter, the workpiece must thicken, elongate radially, or buckle circumferentially.
A more involved process, known as reducing or necking, allows a spun workpiece to include reentrant geometries. If surface finish and form are not critical, then the workpiece is “spun on air”; no mandrel is used. If the finish or form are critical then an eccentrically mounted mandrel is used.
“Hot spinning” involves spinning a piece of metal on a lathe while high heat from a torch is applied to the workpiece. Once heated, the metal is then shaped as the tool on the lathe presses against the heated surface forcing it to distort as it spins. Parts can then be shaped or necked down to a smaller diameter with little force exerted, providing a seamless shoulder.
The basic hand metal spinning tool is called a spoon, though many other tools (be they commercially produced, ad hoc, or improvised) can be used to effect varied results. Spinning tools can be made of hardened steel for use with aluminum, or from solid brass for spinning stainless steel or mild steel.
Some metal spinning tools are allowed to spin on bearings during the forming process. This reduces friction and heating of the tool, extending tool life and improving surface finish. Rotating tools may also be coated with a thin film of ceramic to prolong tool life. Rotating tools are commonly used during CNC metal spinning operations.
Commercially, rollers mounted on the end of levers are generally used to form the material down to the mandrel in both hand spinning and CNC metal spinning. Rollers vary in diameter and thickness depending the intended use. The wider the roller the smoother the surface of the spinning; the thinner rollers can be used to form smaller radii.
Cutting of the metal is done by hand held cutters, often foot long hollow bars with tool steel shaped/sharpened files attached. In CNC applications, carbide or tool steel cut-off tools are used.
The mandrel does not incur excessive forces, as found in other metalworking processes, so it can be made from wood, plastic, or ice. For hard materials or high-volume use, the mandrel is usually made of metal.
ADVANTAGES AND DISADVANTAGES
Several operations can be performed in one set-up. Work pieces may have re-entrant profiles and the profile in relation to the center line virtually unrestricted.
Forming parameters and part geometry can be altered quickly, at less cost than other metal forming techniques. Tooling and production costs are also comparatively low. Spin forming, often done by hand, is easily automated and an effective production method for prototypes as well as high quantity production runs.
OTHER METHODS OF FORMING ROUND METAL PARTS
Include hydroforming, stamping, forging and casting. These other methods generally have a higher fixed cost, but a lower variable cost than metal spinning. As machinery for commercial applications has improved, parts are being spun with thicker materials in excess of 1in (25mm) thick steel. Conventional spinning also wastes a considerably smaller amount of material than other methods.
Objects can be built using one piece of material to produce parts without seams. Without seams, a part can withstand higher internal or external pressure exerted on it. For example: scuba tanks and CO2 cartridges.
One disadvantage of metal spinning is that if a crack forms or the object is dented, it must be scrapped. Repairing the object is not cost-effective.