Stainless Steel Manufacturing

Background

About 200,000 tons of nickel-containing stainless steel is used each year by the food processing industry in North America. It is used in a variety of food handling, storing, cooking, and serving equipment—from the beginning of the food collection process through to the end. Beverages such as milk, wine, beer, soft drinks and fruit juice are processed in stainless steel equipment. Stainless steel is also used in commercial cookers, pasteurizers, transfer bins, and other specialized equipment. Advantages include easy cleaning, good corrosion resistance, durability, economy, food flavor protection, and sanitary design. According to the U.S. Department of Commerce, 1992 shipments of all stainless steel totaled 1,514,222 tons.

Stainless steels come in several types depending on their microstructure. Austenitic stainless steels contain at least 6 percent nickel and austenite—carbon-containing iron with a face-centered cubic structure—and have good corrosion resistance and high ductility (the ability of the material to bend without breaking). Ferritic stainless steels (ferrite has a body-centered cubic structure) have better resistance to stress corrosion than austenitic, but they are difficult to weld. Martensitic stainless steels contain iron having a needle-like structure.

Duplex stainless steels, which generally contain equal amounts of ferrite and austenite, provide better resistance to pitting and crevice corrosion in most environments. They also have superior resistance to cracking due to chloride stress corrosion, and they are about twice as strong as the common austenitics. Therefore, duplex stainless steels are widely used in the chemical industry in refineries, gas-processing plants, pulp and paper plants, and sea water piping installations.

Raw Materials

Stainless steels are made of some of the basic elements found in the earth: iron ore, chromium, silicon, nickel, carbon, nitrogen, and manganese. Properties of the final alloy are tailored by varying the amounts of these elements. Nitrogen, for instance, improves tensile properties like ductility. It also improves corrosion resistance, which makes it valuable for use in duplex stainless steels.

The Manufacturing Process

The manufacture of stainless steel involves a series of processes. First, the steel is melted,

and then it is cast into solid form. After various forming steps, the steel is heat treated and then cleaned and polished to give it the desired finish. Next, it is packaged and sent to manufacturers, who weld and join the steel to produce the desired shapes.

Melting and casting

• 1 The raw materials are first melted together in an electric furnace. This step usually requires 8 to 12 hours of intense heat. When the melting is finished, the molten steel is cast into semi-finished forms. These include blooms (rectangular shapes), billets (round or square shapes 1.5 inches or 3.8 centimeters in thickness), slabs, rods, and tube rounds.

Forming

• 2 Next, the semi-finished steel goes through forming operations, beginning with hot rolling, in which the steel is heated and passed through huge rolls. Blooms and billets are formed into bar and wire, while slabs are formed into plate, strip, and sheet. Bars are available in all grades and come in rounds, squares, octagons, or hexagons 0.25 inch (.63 centimeter) in size. Wire is usually available up to 0.5 inch (1.27 centimeters) in diameter or size. Plate is more than 0.1875 inch (.47 centimeter) thick and over 10 inches (25.4 centimeters) wide. Strip is less than 0.185 inch (.47 centimeter) thick and less than 24 inches (61 centimeters) wide. Sheet is less than 0.1875 (.47 centimeter) thick and more than 24 (61 centimeters) wide.

Heat treatment

• 3 After the stainless steel is formed, most types must go through an annealing step. Annealing is a heat treatment in which the steel is heated and cooled under controlled conditions to relieve internal stresses and soften the metal. Some steels are heat treated for higher strength. However, such a heat treatment—also known as age hardening —requires careful control, for even small changes from the recommended temperature, time, or cooling rate can seriously affect the properties. Lower aging temperatures produce high strength with low fracture toughness, while higher-temperature aging produces a lower strength, tougher material.Though the heating rate to reach the aging temperature (900 to 1000 degrees Fahrenheit or 482 to 537 degrees Celsius) does not effect the properties, the cooling rate does. A post-aging quenching (rapid cooling) treatment can increase the toughness without a significant loss in strength. One such process involves water quenching the material in a 35-degree Fahrenheit (1.6-degree Celsius) ice-water bath for a minimum of two hours.The type of heat treatment depends on the type of steel; in other words, whether it is austenitic, ferritic, or martensitic. Austenitic steels are heated to above 1900 degrees Fahrenheit (1037 degrees Celsius) for a time depending on the thickness. Water quenching is used for thick sections, whereas air cooling or air blasting is used for thin sections. If cooled too slowly, carbide precipitation can occur. This buildup can be eliminated by thermal stabilization. In this method, the steel is held for several hours at 1500 to 1600 degrees Fahrenheit (815 to 871 degrees Celsius). Cleaning part surfaces of contaminants before heat treatment is sometimes also necessary to achieve proper heat treatment.

Descaling

• 4 Annealing causes a scale or build-up to form on the steel. The scale can be removed using several processes. One of the most common methods, pickling, uses a nitric-hydrofluoric acid bath to descale the steel. In another method, electrocleaning, an electric current is applied to the surface using a cathode and phosphoric acid, and the scale is removed. The annealing and descaling steps occur at different stages depending on the type of steel being worked. Bar and wire, for instance, go through further forming steps (more hot rolling, forging, or extruding) after the initial hot rolling before being annealed and descaled. Sheet and strip, on the other hand, go through an initial annealing and descaling step immediately after hot rolling. After cold rolling (passing through rolls at a relatively low temperature), which produces a further reduction in thickness, sheet and strip are annealed and descaled again. A final cold rolling step then prepares the steel for final processing.

Cutting

• 5 Cutting operations are usually necessary to obtain the desired blank shape or size to trim the part to final size. Mechanical cutting is accomplished by a variety of methods, including straight shearing using guillotine knives, circle shearing using circular knives horizontally and vertically positioned, sawing using high speed steel blades, blanking, and nibbling. Blanking uses metal punches and dies to punch out the shape by shearing. Nibbling is a process of cutting by blanking out a series of overlapping holes and is ideally suited for irregular shapes.Stainless steel can also be cut using flame cutting, which involves a flame-fired torch using oxygen and propane in conjunction with iron powder. This method is clean and fast. Another cutting method is known as plasma jet cutting, in which an ionized gas column in conjunction with an electric arc through a small orifice makes the cut. The gas produces extremely high temperatures to melt the metal.

Finishing

• 6 Surface finish is an important specification for stainless steel products and is critical in applications where appearance is also important. Certain surface finishes also make stainless steel easier to clean, which is obviously important for sanitary applications. A smooth surface as obtained by polishing also provides better corrosion resistance. On the other hand, rough finishes are often required for lubrication applications, as well as to facilitate further manufacturing steps.Surface finishes are the result of processes used in fabricating the various forms or are the result of further processing. There are a variety of methods used for finishing. A dull finish is produced by hot rolling, annealing, and descaling. A bright finish is obtained by first hot rolling and then cold rolling on polished rolls. A highly reflective finish is produced by cold rolling in combination with annealing in a controlled atmosphere furnace, by grinding with abrasives, or by buffing a finely ground surface. A mirror finish is produced by polishing with progressively finer abrasives, followed by extensive buffing. For grinding or polishing, grinding wheels or abrasive belts are normally used. Buffing uses cloth wheels in combination with cutting compounds containing very fine abrasive particles in bar or stick forms. Other finishing methods include tumbling, which forces

  

  The initial steel shapes—blooms, billets, slabs, etc.—are hot rolled into bar, wire, sheet, strip, and plate. Depending on the form, the steel then undergoes further rolling steps (both hot and cold rolling), heat treatment (annealing), descaling Ito remove buildup), and polishing to produce the finished stainless steel. The steel is then sent the end user.

  movement of a tumbling material against surfaces of parts, dry etching (sandblasting), wet etching using acid solutions, and surface dulling. The latter uses sandblasting, wire brushing, or pickling techniques.

Manufacturing at the fabricator or
end user

• 7 After the stainless steel in its various forms are packed and shipped to the fabricator or end user, a variety of other processes are needed. Further shaping is accomplished using a variety of methods, such as roll forming, press forming, forging, press drawing, and extrusion. Additional heat treating (annealing), machining, and cleaning processes are also often required.There are a variety of methods for joining stainless steel, with welding being the most common. Fusion and resistance welding are the two basic methods generally used with many variations for both. In fusion welding, heat is provided by an electric arc struck between an electrode and the metal to be welded. In resistance welding, bonding is the result of heat and pressure. Heat is produced by the resistance to the flow of electric current through the parts to be welded, and pressure is applied by the electrodes. After parts are welded together, they must be cleaned around the joined area.

Quality Control

In addition to in-process control during manufacture and fabrication, stainless steels must meet specifications developed by the American Society for Testing and Materials (ASTM) with regard to mechanical properties such as toughness and corrosion resistance. Metallography can sometimes be correlated to corrosion tests to help monitor quality.

The Future

Use of stainless and super stainless steels is expanding in a variety of markets. To meet the requirements of the new Clean Air Act, coal-fired power plants are installing stainless steel stack liners. Other new industrial applications include secondary heat exchangers for high-efficiency home furnaces, service-water piping in nuclear power plants, ballast tanks and fire-suppression systems for offshore drilling platforms, flexible pipe for oil and gas distribution systems, and heliostats for solar-energy plants.

Environmental legislation is also forcing the petrochemical and refinery industries to recycle secondary cooling water in closed systems rather than simply discharge it. Reuse results in cooling water with elevated levels of chloride, resulting in pitting-corrosion problems. Duplex stainless steel tubing will play an increasingly important role in solving such industrial corrosion problems, since it costs less than other materials. Manufacturers are developing highly corrosion-resistant steels in respond to this demand.

In the automotive industry, one steel manufacturer has estimated that stainless-steel usage per vehicle will increase from 55 to 66 pounds (25 to 30 kilograms) to more than 100 pounds (45 kilograms) by the turn of the century. New applications include metallic substrates for catalytic converters, air bag components, composite bumpers, fuel line and other fuel-system parts compatible with alternate fuels, brake lines, and long-life exhaust systems.

With improvements in process technology, superaustenitic stainless steels (with nitrogen contents up to 0.5 percent) are being developed. These steels are used in pulp-mill bleach plants, sea water and phosphoric-acid handling systems, scrubbers, offshore platforms, and other highly corrosive applications. A number of manufacturers have begun marketing such materials in sheet, plate, and other forms. Other new compositions are being developed: ferritic iron-base alloys containing 8 and 12 percent Cr for magnetic applications, and austenitic stainless with extra low sulfur content for parts used in the manufacture of semiconductors and pharmaceuticals.

Research will continue to develop improved and unique materials. For instance, Japanese researchers have recently developed several. One is a corrosion-resistant stainless steel that displays the shape-memory effect. This type of material returns to its original shape upon heating after being plastically deformed. Potential applications include assembly components (pipe fittings, clips, fasteners, clamps), temperature sensing (circuit breakers and fire alarms), and springs. An improved martensitic stainless steel has also been developed for precision miniature and instrument rolling-contact bearings, which has reduced vibration levels, improved life expectancy, and better surface finish compared to conventional materials.

Stainless Steel Stretcher Leveler Machine

What Stainless Steel Leveling is

Over the years, much has been written regarding the principles of leveling, and how to get material flat. In addition to simply getting material flat, more recently there has been a greater emphasis placed on developing technology to assure the material stays flat after subsequent processes such as laser or plasma cutting, welding, and/or other various forming and fabricating operations. All too often, material that appears to be flat doesn’t stay that way. Today, we understand that this is primarily due to the existence of randomly trapped internal stresses. While a flat piece of material may appear to be relaxed and at rest, in reality there is often a “tug of war” of epic proportions being waged right before our eyes. Trapped internal stresses can be introduced in a host of areas or operations. Typically these random stresses are at least in part initially introduced at the mill during the rolling process. Some portions of the strip are worked more than others. In addition, as the material cools, the outer wraps of the coil will cool at different rates than the inner wraps. Subsequent exposure to significant temperature changes can also induce additional changes in the material, such as coils that are stored in a heated environment, shipped during the cold winter months, and subsequently allowed to warm in the next facility. Because the material expands and contracts at different rates, additional stresses can be introduced. The leveling process itself, while having the ability to produce flat material, can also induce randomly trapped stresses. Because no one single process or condition is solely responsible, trapped internal stresses and their consequences are inevitable. While there is a general understanding and acceptance regarding the common principles of leveling, there is considerable debate as to how or if one particular type of Leveler or leveling process is really better than another at specifically addressing the problem of trapped internal stresses and spring back in addition to simply producing flat material. Understanding this tug of war will ultimately reveal clear commonsense reasons as to why some systems are clearly more effective than others.

FLATNESS AND STRESS

While flatness is frequently referred to as a side to side length differential, that is, some portions of the sheet or strip are literally longer in some areas than others, trapped stresses could be described as some portions of the sheet being under tension while other portions are relaxed as illustrated on the stress-strain curve. The portions of the strip labeled C & D are under tension. While flatness and stresses are both clearly related, each problem is unique and must be addressed as such.For leveling, the most important and key issue is the importance of exceeding the material’s yield point in order to affect permanent change in the material. Once you take the material past the yield point, everything before that is forgotten. While this is the basic key requirement for producing flat strip, it is also the key to producing flat material that stays flat.

While the material may appear to be flat and at rest, portions of the strip are hung up on the stress-strain curve and are actually under tension.

In order to get material flat, it is necessary to selectively elongate portions of the strip. If some sections of the strip are longer than others, the only way to get the material to lay flat is to sufficiently elongate the short portions lengthwise to “dimensionally equalize” the material. The goal is to make the strip dimensionally the same length across its entire width.

In order to eliminate spring back, the strip must be “stress equalized” by elongating the material’s entire cross section, top to bottom, and side to side, past its yield point to erase its previous memory. As a result the randomly trapped internal stresses are made into consistently trapped stresses; that is, all the material is allowed to spring back consistently to the same point on the stress-strain curve. Consequently in order to produce flat material that stays flat, it is necessary to both elongate portions of the strip more than others to dimensionally equalize or flatten the strip while also elongating the entire strip enough to exceed the yield in all of the material to stress equalize the strip. Consequently it is important to note that it is possible to stress relieve material without necessarily getting it flat. Conversely it is possible to get material flat without eliminating trapped internal stresses.

ROLLER LEVELERS

Roller Levelers bend the material progressively up and down over rolls of sufficient diameter to stretch the outer and inner surfaces of the material past the yield point. The smaller the roll, the more yielding top to bottom that will occur. The farther the outer surfaces are from the central or “neutral” centerline, the more yielding that will occur. The “neutral” centerline or fiber is an imaginary line or area in the middle of the material’s cross section which neither stretches nor compresses during the bending process. As a result, the material in this neutral area never exceeds its yield point. A Roller Leveler also has the ability to selectively stretch the material from side to side. Roller Levelers incorporate individual backup roller flights or banks that can be vertically adjusted so that the work rolls can be bent during the leveling process. By deliberately bending the rolls, portions of the strip are forced to take a longer path through the machine. As a result, some portions of the strip relative to length are permanently stretched longer while other sections are stretched very little or not at all.

The sheet illustrates a part before leveling. The stress-strain curve illustrates which portions of the sheet are stretched first and the greatest amount while also indicating the portions of the material that have not been stretched past their yield point.

It is also important to note that although portions of the strip have exceeded the yield point, much of the strip has not. Therein lies the problem. While the strip may be dimensionally equalized and allowed to lay flat, portions of the sheet are still in tension, unable to relax. Portions of the strip are in effect “hung up” and still under tension. Although it may appear as if the strip is at rest, portions of the material are actually under tension and being held in place by the surrounding material.Once released by shearing or cutting the material, these areas will simply spring back to their original often out of flat condition or shape.

Today heavy duty Roller Levelers are being promoted as being able to equalize stress. While these units can certainly work the material much harder than conventional Roller Levelers, they cannot exceed the yield of the material throughout the entire thickness of the strip by simply bending the strip over a series of rolls. Nor can they assure that all the stresses in the material have been equalized side to side. Both are essential to producing low stress material. All Roller Levelers are also sensitive to incoming material shape. There is a limit to the amount of shape correction that can be expected by virtue of the amount of roll bend that is possible.

TEMPER MILLS

Today, companies frequently use Temper Mills in Cut-To-Length Lines to produce stress relieved material. As previously discussed, in order to eliminate randomly trapped internal stress, all the material must be elongated or stretched beyond its yield point. A Temper Mill does this through the use of compressive forces. By squeezing the material between two rolls with enough force to reduce its thickness, the strip is slightly elongated. The typical amount of elongation is only 1 or 2%. Although most Temper Mills have the ability to elongate the strip further, doing so can change the material’s properties significantly. Consequently, some end users will limit the allowable elongation their processor can impart due to these changes. While a Temper Mill is often associated with producing flat material, to the contrary, the Mill by itself will not necessarily get the material flat.

By squeezing the material between two rolls with enough force, the strip is slightly elongated.

Although the Mill elongates the entire cross section of the strip, the amount of elongation is essentially the same. For the most part, a Mill cannot selectively elongate the strip which is required for shape correction. As a result, it is possible to stress relieve material without necessarily getting it flat. Conventional Roller Levelers must still be used to flatten the material. This is why you will always find Roller Levelers being used in conjunction with a Temper Mill in Cut-To-Length Lines. However, by virtue of the roller leveling process, stresses can be reintroduced into the strip, and you can “undo” the benefits of the Temper Mill. If the Mill works the strip too hard, it will actually induce shape problems. If the Roller Leveler works the strip too much, the benefits of the Mill will be diminished or eliminated. This system is also sensitive to incoming strip shape. The old saying “junk in, junk out” is still true. While the combination of the Temper Mill and Roller Leveler will produce flat stress equalized material, the two systems are reliant on one another. In addition, the combination is still limited by the capabilities of the Roller Leveler relative to the severity of the incoming shape that can be corrected and the skill of the operators.

STRETCHER LEVELING

Unlike conventional Levelers, no bending is used as part of this leveling process. In-Line Stretcher Levelers operate in conjunction with a Cut-To-Length Line. The strip is first stretched and then subsequently cut-to-length to a specific sheet or blank size. An In-Line Stretcher Leveler consists of a pair of entry and exit frames. These frames are adjustable relative to the desired part or stretch length. When required, each frame grips the material across its width. Large hydraulic cylinders connect the two frames. When pressurized, these cylinders push the frames away from one another. The pressure exerted by the cylinders exceed the collective yield of the material (thickness x width x yield), and the strip is subsequently stretched in the direction of travel. The Line’s Feeding System pulls the material through the Leveler incrementally. Between each Feed cycle, while the material is stopped to be sheared, a portion of the strip is stretched.

While some portions of the strip are stretched more than others, all of the material has exceeded its yield point and will “spring back” consistently.

In addition to the material being sufficiently elongated lengthwise to exceed the yield point in all of the material, top to bottom, and edge to edge, by virtue of the process, portions of the strip are stretched more than others at the same time. In effect, the material’s entire cross section is stretched to a common length or distance; that length being sufficient to both dimensionally and stress equalize the material at the same time. No subsequent processing is required. The end result is material that is more homogeneous and consequently more stable than with other forms of leveling.

Stretcher Leveling Process

While stretcher leveling is one of the oldest and most effective types of leveling ever developed, there is a limited understanding as to how the process works. Typically, there are concerns that the process works the material significantly more than with other types of leveling. Consequently, similar to the Temper Mill, if the material is overworked, this will change the properties of the material. However, while a Temper Mill may elongate the strip 1 or 2%, a Stretcher accomplishes its work with typically only .3 to .5% elongation. Relative to the percentage of elongation required to do its job, the Stretcher has very little impact on the metallurgical properties of the material. In regard to shape correction, if material has an edge wave or center buckle, the strip is longer in this area. Consequently, the short portions of the strip must be elongated to somewhat equal the longest length to allow the material to lay flat. However, regardless of the method used to elongate the strip, each system would need to elongate these areas essentially the same amount to achieve flat material. As a result, the percentage of elongation would be comparable. The Stretcher Leveler performs more overall work due to the fact the entire thickness of the material is elongated. However, no specific portion of the strip is worked more than with any other type of leveling process in order to achieve similar results. The Stretcher is relatively simple to adjust and operates with minimal skill. If the material is not flat, the operator simply increases the amount of stretch until the desired results are achieved. As long as the material lays flat, trapped internal stresses will also be consistent. Stretchers are also far less sensitive to incoming shape. Because Stretchers do not rely on the use of rolls, the ability to level is not limited by the amount of roll bend that is possible. As a result, the finished product is virtually independent of incoming shape.

IN CONCLUSION

Today, a number of companies offer systems that are promoted as producing temper passed or stretcher leveled “quality” material; however as to whether these systems actually achieve similar results are questionable. While there are a number of options available for leveling flat rolled material, each has their own inherent limitation. Consequently, in order to achieve the best possible results with a particular application, it is important to know the limitations of each machine.

Sources: (Mideast Metals FZCO. & Madehow.com)