Processing of stainless steels

In the forging of martensitic stainless steels, especially those with high carbon contents, precautions must be taken to avoid cracking during cooling caused by the martensitic transformation. These steels are cooled more slowly than austenitic steels to a temperature of about 590°C. Forging is recommended in the temperature range of 900-1200 C. 

Alloys with low carbon contents (ferritic or austenitic stainless steels) are suitable for cold deformation such as bending, folding or deep-drawing. In general, austenitic stainless steels show better cold work properties than ferritic stainless steels. 

In the deformation of ferritic stainless steels a temperature of 100-300°C should be used to achieve a better workability. If higher deformation rates are required, an intermediate annealing at 750-800°C should be performed. 

Deformation of austenitic stainless steels results in a much higher work hardening than in ferritic stainless steels. Therefore, higher deformation forces, harder and more wear-resistant tools are required. 

In drawing, excessively high speeds must be avoided. Best results are obtained with a speed of 6-8 m/min. 

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In the industrial processing of stainless steel sheets, two final annealing treatments (after cold rolling) are commonly used ... 

All metal parts exposed to the room are made of stainless steel and motors and transmissions are IP 65 to withstand the eflfect of the cleaning agents. The design also takes into account the special considerations necessary for food processing machinery with regards to easy accessibility to all parts and the lack of corners, edges, pockets or other food traps , so that the mechanical system can be easily cleaned. 

In the batch process which finds occasional use, the steps used in the successive nitrations are similar and include acid mixing, addition of the oil, digesting (cooking) the reaction to completion, cooling and settling the mix, and separating the oil from the acid. The nitrators are made of stainless steel... 

Most manufacturing equipment should be made of stainless steel. The liming tanks, however, can be either concrete or wood (qv). Properly lined iron tanks are often used for the washing and acidification, ie, souring, operations. Most gelatin plants achieve efficient processes by operating around the clock. The product is tested in batches and again as blends to confirm conformance to customer specifications. 

In appHcations as hard surface cleaners of stainless steel boilers and process equipment, glycoHc acid and formic acid mixtures are particularly advantageous because of effective removal of operational and preoperational deposits, absence of chlorides, low corrosion, freedom from organic Hon precipitations, economy, and volatile decomposition products. Ammoniated glycoHc acid Hi mixture with citric acid shows exceUent dissolution of the oxides and salts and the corrosion rates are low. 

Molybdenum improves the corrosion resistance of stainless steels that are alloyed with 17—29% chromium. The addition of 1—4% molybdenum results in high resistance to pitting in corrosive environments, such as those found in pulp (qv) and paper (qv) processing (33), as weU as in food preparation, petrochemical, and poUution control systems. 

The obvious destination for nickel waste is in the manufacture of stainless steel, which consumes 65% of new refined nickel production. Stainless steel is produced in a series of roasting and smelting operations. These can be hospitable to the various forms of nickel chemical waste. In 1993, 3 x 10 t of nickel from nickel-containing wastes were processed into 30 x 10 t of stainless steel remelt alloy (205,206) (see Recycling, nonferrous metals). This quantity is expected to increase dramatically as development of the technology of waste recycle coUection improves. 

The calcium cyanamide feed is weU mixed with the recycled slurry and filtrate ia a feed vessel. The calcium cyanamide is added at a rate to maintain a pH of 6.0—6.5 ia the cooling tank. The carbonation step can be conducted ia a turbiae absorber with a residence time of 1—2 min. After the carbonation step, the slurry is held at 30—40°C to complete the formation of calcium carbonate, after which the slurry is cooled and filtered. AH equipment for the process is preferably of stainless steel. The resulting solution is used directiy for conversion to dicyandiamide. 

Ejectors are available in many materials of construction to suit process requirements. If the gases or vapors are not corrosive, the diffuser is usually constructed of cast iron and the steam nozzle of stainless steel. For more corrosive gases and vapors, many combinations of materials such as bronze, various stainless-steel alloys, and other corrosion-resistant metals, carbon, and glass can be used. 

Stainless Steel There are more than 70 standard types of stainless steel and many special alloys. These steels are produced in the wrought form (AISI types) and as cast alloys [Alloy Casting Institute (ACI) types]. Gener y, all are iron-based, with 12 to 30 percent chromium, 0 to 22 percent nickel, and minor amounts of carbon, niobium (columbium), copper, molybdenum, selenium, tantalum, and titanium. These alloys are veiy popular in the process industries. They are heat- and corrosion-resistant, noncontaminating, and easily fabricated into complex shapes. 

The internals of the reservoir should be coated unless the reservoir is constructed of 300 series stainless steel. API 614 mandates the use of 304L, 321, or 347 stainless steel processed to ASTM A 240. For the critical equipment units, the stainless steel reservoir is a good idea. A decision must be made by the user relative to the general purpose units. A good coating can keep the internals clean and free of corrosion if applied properly. When the idea of stainless steel reservoirs was introduced, it met with immediate resistance, but as the alternatives were considered, it began to gain acceptance. 

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