Winegrowers Supplies - Stainless Steel
is a term coined early in the development of special steels for cutlery
applications. It was adopted as a generic name for these steels and now covers a
wide range of steel types and grades for corrosion or oxidation resistant applications.
Stainless steels are iron alloys (low-carbon steel) to which chromium has been added. The minimum is 10.5% chromium, although amounts greater than 12% are really necessary to add strength and provide its corrosion-resisting properties.
Other alloying elements are added to enhance the structure and properties such as formability, strength and cryogenic toughness. These include metals such as: nickel, molybdenum, titanium, copper.
Non-metal additions are also made, the main ones being: carbon, nitrogen.
The selection of a particular "type" and "grade" of stainless steel must initially meet the corrosion resistance requirements. Additional mechanical or physical properties may also need to be considered to achieve the overall service performance requirements.
Grades: of stainless steel used for winery equipment:-
Their main alloying elements are chromium (typically 18%) and nickel, which increases their corrosion resistance. They are the most widely used family of stainless steels:-
304/304S31 (also known as A2 or 1.4301) and 304L/304S11 (1.4307).
Grade 304 is chromium 18% + nickel 8% (known as 18/8).
316/316S31 (also known as A4 or 1.4401) and 316L/316S11 (1.4404).
Grade 316 stainless steel is chromium 18% + nickel 8% + molybdenum 3% (known as 18/8/3).
It is also known as 'marine grade' due to its increased resistance to chloride corrosion compared to 304.
Stainless steels can also be classified by their crystalline structure, which is determined by their chemical composition. The two grades mentioned above are Austenitic stainless steels, which are non-magnetic. Other types include Ferritic, Martensitic, and Duplex.
The austenitic and ferritic stainless steels used for cutlery and holloware are: 18/8 (18.8 or 18-8), 18/10 (18.10 or 18-10) and 18/0.
Duplex stainless steels are used where combinations of high strength and corrosion resistance are needed. Examples of duplex grades are 2205 / 318S13 (1.4462) and 1.4501.
Surface finishes: are important for more than just aesthetic considerations. When a design engineer chooses stainless steel for a particular purpose, he or she has a number of different grades to select from. There are also various surface finishes to choose from. The decision as to what type of steel is best suited for any given purpose is based mainly on how corrosive the ambient conditions are. However, surface quality or surface finish also affects the material's sensitivity to corrosion and its clean-ability, which is of particular importance in the food industry and health sector. Surface finish characteristics are therefore not merely a question of appearance.
All steel sheet is hot-rolled as a first rolling step, after which it is annealed in order to attain homogeneous material characteristics. Annealing produces an oxide coating on the steel, which is removed by shot blasting and by pickling in an acid bath. As a result of these manufacturing operations, the steel acquires a relatively coarse, matt grey surface with an Ra value in the order of 2-8 µm; the thicker the plate the coarser the surface, and the higher the Ra value.Cold rolled surface finishes:-
1. Annealed and pickled finish - 2D:-
By means of further rolling of the steel in a cold state, both its surface smoothness and thickness accuracy are improved. Since stainless steel is cold hardened, that is, it is hardened by the cold-working process, it has to be annealed after the rolling so as to regain its softness and working characteristics. This is most often carried out in an ordinary air atmosphere, which results in a new coating of oxide that has to be removed by pickling in an acid bath. Steel sheet processed in this way has a semi-matt silver-grey surface, which is known as finish 2D (D=dull), and has an Ra value in the order of 0.2-1.0 µm. This value increases with greater sheet thicknesses. It is used mostly for industrial equipment and for the manufacture of pipes and tubes for the process industry.2. Skin passed finish - 2B:-
3. Bright Annealed finish –
If, after cold rolling, the sheet coil is annealed in a protective gas that prevents oxidisation of the surface (for example, hydrogen and nitrogen from cracked ammonia), pickling is not needed since a bright finish is directly acquired. This surface quality is termed bright annealed finish, BA or 2R. Since the surface has not come into contact with pickling acids, it attains a brighter finish than either of the above examples, and an Ra value of 0.03-0.1 µm. It is used for the manufacture of household equipment and appliances (dish washers, washing machines, etc.).
Bright pickled 2BB surface is a mirror surface finish, providing an alternative to Bright Annealed (BA) finishes on most of the applications where a smooth surface and a bright appearance are needed. Typical applications for the finish 2BB are washing machine drums; inside parts of the dishwashing machines etc. Thanks to the low surface roughness, the finish 2BB is a perfect material for further polishing. Additionally, the smooth surface makes it easy to clean and hygienic.4. Polished finished:-
Surface roughness and structure can also be affected by different polishing techniques, even if no differences are detectable with the naked eye. Wet polishing, for example, produces a much finer surface than polishing with a dry belt. The Ra value could be less than 0.4 µm. The mesh size of the grinding bell and the resultant surface roughness of the sheet (Ra), do not, therefore, have any fixed relation. Wet polishing also produces a cleaner surface than dry polishing, which can be compared to the ploughing of furrows in a field. This means that dirt, bacteria and corrosive substances do not attach as easily to wet polished surfaces.
The ground or polished surface has more or less clearly visible scoring, which also makes scratches from daily wear and tear less visible. A polished surface is also easy to re-create, for example after repairs or welding work, compared with a 2B or BA surface. Polished sheet is used for purposes where quality of appearance and hygiene are very important, such as in commercial kitchens (work surfaces, cooking utensils, dishwashers, freezers), in house-holds (cooking vessels), and in public places (sanitary facilities, hospital equipment, kick-plates).5. Brushed finish:-
6. Patterned (e.g. marbled):-
Patterned sheet is produced by means of extra rolling of cold-rolled brushed sheet coils between special imprinted rolls. In this way, a waffle-type pattern can be obtained on both sides of the sheet. At the same time, the rigidity and strength of the material is increased by almost 50%. This means that it is possible to use a thinner sheet, and thus obtain a lower weight.