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Austenitic Stainless Steels are more difficult to drill or machine than Carbon Steels because of their work hardening characteristics. The more you work them the harder they become. The machinability of the conventional Austenitic grades (types 304 and 316) is about 50% of that of the Free Cutting Carbon Steels.
To successfully drill the Austenitic Stainless Steels a correctly sharpened drill bit is a pre-requisite Cutting speeds must be low (approximately 50% of those used for Carbon Steels), while the feed must be high. The feed must be high enough to ensure that the drill bit is penetrating the work hardened surface layer. Generous volumes of cutting fluid must be applied.
* HRA / No 0 (1C) – Hot rolled and annealed finish. This is a plate finish with the mill scale still present on the surface. This finish is generally only used for high temperature and materials handling applications in thickness greater than 3 mm
* No 1 (1D) – Hot rolled, annealed and de-scaled finish. This is a plate finish with a relatively rough, matt texture, generated by shot blasting and acid pickling. This is an industrial finish with good corrosion resistance and is available in thicknesses greater than 3 mm
* 2B (2B) Cold-rolled, annealed, se-scaled and skin passed finish. This finish is produced by cold rolling a product having a No1 finish to a thinner gauges, annealing and acid pickling. It is then given a light skin pass between polished rolls. The result is a smooth, matt finish with a milky white appearance which is used extensively in the food and beverage industry, as well as for aesthetic applications in architecture and building. It is generally available in thicknesses below 3 mm.
* 2BA (2R) Cold rolled and bright annealed finish. This finish is produced by cold rolling the product using highly polished rolls. The resultant smooth, reflective surface is preserved by annealing in an inert (non-oxidising) atmosphere, thus avoiding the need for acid pickling. It is a smooth, bright and reflective finish available in thicknesses below 2 mm
* No 3 / No 4 (2G) Polished finish. These finishes are undirectional finishes produced by polishing with abrasive belts of varying grit size. These finishes are used for hygienic (food processing) and architectural applications. These finishes are generally available in thicknesses less than 2 mm.
(XX) Denotes finishes as designated in the European (EN) Standards
The final appearance and surface quality of mechanically-finished stainless steel fabrications is dependent on several factors, including :
The optimum choice of finishing equipment, consumable and method will depend on :
Austenitic stainless steel when welded or exposed to a high temperature within the range 450° to 850° C (within the heat affected zone of the weld) are susceptible to a phenomenon called sensitization.
Carbon has a high affinity for Chromium and under these circumstances (high temperatures 450° to 850°C) will combine with the Chromium to form a complex Chromium Carbide. The formation of this Chromium Carbide takes place preferentially at the grain boundaries. A small amount of Carbon locks up a relatively large amount of Chromium, and thus depletes the grain boundary area in Chromium. The steel is then said to be sensitized.
The grain boundaries, now depleted in chromium, have both a lower corrosion resistance and passivating capacity. These areas therefore suffer a preferential attack when exposed to a corrosive environment. This form of corrosion is termed Intergranular Corrosion or Weld Decay.
Yes, Austenitic stainless steels (types 304, 316 etc), are generally non-magnetic in the fully annealed condition, however, they can become slightly magnetic when “cold worked”. This is due to a stress induced transformation of the austenite crystal structure into a martensitic crystal structure. This cold working can occur from various sources: cutting, bending, forming, machining.
Additionally, all austenitic stainless steels have a relatively small percentage of ferrite (a magnetic crystal structure) intentionally introduced into the austenitic structure (typically 3-5%) to aid in the hot workability of the steel. this will also result in a very slight “pull” on a magnet.
All of these will result in some small level of magnetic pull, and are not necessarily indicative of the product not being stainless steel, or Austenitic, or of the product being inherently inferior.