Question : How do I cut Austenitic Stainless Steels?
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.
Question : What are the standard mill finishes for Stainless Steels?
* 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
Question : Abrasives of a machine-applied finish versus a hand-applied finish
The final appearance and surface quality of mechanically-finished stainless steel fabrications is dependent on several factors, including :
- Abrasive type: backing material, grit size, shape and hardness
- Number of finishing steps
- Equipment used
- Type of power supply to the equipment
- How the abrasive is supported (ie, belt or disc support, wheel type and flexibility)
- Surface speed and applied pressure
The optimum choice of finishing equipment, consumable and method will depend on :
- The existing surface condition of the semi-finished fabrication
- The accessibility of the areas to be finished
- The required final visual effect
The abrasives used for grinding and polishing stainless steel fabrications under workshop and on-site-conditions are usually different from those used for finishing coils, sheets or plates in steel mills and service centres where aluminium oxide or silicon carbide abrasives are mainly used.
During the finishing of fabrications, zirconium oxide abrasives are more commonly used in the grit size range 24 to 120. These types of abrasives have better durability under these arduous working conditions than either aluminium oxide or silicon carbide abrasives. For finer grit size finishes, aluminium oxide or silicon carbide can be used.
The properties of the abrasives that determine the final polishing results are :
- The grit size
- Tech size (diameter) for the support discs or wheels and their peripheral speed
- The backing material type and stiffness
- The use of any lubricating grease or oils in conjunction with the abrasive (not normal practice in hand grinding and polishing)
Contrary to the range of abrasives used for coil and sheet polishing, the wear of the grain – and hence the variation in visual aspect of the finished coil – is not such a disturbing feature in abrasives used for manual operations. Not only do manual jobs incorporate a lot of finishing steps using fleece ( which would mask the effect of a wearing grain from pre-polishing), the wear of the abrasives used (eg, wheel) shows a different behaviour to that of large abrasive belts used for coil and sheet polishing.
Care must be taken when selecting your abrasives, and it is suggested that for continuity and for the appearance that at all times you use the same supplier of your abrasives on anyone particular application. Each supplier has a different formula, composition and backing. Changing the supply during a fabrication could result in a slight difference in the finish and appearance.
QUESTION: When should I specify an ‘L’ grade stainless steel?
- When there is no price premium for the low carbon grades, make them your standard choice.
- When manufacturing welded components, particularly in heavy thicknesses.
- ‘L’ refers to low Carbon levels-example types 304L and 316 L.
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.
PREVENTION OF INTERGRANULAR CORROSION
- Always begin fabrication with the material in the fully annealed state.
- Use the stabilised grades of stainless steel. these stabilised grades contain the alloying elements of Ti or Niobium (Nb). These elements have a higher affinity than Chromium for Carbon. They therefore lock up the carbon as Titanium (Ti) or Niobium (Nb). Carbides, leaving insufficient free C to combine with and deplete the Cr.
- Use ‘L’ grades. The “L” grades will sensitize given enough time in the sensitizing temperature range albeit to a lesser degree.
QUESTION: Can Austenitic stainless steels (type 304, 316 etc) be magnetic?
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.
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