BRIGHT ANNEALING TECHNOLOGY
A highly technological heat treat process performed to our stainless steel heating elements by a carefully controlled furnace atmosphere resulting to a clean, smooth, scale free metal surface. During typical and traditional annealing process metholodgy for tubular sheathed heating elements, the heated sheath steel combines with oxygen in the air to form an oxide layer on the steels surface and gets a dark and black colour. In bright annealing, the difference is the stainless steel sheath of heating element is heated in a furnace filled with gases, such as hydrogen or nitrogen, to prevent oxide scale formation. The heating element comes out of the bright anneal furnace with the same surface as it had when it went into the furnace.
Bright annealing is carried out in a furnace full of Hydrogen (H2) at temperatures ranging between 1040 ° C and 1100° C and is followed by a rapid cooling. The Hydrogen is NOT an oxidising agent and therefore no surface oxidation is created and pickling is no longer required after the bright annealing.
The main advantage of this technology, besides a bright and even surface that eases further processing of the tubes, is the improved corrosion resistance of the heating element sheath. Such treatment, carried out at the production process, ensures the complete solution of the possible carbides precipitated at the grain border, thus obtaining an austenitic matrix free of defects. This makes it possible to avoid the dangerous phenomena of intergranular corrosion.
The austenitic structure obtained through bright annealing, is homogeneous with regular grain size; the consequence is an improvement of stainless steel tensile properties, in particular traction and elongation, with an increase of plasticity and a decrease of residual stress.
Extensive tests by BALÇIK examined the difference of annealing processes on tubular heating elements. An atmospheric test was carried out in a marine environment. The environment was chosen for its high temperature, salt concentration and humidity, and the test period lasted 12 months.
Engineers compared tubes from two different production routes:
Conventional typical annealed and bright annealed. The aim of the study was to compare the corrosion resistance of the heating elements, in order to assess the impact of bright annealing technology on the quality of the tube surfaces.
After the test period, each tube was then split into two halves to examine the surface with optical microscopy. It is clear that the bright annealed heating element exhibited the best corrosion resistance. Its still bright and shiny even after one year of marine exposure.
The conventionally typically annealed heating element exhibited inferior corrosion performance, in comparison to the bright annealed tube.