Sensitisation
Sensitisation occurs because Chromium has a higher affinity for Carbon than does Iron. Welding and some heating cycles provide the thermal energy for chromium carbides form in preference and these precipitate within the microstructure. This process causes the steel to become susceptible to corrosion as the matrix areas around the carbide are depleted in Chromium and cannot form the protective oxide film.
Related terms: Carbide, Inter-granular Corrosion, Sensitised, Stabilised Grades
Sensitised
This describes the condition of a stainless steel that, having been welded or otherwise thermally processed, now has chromium carbide particles present in the grain boundaries. In this condition the steel becomes susceptible to corrosion as the matrix areas around the carbide are depleted in chromium and cannot form the protective oxide film.
Related terms: Carbide, Stabilised Grades, Inter-granular Corrosion, Sensitisation
Sheared Edges
The cut edges that are produced when a wide sheet or coil is sheared or slit, by rotary cutters or a guillotine, into narrower widths/lengths.
Sinking, Sink Drawing
The cold drawing of a tube without a mandrel in the bore to control bore dimensions. It enables closer tolerances to be achieved on the outside diameter and increases mechanical properties.
Related terms: Cold Drawing
Skin Passing, Pinch Passing
A cold rolling pass, with minimal reduction of thickness, of annealed sheet or strip. It reduces the tendency to kinks, flats and stretcher strains on subsequent manipulation. Flat material so treated is described as 'skin passed', 'pinch passed', 'non-kinking', 'non-flatting' or 'killed'. The material will also be brighter and have closer tolerances.
Slab
A hot rolled or forged primary feedstock for rolling or forging. The cross-section is rectangular, typically with a width more than twice the thickness.
Related terms:
Slitting
The operation of continuously cutting a wide coil of strip into a narrower strips by rotary cutters. The narrower strips are usually re-coiled after slitting.
Solid Solution
The name given to an alloy, e.g. Iron Chromium, phase which appears, in the microstructure, to be one material. In Iron Chromium solid solutions the Iron is the major constituent and the Chromium, the “alloying element”, atoms occupy places within the iron lattice and modify its properties. Solid solutions are fundamental in metallurgy. Pure metals have definite melting and freezing points, solid solutions have a melting and freezing range.
Solution Treatment, Solution Annealing
A heat treatment process to dissolve precipitated carbides followed by quenching or rapid cooling to retain the carbon into solid solution. This restores the chromium content of the matrix and hence the corrosion resistance.
Related terms: Sensitisation, Inter-granular Corrosion
Springback, Spring Back
The tendency of a metal work-piece that has been deformed to not stay exactly in the shape to which is has been manipulated, but instead to spring back a little. Stainless steels spring back more than mild/carbon/alloy steels and thus need to be over-bent to a greater degree.
Stabilised Grades
Stainless Steels whose composition has been modified by additions of Titanium or Niobium to overcome sensitisation and its consequent problems. The mechanism is that these elements have a higher affinity for Carbon than does Chromium so their Carbides form in preference. The Chromium content of the matrix is therefore not reduced so it corrosion resistance is retained.
Stainless Steel Designations
Stainless steels are governed by a wide range of Proprietary, National, European and International standards. This range of standards has led over time to the use of a number of designation systems. Many people are familiar, and comfortable with, the 3-digit system that originated in the USA and was adopted into former British Standards in which 2xx and 3xx designations were applied to austenitic grades (e.g. 201, 304, 304L) 4xx designations were applied to ferritic and martensitic grades (e.g. 410, 430) In Europe these have now been replaced by BSEN numbers of the type 1.4xxx (e.g. 1.4301) although the American 3-digit system is still widely used.
Stainless Steel
Stainless Steel is the term used to describe an extremely versatile family of engineering materials, which are used primarily for their corrosion and heat resistant properties. All Stainless Steels are alloys of Iron and Chromium, with a minimum Chromium content of about 10.5%, and with varying Carbon content. This Chromium content develops a self-repairing Chromium Oxide surface film which prevents the surface rusting typical of many other types of steel. Higher Chromium content will further enhance corrosion resistance and some stainless grades also contain other alloying elements including Nickel and Molybdenum. Most commonly you will see reference to “austenitic stainless steel”, “ferritic stainless steel”, “martensitic stainless steel”, “duplex stainless steel” and “precipitation hardening stainless steel”.
Straightness Tolerance, Camber
The permitted deviation from a true straight line, or bow, within a specified length of a product. The usual methods to determine such a deviation are with a straight edge and dial gauge or a flat plate and feeler gauges.
Stress Corrosion Cracking
Stress Corrosion Cracking (SCC) is the rapid and often abrupt failure of a normally ductile metal or alloy caused by residual or other stresses in the work-piece and in the presence of specific corroding agents. It is temperature dependent. Austenitic stainless steels are susceptible in chloride environments temperatures above 60°C. Ferritic grades are immune to this form of attack.
Stress Relieving
An annealing heat treatment used to reduce internal stresses in steel after fabrication.
Related terms: Annealing
Stress
Stress is the measure of the forces acting within a deformable body or work-piece when it is under load. These forces will seek to resist any permanent change in shape of the body or work-piece until the yield point strength of the material has been exceeded.
Swaging
A cold forging process in which the dimensions of a tube are altered by blows delivered rapidly against its outside diameter. An internal mandrel is usually used.