Steels for machining oprerations have their machinability improved by the addition during steel manufacture, of small amounts of certain metals or metallic substances and by the control of inclusions.
Nitriding is a surface heat treatment which consists of a surface enrichment of nitrogen allowing an improvement of hardness (wear resistance) and the tribology characteristics. All steels can be nitrided. However, so-called nitriding steels contain alloying elements that allow nitrogen to become soluble or that contribute to hardening.
Direct hardening is a surface heat treatment which consists of the surface enrichment of carbon in the steel. Enrichment by carbon and nitrogen in the steel is called carbo-nitriding. Direct hardening, nitriding and carbo-nitriding are all heat treatments achieved in the hot condition and are generally followed by a temper or by a temper and harden, except for nitriding, in order to obtain the required surface characteristics.
The steel is produced from scrap* melted in an electric arc-furnace, then refined (made to specification by appropriate addition of alloying elements) in a ladle furnace. The three steel works of Ascometal have electric arc-furnaces.
*Scrap : recycled steel from demolition of used products (vehicles, household appliances/white goods ... ), or from new arisings (steel works, forgers, machinists). Scrap is graded in accordance with its shape and composition/chemistry, allowing the steel works to best choose from different scrap mixes to produce a given quality of steel.
Once out of the electric arc furnace, the steel must still be modified by ladle steelmaking or secondary metallurgy to obtain the required characteristics for the final product. The level of alloying elements is adjusted and the amount of impurities optimized. Amongst the main operations of secondary metallurgy are deoxidation, desulfurizing, dephosphorizing and decarburizing.
Liquid steel is cast in an ingot mould. The product, once cooled and stripped from the mould, is called an ingot.
Liquid steel is cast in a suspended base-free mould that is very strongly cooled. At the start of the casting operation a moveable base blocks the mould; once the base and the walls of the mould are sufficiently cooled, the moveable base and the liquid steel as it solidifies are withdrawn continuously between rollers. The product, on exiting the continuous casting machine, is cut into blooms.
Product of large section (round, square or rectangle). The term bloom is normally used when the section is greater than a square having sides of 160mm.
Product of smaller section than bloom (round, square or rectangle).
Product (round, square or rectangle) produced by a continuous caster.
Continuously cast blooms are most often produced for further processing such as hot rolling, in order to produce bars or wire rod.
Product resulting from hot rolling of an ingot or a bloom.
Product resulting from hot rolling of blooms or billets.
Wire rod is a rolled product that is coiled while hot, having a dimension greater or equal to 5mm. The product obtained by drawing or cold rolling of rod is called wire.
Operation of cold finishung without the production of swarf consisting of passing wire rod (or bar) through dies so as to achieve large reductions in section. The products obtained are precision gauged, but their mechanical characteristics are equally modified.
The rolled bar is passed between guides (or rollers) in order to make it straight.
A rolled bar that has been sheared or cropped will have its ends deformed by shearing or by a burr after cropping. Facing is a machining operation that produces a straight and clean end.
Annealing treatments designed to soften the steel or to obtain a favourable microstructure (grain size,shape and type of cristal) to allow machining, shearing, cropping, softening, or several mechanical properties.
Heat treatment followed by a sudden stringent cooling, allowing the steel to get improved mechanical properties. However, as this process leads to excessive susceptibility to buttle fracture, a further tempering is required to give back to steel its ductility.
Non-destructive testing (NDT)
- laser measuring : to gauge dimensional characteristics
- testing (Eddy current) : for surface defects
- ultrasonic testing : for internal soundness (to detect the presence of non-metallic inclusions or porosity in the bars).