Surface Hardening of Steels

1.         Nothing like conventional through-hardening of steel, occasionally it is desirable to retain a relatively tough (relatively a lesser amount of brittle) inner core, coupled with a very hard surface. This would, typically, be required of a component, which is subjected to high forceful stresses, yet also has to resist surface wear and would include:

(a)       Gears

(b)       Camshafts and crankshafts

(c)        Cylinder barrels of piston engines .

2.         Some materials can be ‘case-hardened’ to achieve this aim. Several methods are used, depending on the close relative material and the particular application.

Carburising.    This is the ordinary way of case-hardening low-carbon steels and, basically, consists of heating the metal to something like 900C, while the element is in contact with a carbon-rich medium followed by a suitable heat-treatment. Carbon is normally engaged into the surface of the heated steel and the rate of penetration is approximately 1mm in 5-6 hours. Low-carbon steels are particularly suited to this type of treatment, as it increases the carbon content and hence the hardness in  vicinity. A variety of methods of carburising are used, the most common ones being:

(i)         Pack Carburising. The object is potted in a container containing a carbon- rich (charcoal based) powder and heated in a furnace. The metal is after that quenched in oil (not water-which would cause the hard case to peel off). The depth of the hard skin depends on the length of time that the metal is heated.

(ii)        Gas Carburising. The thing is placed in a basket in a furnace, through which is passed a suitable, carbon-rich gas (e.g. Methane, propane).

(iii)       Liquid Carburising. The object is heated to a appropriate temperature and then immersed in a hot, salt bath at 900C. The salts are typically based on sodium cyanide and the process is often called ‘cyanide hardening’. The metal is quenched in water (not oil-which would react unfavourably with the salts).

 Nitriding.        This process involves the absorption of nitrogen (instead of carbon) into the surface of the steel. Suitable "Nitralloy" steels are essential for this process and they usually contain 1% Aluminium, 1.5% Chromium and 0.2% Molybdenum. A special furnace is used and ammonia gas is circulated through it. The furnace temperature of 500C converts the ammonia into a nitrogen-rich gas and forms hard iron nitride in the surface of the steel.  The container depth, feasible by this process, is less than that by pack carburising, but the key advantage of nitriding is that no hardening or tempering is necessary to attain the final hardness, and no finish machining is required after nitriding. This, relatively low-temperature procedure results in negligible distortion and is much cleaner than the carbon methods. Aircraft piston engine cylinder barrels are particularly suitable for nitriding, as are some crankshaft bearing surfaces and the stems of a few aero-engine induction and exhaust valves. Nitrided surfaces must be protected in opposition to pitting corrosion, usually (as with engine gears and shafts) by keeping the surface oiled.

NOTE:If certain surfaces of a component are not to be case-hardened, it is necessary to guard them during the carburising or nitriding processes, to locally stop the hardening agent from being absorbed. Copper plating, nickel plating or a proprietary paste are normally used in such areas.

(c)        Flame/Induction Hardening.          Unlike carburising and nitriding, flame and induction hardening do not add a hardening agent into the surface of a fundamentally softer material. Instead, they are merely techniques for hardening the surface of material by a `local heat- treatment'. Steels suitable for these processes before now contain sufficient carbon (or other elements) to attain a high degree of hardness if heated and quenched. Only the surface is locally heated (by a flame or electrical induction coil), and the heated surface is then instantly quenched by water jets.  The flame or induction coil is placed so that it only heats the area required to be hardened.

(d)       Other Surface Hardening Techniques.   Case-hardening, there are further methods of producing tough surfaces on metals, such as by electro-plating, welding, bonding, and metal spraying. All generally involve adding a harder surface metal to the parent substance.

Alloying Elements in Steel

       As discussed earlier, iron has few practical uses in its pure state. Adding small amounts of other materials to molten iron, however, dramatically changes its properties. Some of the more ordinary alloying elements comprise carbon, sulphur, silicon, phosphorus, nickel and chromium

Carbon

Carbon is the most common alloying element found in steel. When mixed with iron, compounds of iron carbide form and it is the carbon in steel that allows it to be heat-treated to obtain varying degrees of hardness, strength and toughness.  The greater the carbon content, then the more sympathetic the steel becomes to heat-treatment and, while its strength and hardness increases, its malleability and weld ability decreases.

 Low-Carbon Steel.               Low-carbon steels enclose between 0.1% and 0.3 % carbon and are classified as SAE 1010 to SAE 1030 steels. They are used in such items as locking wire and cable bushings and, in sheet form, they used for low-load applications. Carbon steels weld simply but do not accept heat-treatment very well.

Medium-Carbon Steel.        These steels contain among 0.3% and 0.7 % carbon. The increased carbon assists in heat-treatment while still retaining sensible ductility. Where surface hardness is required  Medium-carbon steels are used for machining or forging.

 High-Carbon Steel. The carbon content of these steels ranges between 0.5% and 1.5 % and this makes them very hard. High-carbon steels are mainly used in springs, files and in most cutting tools.

Sulphur

Sulphur causes steel to be brittle when rolled or forged and so it must be removed during the refining process. If it proves not possible to remove all of the sulphur, then manganese, which is risk-free to the steel can be added to the metal (to form manganese sulphide), the manganese also improves forging by making the steel less brittle during the processes.

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