2.5 Normalizing

Plain carbon steels are normalized by heating them to the proper temperatures. This time we want a finer grain structure in the steel. To achieve this, we have to heat the metal up more quickly and cool it more quickly. The workpiece is taken out of the furnace when its normalizing temperature has been achieved and allowed to cool down in the free air of the heat treatment shop. The air should be able to circulate freely round the workpiece. However, the workpiece must be sited so that it is free from cold draughts. Warning notices that the steel is dangerously hot must be placed around it.

Castings and forgings are produced by high temperature processes. As they cool down and shrink they often develop high internal stresses. Machining tends to partially release these stresses and over a period of time the machined components ‘move’ or distort and become inaccurate. At one time, large castings and forgings were rough machined and left out of doors to ‘weather’ over a period of a year or more. The continual changes in temperature released all the stresses and the components became ‘stabilized’ ready for finish machining. No further movement then took place.

Keeping such a large amount of valuable stock tied up over a long period of time is no longer economically viable and weathering has given way to normalizing. The normalizing process is now frequently used for stress relieving between the rough machining of castings and forgings and the finish machining of such workpieces. This is done to stabilize such workpieces and to avoid ‘movement’ or distortion subsequent to machining. When normalizing is used for the stabilization of castings and forgings, it is sometimes referred to as ‘artificial weathering’.

2.6 Case hardening

Often, components need to be hard and wear resistant on the surface, yet have a tough and strong core to resist shock loads. These two properties do not normally exist in a single steel but are achieved by a process called case hardening. This is a process by which carbon is added to the surface layers of low carbon steels or low alloy steels to a carefully regulated depth. This addition of carbon is called carburizing. After carburizing the component is put through successive heat treatment processes to harden the case and refine the core. This process has two distinct steps. First, the workpiece is heated to between 900◦C and 950◦C in contact with the carburizing compound until the additional carbon has been absorbed to the required depth. Second, the workpiece is removed from the carburizing compound, reheated to between 780◦C and 820◦C and dipped off (quenched) in cold water.

Carburizing

This depends upon the fact that very low carbon (0.1%) steels will absorb carbon when heated to between 900◦C and 950◦C. Various carbonaceous materials are used in the carburizing process.

• Solid media such as bone charcoal or charred leather, together with an energizer such as sodium and/or barium carbonate. The energizer makes up to 40% of the total composition.

• Molten salts such as sodium cyanide, together with sodium carbonate and/or barium carbonate and sodium or barium chloride. Since cyanide is a deadly poison such salts must be handled with great care and the cyanide makes up only between 20 and 50% of the total. Stringent safety precautions must be taken in its use. The components to be carburized are immersed in the molten salts.

• Gaseous media based upon natural gas (methane) are increasingly used. Methane is a hydrocarbon gas containing organic carbon compounds that are readily absorbed into the steel. The methane gas is frequently enriched by the vapours that are given off when mineral oils are ‘cracked’ by heating them in contact with the metal platinum which acts as a catalyst. It is a common fallacy that carburizing hardens the steel. It does not, it adds carbon only to the surface of the steel and leaves the steel in a fully annealed (soft) condition. It is the subsequent heat treatment that hardens the steel.

• The component is raised to red heat using a gas torch and brazing hearth.

• The red-hot component is then plunged into a case-hardening powder. This consists of carbon rich compounds plus an energizer as previously described.

• The component absorbs the powder onto its surface. This ‘carburizes’ the surface of the metal and increases its carbon content.

• The heating and dipping can be repeated several times to increase the depth of carbon infusion.

• Finally, the component is again heated to red heat and plunged immediately into cold water. If the case-hardening powder has done its job, there should be a loud ‘crack’ and any surplus powder breaks away from the surface of the metal.

• The surface of the metal should now have a mottled appearance and it should be hard. The component is now case hardened.

• Because this technique results in only a fairly shallow case, it is referred to as ‘superficial hardening’. The case is not deep enough for finishing by grinding processes, although polishing is permissible.

• Bright-drawn steels do not absorb carbon readily unless the drawn surface is removed by machining. Wherever possible use a casehardening quality steel which is formulated and processed to suit this treatment.

Deep case hardening

Where a deep case is required so that components can be finished by grinding (e.g. surface grinding or cylindrical grinding) the following procedure is required.

• The component is ‘pack-carburized’ by burying it in the carburizing compound in a steel box, sealed with an airtight lid.

• The box is heated in a furnace for several hours depending upon the depth of case required. The carburizing temperature is about 950◦C.

• The box is removed from the furnace and allowed to cool down. The component is removed from the box and cleaned so as to remove any residual powder from its surface.

• The component will be soft and have a coarse grain structure because of the long time for which it has been heated at a high temperature, and its subsequent slow cooling.

• The core of the component will have a carbon content of less than 0.3%. The low carbon steel core of the component is toughened by refining its grain. To do this, the component is heated to 870◦C and then quenched in oil. Since this temperature is well below the carburizing temperature which caused the grain growth and because of the rapid cooling the core will now have a fine grain.

• This rapid cooling will also have the effect of hardening the case. Unfortunately the case will have a coarse grain since it was heated to above its correct hardening temperature.

• To refine the grain of the case and reharden it, the component is heated to 760◦C, and quenched in water. This is the correct hardening procedure for a 1.0% carbon steel which is what the surface of the component has become. This temperature is too low to affect the fine grain of the core.

• Finally the component can be tempered if required.

Localized case hardening

It is often undesirable to case harden a component all over. For example, it is undesirable to case harden screw threads. Not only would they be extremely brittle, but any distortion occurring during carburizing and hardening could be corrected only by expensive thread grinding operations.

Various means are available for avoiding local infusion of carbon during the carburizing process.

• Heavily copper plating those areas to be left soft. The layer of copper prevents intimate contact between the component and the carburizing medium. Copper plating cannot be used for salt-bath carburizing as the molten salts dissolve the copper.

• Encasing the areas to be left soft in fire-clay. This technique is mostly used when pack carburizing. Leaving surplus metal where a soft area is required. This is machined off between carburizing and hardening (dipping off). Although more expensive because of the extra handling involved, it is the most sure and effective way of leaving local soft features.