NORMALISING

Also used to soften and relieve internal stresses after cold work and to refine the grain size and m etallurgical structure. It may be used t o break up the dendritic (as cast) str ucture of castings to improve their m achinability and future heat treatment response or to mitigate banding in rolled steel. This requires heatin g to above the As temperature, holding for sufficient time to allow tem perature equalization followed by air co oling. It is therefore similar to annealing but with a faster cooling rate. Curve 3 in Figure I would give a normalized structure.

THE HARDENING PRO CESSES

Hardening

In this process steels which contain sufficient carbon, and perhaps other alloying elements, are cooled (quenc hed) sufficiently rapidly from above the transformation temperature to produce Martensite, the hard phase already described, s ee Curve 1 in Figure 1.There is a range of quenching media of varying severity, water o r brine being the most severe, through oil and synthetic products to air which is the least severe.

 Tempering

After quenching the steel is hard, brittle and internally stre ssed. Before use, it is usually necessar y to reduce these stresses and increa se toughness by 'tempering'. There will als o be a reduction in hardness and the selection of tempering temperature dictates The final properties. Tempering curves, which are plots of hardness against tempering temperature. exist for all commercial steels and are used to select the correct tempering temperature. As a rule of thumb, within the tempering range for a particular steel, the higher the tempering temperature the lower the final hardness but the greater the toughness. It should be noted that not all steels will respond to all heat treatment processes, Table 1 summaries the response, or otherwise, to the different processes.

Boronised substrates will often require heat treatment to restore mechanical properties. As borides degrade in atmospheres which contain oxygen, even when combined as CO or C02, they must be heat treated in vacuum, nitrogen or nitrogen/hydrogen atmospheres.

 PROCESSING METHODS

In the past the thermochemical processes were carried out by pack cementation or salt bath processes. These are now largely replaced, on product quality and environmental grounds, by gas and plasma techniques. The exception is boronising, for which a safe production scale gaseous route has yet to be developed and pack cementation is likely to remain the only viable route for the for some time to come.

The gas processes are usually carried out in the now almost universal seal quench furnace, and any subsequent heat treatment is readily carried out immediately without taking the work out of the furnace. This reduced handling is a cost and quality benefit.

TECHNIQUES AND PRACTICE

 As we have already seen this requires heating to above the As temperature, holding to equalise the temperature and then slow cooling. If this is done in air there is a real risk of damage to the part by decarburisation and of course oxidation. It is increasingly common to avoid this by ‗bright„ or ‗close„ annealing using protective atmospheres. The particular atmosphere chosen will depend upon the type of steel.

 NORMALISING

In common with annealing there is a risk of surface degradation but as air cooling is common practice this process is most often used as an intermediate stage to be followed by machining, acid pickling or cold working to restore surface integrity.

 HARDENING

With many components, hardening is virtually the final process and great care must taken to protect the surface from degradation and decarburisation. The ‗seal quench„ furnace is now an industry standard tool for carbon, low and medium alloy steels. The work is protected at each stage by a specially generated atmosphere.

Some tool steels benefit from vacuum hardening and tempering; salt baths were widely used but are now losing favour on environmental grounds.