Probably the biggest increase in the use of heat-treatment furnaces in the last year or two has been for light alloys for the aircraft industry, but a close runner-up is found in furnaces for tool-room work. The stringent demands on tools of all descriptions, due to the materials on which they are used, has inevitably led to improved methods of heat-treatment. For whilst the average works executive in the past has been only too pleased to install the most modern machine tools, the equally important equipment in the hardening shop has been cold shouldered and, wherever possible, forgotten.
Just as one cannot produce accurately-machined components without proper machines and tools, so it is asking the impossible to make tools without proper heat-treatment facilities, and electric furnaces are therefore being increasingly used. A brief description of some of the more usual small equipments may be of interest to those who have now reached the more enlightened stage, but who are not fully aware of the variety of types available for the various purposes.
For tool-room work three broad divisions may be made. There is first of all the group of furnaces for temperatures up to 900 deg.-1000 deg. C., which provides the means for normalising and hardening carbon and the alloy steels with relatively low percentages of chromium, tungsten, nickel, etc.
This same group is also used for pre-heating high-speed steels, the hardening of which is carried out in furnaces belonging to the second group, which comprises equipments to operate up to 1350 deg. C.
The third group covers low temperature furnaces, with 700 deg. C. as their maximum operating temperature, and these are used for straight tempering from 200 deg. C. to 400 deg. C, and for secondary hardening of high-speed steel, usually in the range 580 deg. to 620 deg. C.
Each group, of course, may include several different types of furnace, the selection of any particular type being dependent upon the nature of the work to be handled, the volume of work, and the amount of capital allocated for the installation. The first group may consist of furnaces of the “wound muffle " construction, which, whilst not ideal for prolonged use at the top temperatures, and limited also to a certain extent in size, have the outstanding advantage of being low in cost, and are also reasonable in running costs. Their main field is that where the work is all of small dimensions, and where the amount of work does not justify either a more expensive type or the maintenance of the furnace at or about the working temperature throughout the day.
In such cases the ability to heat the furnace up in an hour or so after a tool has been made, and that also at low current consumption, is of utmost value to the smaller firms where tool production does not provide a constant stream of work.
For larger tools, and where greater quantities have to be catered for, the more robust heavy-duty type of electrical furnace is to be preferred. Furthermore, being extremely well insulated, although this type takes longer to heat up, it can be maintained at an even temperature all day at low cost. Automatic temperature control is usually provided, and atmosphere control to maintain a suitable atmosphere in the chamber is generally included. It will be observed that the exterior construction lends itself to cleanliness which is of importance when the furnace is installed where it should be—in the tool-room.
For high-speed steel ranges the most usual form of hardening furnace incorporates carborundum heating elements. Again, for small tools a compact twin-chamber equipment forms an ideal unit, whilst for large tool-rooms, where both size and quantity of tools are greater, two independent furnaces, are used. The one furnace is used for pre-heating, and is the same as that described above, whilst the other is similar but with carborundum elements. The small twin-chamber equipment is usually provided with hand control on both chambers, and as the operator must be present during the hardening, this arrangement offers no disadvantage, but is considerably cheaper than automatic control. Both chambers are mounted on one stand, and the floor space occupied is thereby reduced.
Another type of furnace to be recommended where quantities and sizes of tools are large is the electrode salt bath. With this type of equipment, complete absence of scaling or decarburisation is ensured, the work being heated in salt, which itself is heated by the passage of electric current from the electrodes. The cost of this type, however, precludes its use unless a full day's work can be treated when it has been heated up, or the tools are such that the slightest surface blemish causes them to be scrapped. Numerous instances can be cited, however, where the remarkable results obtained on expensive tools have fully justified the relatively high cost of the installation.
The final group is one covering the treatments which are often regarded as unimportant, but which are really of importance equal to that of the hardening process. The maintenance of a uniform temperature for tempering is imperative, and the failure to realise this is often the cause of much wastage of expensive tools.
Hitherto, oil, salt, and lead baths have been widely used, but since the introduction of forced air circulation, giving the same rapidity of heating as liquid baths, but at far lower cost, their use has gradually diminished.
Made in a number of standard sizes, some of which have been designed specifically for tool-room work, these furnaces provide an extremely uniform temperature without the inherent disadvantages of liquid baths. There is no danger of rusting as may easily occur where salt is used, and the work requires no special cleaning after treatment. Furthermore, as there is only a very low thermal capacity, the temperature may be raised or lowered rapidly, and having a temperature range up to 650 deg. C, this type scores heavily over liquid baths.
A typical electrical furnace is one in which the whole equipment is self-contained, the switchgear, temperature controller and pyrometer being mounted on the furnace framework. This particular furnace has a useful work basket space of 10in. diameter by 10 in. deep, and will therefore accommodate all the tools normally found in the average tool-room. Sizes are also available for quantity production of tools.