HEATING AND FORGING

.Name the fuels used in hand forging

. qualities of a good forge fire

. the correct method of placing stock in fire

For heating metals to be forged, different types of fuels are used. Fuels are combustible materials capable of generating heat energy.

Forge furnaces are fired with petroleum gas, producer gas, oil, coal and charcoal.

For hand forging, the hearths are heated with soild fuels like coal or charcoal with low sulphur content.

For heating tools steels charcoal is preferred as the fire burns more evenly when charcoal is used.

Heating and metal stock

For efficient heating, the way in which the fire is maintained is important.

Thr fire which is maintained on the top surface will be thin and not suitable for heating the metal to carry out forging.

The fire should come from deep in the hearth.

The forge fire should be maintained clean i.e, free from ash, clinkers and dust.

Clinkers is formed as the coke burns. This settles down at the bottom of the hearth and solidifies. Accumulation of clinker will obstruct the tuyere nozzle.

Placing the stock

If the stock is kept on the top of the fire, it will be heated underneath and the top surface will not get heated sufficiently as it is exposed to the atmosphere, and hence gets cooled.

If the work is placed too low in the fire and very near to the tuyere, the cold air may cool one side of the work. This will also lead to oxidation of the steel surface.

The correct position of the stock in the forge fire will be as follows.

The stock is placed away from the tuyere ensuring that the cold air does not fall directly on the stock.

The top surface of the work shoul have a thick layer of fuel.

The work should be covered on all sides with the fuel coal charcol etc.

Forging temparatures

While forging the metal should be heated to temparature when it becomes plastic. The forging temparature for steel depends on its carbon content and the alloying elements.

Heating temparature of steel

The heating temparature for steel are

-low carbon steel 1300°c

-high carbon steel 1100°c

-low alloy steel 1100°c

(Only steel with carbon content up to 1.7% is forgeable )

If the forging is carried out at very low temaprature, the work may develop cracks or will be work hardened. If heated excessively, it will lead to scaling on the outer layers.

A work to be forged may require several heating to complete the forging. But it is always better however to complete the work with the minimum number of heatings.

FORGE AND ITS ACCESSORIES

. function of a forge

. the parts of a forge and state their functions

. identify the forge tools and state their uses

The forge supplies heat for raising the temparature of the metal to be forged.

Parts of a forge and their uses

Hearth

The hearth is the main part where the heating of the metal takes place. It is prepared with refractory (fire) bricks and clay.

Tuyers

The forced air is sent through the tuyers to burn the fuel. The tuyer covered with refractory materials to avoid melting. The air also passes through a cooling tank.

Cooling tank

To keep the tuyer cool always, the cooling tank is filled with cold water.

Air valve

The air valve is used to regulate the supply of air to the hearth

Blower

The blower supplies air to the forge for burning the  coal or coke. The blowers are operated manually or by electric Power.

The blower is fixed independently to each forge or it supplies ait to a number of forges which are controlled by independent air valves.

Hood and chimney

Smoke and  dust are let out through the hood and the chimney. The fumes given off by a coal forge are toxic therefore, good ventilation is essential

Quenching tank

A quenching tank is provided along with the forge. It is filled with water for cooling the hit tools and workpieces. The tank is also used for hardening and Tempering

Coal tank

The coal tank is used for the storing of fuel for the forge. Generally solid fuels are used i.e coal, coke and charkol. Of these, steam coal with low sulphur is the best fuel for the forge. Hence it is known as Smith's fuel.

Forge tools

The SHOVEL is used to shovel coal and to clean the hearth. The RAKE AND POKER are used to remove clinkers from the fire, and to draw fresh fuel over the nozzle.

The SPRINKLER is used to localize the heat. The outer ring of the fire is kept wet so that the heat is concertrated in the middle of the hearth.

HEAT TREATMENT OF HIGH SPEED STEEL

 The necessary of preheating high speed steel

The hardening process of high speed steel

High speed steels get their name from the fact that they may be operated as cutting tools at much higher speeds than is possible with plain carbon tool steels. Since the maximum hardness of high speed steels is  obtained on tempering at high temparatures, they can be operated as cutting tools in the same temparature range without loss of hardness. That is the rise in temaprature due to friction will not reduce the temper of the tool pointas it cuts into the steel. Tool steels are often annealed for softening before machining or forming, and also for obtaining grain refinement.

Annealing

Soak at 900° C for about four hours. Then cool slowly at not more than 20°C per hour to 600°C. It may then be cooled to room temparature in still air.

Hardening

Pre -heat to 850°C slowly to prevent cracking. Then heat rapidly to approximately 1250°C depending upon the alloy used. This rapid heating reduces grain growth. Quench in air blast or oil depending on the size of the component. Because of the high temparature involved, oxidation of the steel can be severe. This can be minimised in a double chamber muffle by using the excess fuel to give a carburising atmosphere.

However, this reduces the combustion efficiency, and there may be some difficulty in reaching salts available, modern practice favours the use of the salt bath furnace.

Secondary hardening 

This is sometimes called tempering However, this term is not strictly true. Not only does secondary hardening increase the toughness of the steel, it also increase the hardness whereas tempering increases the toughness at the expense of hardness. The effect of secondary hardening helps these steels to work effectively at very much higher temparature than plain carbon steel.

Heat treatment of non ferrous metals

Annealing

Like steel non ferrous metals can be softened by heating and allowing to cool. However since they cannot be hardened by rapid cooling, the rate of cooling is comparatively unimportant. In fact copper components are often quenched in water. This not only saves time but also cleans the black oxide film from the surface of the components because of the rapid contraction.

Non ferrous metal annealing temparatures

Metal                              temaprature ° C

Cold working brasses          600-650

Copper                                    650-700

Aluminum                             500-550

Duralumin                            480-500

Hardening

Only a very few non ferrous alloys can be hardened by heating and quenching, like steel and most non ferrous metals are hardened by cold working. As anyone who has tried coppersmithing will konw the metal starts to work harden as it beaten to shape and it must be annealed from time to time to prevent it from cracking.