Flame hardening is a method derived from the old quench hardening method and is used for surface hardening of heat treatable steel.
The name of flame hardening is similar to flame cutting because the use of flame is a distinguishing feature of the process compared to the use of a furnace.
The characteristics of flame hardening are as follows:
In the flame hardening process, the area to be hardened should use a high heat capacity burner to provide a mixture of fuel gas and oxygen.
Therefore, the surface hardening temperature is reached in a short time and a heat jam occurs, that is, more heat is supplied to the surface than the heat that is dissipated into the workpiece.
Because cooling occurs immediately after heating, it prevents heat from penetrating deeper into the wear prone outer layer and hence hardens.
The core of the workpiece is not affected by this heat treatment, while other hardening methods are that the entire workpiece is fully heated in the furnace.
The material must already contain the hardenable ingredient, because the heating time is too short to carburize it.
The geometry of the surface to be hardened must be such that they can be heated by a burner.
From an operational point of view, it provides many advantages.
The burner can be used immediately, without prolonged heating or idle.
The hardening is very fast, and the labor and fuel costs are low.
The distortion is very small because the heat treatment only affects the portion of the surface which is subjected to wear
COMPARISON OF FLAME HARDENING AND OTHER SURFACE HARDENING METHODS
Heat treatment of case hardening:
It requires heating the entire workpiece in a furnace.
Therefore, in the subsequent quenching process, changes will also occur in the grain structure of the workpiece.
In flame hardening, only the outer layer is heated where wear occurs.
Therefore, it greatly reduces heating time and fuel consumption.
For heavy work, tempering is recommended.
If the hardness is limited to the surface in the case of hardening, only steel with a carbon content of not more than 0.25 will not harden or to a small extent can be used.
However, the heat-treatable steel used in flame hardening already contains the composition required for hardening.
The time consuming carburizing process is eliminated.
Due to the high capacity of the burner head, the heating of the burner head always takes only few seconds.
For case hardening the parts that remain soft, they must be protected from the penetration of carbon.
In some cases, excess material will remain in these areas and then be removed after carburization but before quenching.
This requires expense of a second heating, if a uniform hardness is to be obtained, such preparations require time, great care and experience.
All of these are not necessary for flame hardening, as long as the burner can be made to exactly match the shape of the surface to be hardened, so that only the surface can be heated.
Due to the design of the burner, flame hardening is limited to objects of uniform shape.
Case hardening is much less affected by the shape of the workpiece.
For bulky workpieces, case hardening requires a sufficiently large oven, and it is usually very uneconomical to operate, so processing is usually eliminated.
With the use of case quenching, even the largest workpieces can be hardened in a relatively simple way.
With case hardening considerable stress will be generated in a large section.
The cooling rate of the outer layer is much faster than that of the core due to its rigidity, it can prevent the core from completely shrinking, there by generating considerable tensile stress that can exceed the ultimate strength of the material.
The result is a weak spot hardening cracks and fractures or distortions, must be corrected by subsequent straightening or can sometimes be leveled by grinding.
Sometimes doing this will remove the hardened case.
Due to the different composition of the core and the shell, considerable stress will be generated between them, which will cause the hard case to crack.
All these difficulties can be avoided by flame hardening, because the core is not affected by heat treatment and the chemical composition of the case and core is the same.
TECHNIQUE OF THE FLAME HARDENING
In flame hardening one can proceed in such a way that first the total surface to be hardened is brought up to hardening temperature and then quenched.
This is called complete hardening .
The work piece is heated immediately after the burner’s head.
This is called progressive hardening because the work piece is gradually heated along the line.
1. Total surface hardening by the oscillating method.
The small flat or curved surface is heated by a welding torch or the contour burner correctly by vibrating the flame forward until the uniform hardening temperature is reached, and then quenched.
A mechanically operated vibration movement is preferred for larger amounts.
A small cross-section can be recommended and shallow dies when the simultaneous use of two burners.
2. Total surface hardening by the spot or stationary method.
For small areas the burner needs to be stationary without oscillation.
This is called stationary or spot hardening.
This method is also successful when irregular forms of pieces of work excluded all other methods.
Simple fixtures to maintain torches and burners, scissors, shears, clippers, combined pliers and operators to keep the parts in their hand in front of the burner, facilitate fair work.
In addition, if the rest is provided for workpieces, you can easily conserve the hardening rocker arm with an appropriate distance from the burner.
For a large amount, automation is extensively used.
3. Total surface hardening by the spin hardening.
This method is very suitable for short bearing journals on shafts, pins, trunions and axles.
First, the journal is heated with a burner that has a flame length corresponding to the length of the journal while workpiece is rotating until the hardening temperature is reached.
The burner remains close to the workpiece, so the flame collides at right angles to the surface, so the core of the flame is close enough to the surface without tune.
4. Progressive slip hardening.
When hardening round surfaces of larger diameters by the progressive method all around a soft slip results at the point where hardening was started and ended as the already hardened section is tempered by the
heat dissipating ahead of the flame.
Additional burner fails can be used to minimize the width of the slip.
This flame must clean the water that is easily achieved with the appropriate cooling design.
Special attention is required with the materials that tend to become brittle when tempered.
If you need a deeper case, you must use a preheating burner.
Preheating and hardening burners are interlocked so that only the first preheater burner starts.
Flame hardening is well suited for surface hardening of machine parts subjected to local wear as shafts, trunions, pins, gears, track wheels, guide rolls and similar parts.
For tools it can only be used if they are made of machine steel whereas for tool steel it is only limited to exceptional cases.
High speed steels, requiring long heating times for the transformation of the grain structure prevent the use of surface heating as the special advantages of flame hardening become useless.
In spite of the advantages of flame hardening the older methods will remain in use for special fields of application.
It is difficult to control exactly the temperature of the heating of the component.
Overheating of surface layers may take place.
As coarse martensite (due to overheating) may be present, the quality of the case is relatively poor.
The cost per piece on a mass scale production is higher than in the induction hardening.