Flame Hardening – Process, Diagram, Advantages, Application

Introduction to Flame-Hardening:
It is a surface-hardening process in which a thin surface section of a component of hardenable steel is heated rapidly by direct application of high tempe­rature flame to a temperature above its upper critical temperature (i.e., it is austenitized), and then the component is quickly quenched in water or oil, transforming austenite to mar­tensite, while the core remains in an original soft state, normally consisting of ferrite-pearlite structure.

The high-temperature flame is obtained by the combustion of a fuel gas with oxygen or air. There is no change in composi­tion of the steel and therefore, the flame- hardened steel (as in induction-hardening) must have adequate carbon content for the desired surface hardness (i.e., must be hardenable steel). The hardened depth could be between 0.8 to 6 mm, or even more, depen­ding on the fuel gas, air or oxygen, design of the flame head, duration of heating, the hardenability of steel, quenching medium, and method of quenching.

Methods of Flame-Hardening:

The method of flame-hardening depends on the shape, size, composition of the component, area to be hardened, depth of hardened-case desired, number of components to be hardened.

The methods are:

1. Manual Hardening:

In this method, the local heating of the selected area is done by hand with a welding torch, or a suitable flame head to the hardening temperature, and is then quenched in water or oil. The flame head could be either a single orifice or multiple orifices depending on the area to head, the uniform temperature can be ensured over the entire surface to be hardened A spray quench may be used for hardening.

2. Spinning Method:

This method is suitable for components having rotational symmetry-round or even semi-round components such as wheels, cams, and gears.

3. Progressive Hardening:

This method is used to harden large areas which cannot be easily hardened by manual method.

4. Progressive Spin Hardening:

This method is commonly used for long parts such as shafts and rolls. Here progressive hardening is combined with spin hardening (Fig. ) i.e., the component rotates as well as moves axially. The rotation speed is around 3-12″/minute), but the flame heads traverse the roll or shaft from one end to the other. The speed of rotation and axial motion is important for getting a uniform hardened layer.

flame hardneing
flame hardening

The rotation of the component throws water tangentially, and the higher speeds may interfere with the heating up process. But at low speeds, soft spiral bands may form. After progressive heating, quench jets cool the heated section.

Flame Hardening Process Woking : 

The surface to be case hardened is heated by means of an oxyacetylene torch for sufficient time and Quenching is achieved by sprays of water which are integrally connected with the heating device. The heating is generally accomplished for sufficient time so as to raise the temperature of the surface of the specimen above the critical temperature. As the temperature desired is achieved immediately, spraying of water is started. In mass production work, progressive surface hardening is carried out where it is arranged to have the flame in progress along with quenching.

Flame Hardening Diagram : 

flame hardening Diagram
flame hardening Diagram

Operating Variables for Flame-Hardening:

The skill of the operator is the main factor in the success of the flame hardening.

The other main operating variables to be skillfully controlled are:

(i) The distance between the inner cone of the flame and the surface of the component.

(ii) The speed of travel of the flame-head or the component.

(iii) Oxygen to fuel gas ratio in the mixture.

(iv) Flame velocities.

(v) Type, and angle of the quench.

Applications of Flame-Hardening:

Flame-hardening is used because:

1. Parts are very large, making conventional heating and quenching impracticable, or uneconomical, such as large gears, rolls, etc.

2. Only a small region of the component needs heat treatment, or if the whole surface is heated, it may be detrimental to the function of the part, such as wearing surfaces of cams.

3. Exact dimensions or its control is impracticable or difficult by normal through hardening. For example, flame hardening of teeth of very large gears shall not disturb the dimensions of gears.

4. Cheaper steels could be used if flame hardened. Also, the parts are hardened with little oxidation and decarburization.

 Advantages and Disadvantages of Flame-Hardening:

Advantages Of Flame Hardening : 

1. The process is simple.

2. A large number of steels could be flame-hardened.

3. Flame hardening can be done of any shape, like flat, circular, irregular shapes, etc.

4. Very large forgings and castings can be flame hardened where other methods, even induction hardening fails or are uneconomical.

5. Mass distortion is absent.

6. The equipment cost is much less than for induction hardening.

7. It is faster than carburizing or nitriding etc.

8. The flame could be directed to harden sections deep inside components.

9. A wider depth of hardening can be obtained.

10. It is cheaper if only a few parts are to be surface hardened as compared to induction hardening.

11. Selective hardening is easily obtained.

12. Cheaper steels could be used.

Disadvantages of Flame Hardening :

1. It is difficult to control exactly the temperature of the heating of the component.

2. Overheating of surface layers may take place.

3. As coarse martensite (due to overheating) may be present, the quality of the case is relatively poor.

4. The cost per piece on a mass scale production is higher than in the induction hardening.

5. It is difficult to adjust the case depth exactly.

6. Explosive fuel gases have to be used cautiously.

7. Some oxidation or decarburization may occur as compared to induction hardening.

Difference between Flame Hardening and Induction hardening : 

Sr. no.Flame HardeningInduction Hardening
1.Material is heated with oxyacetylene flame at a required temperature, and then it is followed by water spraying.Material is heated by using high frequency induced current and then it is followed by water spraying.
2.Low equipment and maintenance cost.High equipment and maintenance cost
3.Holding time is required.Due to very fast heating, no holding time is required.
4.Oxidation & decarburization is minimum.No scaling & decarburization.
5.Overheating can damage parts.Damage of the overheating of metal can be avoided.
6.Irregular shape parts can be flame hardened.Irregular shape parts are not suitable for induction hardening.
7.Skilled labor is requiredIt can be carried out with unskilled labor
8.Flame hardening requires more care in the control of temperature.Easy control of temperature by control of the frequency of supply voltage.
9.Applications:-Large gear shafts, lathe ways, spline shaft etcApplications:-Piston rods, cams, shafts etc
• distance between flame & workpiece.
• gas pressure,
• flame or work travel rate,
• type, volume, and application of quench.
• any shaped parts are suitable for
• induced voltage
• flow of current
• resistance offered by work
• shape and design of coil &
• rate of heating.
• irregular shaped parts are

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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