Case hardening

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WHAT IS CASE HARDENING?

The heat-treatment processes described thus far involve microstructural alterations and property changes in the bulk of the material or component by means of through hardening.

It is not desirable to through harden parts, because a hard part comes up short on vital toughness for these applications; a little surface break could propagate quickly through such a part and cause absolute failure.

In many cases, however, alteration of only the surface properties of a part (hence, the term surface or case hardening) is desirable.

This method is particularly useful for improving resistance to surface indentation, fatigue, and wear.

Typical applications for case hardening are gear teeth, cams, shafts, bearings, fasteners, pins, automotive clutch plates, tools, and dies.


Several case-hardening processes are available:

Carburizing (gas, liquid, and pack carburizing).

Carbonitriding.

Cyaniding.

Nitriding.

Boronizing.

Flame hardening.

OUTLINE OF HEAT TREATMENT PROCESSES FOR SURFACE HARDENING:

1.PROCESS CARBURIZING:

Metal hardened: Low carbon steel (0.2% C) there is alloy steel (0.08-0.2%C)

Element added to surface: C

PROCEDURE:

Heat steel at 870-950°C( 1600-1750°F)  in the atmosphere of carbonaceou gases (gas carburizing) or carbon containing solids
(pack carburizing).

GENERAL CHARACTERISTICS:

A hard, high carbon surface is produced.
Hardness 55 to 65HRC. Case depth <0.5 to 1.5 mm (< 0.020to 0.060 in.). Some
distortion of part during heat treatment.

APPLICATIONS:

This is used in gears, cams, shafts, bearings, piston pins, sprockets, clutch plates.

2. PROCESS CARBONITRIDING :

Metal hardened:

Low carbon steel is metal hardened.

Element added to surface are:

C & N are added to surface.

PROCEDURE:

Firstly steel is heated at 700-800°C (1300-1600°F) in an atmosphere of
carbonaceous gas and ammonia. Then after this it is quench in oil.

GENERAL CHARACTERISTICS:

Surface hardness 55 to 62 HRC. Case depth 0.07 to 0.5 mm (0.003to 0.020 in.). Less distortion than in carburizing.

APPLICATION:

It generally used in the bolts , nuts , gears.

3. PROCESS CYANIDING :

Metal hardened:

Low carbon steel (0.2% C) there is alloy steel (0.08-0.2%C).

Element added to surface :

Here also C & N are added to surface.

PROCEDURE:

Heat steel at 760-845°C ( 1400-1550°F)  in a molten bath of solutions of cyanide (e.g., 30%sodium cyanide) and other salts.

GENERAL CHARACTERISTICS:

Surface hardness up to 65 HRC. Case depth 0.025 to 0.25 mm (0.001 to 0.010 in.). Some distortion.

APPLICATION:

It is used in bolts, nuts, screws and small gears.

4. PROCESS NITRIDING :

Metal hardened:

Steel ( 1% Al, 1.5% Cr, 0.3% Mo) alloy steel, stainless steel and high speed tool steels are there.

Elements added to surface:

In this only N is added.

PROCEDURE :

The steel is heated at 500-600°C (925-1100°F) in an atmosphere of ammonia gas or mixtures of molten cyanide salts. There is no further treatment.

GENERAL CHARACTERISTICS:

Surface hardness up to1100 HV. Case depth 0.1 to 0.6 mm (0.005 to 0.030 in.) and 0.02 to 0.07 mm (0.001 to 0.003 in.) for high speed steel.

APPLICATION:

It is useful in the gears, shafts, sprockets, valves, cutters, boring bars, fuel-injection pump parts

5. PROCESS BORONIZING :

Metal hardened: Here is only steel.

Elements added to surface is:

Here only B element is added.

PROCEDURE:

In this the part is heated using boron containing gas or solid in contact with part.

GENERAL CHARACTERISTICS:

This is extremely hard and wear resistant surface. Case depth 0.025 to 0.075 mm (0.001 to 0.003 in.).

APPLICATION:

This is only used for tool and die steels.

6.  PROCESS FLAME HARDENING :

Metal hardened:

Medium carbon steels cast irons.

Elements added to surface:

None of element is added to this surface.

PROCEDURE:

In this firstly the surface is heated with the help of oxyacetylene torch, and then quenched with water spray other quenching methods.

GENERAL CHARACTERISTICS:

Surface hardness 50 to 60 HRC. Case depth 0.7 to 6 mm (0.030 to 0.25 in.). Little distortion.

APPLICATION OF PROCESS FLAME HARDENING:

Mostly it is used in gear and sprocket teeth, axles, crankshafts, piston rods, lathe bed sand centers.

Basically, these are operations in which the component is heated in an atmosphere containing elements (such as carbon, nitrogen, or boron) that alter the composition, microstructure, and properties of surfaces.

For steels with sufficiently high carbon content, surface hardening takes place without using any of these additional elements.

Only the heat treatment processes described are needed to alter the microstructures, usually by flame hardening process.

Laser beams and electron beams are used effectively to harden both small and large surfaces, such as gears, valves, punches, and locomotive cylinders.

These methods are also used for thorough hardening of relatively small parts.

The main advantages of laser surface hardening are close control of power
input, low distortion, and the ability to reach areas that would be inaccessible by other means.

Capital costs can be high, however, and the depth of the case-hardened layer is usually less than 2.5 mm (0.1 inch).

Because case hardening is a localized heat treatment, case-hardened parts have a hardness gradient.

Typically, the hardness is maximum at the surface and decreases below the surface, with a rate of decrease that depends on the composition of the metal and the process variables. Surface hardening techniques can also be used for tempering, to modify the properties of surfaces that have
been subjected to heat treatment.

Various other processes and techniques for surface hardening, such as shot peening and surface rolling, improve wear resistance and other characteristics.

ADVANTAGES:

Mostly it is used to creating a more durable product.

Case hardening increases wear resistance of the metal.

This also increases lifetime of objects.

The metal is more flexible.

Steel is tougher and stronger.

DISADVANTAGES:

Austenitic stainless steels, like 316 and 304, are known to offer the benefits of corrosion resistance.

However, disadvantages include a limited wear resistance and increased risk of galling.

APPLICATIONS:

Generally it used for the shackle of a lock, where the outer layer is hardened to be file resistant.

Mechanical gears, where hard gear mesh surfaces are needed to maintain a long service life while toughness is required to maintain durability and resistance to catastrophic failure.

CONCLUSION:

We have covered all the important concepts related to case hardening process. Hope you all are crystal clear with understanding all the concepts mentioned here. If you have any questions please use the comments section to get in touch with us. Till then have fun and always keep reading!

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