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rinsing is necessary. Cold water rinsing is recommended. Parts should be kept wet 

between stages, and delays before subsequent processing should be kept to a 

minimum. Alkaline Cleaning is the least expensive method and it also can be used in 

cleaning all cutting fluids [20]. 

 

2.3 Surface hardening methods 

Surface hardening is used to improve the versatility of certain steels by producing 

combinations of properties not readily attainable in other ways. For many applications, 

wear and the most severe stresses act only on the surface of a part. So surface 

hardening is the process to improve the wear resistance of parts without affecting the 

softer, tough interior of the part. Therefore, the part maybe fabricated from a low- or 

medium-carbon steel, and is surface hardened by a final heat treatment after all other 

processing has been accomplished. This combination of hard surface and resistance to 

breakage upon impact is useful in parts which must have a very hard surface to resist 

wear, and a tough interior to resist the impact as well. [1] Further, the surface 

hardening of steel has an advantage over through hardening because surface 

hardening also reduces distortion and eliminates cracking which happens through 

hardening, especially in large section. There are two distinctly different ways to the 

methods for surface hardening:   

 



 

Methods that involve an intentional buildup or addition of a new layer   



 

Methods that involve surface and subsurface modification without any intentional 

buildup or increase in part dimensions [2] 

 

The surface hardening method used in this study focuses on the second set of methods, 

which is divided into diffusion methods and selective hardening methods. These are 

two major approaches to surface hardening techniques. First approach changes 

surface composition and includes the applications of such techniques as carburizing, 

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nitriding, and carbonitriding. The second one does not change composition and 

consists of hardening the surface by flame or induction heating. 

 

The selective hardening of steel surfaces is typically achieved by localized heating 

and quenching, without any chemical modification of the surface. However, selective 

surface hardening can also include chemical modification by such techniques as ion 

implantation and selective carburization. The more common methods currently used 

to harden the surface of steels include flame and induction hardening. However, each 

of these methods has shortcomings that can prevent its use in some applications. For 

example, the disadvantage of flame hardening include the possibility of part distortion, 

while induction hardening requires very small part-to-coil distances, which must be 

precisely maintained. [2] 

 

Surface hardening by diffusion will modify the chemical composition of the surface 

with several variations of hardening species like carbon, nitrogen, or boron. The 

diffusion methods allow effective hardening of the entire surface of a part and are 

generally used when large numbers of parts are to be surface hardened. The basic 

process is thermochemical because some heat is needed to enhance the diffusion into 

the surface of the part. The depth of diffusion exhibits time-temperature dependence 

as shown in eqn. (1). 

 

Time

K

 

 

 



depth

case

 (1)

 

where the diffusivity constant, K, depends on temperature, the chemical composition 

of the steel, and the concentration gradient of a given hardening species. 

Concentration gradients depend on the surface kinetics and reactions of a particular 

process. The diffusivity constant increases as a function of absolute temperature. [21]   

 

Diffusion methods include different hardening species and process methods which are 

used to transport the hardening species to the surface of the part. These processes 

variations generate differences in case depth and hardness as shown in Table 1.    It is 

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also important to discriminate between the total case depth and the effective case 

depth. The effective case depth is usually about two-thirds to three-fourths of the total 

case depth. [22] 

 

Table 1 Typical characteristic of diffusion treatments [22] 

Process 

Process 

temperature (C) 

Typical case depth 

Case hardness 

(HRC) 

Carburizing 

Gas 815-980 

75um-1.5mm 

50-63 

Liquid 815-981 

50um-1.5mm 

50-65 

Vacuum  

815-1090 

75um-1.6mm 

50-63 

Nitriding 

Gas 480-590 

125um-0.75mm 

50-70 

Salt 510-565 

2.5um-0.75mm 

50-70 

Ion 340-565 

75um-0.75mm 

50-70 

Carbonitriding 

Gas 760-870 

75um-0.75mm 

50-65 

Liquid 760-871 

2.5-125um 

50-65 

Ferrite nitrocarburizing 

565-675 

2.5-25um 

40-60 

 

 

Nitriding 

 

Nitriding is a surface hardening heat treatment in which the nitrogen content of the 

surface of the steel is increased by exposure to an appropriate atmosphere at a 

temperature in the ferrite phase field. Hardening can be accomplished with a 

minimum of distortion and excellent dimensional control, because nitriding does not 

involve heating into the austenite phase field and a subsequent quench to form 

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