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37

 

Alkaline cleaning 

Alkaline cleaning was selected to remove the oil contamination from the sample. The 

alkaline cleaner was diluted in hot water which temperature was about 80°C, in which 

5 vol% cleaner was used. The samples were dipped in the solution for a few minutes 

at room temperature, rinsed in distilled water twice for several minutes each time, 

dried and kept in cool and dry environment. Visually, the surfaces of the sample were 

shiny after cleaning. 

 

Nitriding Experiments 

After cleaning, the samples were placed in the nitriding furnace at the Bodycote [12] 

facility in Worcester. The reaction temperature has an effect on the ammonia 

dissociation and diffusion rates and reaction progress is determined by the residence 

time. The furnace was flushed with nitrogen and the reaction temperature was 

established.. Furnace residence time started when ammonia was introduced into the 

system.  

 

The nitriding process done by Bodycote is double-stage process. In the first stage of a 

temperature of 526°C (980°F) is used, and the dissociation rate ranges from 24 to 

28%. This process produces a brittle, nitrogen-rich layer known as the white nitride 

layer at the surface of the nitrided case. The second stage proceeds at the nitriding 

temperature which may be increased to 549°C (1020°F) and the dissociation rate in 

the second stage is increased to 78 to 80%. External ammonia dissociation is 

necessary for obtaining the required higher second-stage dissociation. The principal 

purpose of double-stage nitriding is to reduce the depth of the white layer produced on 

the surface of the case. 

 

 

 

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Measurements 

Weight test 

The weight of samples was tracked during these experiments. The weight of samples 

before and after rust process has been recorded and subtracted by original weight to 

obtain the rust gain during rust forming step by using the lab balance, Mettler H54AR.   

 

Hardness test 

Surface hardness was measured with Wilson Rockwell Hardness Tester.

 

The Rockwell 

scale characterizes the indentation hardness of steel through the depth of penetration 

of an indenter, loaded on a sample and compared to the penetration in some reference 

material. The HR

c

 was measured on each sample for 5 times.   

 

Microstructure

 

Surface composition and layer thickness after nitriding was analyzed qualitatively and 

quantitatively by using an optical microstructure and scanning electron microscope 

(SEM). The SEM signals result from interactions of the electron beam with atoms at 

or near the surface of the sample. SEM micrographs have a very large depth of field 

yielding a characteristic three-dimensional appearance that is useful for understanding 

the surface structure of a sample. A wide range of magnifications is possible, ranging 

from about 25X (about equivalent to that of a powerful hand-lens) to about 250,000X, 

approximately 250 times the magnification limit of the best light microscopes.   

 

 

 

 

 

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39

3.3 Results and Discussion

 

Flux of nitrogen vs. contaminants   

The total flux of nitrogen atoms that have diffused in the work-piece can be presented 

from the equation of the change of the total weight gain by the steel over the surface 

area by nitriding time. The flux of nitrogen J was calculated by using the equation 1. 

t

A

M

J







 

(1)  

 

where  J is the nitrogen flux (g/cm

2

*s),  ΔM is the total weight gain (g), A is the 

surface area (cm

2

) and t is the carburizing time(s). 

 

As seen in Figure 11, the total flux for the samples including the as-polished samples 

was plotted along with contaminated samples. In the case of the samples that were 

contaminated in the cutting fluid and rust preventive oil, the total flux did not show a 

significant difference with the one of as-polished samples. In the contrast of the rust 

samples, the difference for the flux can be seen in Figure 9. Furthermore, the more 

time the samples were allowed to oxidize, the lower total flux of nitrogen, as shown in 

Figure 9 and table 3. For this reason, the following experiments will be focused on the 

effect of rust on the gas nitriding. 

 

 

 

 

 

 

 

 

 

 

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40

 

Table 3 The total flux of nitrogen vs. Contaminants 

Samples 

Weight gain 

after nitriding 

(g) 

J(g/cm

2

 hour) 

As-polish 

0.02229 8.67E-05 

0.02178 8.91E-05 

Rust 

2 days 

C 0.02253 7.89E-05 

NC 0.01897  6.64E-05 

4 days 

C 0.00962 4.77E-05 

NC 0.0137  3.37E-05 

7 days 

C 0.02338 8.16E-05 

NC 0.00048  1.69E-05 

Rust preventive oil 

C 0.02432 8.49E-05 

NC 0.02388  8.48E-05 

Hot cutting fluid 

C 0.02546 8.88E-05 

NC 0.02491  8.70E-05 

*C: cleaned sample; *NC: non-cleaned sample 

 

 

Figure 9 The total flux of nitrogen vs. contaminants 

 

 

0

1

2

3

4

5

6

7

8

9

10

As polished

2 days rust

4 days rust

7 days rust

HC

R

Flux

 g/cm

2

*1

0

5

Contaminants

NC

C

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