Microsoft Word WendiThesis docx

 

33

3.2 Experimental procedure 

Sample preparation and test matrix 

The Nitralloy-135 alloy was used in this study. It is a modification of AISI 4140 

specifically designed to be nitrided for high surface hardness and abrasion resistance 

to develop the case-hardened products, such as gears, bushings and pinions. [8] The 

composition of the material is presented in Table 1. 

Table 1 Composition of the Nitralloy-135 in wt% 

C 

Mn 

Si 

Cr 

Mo 

Al 

Fe 

0.4 

0.6 

0.3 

1.6 

0.35 

1.15 

balance 

This alloy is supplied for nitriding in a prehardened condition, about 26/30 HRC. 

Further heat treatment is generally not required. The nitriding treatment is undertaken 

after all grinding and polishing on the part has been completed. [8] 

 

Disks used in this experiment were about 1.43 cm in diameter and 0.56 cm in 

thickness with an identifying mark on the side. The schematic of the sample and its 

loading configuration for the nitriding experiments can be seen in Figure 2. 

         

Figure 2 Sample shape and workload used for the nitriding experiments 

 

Four sample sets were settled for characterization using several analysis 

measurements. Sample set 1 was ground by using 240 grit SiC paper to create a 

standard surface finish. Sample set 2 was placed in a controlled environment of 25℃ 

and 80% humidity to form rust. The times that were chosen are 7 days, 4 days and 2 

---

 

34

days. Sample set 3 was contaminated in cutting fluid (Castrol Clearedge 6519). 

Sample set 4 was contaminated in rust preventive oil (Houghton Rust Veto 4225). All 

the sample sets can be seen in table 2. 

 

Table 2 Test matrix for the effect of contaminant on the heat treatment performance 

test  

Material Contaminant 

No.of 

trials 

Nitralloy-135 

As-polished 4 

Rust 

2 days 

2 

4 days 

4 

7 days 

2 

Cutting fluid 

2 

Rust preventive oil 

2 

 

The experimental plan flowchart is shown in Figure 3. The samples which were 

polished were divided into two groups for each contamination: cleaned and 

non-cleaned. The samples were nitrides in the same batch to ensure the same nitriding 

condition. The workload used in nitriding experiment is shown in Figure 4. Each 

sample was weighed before and after the nitriding using a Mettler AB54-S scale to 

calculate the nitrogen uptake and the total flux into the steel. The surface 

hardness-HRc and Vickers hardness were also measured. One sample for each 

condition was heat treated and analyzed. By comparing the as-polished and 

non-cleaned samples, the effect of contamination on gas nitriding can be studied. By 

comparing the cleaned and non-cleaned samples, the cleaning method for 

contamination removal can be evaluated. 

---

 

35

 

 

Figure 3 The flowchart for the experiment 

. 

Contamination methods 

Rust 

As evident in Figure 4, the samples were placed in a controlled tank of 25℃ and 80% 

humidity to form rust. The corrosion times which were chosen are 7 days, 4 days and 

2 days, as presented in Table 2. 

 

 

Figure 4 the sketch for the experimental set-up. 

 

Cutting fluid 

The cutting fluid used in the experiment was Castrol Clearedge 6519 [9], a high 

Thermo-Hygrometer  

Humidifier 

---

 

36

performance cutting and grinding fluid. It is chlorine free and formulated to eliminate 

time consuming maintenance problems associated with traditional semi-synthetic 

coolants, particularly regular additive additions, dumping and recharging, and 

cleaning foam-outs. It can be used in aluminum and ferrous alloy machining. The 

emulsion is 5% Concentrate + 95% tap water, which has the pH value of 9. The used 

cutting fluid was taken from the WPI machine shop is CNC machines. 

 

The samples contaminated with cutting fluids were prepared as follows – immersed in 

room temperature oil and then heated to 300

o

C for five minutes, then immediately 

immersed in the fluid to simulate the cutting condition.   

 

Rust preventive oil 

The rust preventive oil used in the experiments was Houghton Rust Veto 4225 [10]. It 

is water displacing, light solvent-based rust preventive that leaves a soft film to 

provide good long term indoor protection [11]. It can release displaced water within 

minutes and leave a very thin transparent protective film. It can provide indoor 

protection for steel, if parts are packaged. It contains mineral oil and paraffin wax. 

 

To apply the rust preventive oil contamination, the immersion was performed at room 

temperature. New rust preventive oil was used in this experiment. 

 

Cleaning methods 

Acid cleaning 

Acid cleaning was selected due to its ability to remove the rust from sample. It is 

effective to remove light rust, such as the rust forms on ferrous metal in storage under 

high humidity. Hydrochloric acid cleaner: 50 vol% HCl was used as the cleaner. The 

samples were dipped in acid cleaner for 1min at room temperature, rinsed in distilled 

water twice for 3 min each time, dried and kept in a cool and dry environment. 

Visually, the brown rust layer disappeared after immersion. 

---