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- Bu səhifədəki naviqasiya:
- Cross-section hardness vs. corrosion time
- Appendix CHTE Cleaning Project Questionnaire Result
- 2. What type of surface contaminants is on your steel work piece after annealing/normalizing/carburizing
- 3. What type of surface contaminants is on your steel work piece after quenching/cooling
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The weight of samples before and after the rust process was recorded and subtracted by the original weight to obtain the rust gain during the rust forming step. Weight increased when time increased, shown in Figure 10. The thickness of the rust was calculated by equation 2. A m Thickness (2) Where m is the weight gain after rust (g),
is the density of the rust, A is the surface area (cm 2 ).The thickness of the rust vs. Corrosion time is shown in Figure 11. The weight after nitriding was also measured. The weight gain after nitriding was observed to decrease as the rust layer becomes thicker, as shown in Figure 12. The blue diamond represents the weight difference between rusted sample and nitrided sample. Compared to 0.0024 - 0.0034 mg/cm 2 weight gain for other rusted samples, only 0.0001 mg/cm 2 of nitrogen diffused into the 7 days rust sample. Because the rust layer prevents the nitrogen absorption, less nitrogen was diffused into sample when the rust layer thickness increased. The green triangle in Figure 12 shows the weight gain between as received sample and nitrided sample, which got the same conclusion: less nitrogen is diffused into sample when the rust layer thickness is increased. After cleaning, the weight gain varies in a small range, from 0.0039 - 0.0041 mg/cm 2 , except for the sample in 4 days rust set. The rust layer has been removed effectively by the acid cleaner. 42
Figure 10 Average weight gain of rust sample due to rust vs. time curve.
Figure 11 The thickness of the rust vs. corrosion time
0 0.5
1 1.5
2 2.5
3 3.5
4 4.5
2 days Rust 4 days Rust 7 days Rust
C NC 43
Figure 12 Average weight gain/cm 2 of rust sample after nitriding vs. corrosion time. Hardness Surface hardness vs. corrosion time HR c was measured and plotted with corrosion time, which has been shown in Figure 13. The hardness of samples is between 55 and 57. The non-cleaned samples have lower hardness when compared to clean samples. 0 0.0005 0.001 0.0015
0.002 0.0025
0.003 0.0035
0.004 0.0045
0 2 4 6 8
e igh t ga in /S u rfa ce ar ea (g /c m 2 ) Corrosion Time NC‐R
C NC‐A
As‐polished 44
Figure 13 Surface hardness vs. corrosion time.
The Vickers hardness test was used in testing the cross-section micro hardness. The samples were cut, polished with 800 grit sand paper and then a 200gf load and 15s loading time was used in Shimadzu HMV-2000 Micro Hardness Tester. The measurements started at 100µm depth from the edge, and an increase of 100µm per measurement was taken at 100-1000µm depth, 200µm per measurement was taken at 1000µm-2000µm. Figure 14 shows the diamond shape made by the indenter with a square-based pyramid and an angle of 136° between opposite faces.
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Figure 14 The optical image of the diamond shape holes made by the indenter under 200gf for 15s for Vickers Hardness test.
The results of the Vickers hardness were collected and plotted in Figure 15-17. In the case of 2 days rust samples, the hardness variation can be seen clearly from 100 um to 500 um case depth. For the range from 500 um to 1000 um, there is no significant hardness difference for the samples. In Figure 15, the hardness for the non-cleaned sample is smaller than the one of cleaned sample and as the polished sample. The large difference was found about the 400 um case depth. In the case of 4 days rust samples, the hardness variation can also be seen from 100 um to 500 um case depth in Figure 16. After the 500 um case depth, there is no significant hardness difference. As it can be seen in Figure 17, there is large difference between the non-cleaned sample and cleaned sample from the 100 um and 500 um case depth. After the 500 um case depth, there is the same situation for the hardness because the depth for the nitrogen diffusion layer is about 500 um from the surface of case.
As shown in Table 4, the cross-section hardness of non-cleaned sample is lower than cleaned sample. The total and effective case depths data, given in Table 4, were obtained from the micro hardness data (to 420HV). Comparing data from the Figure 15-17 and Table 4, the data clearly suggests that nitriding the parts of the same steel grade in the same workload, and therefore, the same nitriding process parameters will be affected by the rust layer.
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Figure 15 The HV profile for the samples rusted for 2 days
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Figure 16 The HV profile for the samples rusted for 4 days
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Figure 17 The HV profile for the samples rusted for 7 days
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Table 4 Case depth based on microhardness measurements.
Case depth for Cleaned sample (um) Case depth for Rusted sample (um) Rust time 420HV Rust
time 420HV
7days 400 7days 315 4days 400 3days 400 2day 480 1day 370 As-polished 410 --
-- Micrographs Figures 18-21 present the photomicrographs of nitralloy-135 steel after nitriding. Cleaned and rusted samples both show fine grains in the edge due to the nitriding treatment. The same grain boundary condition was observed for all samples. The difference of the diffusion layer between cleaned and non-cleaned samples can be clearly seen from the Figure 23-25 which are micrographs under magnitude 10 X magnification. The same result can also be concluded from Figure 9: the more time the sample was corroded, the lower was the flux of total nitrogen. The diffusion layer and white layer were identified by red arrows in Figure 18. The thickness slight variation of the diffusion layer for each sample can be seen in Figure 22. For each non-cleaned sample, the more time the sample was rusted, the lower thickness of the diffusion layer the micrograph showed.
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(b) 50X Figure 18 Photomicrograph of the as-received #135 steel’s edge, etched with 2% nital solution. (a) the photomicrograph 10X magnification. (b) the photomicrograph 50 X magnification
Case depth
White layer 51
(a) cleaned (b) contaminated Figure 19 Photomicrograph of the 2 days rust samples, etched with 2% nital solution.
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(a) cleaned (b) contaminated Figure 20 Photomicrograph of the 4 days rust samples, etched with 2% nital solution.
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(a) cleaned (b) contaminated Figure 21 Photomicrograph of the 7 days rust samples, etched with 2% nital solution.
Figure 22 The thickness of the diffusion layer for each sample.
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(a) cleaned (b) contaminated Figure 23 Photomicrograph of the 2 days rust samples 10 X, etched with 2% nital solution. (a) sample cleaned before nitriding (b) sample non-cleaned before nitriding
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(a) cleaned (b) contaminated Figure 24 Photomicrograph of the 4 days rust samples 10 X, etched with 2% nital solution. (a) sample cleaned before nitriding (b) sample non-cleaned before nitriding
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(a) cleaned (b) contaminated Figure 25 Photomicrograph of the 7 days rust samples 10 X, etched with 2% nital solution. (a) sample cleaned before nitriding (b) sample non-cleaned before nitriding
Figure 26 presents the scanning electron microscopy (SEM) image of Nitralloy-135, which shows the white layer and diffusion layer. To identify the type of scales on the nitralloy-135, X-ray diffraction (XRD) pattern had been collected by Panalytical Inc [13]. The peaks of epsilon-iron nitride (Fe 3 N), and gamma-iron nitride (Fe 4 N) were
identified as shown in Figure 28
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Figure 26 SEM image of nitrided #135 alloy etched by 2% nital.
Figure 28 XRD diffraction pattern of nitralloy-135 surface after gas nitriding
0 1000 2000
3000 4000
5000 6000
7000 8000
9000 10000
20 40 60 80 100
Fe 3 N (101) Fe 4 N (100)
Fe 3 N (111) Fe 4 N (111)
Fe 3 N (002) Fe 3 N (112)
Fe 3 N (113) Fe 4 N (200)
Fe 4 N (220) 58
1.
D. MacKenzie, FASM, M. Fretz and D. Schuster, Cleaning for heat treating, Heat Treating Progress, October 2008 2.
Pressures and Elevated Temperature: Impact on Gas Nitriding. Surface Engineering, 2000, 16(3):p263-268 3.
composition on Gas nitriding mechanism”, Surface engineering, vol 13, p 251-256, 1997 4.
OH, 1992 5.
J. Darbellay, Gas nitriding: An Industrial Perspective, MSE 701 – March 22,2006 6.
K. Jack, Nitriding, Proceedings of the Metal Society, Heat Treatment, London, 1973
7.
B. Haase, J. Dong and J. Heinlein. Surface oxidation of steels at low pressures and elevated temperature, Impact on Gas nitriding, 2000, p263-268 8.
Product data, Nitriding steel #135, Tool steel and Specialty Alloy Selector, website 9.
FluidCastrol, website 10.
Product data, Houghton- Rust Veto 4225, Houghton, website. 11.
online datasheet 12.
Private community: Bodycote Thermal Processing, Worcester, MA, 01609 13.
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Summary Gas nitriding is a complex process which leads generally to increased surface hardness, and improved wear behavior and corrosion resistance. One problem is that the process is not yet fully understood, especially the influences of surface contamination. The experimental results presented indicate that there is a strong influence of surface contaminants on the rate of nitrogen acceptance. The effects of rust layer on the hardness were experimentally investigated. The results are summarized as below:
The surface hardness (R c ) didn’t show a significant difference between the heavily rusted sample and clean sample. The hardness of samples nitrided at 526
o C and 549 o C for 50 hrs is between 55 and 57 HR c .
Acid cleaning can remove the rust layer effectively. Hydrochloric acid cleaner: 50 vol % HCl is used. Rinsing completely in distilled water is necessary to remove the cleaner residue.
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CHTE Cleaning Project Questionnaire Result 1.
What type of surface contaminants do you find on your steel work piece before heat treatment (from upstream operations)? *Other: Cutting fluids, Lubricants, Soap, Drawing compounds, Fine blanking oils, NaOH, KOH, Borates, and Silicates Salt. **What must be removed: Hard water, NaOH, KOH, Borates, Silicates Salt, Oil, Chips, Scale from forging?
*Other: quench oil, 0 1 2 3 4 5 Dust Oxides
Carbon deposits Chips
Other 61
3. What type of surface contaminants is on your steel work piece after quenching/cooling?
*Other: Rust Preventive, Boron. **What must be removed: Salt, Oil, Chips, Carbon deposits, [4] According to the survey result, the most common kinds of surface contamination before the heat treating process are oil, rust preventive oil and cleaner residue. After annealing, normalizing or carburizing, carbon deposits are the most important surface contaminants. The oil and carbon deposits are left after quenching. Yüklə 284,05 Kb. Dostları ilə paylaş:
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