Blackening Technology of Bearing Stainless Steel

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Blackening Technology of Bearing Stainless Steel

Time : March 2, 2024

Abstract: The blackening process of bearing stainless steel 9Cr18 was studied. Adding electrolysis process to the blackened 9Cr18 steel bearings significantly improves their corrosion resistance without reducing their hardness. SEM and AES analysis showed that the corrosion resistance was due to the obvious increase of Cr/Fe ratio in the surface facial mask.

Keywords: 9Cr18 bearing stainless steel; Wear resistance; Corrosion resistance; Blackening treatment; electrolysis

 

  1. Introduction

With the rapid development of the world economy, higher requirements have been put forward for the corrosion resistance and surface aesthetics of bearing stainless steel. For example, for bearings made of 9Cr18 stainless steel used as fishing gear, in order to be applied in the ocean and improve the success rate of fishing, while maintaining its original wear resistance (hardness), it is required to not produce rust spots and maintain a uniform black color after 96 hours in artificial seawater.

 

Although using heat treatment methods can achieve better color, the hardness and wear resistance of bearings will be greatly reduced. Although using coatings can achieve satisfactory black color and maintain the original hardness, the bonding force is poor (mechanical bonding), and the coating will fall off shortly after use, causing a significant decrease in the corrosion resistance of bearings and quickly scrapping.

 

In order to maintain the original hardness (wear resistance) of bearing stainless steel while maintaining a uniform black appearance and high corrosion resistance, this work studied the chemical blackening technology of bearing stainless steel.

 

  1. 1.Test method

1.1 Materials

Using 9Cr18 stainless steel, its chemical composition is (mass fraction): C 1.03%, Mn ≤ 1.00%, Si ≤ 1.00%, P 0.03%, S 0.005%, Cr 17.5%, Fe residue.

 

1.2 Process screening

Using a mixture of sulfur and chromic acid, containing 100g/L H3PO4900g/L H2SO4100g/L CrO3, with a time of 100 minutes and a temperature of 80 ℃, the obtained film has a dark color and a hardness (HRC) of 56-57.

 

1.3 Salt spray test

According to the test method in GB/T 10125-1997, 96 h continuous spray test is conducted under the condition of (50 ± 5) g/L NaCl solution, temperature (35 ± 2) ℃, pH 6.5~7.2.

 

1.4 Water resistance test

According to the GB/T 1733-1993 test method, conduct water resistance tests on the above three samples in deionized water at (42 ± 2) ℃.

 

  1. 2.Test results

2.1 Salt spray test results

The black 9Cr18 stainless steel bearings treated with the above process were subjected to salt spray testing with untreated bearings, and the results are shown in Table 1.

 

Table 1 Comparison of corrosion resistance between blackened and non blackened 9Cr18 bearings

Bearing 12h 24h 48h 96h
9Cr18 Rust spots appear Increased rust spots Obvious rusting Severe rusting
Blackening 9Cr18 Rust free spots Rust free spots Rust free spots many rust spots

 

The test results indicate that although the salt spray test time for the blackened 9Cr18 stainless steel bearings has been significantly increased, there is still a certain gap compared to the requirements of the 96h salt spray test. Therefore, a more corrosion-resistant treatment method is adopted, which is blackening treatment+electrolysis treatment, that is, electrolysis is carried out in a solution of H2SO4-CrO3-H2O additive A. The results of the three states of bearings in the salt spray test are shown in Table 2.

 

Table 2 Comparison of Salt Spray Resistance Tests for 9Cr18 Bearings with Three Surface States

Bearing 48h 96h 120h
9Cr18 Obvious rusting Severe rusting Severe rusting
Blackening 9Cr18 Rust free spots many rust spots a large number of rust spots
Blackening 9Cr18+point solution Rust free spots Rust free spots Rust free spots

 

 

2.2 Water resistance test results

The test results are shown in Table 3. It can be seen that the 9Cr18 stainless steel bearings treated with blackening and electrolysis have good corrosion resistance in water and salt spray tests, which can meet the requirements of users.

 

Table 3 Comparison of Water Resistance of 9Cr18 Bearings with Three Surface States

Bearing 48H 96H 120H
9Cr18 very few rust spots a few rust spots a few rust spots
Blackening 9Cr18 Rust free spots Rust free spots Rust free spots
Blackening 9Cr18+point solution Rust free spots Rust free spots Rust free spots

 

2.3 Analysis

The EDAX energy spectrometer analysis spectra of three different surface states of 9Cr18 stainless steel bearings are shown in Figure 1. It can be seen that the Cr/Fe ratio of three different surface facial mask has changed greatly. The Cr/Fe of 9Cr18 stainless steel without blackening treatment is less than 1, while the Cr/Fe of 9Cr18 stainless steel with two blackening treatments is greater than 1.

Figure 1 EDAX energy spectrum analysis results of 9Cr18 stainless steel bearings with three different surface states

 

Auger electron spectroscopy analysis shows that the Cr/Fe ratio in the surface facial mask of 9Cr18 stainless steel is about 0.5, while the Cr/Fe ratio in the two blackened surface facial mask of 9Cr18 stainless steel is about 1.5, which is consistent with the analysis results of scanning electron microscopy energy spectrum analysis. In addition, the results of Auger electron spectroscopy analysis show that the thickness of the surface facial mask layer of the electrolytic corrosion resistant 9Cr18 stainless steel is significantly greater than that of the non electrolytic blackened 9Cr18 stainless steel, which indicates that the corrosion resistance of the former is better than that of the latter.

 

  1. 3.Conclusion

(1) The blackened 9Cr18 stainless steel bearings can significantly improve their corrosion resistance.

 

  • The corrosion resistance of 9Cr18 stainless steel bearings treated with blackening and electrolysis has been further improved.

 

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