Bolts also grow black spots?

At present, the standard parts on the market mainly include carbon structural steel, alloy structural steel, stainless steel and copper. Bolt fasteners are the most used type of products. For refined fastener threads such as bolts and nuts, 6H/6g is recommended in the standard. Bolts and nuts are often made of carbon structural steel and alloy structural steel. High-grade fasteners must undergo heat treatment. However, it is found that black spots on the surface of some products that have undergone heat treatment and quenching affect the appearance quality and surface treatment, which is analyzed to be caused by cleaning liquid.

The factors of temperature, concentration and cleaning time of cleaning solution before heat treatment were followed up. The test results of 1 to 4 in Table 1 show that the temperature of the cleaning fluid before heat treatment will affect the cleaning effect of the bolt, and when the concentration of the cleaning fluid before heat treatment is 0.10, the optimal temperature of the cleaning fluid should be higher than 60 degrees C. The test results of No. 5-7 in Table 1 show that when the concentration of the cleaning solution before the heat treatment is 1.0%, the cleaning solution at a temperature of 50-70°C meets the surface quality requirements; at this time, the temperature of the cleaning solution can be appropriately reduced to 50°C.
 

Table 1 A total of 8 groups of test results show that the cleaning fluid after heat treatment quenching is the main reason for the black spots produced by bolt quenching, and the cleaning fluid after heat treatment quenching has a service life of 1 month, which will make the bolt surface produce spots that are difficult to remove, resulting in defects such as residual heat blackening and redness.

The composition analysis of the black spot on the bolt surface was carried out by direct reading spectrometer, and it was confirmed that there were O, S, and Cl elements at the black spot, while no O, S, and Cl elements could not be observed for the bolt without the black spot under normal conditions, which indicated that the black spot was caused by oxidation on the surface during a certain process in the heat treatment furnace. In particular, the bolts come out of the oil tank and enter the cleaning tank for cleaning. The purpose of cleaning is to clean the quenching oil on the bolt surface to ensure that the fastener surface is clean and free of oil. Because the tap water in the cleaning tank has a long replacement cycle and has been maintained at 50~70 ℃, some additives of the rapid quenching oil are also water-soluble, which leads to a series of chemical reactions in the water in the cleaning tank.

The water-based cleaning agent used in the test is generally composed of anionic surfactants, three carboxylic acids, organic amines, complexing agents, dispersants, builders, stabilizers, etc., and the best use temperature is 60~70 ℃. The best process effect of bolt cleaning is to ensure that the surface of the bolt reaches a certain degree of cleanliness; the bolt cannot rust within a week before the furnace is installed.

The following reactions exist in the deteriorated cleaning fluid after laboratory analysis:
Fe CI = Fe CI-
Fe CI-32O = FeOH- H CI-
FeOH- H = Fe2- H2O 
2H2S 2e = H2 2HS- 

The cleaning ability of the deteriorated cleaning liquid is reduced, and the incomplete cleaning of the quenching oil on the bolt surface leads to large oil fume in the tempering furnace, which is harmful to human health. Deteriorated cleaning fluid is highly corrosive, and the cleaning fluid remaining on the surface of the bolt causes black spots on the bolt. Sometimes sandblasting or shot blasting is still difficult to remove when the surface is cleaned, which affects the appearance quality and surface treatment.

After quenching, there was no black spot after cleaning in the cleaning tank with thoroughly changed water. A month later the black spots appeared again. After thoroughly cleaning the cleaning tank, replace the water and configure the cleaning fluid, and the black spots of the bolts will disappear again. In short, the black spot on the bolt surface is a quality defect of the bolt surface caused by the corrosion of the modified cleaning solution after quenching. This kind of local defect affects the appearance quality, and the more harmful is to affect the service life of the product, and it is an important factor that causes the pitting corrosion of some bolts.

Source: Golden Spider Fastener Web