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The blackening treatment of steel parts is also known as bluing. The common methods for blackening nowadays are traditional alkaline heating blackening and the more recent room-temperature blackening. However, the normal-temperature blackening process does not work very well on low-carbon steel. Alkaline blackening is better for A3 steel. Alkaline blackening can be further divided, with a difference between one type of blackening and two types of blackening. The main components of the blackening solution are sodium hydroxide and sodium nitrite. The temperature tolerance required for blackening is relatively large; a surface of good quality can be obtained between 135 degrees Celsius and 155 degrees Celsius, with the only difference being the amount of time required. In practical operation, it is important to pay attention to the quality of rust and oil removal before blackening the workpiece, as well as the passivation and oil immersion after blackening. The quality of the blackened surface often varies depending on these processes. Metal “blueing” solution: uses alkaline oxidation or acidic oxidation methods ； A layer of oxide film is formed on the metal surface to prevent it from corroding; this treatment process is known as “blueing”. The oxide film formed on the surface of ferrous metals after \"blueing\" treatment consists mainly of ferric tetraoxide on the outer layer and ferrous oxide on the inner layer. I. Alkaline oxidation method – “blueing” solution 1. Formula: 50–100 grams of sodium nitrate, 600–700 grams of sodium hydroxide, 100–200 grams of sodium nitrite, 1000 grams of water. 2. Preparation method: After measuring the ingredients according to the formula, add them one by one under stirring until they are dissolved and the mixture is well mixed. 3. Instructions: (1) The metal surface must be thoroughly cleaned and dried before the “firing basket” treatment can be carried out. (2) The conditions for performing the \"blueing\" treatment on metal components are related to the carbon content in the metal. The temperature of the blueing solution and the treatment time of the metal components in it can be found in the table below. Carbon content in metal (%) Working temperature (°C) Treatment time (minutes) Start End >0.7 135-137 143 10-300.5-0.7 135-140 150 30-50