At present, the rapid quenching oil still plays a leading role in the cooling medium of the fastener heat treatment. The use temperature of the quenching oil is increased, which will significantly accelerate the aging reaction rate. When the oil temperature is increased by 10°C, the chemical reaction rate is increased by about 2 to 4 times. In principle, the use temperature of the quench oil is 60 to 80°C. The use of quenched oil heating and repeated contact with hot workpieces leads to quenching oil oxidation and aging, which is the greatest threat in the production.
The long-term hardening operation of bolts and nuts causes many complicated chemical changes in the quenching oil, which causes a significant change in the properties of the oil, and also causes the quenching oil to age continuously, including quenching oil hydrocarbons that thermally decompose volatile components or gases and Oxidation of some compounds, the polymerization of oxidation products will lead to an increase in the viscosity of the oil, which is the mechanism reaction of quenching oil aging.
At the same time, the oil contacts the hot surface of the workpiece and is heated to a higher temperature, causing chain scission and secondary reactions to produce decomposition products. The oxidized hydrocarbons are all in a highly unstable state and will polymerize to form grease. Even serious consequences such as contaminating the workpiece and fouling the pipeline occurred. Obviously, the mechanism of oxidation of hydrocarbons during the use of quenched oils should be considered.
Why does the rapid quenching oil age for a long time? This is the reason why the quenching oil continuously oxidizes during use, and the main cause of sludge and stain on the surface of the workpiece.
With the increase of the use time, the oxidation of the oil becomes more and more serious, the viscosity further increases, and the sludge further increases. These all reduce the cooling performance of the quenching oil, resulting in soft spots or even hard quenching of the workpiece. The metallographic examination shows that Micro-structure: Carbon-steel mid-block bulk floes increase; lower bainite is present in alloy steel. At this time, the essence is that the quenching oil undergoes serious oxidation during use.
As the quenching oil oxidizes during use, the acid content of its carboxyl (SUO-containing carbon, oxygen, and hydrogen COOH groups is called carboxyl) will gradually increase, so the oil's acid value can be measured to measure its aging. Degree. After the quenched oil is continuously used for more than 2 years, the acid value is continuously increased. When the acid value reaches about 1.0 to 1.5 mgKOH/g, the rate of carboxylic acid generation is significantly increased.
After quenching oil aging, its cooling curve is reflected in the steam film stage shortened, the maximum cooling rate increases, the cooling curve as a whole moves to the upper right, high temperature cooling speed, the workpiece hardenability decreased. An acid value of 1.5 mgKOH/g is a quantitative indicator of the aging of the quench oil. It is also the basis for the oil change. Normally, the oil should be replaced within 1 year.
Measures to prevent aging of quenched oils include: The use of hydrorefined base oils and composite antioxidants can effectively improve the oil's antioxidant performance. In order to improve the anti-aging performance of quenched oil, the following matters should be taken into account when using quenching oil: make the oil in a circulating state, filter it in time, prevent water inflow, do not change oil partially, reduce the use temperature of quench oil as much as possible, and reduce pollution.
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