Selection of carrier liquid in wet magnetic particle testing
The liquid used to suspend magnetic particles in wet magnetic particle testing is called carrier or carrier, which is generally divided into oil-based carrier and water-based carrier.
·Oil based carrier
(1) The oil-based carrier liquid for magnetic particle testing is kerosene with high flash point, low viscosity, no fluorescence and no odor.
Flash point refers to the lowest temperature at which the vapor produced by the volatilization of flammable substances in the air can be burned. If the flash point of the oil is low, the magnetic suspension is easy to be ignited, which will cause burns to the operators and damage to the flaw detection equipment.
Viscosity is a measure of internal friction in the flow of a liquid. The viscosity decreased with the increase of temperature. The viscosity of oil is divided into dynamic viscosity and kinematic viscosity.
Dynamic viscosity is a measure of the internal friction force of a liquid flowing under a certain shear stress. Its value is the ratio of the dynamic viscosity of a liquid at the same temperature to its density. It is expressed in Pascal seconds (PA? S) in the international system of units (SI). The customary unit is centipa, 1cpa = 10-3pa · s = 1MPa · s
Kinematic viscosity is a measure of friction force when liquid flows under the action of gravity. Its value is the ratio of dynamic viscosity of liquid to its density at the same temperature. It is expressed in m2 / s in international system of units (SI). It is customary to use CST as a unit, 1cst = 10-6m2 / S = 1mm2 / s.
In a certain temperature range, especially at a lower temperature, if the viscosity of the oil is small, the fluidity of the magnetic suspension is good and the detection sensitivity is high. However, if the viscosity of the oil is too small, the suspension of the carrier liquid is poor, and the magnetic particle is easy to precipitate, which also affects the detection sensitivity, so the viscosity of the oil should be controlled in a certain range.
(2) Kerosene carrier liquid with low flash point shall not be used for oil-based carrier liquid in Mt.
(3) Oil based carrier fluid is preferred for the following applications:
1) Some iron-based alloys that should be strictly prevented from corrosion (such as some bearings and bearing sleeves that are finished).
2) Where water may cause electric shock.
3) Some high strength steels that can cause hydrogen embrittlement or corrosion by immersion in water.
·Water based carrier
Water can not be used as carrier liquid alone, so when water is used as carrier liquid, it is necessary to add wetting agent, antirust agent and defoamer, if necessary, to ensure that water-based carrier liquid has proper wettability, dispersibility, corrosion resistance, defoaming and invisibility.
(1) Wettability refers to that the magnetic suspension should be able to rapidly wet the surface of the workpiece. The appropriate wettability can be determined by "water break test", and the pH value should be controlled between 8 and 10.
(2) Dispersibility, that is, the magnetic powder can be evenly dispersed in the water-based carrier liquid, and the magnetic powder does not agglomerate in the effective service life.
(3) Corrosion resistance, i.e. no corrosion to workpiece, equipment and magnetic powder itself.
(4) defoaming performance requirements, in a relatively short period of time to automatically eliminate the foam in the water base carrier, in order to ensure detection sensitivity.
(5) The stability is that the performance of water-based carrier does not change during the specified storage period.
The advantages of using water as carrier liquid are that water is nonflammable, low viscosity, wide source and low price. However, it is not suitable for some high strength alloy steels which can cause hydrogen embrittlement or corrosion when immersed in water.
Attention should also be paid to the following when selecting carrier liquid:
Although kerosene is used as the flaw detection carrier liquid, it will not produce rust, but it has peculiar smell, and the operation time is a little longer. When the oil temperature rises or the flaw detection is carried out in a high temperature environment, a pungent smell will be emitted, which will have an impact on the surrounding environment and operators. The kinematic viscosity of kerosene itself is not very good, and other oil (such as transformer oil) can be added for lifting during the non fluorescent magnetic particle testing High kinematic viscosity; the flash point of kerosene is not too high. In the normal operation process, if the flaw detector is arcing, the probability of fire is very high, increasing the risk of operation.
Water is widely used as flaw detection carrier fluid because of its convenient material, but water is used as flaw detection carrier fluid, which has relatively high requirements on the surface of the workpiece; if there is oil on the surface of the workpiece before flaw detection, it must be cleaned before flaw detection; there is water residue on the flaw detection equipment and the workpiece, which will cause corrosion of different degrees, and rust inhibitor needs to be added to the water when flaw detection of the workpiece with high rust prevention requirements; because of water The kinematic viscosity is not very good, and the effect of using water for high-precision workpiece flaw detection as flaw detection carrier is not very good.
Therefore, when selecting magnetic particle carrier fluid, we should consider the advantages and disadvantages of oil-based carrier fluid and water-based carrier fluid, and then select the appropriate carrier fluid according to the tested workpiece and testing requirements.