The time required for the heat transfer and tissue transformation of the workpiece not only saves energy and improves labor productivity, but also reduces or eliminates defects such as oxidation and decarburization generated during the heat preservation process of the workpiece, which is beneficial to the improvement of product quality.

27SiMn steel is steel for mining hydraulic props cylinders. It is prone to axial and radial deformation during quenching, and some cylinders are scrapped due to difficulty in straightening, resulting in great waste. Practice has shown that shortening the heating and holding time can effectively reduce the deformation of the cylinder. Under the condition of “zero insulation”, the influence of quenching temperature on the microstructure and properties of 27SiMn steel has practical significance for promoting the “zero insulation” heat treatment technology of the cylinder, optimizing the heat treatment process parameters of the cylinder and improving the quality of the hydraulic prop. In this paper, the influence of quenching temperature of “zero insulation” on the microstructure and properties of 27SiMn steel was studied, and the possibility of “zero insulation” quenching of cylinders was discussed.

In the experimental temperature range, the higher the quenching temperature, the coarser the austenite grains, the reason is: the velocity of austenite grain growth VD0exp(-Q/KT),: D0: constant; Q: activation energy; K: Boltzmann constant; T: heating temperature; as the temperature increases, the activation energy Q of atomic diffusion decreases, the velocity V of grain growth increases, and the austenite grains gradually become coarse. From the experimental results, the 27SiMn steel is quenched when heated to the complete disappearance of ferrite ( (this test is 930C left stone), with the best toughness. The reason is that the mechanical properties of the steel are affected by two aspects: on the one hand, as the heating temperature increases, the ferrite is completely dissolved in the austenite, and all the martensite structure is obtained after quenching, and at the same time, the alloy carbide Gradual decomposition, the dissolution of alloying elements C, Mn, Si in austenite increases, and the solid solution strengthening effect is strengthened. These two factors lead to an increase in the hardness of steel and an decrease in elongation; on the other hand, as the quenching temperature increases The austenite grains are gradually grown to obtain a coarse martensite structure ((e)), which causes the strength, hardness and elongation of the steel to decrease.

High, and sometimes a layer of grounded metal shield is inserted between the two windings. The secondary voltage cannot exceed 50V. PELV can be used in places where extra low voltage is required, or in consideration of more safety factors, but it cannot be used in the above high-risk places. Its safety concept is similar to SELV, but its secondary circuit can be grounded at one point.

Unless the PELV-powered equipment is within the equipotential bonding range, the risk of direct contact is generally required when applying PELV. The nominal voltage should not exceed 25V, and the electrical equipment can only be used in normal dry places, and the human body and Equipment should not have a large area of ​​contact. In all other cases, if direct contact is not prevented, the maximum allowable value of the voltage is only 50V or less due to functional reasons, but it does not fully meet the requirements of SELV or PELV, and should be based on site conditions and the purpose of the circuit. And in accordance with the reasonable measures specified in IEC60364-4-41 to prevent direct contact and indirect contact hazards.

With the development of social economy, industrial and agricultural production put forward higher requirements for the safety and reliability of electricity consumption. In each power supply network, the technical measures for electric shock protection are only systematic, tidy and reliable, in order to play the protective role. Any one of them will be neglected, and it will inevitably cause harm to the person and property. connection).

If a second fault occurs, the current is automatically shut off by the overcurrent protector, as is the case with the second failure of the IT system.

24 Use ungrounded local equipotential bonding Within this measure, all exposed conductive parts, including the floor, are connected with appropriate large-section conductors so that no significant potential difference occurs between any two points. Damage to the insulation of the live conductor and the electrical metal casing will cause the potential of the entire "equal-potential cage" to rise relative to the ground voltage, but no fault current will occur.

Under this circumstance, the person entering the room will be in danger (at this time, the person is stepping on the charged floor), and precautions must be taken to protect the human body from electric shock (such as laying a non-conductive floor in the room).

3. Protection against both direct and indirect contact 3.1 The residual current is used to protect the current through the current loop of the main circuit of the protector. When the human body comes into contact with a live conductor or the protected line and equipment is damaged, residual current occurs. For direct contact with electric shock protection, the value of 30mA and below is used as the operating current of the residual current protector; for indirect contact protection, the ratio of the contact voltage limit to the grounding resistance is used as the operating current of the device (ie 2-1) .

3.2 Using a special low voltage system Extra low voltage system is powered by a safe voltage. It is divided into SELV (Safety Extra Low Voltage), PELV (Protective Extra Low Voltage) and FELV (Functional Extra Low Voltage).

Extra low voltage SELV safety measures are used where there is a serious risk of electrical equipment operation. Under normal conditions, if the SELV is below 25V, direct contact protection is not necessary. This measure is powered by a very low voltage of the secondary winding of a special transformer that conforms to national or international standards. The level of the voltage pulse between the primary and secondary windings is very high (upper page 362). The “Zinc Insulation” quenched structure of 27SiMn steel is a typical lath martensite morphology, and the martensite lath bundle is very small. The reason is that, besides the fine austenite grains, it is related to the uneven distribution of carbon concentration in the austenite grains. There are a large number of carbon concentration microdomains in the uneven austenite, and the Ms points in the microdomains are different. During the quenching and cooling process, the low carbon concentration microdomain with high Ms point first forms martensite; the high carbon concentration microdomain with low Ms point, then martensite is formed. The transformation of martensite has non-isothermal characteristics, and martensite sheets cannot grow through different carbon concentration domains due to different formation temperatures. Therefore, the martensite lath is further refined, and this refinement will inevitably lead to an increase in the strength and hardness of the steel, which is consistent with the result.

4. The "zero insulation" heat treatment process parameters of the cylinder body are based on the test. According to the technical requirements of the cylinder block, the heat treatment process parameters of the cylinder under the condition of "zero insulation" are: (900 ± 0) C quenching, (610 ±0)C tempered. Compared with the conventional heat treatment process, the quenching temperature is increased and the tempering temperature is greatly improved. According to the above

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