The hydraulic cylinder is an important actuator in the hydraulic system of the construction machinery. It is used to perform reciprocating motion and drive the construction machinery to perform various functions. The piston rod is a key transmission force component with high technical requirements for connecting the piston and the working part in the hydraulic cylinder. The working rod needs to bear a large tensile stress. Therefore, the piston rod must have sufficient strength, rigidity and toughness, and at the same time It is easily eroded by abrasive particles and must have high wear resistance.
The long cylinder piston rod is made of 45 steel. The processing route is: forging material (rolled material) - blanking - quenching and tempering - straightening - machining - surface quenching, tempering - straightening - head welding - machining - Grinding - stress relief annealing - polishing - hard chrome plating - polishing - cleaning - assembly. Processing high-quality hydraulic cylinders to identify Titanium machinery, professional quality assurance, piston rod heat treatment is the key process to ensure the internal quality and mechanical properties of the piston rod, the quality of heat treatment is directly related to the life and reliability of the entire hydraulic system, if heat treatment Improper, causing early breakage during the use of the piston rod, and lightly destroying other parts, which may cause damage to the entire equipment and casualties.
First, quenching and tempering
The purpose of quenching and tempering treatment is to make the piston rod have comprehensive mechanical properties with good matching of strength, hardness, plasticity and toughness. The internal structure is uniform and fine tempered sorbite, which is ready for subsequent surface quenching. The long cylinder piston rod has a length of 3800-4200 and a diameter of Φ90-Φ110mm. Therefore, the heating equipment adopts a 150KW well type resistance furnace or a 600KW suspension type continuous resistance heating furnace, and the temperature is controlled by two zones.
Heat treatment process parameters: In the well type furnace, 4 furnaces are suspended, and the quenching heating temperature is 830±10°C. After heat preservation for 160 minutes, the furnace is quenched twice, each time quenching 2, using circulating cooling water to cool, quenching and cooling. Swing to ensure maximum cooling evenly, cooling to about 100 ° C (the rod steam but not foaming) effluent into the well tempering furnace temper.
Then, 4 tubes were heated at 550±10°C, tempered for 190 minutes, and then chilled. After the above-mentioned process quenching and tempering treatment, the performance is unstable, the hardness fluctuates between 210-255HBS, and the hardness of the upper, middle and lower pipe rod of the same piston rod also differ greatly. And sometimes there are individual furnace hardness failure or low strength, need to be repaired. The quenching deformation is relatively large, which increases the difficulty of subsequent alignment and machining. Due to the poor hardenability of 45 steel, the metallographic observation of its internal structure is not a single uniform tempered sorbite, but a large piece of free iron body in its core, and there are mesh iron bodies and Wei's organization in individual parts. .
In order to solve the above problems, we use the suspension continuous heat treatment quenching furnace for quenching heating, each mounting 2, after heating and heat preservation, the furnace is automatically quenched, and each section is shot to ensure uniform heating. Considering that the Ac3 temperature of 45 steel is 770-780 °C, in order to refine the grain and reduce the deformation as much as possible, we use the 790±10 °C sub-temperature quenching process to refine the austenite grains and obtain fine and uniform after quenching. Lath martensite to enhance the toughness of the piston rod.
In order to further reduce the deformation and improve the quenching liquid cooling uniformity, we add 5%-10% quenching additive in tap water. When quenching, the circulating water pump is also used to force the coolant to circulate and cool. The tempering is still heated by 550±10°C, the beat is the same as the quenching beat, and the water is cooled after tempering to suppress the second type of temper brittleness. After the above process is improved, the internal structure is uniform fine tempered sorbite, and the bulk or reticular ferrite and Wei's structure are removed, and the hardness is uniform and stable.
Second, surface quenching
During the working process of the piston rod, the movement is frequent and the force is complicated. Therefore, in addition to the requirement of good comprehensive mechanical properties to prevent deformation and fracture, the surface is required to have high wear resistance, and the surface hardness requirement is 58-62HRC, which is often used in the production process. Use high frequency quenching treatment. The quenching equipment adopts a semi-automatic continuous quenching heating furnace, and the piston rod is transported by the feeding machine to the conveyor belt of the quenching furnace, and is heated and quenched by feeding the belt horizontally and uniformly into the induction coil.
The key to surface quenching is the choice of induction coil fabrication and quenching process parameters. The induction coil is made of 10*8mm rectangular copper tube, and is designed as a double-layer structure placed side by side. The front and rear rings are separated by a certain distance. The front ring does not make a water spray hole, and the front ring is used for preheating. The water hole with a hole diameter of 0.9mm in the rear circle is evenly distributed along the circumference, and the water spray angle is designed to be 36 degrees; the inner diameter of the induction ring is 4-5 mm larger than the outer diameter of the piston rod. The quenching heating anode voltage PV is 11-12KV, the anode current PA is 1.9-2.2A, and the gate current GA is 0.38-0.40A. The piston rod moving speed is controlled by adjusting the rotation speed of the piston rod feeding frame to heat the piston rod surface to After 890-910 °C, the water spray is cooled.
The quenching liquid is diluted with special quenching liquid and 50-60% water. The water spray pressure is maintained at about 0.1 MPa. By controlling the water spray cooling time, the temperature of the product after water cooling is controlled, and the residual heat is used to temper. After testing, the depth of the hardened layer is 2-3 mm, the surface layer is tempered martensite, the core structure is tempered sorbite, the surface hardness is 58-60HRC, and the core hardness is 210-230HBS.
Third, stress relief annealing
Since the surface quenching is tempering by using the residual heat of the product itself, the tempering temperature is gradually reduced, and there is no complete tempering and heat preservation process, the tempering time is short, the stress elimination is incomplete, and there is a large residual stress in some products. In the subsequent grinding process, if the internal stress of the grinding is superimposed on the residual internal stress of the surface quenching, when the stress exceeds the tensile strength of the material, stress cracks may occur on the surface of the piston rod if the stress does not exceed the tensile strength of the material. It exists in the form of residual stress in the product, causing cracking of the coating due to redistribution of stress during subsequent chrome plating or use, causing cracks in the chrome plating layer. Therefore, the piston rod must be annealed before chrome plating to eliminate the internal stress generated by grinding and surface treatment. The stress relief annealing heating temperature is 200-230 ° C, and after 190 min of heat preservation, the furnace is cooled to 160 ° C to be air-cooled.
Fourth, the conclusion
After the quenching and tempering treatment, the piston rod obtains good comprehensive mechanical properties and is ready for the subsequent surface treatment. The purpose of surface hardening is to obtain a high surface hardness to support the surface chrome plating layer, thereby improving the wear resistance and corrosion resistance of the piston rod. The stress relief annealing is arranged before the electroplating after the grinding process to completely eliminate the residual stress inside the piston rod, improve the plating quality and the product qualification rate, and improve the service life of the piston rod.