Ball screw pair is a mechanical element composed of screw, nut, ball and other parts.It will rotate motion, into linear motion, or linear motion into rotary motion, has the advantages of high transmission efficiency, high positioning accuracy, transmission reversibility, long service life and good synchronization performance, so it is widely used in various industrial equipment, precision instruments and precision CNC machine tools.In recent years, the ball screw pair, as the linear drive unit of CNC machine tools, has been widely used in the machine tool industry and greatly promoted the development of the machine tool industry.
When the ball screw pair is used on all kinds of equipment, due to different loads and different stress sizes, the screw often bears bending, torsion, fatigue and impact, and at the same time bears strong friction in the rotating part, so the main form of damage is wear and fatigue failure.Therefore, when the screw is designed and manufactured, it must have the inherent performance requirements such as high strength and toughness, high surface hardness and wear resistance, and high dimensional stability.In particular, large ball screw (diameter ≥80 mm), due to the use of large load (dynamic and static load up to nearly 1000KN), so in the toughness, surface hardness and wear resistance and other aspects of the higher requirements.At present, domestic manufacturing enterprises generally use GCr15 steel, after the spheroidizing annealing treatment or quenching and tempering treatment and other pre-heat treatment, surface induction quenching heat treatment, to meet the internal performance requirements of the ball screw.
At present, medium frequency induction quenching is generally used for large ball screw.In the production, it is often found that after the medium frequency quenching (tempering) of the lead rod after grinding the thread, the magnetic flaw inspection, often appear on the arc of the thread raceway axial or network cracks, even in the process of grinding the thread can be found only with the naked eye, resulting in the lead rod scrap.This not only causes direct economic losses to the enterprise, but also because of the many factors that cause the problem, which brings greater pressure to the front-line operators of the enterprise production.The author has been engaged in the heat treatment of ball screw for a long time. Through the failure analysis and process traceback of a large number of large screw with grinding cracks, the causes of such cracks and the control measures are summarized, and the validity is confirmed through mass production.
Reason analysis of grinding crack of screw after medium frequency quenching
1. Inadequate raw materials
The main performance is that the mesh carbide grade of GCr15 material is out of standard or the spheroidized annealing structure is not qualified (there are lamellar pearlite).Through the analysis of the carbide inhomogeneity and microstructure of the cracked lead screw, it is found that the net carbide grade is out of standard or the spheroidized annealed microstructure is unqualified for about 40% of the total lead screw.The uneven distribution of surface hardness and internal stress on the screw surface after induction quenching is caused by the inhomogeneity of carbide, and the internal stress in the part with more concentrated carbide is also more concentrated.When the screw is grinding, because the internal stress in this part exceeds the yield strength of the material, the grinding crack will occur.The presence of flake pearlite will result in coarse grain size after induction quenching on the surface of screw, which will reduce the yield strength of steel. Grinding cracks will occur in parts where the internal stress exceeds the yield strength of the material during screw grinding.
2. Bad medium frequency quenching and heat treatment of lead screw
It is mainly manifested as high quenching temperature or insufficient tempering.According to the analysis and statistics, the lead screw caused by the grinding crack accounts for about 20% ~ 30% of the total.
When large ball screw is quenched in medium frequency, the output power of medium frequency is high and the quenching speed is too slow, which may make the temperature of the screw quenching high, and the martensite structure level of the screw after quenching is higher than the upper limit (martensite level 5), and even may exceed the limit (martensite level ≥5).Coarse martensite structure will reduce the steel 40 %.The process parameters of screw grinding are not standard, and the grinding heat generated during grinding causes "secondary tempering" on the surface of screw.Moreover, the grinding heat even makes the surface temperature of the screw rise to the "quenching temperature" of the screw material. Under the cooling action of the grinding fluid, "secondary quenching" is formed on the surface of the screw, resulting in coarse surface grains, reducing the yield strength of the steel, and causing cracks on the surface of the screw.After the quenching of large ball screw, the hardening layer is deeper, the internal stress (including thermal stress and structural transformation stress) is larger, the tempering is insufficient (low tempering temperature or short tempering time), and the internal stress elimination is incomplete.After the screw is quenched and tempered, the residual internal stress in the screw is superimposed with the grinding stress generated during grinding. When the superimposed stress exceeds the yield strength of the steel, cracks will be formed on the screw surface.
3. The process parameters of lead screw grinding are not standard. The lead screw that causes the grinding crack accounts for about 30% ~ 40% of the total.The process parameters of screw grinding are not standard, and the grinding heat generated during grinding causes "secondary tempering" on the surface of screw.Moreover, the grinding heat even makes the surface temperature of the screw rise to the "quenching temperature" of the screw material. Under the cooling action of the grinding fluid, "secondary quenching" is formed on the surface of the screw, resulting in coarse surface grains, reducing the yield strength of the steel, and causing cracks on the surface of the screw.
II. Control measures
1. Carbide inhomogeneity of raw materials and control of spheroidizing annealing structure
At present, domestic GCr15 material procurement refers to GB/ T18254-2002 "high carbon chromium bearing steel" implementation.Standard 5.10.1 stipulates that the carbide inhomogeneity: for the diameter of spheroidized annealed steel greater than 60 ~ 120mm, the carbide network shall not be greater than grade 3;The carbide network for spheroidized annealed steel with diameter greater than 120mm shall be stipulated by the agreement of the buyer and the buyer.Standard 5.9.2 for spheroidized annealing structure stipulates: the spheroidized annealing round steel, wire rod ≤60mm, all sizes of the steel tube spheroidized annealing microstructure qualification level is 2 ~ 4;The microstructure of > 60mm spheroidized annealed steel shall be specified by agreement between the supplier and the buyer.
In the actual production, due to the large volume of steel production, there is a small amount of carbide non-uniformity of the steel, the microstructure of > 60mm spheroidized annealed steel is difficult to completely reach the 2 ~ 4 grade qualified.Therefore, the use of units need to enter the factory of steel physical and chemical inspection.For the steel with poor non-uniformity of carbide, "forging → normalizing → spheroidizing annealing" treatment must be carried out.If the microstructure of the spheroidized annealed steel is found to be unqualified, the spheroidized annealed steel must be processed again until the carbide inhomogeneity of the steel and the spheroidized annealed steel is qualified before production.
2. Induction quenching process control
Selection and control of quenching inductor.Quenching inductor is the key component of induction quenching equipment and the key parameter of quenching process.The gap between the inductor and the workpiece to be hardened (the lead screw) determines the "heating efficiency" of the inductor and the actual heating power of the workpiece surface.Especially for large ball screw of GCr15 material, the surface heating temperature of the screw is generally "upper limit temperature" (generally about 880 ℃) because the depth of the hardened layer is required to be deep. If the gap between the inductor and the screw becomes smaller, the "heating efficiency" of the inductor will also be improved.Therefore, working under the original quenching parameters, the actual quenching temperature of the lead rod will be higher.The martensite level obtained after quenching is naturally higher.Therefore, the gap between the inductor and the lead rod must be strictly monitored and controlled.The large screw quenching inductor generally adopts the ring-through type or the half-ring floating type.When ring through inductor is used, the size of inductor needs to be checked regularly, and the inductor must be repaired or replaced if the deviation of > 2mm;With the semi-ring floating inductor, it is necessary to check the positioning block thickness of the gap between the fixed inductor and the workpiece regularly. When there is a large wear (>1mm), the positioning block must be replaced in time.
Periodic verification of quenching process parameters.Because the existing induction quenching equipment generally uses electrical parameters and other indirect parameters (current, voltage, output power, relative moving speed) to control the thermal parameters (heating temperature, heating time), the stability of the equipment has a greater impact on the quality of screw quenching.Therefore, when the equipment (including quenching inductor) is overrepaired or the electrical components are replaced, the quenching process parameters need to be re-verified.At the same time, in the normal production process, the original quenching process parameters must be verified regularly to ensure the long-term effectiveness and controllability of the production process.Ensure that the lead screw is sufficiently tempered after quenching.Through a large number of tests, we found that the secondary tempering process of "160 ~ 180 ℃/ 8h/ air cooling" can effectively release and eliminate the internal stress caused by the induction quenching process of large screw, and greatly reduce the cracking rate after grinding.
3. Grinding process control
The methods of "reducing the grinding amount of each feed, multiple feed" and "grinding-stable surface temperature-grinding" can effectively reduce the grinding heat and grinding stress on the surface of the lead screw, and prevent the phenomenon of "secondary quenching" or "secondary tempering" during the lead screw grinding, so as to avoid the generation of "grinding crack".
3. Validity confirmation
From March to October 2006, we took the above measures for 586 large screw (among which, 504 pieces of 80mm;53 pieces of 100mm;The process control and inspection were carried out, and no grinding cracking occurred.The strength and toughness of the lead screw grinding in the internal stress exceeds the yield strength of steel parts to produce grinding cracks.