Zhengzhou Gou's tells you about the defects and improvement measures of screw heat treatment

The lead screw is an important part of the cutting machine tool. The quality of each process will directly affect the quality of the lead screw, which in turn affects the accuracy of the cutting machine tool. The heat treatment of the screw requires complex processes, high quality requirements, and many influencing factors. Today, we will briefly introduce the defects and improvement measures of screw quenching heat treatment.

(1) The elongation caused by quenching is too large. For screws that are hardened after grooving, when determining the pitch size and allowance for machining, the elongation during quenching should be taken into account. Otherwise, the cumulative error will be too large, resulting in excessive grinding on one side of the thread and excessive grinding on the other side. Failure to do so will result in irreparable waste. There are two measures to solve this problem:

① When grooving during machining, compensate for the elongation in advance.

② Use the method of polished rod quenching and thread grinder grooving. The quality of this process is reliable, but the grinding workload is large. Grinding annealing must be prevented. It is suitable for alloy tool steel with good hardenability.

(2) Cracks in the workpiece. Precision screws of alloy tool steel are generally cooled with oil or nitrate during quenching. The possibility of quenching cracks is very small. The screws with surface induction quenching are also spray-cooled with polyvinyl alcohol (or polyether) aqueous solution. Greatly reduces the sensitivity to cracks; the most common occurrence of cracks is in the grinding process. There are many factors leading to grinding cracks, such as:

① Quenching and cooling are too intense and tempering is not timely or insufficient

②Defects in raw materials due to metallurgical processes

③Improper grinding specifications, such as large feed amount, insufficient cooling, etc.

④ The grinding wheel is too fine, the hardness is too high, the grinding wheel becomes dull, and the sand grains do not maintain sharpness, causing the grinding wheel to be squeezed and causing severe heating. The surface of the workpiece instantly reaches more than 860°C, and secondary quenching occurs due to the chilling effect of the coolant. , causing cracks.

⑤ The network carbide is serious, resulting in poor thermal conductivity of the workpiece and serious tendency to wear and crack.

⑥ Coarse grains caused by excessive heating temperature

(3) The metallographic structure of pre-heat treatment is poor. Mainly refers to the presence of network carbides or band carbides and poor spheroidization. If the raw material of the screw (or the modified forged blank) is pressed and processed, if the forging stop temperature is too high, the cooling is too slow, etc., and the network carbide is still greater than level 3, it must be removed and spheroidized. Annealing is performed to obtain uniform and stable spherical pearlite. Reticular carbides can reduce the surface smoothness of the workpiece, and the accuracy is difficult to meet the design requirements. The tool wear is serious during processing. During quenching and heating, the heating temperature must be increased or the holding time must be extended to achieve the homogenization of austenite. This will undoubtedly increase the deformation of the workpiece.

The spheroidization level of pearlite is divided into levels 2 to 4 or 2 to 5. This refers to the proportion of granular pearlite to all pearlite (or the ratio of granular pearlite to lamellar pearlite), because spheroidization is more suitable. The temperature is above and below the Al line, and a certain holding time at this temperature can meet the process requirements.

(4) The dimensional stability of the workpiece is poor after quenching. Precision hardened screws made of alloy tool steel will undergo dimensional deformation after quenching, grinding to finished size, and long-term storage at room temperature. This is mainly due to dimensional expansion caused by the transformation of retained austenite into martensite.

For alloy tool steel screws that require high hardness values (above 61HRC), because the tempering temperature cannot be very high, the amount of retained austenite is large, which will inevitably have an adverse effect on the dimensional stability of the workpiece, so it should first be Under the premise of affecting the hardness, increase the tempering temperature and appropriately extend the holding time to obtain the minimum residual austenite content. A more effective method for this kind of workpiece is to carry out cold treatment at about 70°C for 2 hours to reduce the amount of retained austenite to less than 1%-2%.

The screw is one of the important parts of various machine tools. It is the most common component on machine tools that converts rotational motion into linear motion or converts linear motion into rotational motion and positioning functional components. It must have high precision dimensions. and dimensional stability. Zhengzhou Gao briefly talked about the defects that may be encountered during the heat treatment process of the screw and some improvement measures. I hope it will be helpful to you.