Research on Process Improvement of CNC Precision Parts Machining-Tooth Surface Carburizing Parts

Part analysis

Gear parts can be pided into motion transmission gears and power transmission gears according to their functions. Power transmission gears are often cemented to obtain a high hardness and high wear resistance surface, while the core still retains plasticity and good toughness so that the parts can Withstand a certain impact load. Compared with nitriding, the advantage of carburizing hardening is that the depth of the layer is larger, allowing larger tolerances to be reserved to finish the tooth shape. It has been widely used in gears, shaft pins and other parts of our transmission system.

An output gear manufactured by our factory belongs to the typical external tooth + internal spline short shaft type parts (see Figure 1), material grade S82 (low carbon alloy structural steel), involute outer tooth diameter section 18, number of teeth 39, The pressure angle is 25° and the accuracy grade is AGMA8 (equivalent to 7 grades specified in GB10095). The inner involute spline has 16 teeth, the diameter is 20/40, the pressure angle is 30°, and the ANSI standard rounded root side is matched with 7 levels of precision.

Research on Process Improvement of Tooth Surface Carburizing Parts

This part requires the gear tooth top, tooth surface, root and tooth side to harden to more than 700 HV (HRC ≥ 62), and the remaining surfaces are not carburized. When designing the process plan for the first time, the hardness of the non-carburized surface and the core after quenching and tempering reaches HRC 42-47, and the ultimate hardness of the parts that can be handled by the powder metallurgy tool of our factory is no more than HRC42. Therefore, the traditional process of "copper-carburizing-removing copper" is used in the process of finishing the process. After the tooth blank is finished, the internal spline is inserted. After the copper is plated, the copper layer on the top and the flank of the tooth is removed. The grinding tooth allowance is reserved, and the non-carburized surface is protected from the activated carbon by the copper layer during the heat treatment process, as shown in the following figure:

After the first batch of products were processed, the pass rate was found to be less than 30% during the sampling test. The results of the measurement report of the dental cutting process before the heat treatment were all qualified. The reasons for the analysis leading to the out of tolerance are:

a) After the heat treatment, the two steps of the center hole and the outer grinding are carried out, and the reference variation is measured.



b) The various internal stresses generated during machining and heat treatment are higher than the yield strength of the material, and the stress release causes irreversible plastic deformation of the part.



There are several ways to deal with the above two effects:



1) Reasonably distribute the cold machining dimensional tolerance, properly improve the internal spline machining accuracy, and compensate for the heat treatment distortion with rich size and shape accuracy.



2) Re-stressing means such as circulating heat preservation and ice-cooling treatment are used to eliminate residual stress generated in machining as much as possible.



3) Strictly control the heating rate during the heat treatment, use lower carburizing and quenching heating temperature to reduce the thermal stress generated during the heat treatment; and the surface carbon concentration and carburizing layer of the gear without affecting the quality of carburizing The depth is controlled in the lower limit range.



4) Use special fixtures to increase the rigidity of the gear to reduce the degree of heat treatment distortion.



Based on this, we designed a second set of schemes, after the heat treatment, the finishing process benchmark and the measurement standard, and adjusted the splicing key process to the heat treatment, as shown in the following figure:



Research on Process Improvement of Tooth Surface Carburizing Parts



When the second batch of products was processed to the spline-keying process, the operator raised the problem that the spline inserting knife was abnormally chipped--the first batch of inserting knives capable of continuously processing 15 parts during the trial production, and the front knives appeared when only 4 pieces were processed in the batch. Face wear and chipping.



We verified that the base material and coating material of the tool design were correct. It is speculated that the main reason for the chipping of the gear insert is that the surface of the inner hole leaks carbon, and there is a high hard tissue locally.



There are two reasons for analyzing the reasons:



a) The size of the part is small. It is difficult for copper ions to diffuse to the bottom of the inner hole during copper plating, and there is no copper layer on the surface of the bottom of the hole.



b) The copper layer of the inner hole is uneven or the copper layer peels off during processing.



Since then, we have tried protective measures such as protective coatings on the inner hole. However, due to the high fluidity of the coating and the difficulty of operation, the improvement effect is not satisfactory.



Improved design



In recent years, our factory has deepened the docking and subcontracting mode. We have a more systematic and comprehensive understanding of the carbide tool. In the process of continuous upgrading of the tool supplier technology, the hardness limit of the insertable parts of our factory is gradually increased to HRC52~55. range. By combing the on-site realistic record of the output gear trial stage, we innovated the use of margin protection instead of the traditional copper plating protection in the latest improvement. The program flow is shown in the following figure:



Research on Process Improvement of Tooth Surface Carburizing Parts



The improvement idea of the new scheme is to reserve machining allowance on the non-carburized surface before heat treatment. After the whole part is carburized, the hard car removes the margin. In the state of HRC42~47 (core hardness after heat treatment), the spline precision is guaranteed. Although the scheme increases the manufacturing difficulty, the use of reliable tools can ensure the quality of the internal splines, eliminate the copper plating and copper removal process, and the production process is more streamlined, which greatly optimizes the uniformity of the process benchmarks before and after the heat treatment.



In order to prevent the secondary deformation of the part caused by the release of the cutting stress after finishing the car, the thickness of the protective margin should be as small as possible. Therefore, it is necessary to grasp the change rule of the hardness of the surface of the part to the core. We collected some of the out-of-tolerance parts in the trial stage and made a test piece (see figure) along the radial section of the gear for testing.

Research on Process Improvement of Tooth Surface Carburizing Parts



As can be seen in the figure, the carburizing treatment only changes the carbon content of the surface layer metal. After the quenching and low temperature tempering treatment, there is a hardness transition layer between the carburized layer and the part matrix structure (the closely arranged tempered horse) Clan organization). Taking into account the current extreme hardness of the spline spline and the economic cost of the tool. We used the microhardness method to measure the vertical distance from the surface to HV=463 (HRC47) for analysis of the maximum depth of the hardened layer.

Research on Process Improvement of Tooth Surface Carburizing Parts

Figure 6 shows the trend of hardness change near the top surface of the tooth



Our factory uses toluene as a carburizing medium in gas carburizing, using methanol as a diluent, the carburizing medium decomposes at the carburizing temperature and the following reactions occur:

Research on Process Improvement of Tooth Surface Carburizing Parts

Note: [C] in the formula refers to an activated carbon atom produced in a chemical reaction.

Research on Process Improvement of Tooth Surface Carburizing Parts

Table 1 carburizing process parameters of this part



During the detection process, we found that the depth of the hardened layer showed a regular change in the direction of the tooth shape—the hardening depth of the root portion was the smallest, the hardened layer near the top of the tooth was the thickest, and the hardened layer of the tooth near the indexing circle was between the two. . Through analysis, it can be concluded that after the austenite structure of the carburized surface of the gear absorbs the activated carbon atoms, the carbon concentration increases, and the difference in carbon concentration between the surface of the part and the core material forces the carbon atoms to diffuse internally, but due to the contour of the gear surface The specificity, the diffusion rate of activated carbon atoms in different parts is different, the reason is that the number of activated carbon atoms in contact with each surface of the gear is different - the surface contact amount is the largest near the tooth tip, the tooth surface contact is the second, and the tooth root surface is similar to the concave surface. The bottom of the cavity, so the amount of contact activated carbon atoms is the least (see Table 2).

Research on Process Improvement of Tooth Surface Carburizing Parts

Table 2 depth coefficient of carburized layer on gear surface



In the carburizing process, the protective margin should cover all non-carburized surfaces uniformly and continuously, so when determining the margin thickness, select the data collected from the tooth surface near the index circle. The formula is:



Remaining thickness ≥ tooth surface carbonization layer depth + hardness transition layer maximum depth



It can be seen from Fig. 6 that the vertical distance from the tooth surface to HV = 463 is concentrated in the range of 1.3 ± 0.1 mm, and the axial and radial margins are treated at 1.5 mm in practical use.



Hard car design and cutting parameters



3.1 Hard Car Technology Features and Analysis



After the carburizing and quenching of S82 steel, the tensile strength (ób) is increased to more than 2270Mpa. Compared with the soft state turning, the cutting force of the hard car system is increased by about 2 times. Improving system rigidity is the first problem we face.



The output gear structure belongs to a regular revolving body. The efficiency of turning out the outer circle and the end face protection margin is the highest. However, due to the limitation of the inner hole size, the Φ12 or higher size boring tool is not suitable for this part, so it is preferred to remove the inner hole allowance of the part. Drilling to avoid tremors due to insufficient rigidity of the boring bar.

Research on Process Improvement of Tooth Surface Carburizing Parts

Figure 7 Comparison of the structure of the front and rear parts of a hard car



The instantaneous impact of the blade when contacting the part is the main factor affecting the life of the tool. Considering that the surface hardness of the part reaches HRC62 or above, heavy-duty low-speed cutting can be used to reduce the impact of the impact on the tool and the machine tool. The tool elongation should be controlled in the knife. Within 0.5 times the length of the rod, and minimize the overhang to increase rigidity. The short and thick structure of the part and the small aspect ratio are very suitable for the clamping method of the outer centering and end face positioning. In order to minimize the overhang when the parts are clamped, we designed the turning sequence shown in the figure:

Research on Process Improvement of Tooth Surface Carburizing Parts

Figure 8 hard car positioning method and processing steps



The material from the surface layer to the core is high-carbon, medium-carbon, low-carbon tempered martensite structure. The turning insert requires high edge wear resistance and large depth of cut resistance to cope with variable load continuous cutting. Moreover, in order to reduce the vibration of the blade when switching between different tissue materials and reduce the influence on the surface quality of the part, an outer round bar with anti-vibration design should be used.



By looking up the product catalog, we initially developed two sets of turning solutions. The surface allowance was pided into 3 turnings - the roughing machine removed most of the hardened layer on the surface, the cutting depth was 1 to 1.2 mm, and the semi-finished car evenly distributed the surface allowance to 0.1. Mm, fine car strictly controls the surface roughness and guarantees the dimensional accuracy of the parts:

Research on Process Improvement of Tooth Surface Carburizing Parts

Table 3 comparison of outer blade solutions



Maximizing system rigidity means minimizing the overhang of parts and tools, and arranging the parts of the hard car after heat treatment to have a small aspect ratio (L/D).



Water-based cutting fluid is still used in the turning process. In order to enable the cutting fluid to reach the cutting edge cutting area quickly, we increase the pressure value of the coolant circulation system. The high-pressure cutting fluid can also effectively reduce the cutting accumulation and improve the surface quality of the parts. .



This part of the hard car is more inclined to stable clamping, blade clamping method. There are three kinds of blade clamping methods commonly used in our production site. The characteristics of the C-clamping system are more consistent with the cutting conditions of this part.

Research on Process Improvement of Tooth Surface Carburizing Parts

Table 4 Comparison of the clamping method of the outer circular blade



After two batches of in-process comparison verification, A-single blade can process 5-6 pieces, single-piece blade processing upper limit 13 pieces; B plan single-blade can process 7 pieces, single-piece blade processing upper limit 15 pieces, and finally choose B plan.



After removing the protective margin, the exposed base material has been transformed into a uniform tempered martensite structure, reaching the core hardness HRC42-47 of the part, which is not difficult to process, and the universal blade can fully meet the cutting conditions.



It has been proved by practice that the scheme of removing the protective margin of the hard car is feasible, the tool life is ideal, and the surface roughness, dimensional accuracy and position tolerance of the finished part meet the requirements.



Hard pinion and its cutting parameters



In the past, our factory used powder metallurgy as the base material of the pinion cutter. The hardness of the parts that can be processed by such tools is not more than HRC42. In order to meet the needs of the subsequent mass production of this part, this process improvement has customized the carbide inserting cutter made by DATHAN Company, and the coating material is TiAlN.



Compared with powder metallurgy tools, carbide inserting cutters have better wear resistance, but the bending strength and impact toughness are poor. Therefore, the cutting part adopts a large radial negative rake angle to improve the impact resistance of the gear shaping cutter. Anti-cracking ability.



The cutting parameters are shown in the following table:

Research on Process Improvement of Tooth Surface Carburizing Parts

Hard-tooth cutting parameters in Table 5



The tool has been machined with 52 pieces, and the tool is in good condition. According to the current tool wear rate, it is expected that the grinding cycle can reach more than 80 pieces.



Other measures to improve manufacturing process



After carburizing and quenching and tempering, the ice cooling treatment is equivalent to the secondary normalizing of the parts, which can further reduce the supersaturated martensite in the quenching process, reduce the lattice distortion, reduce the structural stress, and reduce the heat treatment deformation.



After the surface of the part is carburized, the hypereutectoid layer on the surface of the 5μm depth contains dense carbides, which form a very thin hard shell layer after quenching and tempering. The tip of the turning tool is most susceptible to damage when it contacts the surface layer. Therefore, the hard car should be kept as continuous as possible when removing the protective margin, and the surface to be machined should avoid structures such as relief holes and angular positioning holes.



The hard bore requires a large cutting force, and the torsion and tangential forces of the boring bar are multiplied. The tool should be concentric with the part or slightly higher than the center of the part, avoiding the distortion caused by the cutting force affecting the dimensional accuracy of the part.



In practical application, in order to further extend the life of the drill bit, we inject protective coating into the blind hole of the part. After drying for a short time, the coating will infiltrate with the surface of the inner hole and produce certain adhesion. After heat treatment, the sand can be completely removed. With the same turning conditions, the bit sharpening cycle can be increased by 1.5 to 1.8 times.



After this improvement, we compared the gear parts with such carburizing requirements, and summarized the necessary conditions for the applicable margin protection method - the part carburized zone and the non-carburized zone should have a reasonable boundary structure to the part. For example, both ends of the gear are designed with end face grooves for weight reduction.



in conclusion



The improved scheme has been verified by 4 batches of 52 parts. It is feasible to remove the protective margin for heavy-duty low-speed hard cars. The cutting tool has good cutting condition and the spline in the parts is 100% qualified. The process flow is shortened to 2/3 of the previous improvement plan, and the elimination of copper plating and copper removal processes further reduces the manufacturing cost. This program has a reference value for parts with carburizing requirements similar to the tooth surface.