Influence factors of gear parts deformation and heat treatment process

Abstract: This paper analyzes the various influencing factors of gear heat treatment deformation, and points out that the heat treatment deformation of gear parts is mainly affected by many factors such as part structure, material, forging, machining, heat treatment technology, and equipment.

A brief introduction of carburizing heat treatment

Shaft and is commonly used in automobile gear after forging, normalizing, machining, carburizing and quenching and tempering heat treatment, get play for high hardness of carburized layer and the core for the organization, has good comprehensive mechanical properties of these organizations, as well as the residual stress after quenching on the mechanical property of shaft and gear has a decisive role. At present, carburizing heat treatment is widely used in our company, and it is also a relatively mature heat treatment process. The purpose of carburizing is to get a high carbon surface layer, as well as a low carbon core, in order to ensure the high plasticity and toughness of the core, high surface hardness, improve the hardness, wear-resistance, and fatigue strength of the workpiece.

Analysis of heat treatment deformation

1. Factors influencing the deformation of heat treatment

At the same time as heat treatment, the shape and size of the parts must be changed, which is the result of the joint action of tissue stress, thermal stress, and gravity. Both the structural stress and the thermal stress are heat treatment stress, and the structural stress refers to the stress caused by the different transformations of the different parts during the heat treatment due to the different cooling of each part. The thermal stress is the stress caused by the uneven expansion of heat and contraction of cold caused by the temperature difference of each part of the workpiece. During quenching, there are mainly two kinds of deformation of parts: the deformation of geometric shape, mainly the deformation of size and shape, caused by quenching stress; The deformation of the volume is mainly the expansion or reduction of the workpiece volume in proportion, which is caused by the change of specific volume during the phase transition.

There are lots of factors affecting parts heat treatment deformation quenching process is just released the potential stress deformation of the parts, and the deformation potential stress is accumulating in the whole process of parts processing, can be summarized as the chemical composition of the material, forging process of the forging temperature, cooling speed after forging, machining process of feed rate, cutting speed, feed quantity, the clamping way, in the process of heat treatment heating speed, cooling speed, heating temperature, and other factors. The heat treatment process is the final process, and all the upstream processes will bury the seeds for the heat treatment deformation of the parts. Therefore, the study of heat treatment deformation cannot solely study the heat treatment process itself but should focus on the structure of the parts, materials, and all processing procedures of the parts.

2. Annealing process

Processes in which metals out of equilibrium are heated to a higher temperature, held for a certain period of time, and then slowly cooled to produce tissue close to equilibrium are collectively referred to as annealing. The purpose of annealing is to uniform chemical composition, improve mechanical properties and process properties, eliminate or reduce internal stress, and provide suitable internal structure for the final heat treatment of parts.

3. Complete quenching process

Subeutectoid steel or its components are heated to a temperature above Ac3 and then cooled at a cooling rate greater than the critical cooling rate to obtain martensite structure. The heat treatment to improve the strength, hardness, and wear resistance is called complete quenching.

Three, parts heat treatment deformation test example

Our company produces a kind of gear parts. The technological process of this part is blanking → forging → normalizing → grinding → hobbing → inserting spline → shaving → carburizing → quenching → tempering → shot blasting → edge grinding → end face and inner hole of the car. The material is 8620RH, heat treatment technology requirements are: quenching layer depth 0.84 ~ 1.34mm, surface hardness 58 ~ 63HRC, heart hardness 30 ~ 45HRC, metallographic structure in line with TES-003 standard.

Parts production is bigger, the string is put type preparation, the big (219.2 ~ 219.45 mm) in diameter, wall thickness is thin (27.05 mm), and the structure is not completely symmetrical, namely the Bend side of small diameter hole, and Aside face of large diameter internal spline, lead to the two end face structure parts A and B (end) the characteristics of the trend of thermal deformation is inconsistent.

At the end of 2016, the runout error of Bend face (runout value of hot back end face ≤0.06mm is required by the process) of the finished product after sudden heat treatment, thus resulting in the runout error of the drum shape and tooth direction Angle. For the remaining parts in this batch that have been forged and processed by the hot front machine, the temporary test and fine-tuning process are carried out in the process of heat treatment to control the thermal deformation to the maximum extent and reduce the failure rate of the parts.

1. Original heat treatment technology of parts

The original heat treatment equipment used for the parts is AICHELIN42 station ring rotary bottom continuous furnace, set pre-oxidation, carburizing, quenching, cleaning, tempering in one. Carburizing USES nitrogen and methanol as the basic atmosphere, acetone as the enrichment agent, according to the theory of nitrogen-methanol atmosphere, the supply ratio is methanol: nitrogen =1L/h:1.1m3/h, CO content value instrument set 20%. The original heat treatment process is as follows: pre-oxidation → carburizing → oil quenching → cleaning and tempering.

2. Temporary test, fine-tuning process, and result analysis of heat treatment process

(1) Add the annealing process and result analysis

High temperature annealing is adopted, the annealing process is set as 400℃ for 2h, air cooling with furnace cooling to 350℃, and then carburizing and quenching. The end hop values before and after heating were measured one-to-one.

The average thermal deformation of the end slip of annealed carburizing parts is 0.033mm, but the test data is too small, so it is only for reference.

(2) Adjust the quenching mixing parameters and analysis of the results

The premise condition of fine-tuning heat treatment process parameters is to ensure that the parts meet the requirements of the design of the heat treatment technical indicators. For general quenching process, the most ideal state of finished parts with a fast mixing speed setting time martensite phase transformation, and in the next slow stirring speed setting time to reduce the cooling speed in order to reduce the deformation of the heat bilges cold shrink, such ability in ensuring complete heat treatment technical indicators at the same time to reduce the thermal deformation of parts. Reduce the fast quenching stirring time to 45s and the slow quenching stirring speed to 700r/min, and adjust the quenching stirring.

After adjusting the stirring, the measurement results of the end hop of the parts before and after heat treatment show that the average hot deformation end hop of the parts after adjusting the quenching mixing parameters is 0.057mm, which is less than the average hot deformation end hop of the parts after using the original quenching mixing parameters. However, most of its hot back end hops exceed the technical requirements, and the standard deviation of hot back end hops is 0.015.

(3) Add annealing process + adjust quenching mixing parameters and analysis

Based on the above two kinds of the test (increase after annealing process less thermal deformation end jump (0.033), and adjust the mixing parameters after hot deformation quenching end jump high volume (0.057), small discrete (0.015)), the two methods used in the batch parts at the same time, the parts prior to annealing, after using adjusted mixing parameters for carburizing and quenching, observe side jump thermal deformation parts.

A. Using high temperature tempering furnace for annealing process + quenching mixing parameters: using high temperature tempering furnace for annealing process, then stir with adjustment of quenching parameters for carburizing and quenching process, before and after heat-treatment side jump measurement results as shown in figure 5, shows hot back-end jump conform to the requirements of the process, the average amount of thermal deformation end jump of 0.034 mm, hot back-end jumping standard deviation of 0.018.

B. The annular furnace is used for the annealing process + adjusting quenching and stirring parameters: considering the logistics transport problem, the heat treatment process is further optimized, an annealing process is carried out in the pre-oxidation zone of the annular furnace, according to the annealing process requirements, carburizing is carried into the main furnace after holding temperature at 400℃ for 2h, and stirring parameters are adjusted at the same time. The measurement results of the end hop before and after the heat treatment of the parts show that the thermal back hop conforms to the technological requirements. The average thermal deformation end hop value is 0.036mm and the standard deviation of the thermal back hop is 0.017.

C.For more than 4 kinds of the fine-tuning process with the original process comparing with the results and its thermal deformation, known only in the process of quenching mixing parameters remedy hot back-end jump value is bigger, is greater than the increase in annealing + quenching mixing parameters of process types, choose after class remedial process types, from considering the feasibility of manufacturing, annular furnace oxidation zone annealing + stirring parameter adjustment process type is better than that of the high temperature tempering furnace annealing + stirring parameter adjustment process type.

Adopting the optimized remediation technology: the annular furnace pre-oxidation zone annealing + stirring parameter adjustment, the production of the remaining parts of this batch, the defective rate of parts from the sudden 30% to 6%, greatly reducing the defective rate, effectively reducing the economic loss for the company.

D. Conclusion

Adding the annealing process and adjusting quenching parameters after carburizing and quenching can effectively improve the deformation of parts after heat treatment and provide a feasible remedial technology for similar problems later. But the heat treatment deformation of the parts is not only by heat treatment process adjustment can be completely solved, before the heat treatment of each process will cause a certain impact on the deformation of the final heat treatment, the final product conformity needs all processes to coordinate, cooperate with each other, to find the appropriate process, in order to improve the qualified rate of parts, to ensure the quality of the product.

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