Distortion Problems in Vacuum Heat Treatment of Ultra-High Strength Steel Shoulder Shafts

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Shoulder shaft parts are important parts used in the aircraft industry. The parts are composed of rotary shaft, fork-shaped notch and intersection hole and other structural parts, which are relatively complex fork-ear connection shaft parts. Such parts are not only important characteristics of each shaft diameter and coaxiality, but also the dimensional accuracy, symmetry and perpendicularity of the notch at the other end of the shaft diameter. It has always been a feature that is extremely difficult to guarantee and has become the key to the high-quality delivery of shoulder shafts.

In the process of vacuum heat treatment, due to improper process, the distortion of the parts will be higher than expected, which will lead to difficulties in further processing and application, so the appropriate process selection is particularly important.

Brief process flow:

The technological process of the part is: free forging blank → machining → stress relief tempering → machining → quenching, tempering → machining → low temperature tempering → surface treatment.

The factors affecting the quenching distortion of the shoulder shaft are as follows.

1) The installation is unreasonable. There is an angle between the center of gravity of the shoulder shaft and the hanging point of the part and the line of gravity action. When the shoulder shaft is heated and insulated, plastic deformation occurs under the action of gravity due to the relatively large self-weight of the shoulder shaft.

2) The shape of the shoulder shaft is complex. During the previous machining process, the uniform distribution of stress on the surface of the part is destroyed, and there is a large residual stress. During the heating process of the shoulder shaft, the superposition of the residual stress and the thermal stress causes the shoulder shaft to be distorted.

3) Improper process parameters. When the shoulder shaft is heated, the temperature difference between the inside and outside of the part is large, and the shoulder shaft is distorted under the action of thermal stress.

4) When the furnace is installed, the mounting distance of the shoulder shaft is different, the heating and cooling of different parts of the same shoulder shaft are uneven, and the shoulder shaft is distorted under the action of tissue stress and thermal stress.

5) During the cooling process of the shoulder shaft, due to the complex shape, uneven cooling of different parts of the same part, and asynchronous tissue transformation, distortion occurs under the action of tissue stress and thermal stress.

The solution to the distortion problem:

1) Design and manufacture tooling, limit the degree of freedom of the shoulder axis, and ensure that the angle between the center of gravity of the part and the suspension point of the part and the line of gravity action is close to zero.

2) Optimization of heat treatment process parameters. The stress relief tempering process is added, the shoulder shaft is cooled to room temperature and then reloaded for heating, and the hanging spacing between parts is strictly controlled to be uniform, and the 670 ℃ holding time of the first stage of quenching heating is extended to ensure that the temperature difference between the inside and outside of the parts is reduced.

3) Adjust the hanging direction of the shoulder shaft. With the shoulder taper on top, change the direction of the quench oil into the semi-closed cavity.

4) Change the cooling method. When the internal and external cooling of the semi-closed cavity of the part is changed to external cooling, since the external shape of the part is almost symmetrical, the cooling rate of each part is basically the same, and the internal stress generated by the quenching of the shoulder shaft is relatively uniform, so the quenching distortion generated by the shoulder shaft will be distorted. little.

In addition to the appropriate process selection, the choice of vacuum furnace also determines the quality of parts processing. A vacuum oil quenching furnace with uniform heating and sufficient oil quenching process can ensure the quality of the finished parts. Gehang Vacuum’s VOGQ series vacuum oil quenching furnaces have been widely recognized in the industry based on more than ten years of manufacturing and development experience. The product line includes two-chamber, three-chamber, horizontal and vertical models, and its main features are:

1. The overall layout is compact, reasonable and beautiful.

2. Octagonal furnace furnace heating structure, good uniformity of furnace temperature.

3. The heating element adopts the unique non-ceramic support technology, which can be used for 5 years without worrying about the decline of insulation performance.

4. The internal material truck adopts a superimposed integral telescopic structure, frequency conversion speed regulation, stable material transfer, and executes the “full-fast-slow” running rhythm, and the transfer time is short.

5. Internal heat exchanger and forced air circulation cooling method, the jet gas is uniform and the flow rate is large, and the cooling speed is fast.

6. The capacity of the quenching oil tank is large, and the hot oil is cooled by the external circulation of the hot oil pump and the heat exchanger, and the cooling of the quenching oil is better.

7. Double selection of oil stirring and oil spray cooling methods, suitable for different types of parts, small deformation of the workpiece.

8. Among the products of the same specification, the furnace capacity is large and the efficiency is high.

In addition to standard furnace models, Gehang Vacuum’s engineering team also provides customized services. Through their rich experience, we can not only carry out installation and after-sales, but also help customers solve technical difficulties, so as to be easy to use, fast to manufacture, and conducive to the efficient operation of the factory.