Fixture Hardening of Large Components
Given the ever-increasing trend toward renewable energy, the production of large gears and bearing rings – such as those used in wind turbines – is gaining in importance.
Until only a few years ago, such large components were usually custom manufactured to order. Today, they are often produced in small quantities. Requirements such as cost reduction, quality improvement and process automation are the result.
In the case of case-hardened transmission and roller-bearing parts, case depths of several millimeters are currently required in order to be able to compensate for the distortion created during conventional quenching (out-of-roundness of several millimeters) by means of time-consuming and expensive subsequent grinding processes. Apart from the high reworking costs, the large carburization depths cost the manufacturer dearly due to exorbitant furnace holding times.
As a leading manufacturer of fixture-hardening facilities, HEESS decided to analyze the processes involved in the fixture hardening of large gears and bearing rings in greater detail in a joint research project with the Bremen Institute of Materials Science (IWT). The findings from this project have made it possible to selectively counteract the creation of form changes such as out-of-roundness and conicity during the quenching process, resulting in significantly reduced distortion.
For implementation of these requirements, a hardening machine was designed (Fig. 1) that allows workpieces with an outside diameter of up to 1,500 mm (59 inches) to be processed. During the early stages of the design process, IWT Bremen provided simulations of the flow around the part (Fig. 2).
The resulting findings were incorporated in the design of the machine and tool. This allowed a uniform flow around the part to be achieved.
The movable machine table to which the fixture is attached is mounted on the lower machine part, which is designed as a basin. This is provided with several inlet ports for the quenching oil.
The hardening fixture (Fig. 3) consists of a base plate on which lamellas are pressed radially from the inside against the part to be hardened by means of a cone. To fix the part axially, two independent hold-down devices can be used. By varying the cone and/or lamellas, the tool can be used for a wide range of parts. After the hot part has been inserted, the machine table will move in. The carrier plates with the hold-down devices and hydraulic cylinders in the upper part are then lowered, and the cylinders apply the pre-set hydraulic pressures required to reach the desired expansion and hold-down forces to the cone and hold-down devices.
During the subsequent controlled and reproducible quenching process, a limitation bell mounted to the carrier plate restricts the flow space around the part, reducing the filling time. The quenching oil flows past the part on the inside, the outside and underneath through grooves in the base plate of the tool. The flow rate around the part is so high that very uniform cooling across the whole part height is realized. The inside to outside distribution ratio of the quenching oil can be set at the optimum rate for each part.
In a test series, six rings made of 18CrNiMo7-6 with an outside diameter of 1,100 mm (43.3 inches), inside diameter of 800 mm (31.5 inches) and height of 250 mm (9.8 inches) were produced. Five rings were carburized. The sixth ring was fitted with thermocouples that were placed in various bores close to the edge inside and outside, top and bottom as well as in the center of the ring cross section (Fig. 4).
This ring was used in preliminary tests to measure the temperature variation as a function of the position during cooling. Based on these cooling curves, it was possible to determine, for example, that for this ring geometry the cooling can be designed to be more uniform by setting an oil-flow volume of 60% across the inside and 40% across the outside of the ring (Fig. 5). This is due to the fact that the outside of the ring provides a larger surface for heat transfer than the inner side.
It was assumed that more even cooling also results in lower distortion, and this setting was therefore chosen for further tests. Before heat treatment and after each change in condition, each of the rings was measured inside and outside on three levels across the part height using a 3-D measurement device. The inside and outside diameter and the out-of-roundness were determined from these measurements. The differential between the upper and the lower diameter was then calculated as measure of the conicity.
In order to better analyze the process over time, in-situ radius measurements were also made at seven positions. Pneumatic cylinders with integrated displacement measurement systems were used for this purpose. This allowed the change of diameter, roundness and conicity over time during cooling to be recorded for subsequent analysis.
The maximum out-of-roundness of the parts before heat treatment was around 0.34 mm. The evaluation of the conicity yielded a maximum value of 0.8 mm. After fixture quenching, the roundness values after heat treatment were 0.64 mm maximum. The conicity change was approximately 0.7 mm in the worst case and 0.2 mm in the best case, depending on the pressing pressure and flow. This compares to out-of-roundness of 3-5 mm, which normally occurs in parts of this type during free quenching. This shows that rings with good roundness at the start of the process can usually be kept round during the quenching process.
Is repair possible?
The question arises, however, whether it is possible to “fix” rings with poor roundness values (several millimeters) by means of fixture hardening. To investigate this question, a ring was intentionally deformed using the hardening machine, resulting in a distortion of 3.3 mm (0.13 inches). During the following test, the ring was then hardened in the fixture. The roundness of the previously deformed ring could be improved from 3.3 mm roundness to 1.9 mm (0.075 inches) during this test. This shows that it is possible to round deformed rings within certain limits (Fig. 6).
The mechanism creating this effect is that only a few lamellas initially rest against the part in the case of a deformed ring. The total force focuses on these very few lamellas until finally all lamellas come to rest against the part, which will then provide a targeted holding force during quenching.
It has not been established what proportion of this roundness improvement can be achieved in the hot condition before quenching starts by exceeding the yield point or through creep and what proportion is achieved later during the cooling process assisted by the transformation plasticity. Further investigations are planned in this regard.
Another factor influencing distortion is the design of the quench-oil flow around the part. Due to the conical design of the expanding cone, a flow profile is created in the space between the lamellas and the inside of the part. This profile is characterized at the lower inner side by an initially slow flow and a fast flow at the upper inner side (decreasing flow cross section from the bottom to the top). The flow can be actively controlled by using guide plates with different shapes between the lamellas, optimizing the conicity.
These trends, which have been confirmed in tests, coincided with practical observations experienced by industrial users of HEESS production machines.
In conclusion, the following findings could be determined from the research project.
- Fixture-hardened rings reproducibly demonstrated significantly better roundness values than the values reported from free quenching.
- Deformed rings can be rounded within certain limits by means of fixture hardening.
- Using a quench press makes it possible to reduce scrap parts.
- Deformed parts can be processed into good parts.
- The interaction between optimized flow and specific force application has a positive influence on the roundness.
- Besides roundness, conicity changes can also be influenced during fixture hardening. Both the expanding force and the flow control during quenching have a major impact on the result.
- A vast cost reduction can be achieved using a quench press.
- The investment for furnaces and grinding machines can be reduced drastically.
Details of the data presented are available on request.