Advanced Distortion Control for Case Hardening of Transmission Components

Distortion control is one of the major challenges in modern manufacturing. Distorted gear components cause noise in the transmission and may even create problems during transmission assembly. Especially battery-operated electric vehicles (BEV) and other electrified vehicles (such as Hybrids) require a low-noise transmission with high-precision components.

Distortion has a strong cost impact because distorted components often need hard machining after heat treatment. Better control of distortion means:

less cycle time per part in hard machining,

less hard-machining capacity needed, and

less tooling cost for hard machining.

With excellent control of distortion for some applications, hard machining can be eliminated. For some other applications, the need for a cost-intensive press quench can be eliminated if excellent distortion control is established.

Distortion Mechanisms

The plastic deformation of metallic components during heat treatment is called distortion. Distortion occurs if the stress in the material exceeds the yield stress of the material. During case hardening the components are exposed to high temperatures ranging from 880°C to 1.050°C and the yield stress decreases strongly with increasing temperature of a component. Three different types of stress in the material need to be distinguished:

Residual stresses (they are induced before heat treatment by casting, forging, machining, etc. [Ref. 1]).

Thermal stresses (they are caused by the temperature gradient while heating and quenching).

Transformation stresses (they are caused by the transformation from ferrite to austenite during heating and transformation from austenite to martensite/bainite during quenching).

These three types of stresses overlay with each other and add up to the total stress in the component. They are influenced by part geometry, steel grade, casting, forging, machining, etc. and they are influenced by heat treatment. If the total stress in the component exceeds the yield stress, then plastic deformation (distortion) of the component takes place. The chronology and the height of the three types of stresses leading to distortion are dependent on numerous influencing factors, see Figure 1.

When analyzing distortion, it should be distinguished between size change and form change. Size change refers to the homogenous growth or shrinkage of the treated component while maintaining its shape (e.g., the homogenous growth or shrinkage of the diameter or the length of the component). Form change refers to a change in the shape of the part (e.g., roundness of a gear, bending of a gear shaft, or deformation of gear tooth geometry).

All carburized components will have some size change due to a transformation in the microstructure from ferrite into martensite. The size change must be controlled with green machining. For example, if an outside diameter grows 10 microns during heat treatment, it should be machined 10 microns smaller before heat treatment.

There are many different characteristics impacted by form change. However, it helps to better understand form change by simplifying distortion into the two main parameters flatness and roundness.

Form-change for shafts is mainly straightness. When analyzing gears, flatness can be determined by the amount of “helix variation,” or “lead variation.” Roundness is a measurement of “circularity.”

The helix average also changes during heat treatment. The helix average changes in a minus direction, meaning the tooth is unwinding. For instance, the helix angle might be 15 degrees in the green state, but it may change to 14 degrees after heat treatment. This must be compensated for with green machining.

Low-Pressure Carburizing in Combination with High-Pressure Gas Quenching

Heat treatment distortion can be significantly reduced by applying the technology of Low-Pressure Carburizing (LPC) and High-Pressure Gas Quenching (HPGQ). LPC is a case-hardening process performed at a pressure of only a few millibars using acetylene as the carbon source. During HPGQ the load is quenched using an inert gas stream instead of a liquid quenching media. Usually, nitrogen or helium are used as quench gas (Refs. 2, 3, 4).

HPGQ offers significant potential to reduce heat treatment distortion. Conventional quenching technologies such as oil or polymer quenching exhibit nonhomogeneous cooling conditions. Three different mechanisms occur during conventional liquid quenching: film-boiling, bubble-boiling, and convection. Resulting from these three mechanisms, the distribution of the local heat transfer coefficients on the surface of the component is very nonhomogeneous. These nonhomogeneous cooling conditions cause high thermal and transformation stresses in the component and subsequently distortion. During HPGQ only convection takes place which results in much more homogenous cooling-conditions, see Figure 2 (Refs. 5, 6). Significant reductions of distortion by substituting Oil-quench with HPGQ have been published (Refs. 7, 8).

Another advantage of HPGQ is the possibility to adjust the quench intensity exactly to the needed severity by choosing quench pressure and quench velocity. Typical quench pressures range from 2 bar to 20 bar. The gas velocity is controlled by a frequency converter. Typical gas velocities range from 2 m/s to 20 m/s. Quench pressure and gas velocity are chosen depending on the part geometry and the steel grade of the component to achieve optimum results.

Strategies for Distortion Control When Applying HPGQ

As described above, the gas quenching process offers two major advantages when compared to liquid quenching in terms of distortion control:

More homogenous heat transfer coefficient around the surface of the quenched component.

The flexibility to tailor the quench intensity specifically for the needs of the quenched component.

To fully exploit the benefits of HPGQ it is important to optimize the design of the heat treatment fixtures. The fixture should provide a horizontal loading of the components and should allow a homogenous gas flow around the treated components during quenching as much as possible. Figure 3 shows an example of a fixture made of carbon-reinforced carbon (CFC).

In addition, the HPGQ—process offers more options for further reduction of heat treatment distortion. These process modifications are explained in the following.

Dynamic Quenching

Dynamic Quenching is a process where the quenching parameters gas pressure and/or gas flow velocity are stepwise varied during quenching, see Figure 4. This process is typically divided into three steps (Ref. 9):

High quenching severity until a certain part temperature is reached.

Quenching severity is reduced for a set time to allow for temperature equalization in the part.

Quenching severity is increased again until the end of the quenching process.

The control system in the quenching chamber allows to control the different quenching steps of “dynamic quenching” in a very accurate way with good reproducibility. Optimum results are achieved when using helium. The light-quenching gas helium can be decelerated and accelerated very precisely for optimum distortion control.

The application of Dynamic Quenching leads to a reduction of thermal stresses during quenching and thus it offers the potential to reduce heat treatment distortion for certain applications. In addition, a positive effect on fatigue properties can be achieved as well by applying this process modification.

Reversing Gas Flow

High-pressure gas quenching is typically performed with a flow direction from top to bottom through the load. However modern gas-quenching chambers offer the possibility to reverse the direction of the gas flow during quenching. Reversing gas flow means that the flow of gas is alternated back and forth from top-to-bottom and bottom-to-top. By alternating the gas flow direction, there is less difference in the cooling curves of parts placed in different layers. This reduces the variation of distortion inside the load.

A schematic view of a quench chamber with reversing gas flow is shown in Figure 5. To allow for the alternating flow direction, the chamber is equipped with flaps that are operated pneumatically. Depending on the setting of the valves, either top-to-bottom or bottom-to-top flow direction is put into effect. The alternation of the flow direction is time controlled.