Residual stresses from machining

Machining and fixturing before heat treatment are among the potential sources for distortion and residual stress.

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In this column, we will discuss the sources of distortion and residual stresses from machining operations. The problem of residual stresses and distortion is universal across industry. It is a problem which can affect fit and finish of the final product, creating non-value added manufacturing steps, and can adversely affect the life of a product from shortened fatigue life.

Introduction

Distortion is not limited to heat treatment, but each step in the manufacturing process contributes to distortion. The sources of distortion follow the manufacturing process chain. Looking at distortion, the primary sources are volume changes from phase transformations and precipitation, or from deformation from either plastic or elastic deformation [1]. These types of residual stresses and distortion are usually the result from heat treatment or thermal processing.

The literature is filled with papers trying to understand and model the distortion of components. Modeling has been used to understand the heat-treatment process and the entire process chain. However, modeling is usually used to model the component under perfect conditions — new tooling, homogeneous steel, uniform heat-treating conditions and quenchant. Rarely are sensitivity studies performed to look at the expected variation of distortion due to manufacturing variation.

Literature Review

In the typical manufacturing process to manufacture a steel or aluminum part, incoming material is purchased; shaped by forming, forging, or casting; machined; heat treated; and finish processed (final machining, plating, or painting). This is illustrated in Figure 1.

Figure 1: Typical fabrication sequence or process chain of a generic steel component.

There are many sources of distortion — many of which have been described earlier. A fishbone chart illustrating some of the common sources of distortion is shown in Figure 2. It is not the purpose of this article to review all the possible sources and permutations of distortion, but to highlight some of the important factors from machining.

Figure 2: Fishbone diagram showing the many factors affecting distortion and residual stresses in a fabricated component.

Machining

Prior operations such as machining create residual stresses at the surface. This stress is relieved during heat treatment, resulting in distortion.

During turning or milling operations, the workpiece is held fixed by a chuck or jaws. The method of holding the workpiece can result in residual stresses. In one study [2], 100Cr6 (SAE 52100) bearing rings were evaluated on the influence of clamping mechanism, cutting speed, depth of cut, and feed rate on the roundness. Two different types of clamping mechanisms were used: a mandrel clamp and segmented jaws.

The study showed (Figure 3) that the residual stresses in the tangential direction of the two rings for the different clamping methods were nearly identical with a mean residual stress of 600 MPa. The segmented jaws show a periodicity of three around the circumference, with the tangential residual stress varying between 500 and 700 MPa.

Figure 3: Residual stress distribution around a ring circumference depending on the clamping technique used.

The segmented jaws had a segment of approximately 120°; however, the real contact was limited to a single point in the middle of each segment. Therefore, the clamping forces are induced at three locations 120° apart. This results in the ring bulging at these locations. The mandrel supports the bearing uniformly around the circumference, resulting in a uniform residual stress.

The influence of cutting parameters on residual stresses on machining 100Cr6 were also investigated by Grote [2] for rings that were clamped by a mandrel. Four cutting parameters were examined: cutting speed, nose radius, depth of cut and feed rate. The influence of the different parameters is shown in Figure 4. The parameters used for the DOE is shown in Table 1.

Figure 4: Results of DOE conducted by Nowag, et al [2], on 100Cr6, using a TiN tool and a 3 percent emulsion coolant.
Table 1: Levels used by Nowag et al, examining the effects of machining parameters on the external turning of 100Cr6 (SAE 52100).

It was determined that low feed rates contributed to high tangential residual stresses while high cutting speeds and depth of cut contributed to high mean tangential residual stresses.

Further work was conducted by Grote [3], comparing the residual stresses generated by different methods of clamping a 100Cr6 work piece during internal turning.

Funatani [4] examined the influence of shaving thickness on the error over-ball diameter. As the shaved thickness increased, the error over-ball diameter increased (Figure 6). It was found that duller tools resulted in greater cold work (Figure 7), and increased distortion before and after heat treatment. The paper established limits on tool wear before a tool was changed.

Figure 5: Examination of jaw type on the out of roundness during internal turning of a 100Cr6 (SAE 52100) component. Machining accomplished using a 3 percent oil emulsion coolant and Ti-N coated carbide machine tool [3].
Figure 6: Effect of shaving thickness on the over-ball diameter (OBD) [4].
Figure 7: Different depth of cut using worn tool, showing increased cold work at the surface [4].

Conclusions

In this short article, we have demonstrated that the machining parameters, including the method of holding the workpiece, can have a large effect on the residual stress and distortion of a machined component. While heat treatment is usually the largest component of distortion, prior operations will contribute to the resulting distortion of a part. By performing stress-relief after each operation, the size and amount of distortion can be determined. Steps can then be taken to eliminate the residual stresses from each operation, resulting in a distortion-free part.

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References

  1. H.-W. Zoch, “From Single Production Step to Entire Process Chain – the Global Approach of Distortion Engineering,” in Proc. 1st intl. Conf. Distortion Engineering, IDE 2005, Bremen, Germany, 14-16 September, 2005.
  2. L. Nowag, J. Solter, A. Walter and E. Brinksmeir, “Effect of Machining Parameters and Clamping Technique on the Residual Stresses and Distortion of Bearing Rings,” in IDE 2005, Bremen, Germany, 2005.
  3. C. Grote, E. Brinksmeier and M. Garbrecht, “Distortion Engineering in Turning Processes with Standard Clamping Systems,” in IDE 2008, Bremen, Germany, 2008.
  4. K. Funatani, Toyota Technical Review, vol. 18, no. 1, pp. 11-15, 1967.