Controlling Residual Stress and Retained Austenite for Better Spring Life

Springs add a bounce to life – until they don’t. Premature failures are often correlated to detrimental (usually tensile) residual stresses or less-than-optimal retained austenite content. Fortunately, residual stresses and retained austenite can be controlled during the manufacturing process. They can be verified using X-ray diffraction.

Residual Stresses

Springs, by their nature, are subjected to cyclic stresses, or fatigue stresses. Theoretically, if the loading stresses stay below a fatigue stress limit, then the part will have an infinite fatigue life. The problem occurs when residual stresses are not factored into the equation. Residual stresses, the stresses that remain after all external loads are removed, add to loading stresses to give a total stress. Thus, compressive residual stresses typically reduce the total stresses while tensile residual stresses increase the total stress. There is a strong correlation between compressive residual stresses and improved fatigue life. Tensile stresses, on the other hand, almost always lead to premature failure.¹

Putting compressive residual stresses into springs can be done during the manufacturing process. Shot peening, thermal treatments and work hardening techniques can be used to put beneficial compressive stresses at and near the outer surfaces to help prevent crack initiation and propagation. Stresses must balance through the thickness of a part. Low interior tensile stresses can be offset by the surface and near-surface compressive stresses. This stress profile represents an ideal stress state for good spring life.

Shot peening is a common practice to improve spring life by imparting compressive stresses to the outer surfaces. The process puts a relatively high compressive stress at the surface and an even higher compressive stress just beneath the surface. These compressive stresses then fade to neutral and tensile stresses with depth into the interior. Properly done, shot peening improves spring life.²

How can we know that shot peening or other techniques have resulted in beneficial stresses? We can measure them using several techniques, Of these techniques, X-ray diffraction has many advantages. First, the X-ray diffraction technique is a direct measurement of strain. (Strain is converted to stress by multiplying by material constants.) Other techniques, except for neutron diffraction, are indirect techniques that measure the effects of strain/stress.

Secondly, the X-ray diffraction technique can quickly and non destructively measure surface stresses. Sub-surface measurement and measurements on complex geometries may require material removal and/or sectioning. However, research has shown that relative compressive depths may be discerned non destructively by comparing the residual stresses and associated diffraction peak widths.³

Retained Austenite

Austenite is a phase in steel with different properties, compared to other phases in steel such as martensite. The different properties are due in part to the different crystal structures associated with the phases. Because of these crystal structure differences, a volume expansion occurs when austenite transforms into martensite. Thus, significant amounts of retained austenite are undesirable when dimensional tolerances are critical. Also, austenite has a lower yield stress compared to that of marten site or tempered martensite. Springs often require the superior mechanical properties associated with martensite, which can be improved through tempering. Tempered martensite retains much of the strength characteristics while being less brittle compared to martensite.

Low retained austenite content is associated with better fatigue characteristics. Fine-grained austenite interspersed within tempered martensite prevents or retards nucleation/initiation of fatigue cracks until very high stresses are realized.⁴ Certain thermal treatments and mechanical processes like shot peening have been shown to reduce retained austenite.⁵ Thus shot peening improves fatigue life by reducing retained austenite and imparting compressive residual stresses.

Like residual stresses, retained austenite can be measured using X-ray diffraction techniques. In this case, the volume percentage of retained austenite is proportional to the integrated intensities of the diffraction peaks relative to the integrated intensities of the martensite peaks. ASTM E975 -13 Standard Practice for X-Ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation describes the processes required for precise retained austenite measurements. The X-ray diffraction technique is the preferred method for low amounts ( <15 percent) of retained austenite.⁶

The Bottom Line

Fatigue is the prevalent failure mode in springs. Adding compressive residual stresses and reducing retained austenite have been shown to improve fatigue life in springs. These improvements can be accomplished in the manufacturing process. X-ray diffraction measurements are a popular method to verify successful manufacturing. Both residual stresses and retained austenite content can be determined using X-ray diffraction. These measurements can be made quickly, precisely and safely on the manufacturing floor, in the lab, or in a field environment.

Verifying the presence of compressive residual stresses and low amounts of retained austenite provides good return on investment by helping to prevent spring failures. In turn, springs with optimized properties operate at a superior level, allowing you to produce a superior product.

Article reprinted with permission of Springs magazine.

1. Noyan and Cohen, “Residual Stress Measurement by Diffraction and Interpretation,” Springer-Verlag, 1987.
2. “Shot Peening Applications,” Metal Improvement Company, Inc., 2001.
3. Matlock, Snoha, and Grendahl, “Using XRD elastic and plastic strain data to evaluate the effectiveness of different cold-working techniques in aerospace materials,” Powder Diffraction Suppl. 24, June 2009.
4. Smith, Debra Lynn, “The Effect of Cryogenic Treatment on the Fatigue Life of Chrome Silicon Steel Compression Springs” (2011). Dissertations (2009·), Paper 123, Marquette University.
5. Toshiya Tsuji et al, “Influences of Mechanical Properties and Retained Austenite Content on Shot-peening Characteristics,” Conference Proceedings 2011: ICSP-11, South Bend, Indiana
6. ASTM E975 • 13 Standard Practice for X-Ray Determination of Retained Austenite in Steel with Near Random Crystal-lographic Orientation. ASTM International, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania