Effect of Metallurgical Variables on Fatigue

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Introduction to Fatigue and Metallurgical Sensitivity

The fatigue properties of metals are quite structure-sensitive. However, at the present time there are only a limited number of ways in which the fatigue properties can be improved by metallurgical means. By far the greatest improvements in fatigue performance result from design changes, which reduce stress concentration and from the intelligent use of beneficial compressive residual stress, rather than from a change in material. Nevertheless, certain metallurgical factors must be considered to ensure the best fatigue performance from a particular metal or alloy.

Fatigue tests designed to measure the effect of specific metallurgical variables, such as special heat treatments, on fatigue performance typically use smooth, polished specimens under completely reversed stress conditions. It is generally assumed that any changes in fatigue properties due to metallurgical factors will also occur to a similar extent under more complex fatigue conditions, such as with notched specimens under combined stresses.

Relationship Between Fatigue and Tensile Properties

Fatigue properties are frequently correlated with tensile properties. In general, the fatigue limit of cast and wrought steels is approximately 50 percent of the ultimate tensile strength. The ratio of the fatigue limit (or the fatigue strength at 10⁶ cycles) to the tensile strength is called the fatigue ratio.

Several nonferrous metals such as nickel, copper, and magnesium have a fatigue ratio of about 0.35. While these correlations are convenient, it should be clearly understood that these constant factors between fatigue limit and tensile strength are only approximations and hold only for the restricted condition of smooth, polished specimens tested under zero mean stress at room temperature.

For notched fatigue specimens, the fatigue ratio for steel typically ranges from 0.20 to 0.30. However, as yield strength increases through various strengthening mechanisms, the fatigue limit usually does not increase proportionately. Most high-strength materials are fatigue-limited.

Microstructural Influences on Fatigue Behavior

Several parallels can be drawn between the effect of certain metallurgical variables on fatigue properties and their effect on tensile properties. The effect of solid-solution alloying additions on the fatigue properties of iron and aluminum closely parallels their effect on tensile properties. Gensamer demonstrated that the fatigue limit of a eutectoid steel increased with decreasing isothermal-reaction temperature in the same fashion as did the yield strength and tensile strength.

However, fatigue properties exhibit greater structure sensitivity compared to tensile properties. This is evident in tests comparing the fatigue limit of a plain carbon eutectoid steel heat-treated to coarse pearlite versus spheroidite of the same tensile strength. Despite identical tensile strengths, the pearlitic structure resulted in a significantly lower fatigue limit due to the higher notch effects of the carbide lamellae in pearlite.

Evidence suggests that high fatigue resistance can be achieved by homogenizing slip deformation to avoid local concentrations of plastic deformation. This aligns with observations that fatigue strength is directly proportional to the difficulty of dislocation cross slip.

Materials with high stacking-fault energy permit dislocations to cross slip easily around obstacles, promoting slip-band formation and large plastic zones at crack tips. Both phenomena facilitate the initiation and propagation of fatigue cracks. In contrast, materials with low stacking-fault energy make cross slip difficult, constraining dislocations to move in a more planar fashion. This limits local concentrations of plastic deformation and suppresses fatigue damage.

While this concept aids in understanding fatigue mechanisms, practical limitations exist in controlling fatigue strength by altering stacking-fault energy. A more promising approach involves controlling microstructure through thermomechanical processing to promote homogeneous slip with many small regions of plastic deformation rather than fewer regions of extensive slip.

Grain Size Effects and Heat Treatment Optimization

The influence of grain size on fatigue life varies depending on the deformation mode. Grain size has its greatest effect in the low-stress, high-cycle regime where stage 1 cracking predominates. In high stacking-fault-energy materials (such as aluminum and copper), cell structures develop readily and control stage 1 crack propagation. Thus, the dislocation cell structure masks the influence of grain size, making fatigue life at constant stress insensitive to grain size. However, in low stacking-fault-energy materials (such as alpha brass), the absence of cell structure due to planar slip causes grain boundaries to control the cracking rate. In this case, fatigue life is proportional to grain diameter.

In general, quenched and tempered microstructures provide optimal fatigue properties in heat-treated low-alloy steels. However, at hardness levels above approximately Rc 40, a bainitic structure produced by austempering yields better fatigue properties than a quenched and tempered structure of equivalent hardness. Electron micrographs indicate that the poorer performance of quenched and tempered structures results from stress-concentration effects of the thin carbide films formed during martensite tempering.

For quenched and tempered steels, the fatigue limit increases with decreasing tempering temperature up to hardness levels of Rc 45 to Rc 55, depending on the specific steel. Fatigue properties at high hardness levels are extremely sensitive to surface preparation, residual stresses, and inclusions. Even trace amounts of decarburization on the surface may drastically reduce fatigue properties. Similarly, small amounts of non-martensitic transformation products can cause appreciable reductions in the fatigue limit. The influence of small amounts of retained austenite on fatigue properties of quenched and tempered steels remains inadequately established.

Inclusion Effects and Directional Properties

Research indicates that below a tensile strength of approximately 200,000 psi (~1400 MPa), the fatigue limits of quenched and tempered low-alloy steels with different chemical compositions are roughly equivalent when tempered to the same tensile strength. This generalization applies to fatigue properties determined in the longitudinal direction of wrought products. However, tests have shown that the fatigue limit in the transverse direction of steel forgings may be only 60 to 70 percent of the longitudinal fatigue limit. Practically all fatigue failures in transverse specimens originate at nonmetallic inclusions.

Near-complete elimination of inclusions through vacuum melting produces a considerable increase in the transverse fatigue limit. The reduced transverse fatigue limit in steels containing inclusions is generally attributed to stress concentration at inclusions, which can be quite high when elongated inclusion stringers are oriented transverse to the principal tensile stress.

However, the persistence of appreciable anisotropy in fatigue limit even after nearly complete elimination of inclusions through vacuum melting suggests other factors may be important. Further investigations have revealed significant changes in the transverse fatigue limit that cannot be correlated with changes in the type, number, or size of inclusions but are produced by different deoxidation practices. Transverse fatigue properties appear to be among the most structure-sensitive engineering properties.

Interstitial Elements and Fatigue Limit

The existence of a fatigue limit in certain materials, especially iron and titanium alloys, has been shown to depend on the presence of interstitial elements. The S-N curve for a pure metal typically follows a monotonic function with N increasing as stress decreases. The introduction of a solute element raises the yield strength, making slip band initiation more difficult, thereby shifting the S-N curve upward and to the right. If the alloy has suitable interstitial content to undergo strain aging, an additional strengthening mechanism comes into play.

Since strain aging is not strongly dependent on applied stress, a limiting stress exists at which balance occurs between fatigue damage and localized strengthening due to strain aging. Enhanced strain aging, whether from higher interstitial content or elevated temperature, raises the fatigue limit and causes the break in the curve to occur at a lower number of cycles. In quenched and tempered steels, which do not normally exhibit strain aging in tension tests, the existence of a pronounced fatigue limit presumably results from localized strain aging at the crack tip.