Influence of process parameters for induction hardening on residual stresses

The dislocation density and distribution induced by tensile deformation in single crystals of silicon, aluminum and gold and by tension-compression cycling in aluminum single crystals and Al 2024-T3 alloys were studied by X-ray double-crystal diffractometry. The measurements of dislocation density were made at various depths from the surface by removing surface layers incrementally. In this way, a propensity for work hardening in the surface layers compared to the bulk material was demonstrated for both tensiledeformed and fatigue-cycled metals. Analysis of the cycled Al 2024 alloy as a function of the fraction of fatigue life showed that the dislocation density in the surface layer increased rapidly early in the fatigue life and maintained virtually a plateau value from 20 to 90 pct of the life. Beyond 90 pct the dislocation density increased rapidly again to a critical value at failure. Evaluation of the dislocation distribution in depth showed that the excess dislocation density in the bulk material increased more gradually during the life. Using deeply penetrating molybdenumK
 α radiation, capable of analyzing grains representative of the bulk region, the accrued damage and the onset of fatigue failure could be predicted nondestructively for 2024 Al, cycled with constant stress as well as with variable stress amplitude. The dislocation structure produced in the bulk by prior cycling was unstable when the work-hardened surface layer was removed. It is proposed that the deformation response of the bulk material is controlled by the accumulation of dislocations and associated stresses in the surface layer.

Dislocation distribution and prediction of fatigue damage

The hardening mechanism of Cr5 backup roll material induced by rolling contact fatigue was studied. X-ray diffraction, Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to investigate the effect of microstructure alternations induced by fatigue on the hardness change. Remarkable change was observed at the half value breadth of X-ray diffraction under microstructure alternates which resulted from rolling contact fatigue. The OM photos showed obvious white etching areas (WEAs) in the layer influenced by fatigue, reflecting that some kind of content or structure with high etching resistance was generated in the layer influenced by fatigue. Microstructure changes such as nano-sized carbides precipitation and nanocrystalline formation induced by fatigue were observed in the site where the peak of half value breadth of X-ray located. These changes of microstructure should be considered in the mechanism for hardening of Cr5 backup roll material due to rolling contact fatigue.

Hardening mechanism of Cr5 backup roll material induced by rolling contact fatigue

It is well known that X-ray diffraction is one of the most powerful means for investigating the microscopic structure of crystalline materials. X-ray diffraction is advantageous when it is applied to metallic materials; it responds very sensitively to changes in the metal's crystalline structure. Another characteristic advantage of the X-ray-diffraction approach is its nondestructive nature in the measurement of crystalline-material parameters, enabling us to observe the process of mechanical phenomena of metals, such as fatigue and creep.

X-ray-diffraction approach for studies on fatigue and creep

The origins of residual stress and changes during fatigue are discussed, A new mechanism for fading is proposed, Pracitcal (destructive and non-destructive) methods for measuring this stress are critically reviewed. Each technique has major problems requiring further study, but acoustic, magnetic and x-ray methods are all poised for more widespread use.

The Nature of Residual Stress and its Measurement

It was successful to apply the X-ray microbeam technique for observing fatigue phenomena of annealed low-carbon steel. The following conclusions were obtained : (1) Misorientation and lattice strain increase quickly in the early period of fatigue process, and subsequently they continue to increase till initiation of a crack. Formation of subgrains proceeds relatively slowly in the early period, and it becomes thereafter a main process of fatigue. Formation of subgrains is not observed for fatigue below fatigue limit. (2) Density of excess dislocation of one sign in subboundaries for fatigue is much smaller than those for uniaxial tension and rolling. (3) It is considered that the reason why the half-value breadth increases during fatigue is not the formation of subgrains but is mainly the increase of lattice strain. (4) Schematic diagram of deformation in a grain for fatigue is suggested. The formation of subgrains is closely related to fatigue fracture.

X-Ray Investigation on Fatigue Fracture of Notched Steel Specimen : Observation of Fatigue Phenomena of Annealed Low-Carbon Steel by X-Ray Microbeam Technique

X-Ray Study on Fatigue of Mild-Steel -On the Feature of Fatigue Fracture Mechanism in Comparison with Other Modes of Fracturing-

Fatigue Damage in Coarse Al Crystals

Fatigue fracture occurs in any kinds of engineering materials, cold-rolled ones, heat-treated ones, etc. It is considered to be very significant to find out a monotonously varing parameter which represents fatigue progress of these materials, in understanding the mechanism of fatigue fracture as well as in the prediction of fracture by fatigue. For this purpose, the authors have set about a series of studies on microstructural change during the fatigue process of low-carbon steel specimens, which are treated in different manners before fatigue test, by means of back-reflection X-ray microbeam diffraction technique. In the previous papers, the fatigue process preceding macro crack initiation of annealed or cold-rolled specimens was investigated.In this paper, the specimens were initially quench-aged and fatigue process preceding macro crack initiation of these specimens was studied. It was intended to find out the feature characterizing the fatigue process in three kinds of materials, the annealed, the cold-rolled and the quenchaged steels and their features are briefly described in articles (1)-(4). The causes for the increasement in fatigue strength of quench-aged steel was discussed and the results are summarized in article (5).(1) In the early stage of fatigue process, the micro lattice strain range within a grain changes notably. The total misorientation β increases while the subgrain size t decreases rather slowly. As fatigue proceeds, the micro lattice strain range within a subgrain decreases and t continuous to decrease more slowly, although β takes nearly constant value. The width of subboundaries are narrowed. As macro crack initiation is coming near, the changes of these quantities begin to diminish. Therefore, the rearrangement of dislocations to develop substructure is a chracterizing feature of the later stage of fatigue process.The degree of the development of substructure can be used as one of the crystallographic parameters which express the degree of progress in "fatigue damage" of cyclically stressed specimens with any initial state.(2) Excess dislocation density in a subgrain at the number of stress cycles of macro crack initiation takes the small value of 2-5×108 lines/cmcm2. The amount of increase in excess dislocation density at subboundaries during the fatigue process till crack initiation is of the same order notwithstanding the variety of initial state.(3) During the fatigue process of specimens cyclically stressed above fatigue limit, well-developed substructure is observed in severely deformed grains at the same stage as the appearance of persistent slip bands. A crack takes place, at last, at the place of some persistent slip bands. On the other hand, the diffraction patterns reflected from grains without persistent slip bands do not show a notable difference from the initial patterns. From these points of view, it can be said that well-developed substructure plays an important role in crack initiation.(4) The degree of fatigue progress in cyclically stressed specimens is observed to differ from grain to grain, even if these grains are located in the same irradiated area of X-ray microbeam. This implies that there is no remarkable interaction between the adjacent grains, at least until the macro crack initiation of high cycle fatigue.(5) The structural causes for the improvement in fatigue strength are listed as follows; the initial locking of dislocations, the highness of frictional stress and the rapidness of work-hardening rate. These facts reveal the difficulty of dislocation movement mainly in the early stage of fatigue process.

https://www.jstage.jst.go.jp/article/jsms1963/15/159/15_159_879/_pdf

X-Ray Investigation on Fatigue Fracture of Notched Steel Specimen

The 0.45% carbon steel specimens, of which surface layers were perfectly quenched in various depth by induction hardening, were examined in the rotating bending fatigue test machine to ascertain the relation between fatigue endurance limit and residual stress. The result shows that both hardness pattern and residual stress together are not sufficient to be adequate to clarify the interrelations among the endurance limit, hardened depth and residual stress. Some more factors are required for the purpose. Therefore investigations must be made in that line.

https://www.jstage.jst.go.jp/article/jsms1963/18/195/18_195_1100/_pdf

Kenkichi Hayashi

Papers

X-Ray Investigation on Fatigue Fracture of Notched Steel Specimen : Observation of Fatigue Phenomena of Annealed Low-Carbon Steel by X-Ray Microbeam Technique

X-Ray Study on Fatigue of Mild-Steel -On the Feature of Fatigue Fracture Mechanism in Comparison with Other Modes of Fracturing-

Fatigue Damage in Coarse Al Crystals

X-Ray Investigation on Fatigue Fracture of Notched Steel Specimen

The Effect of Hardened Depth and Residual Stress on Fatigue Strength of Induction Hardened Steel