22 lines
3.5 KiB
TeX
22 lines
3.5 KiB
TeX
Surface heat treatment enhances the longevity of hardenable steel parts by affecting the treated volume in multiple ways:
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On the one hand, the material becomes harder and thus experiences less wear and surface damage during operation.
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The hardness is quite easy to measure, in essence requiring only a polished surface and a hardness testing machine.
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Surface measurements leave only microscopic indents and count as non-destructive testing, while cutting samples for in-depth testing does not alter the hardness when done carefully.
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Thus, it is a preferred measure for quality control in a great many heat treatment facilities.
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On the other hand, if the treatment is applied correctly, a residual stress field is imparted on the workpiece that compresses the surface regions and thus inhibits crack initiation and growth, thereby increasing the fatigue resistance of the part.
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The residual stress distribution of heat treated parts is of great interest to manufacturers, since an unfavorable result may lead to a superposition of internal and external stresses during service, which greatly reduces the life span without any visible faults at production.
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Unfortunately, an internal balanced force is impossible to measure directly and only its effects may be observed as residual elastic strain, as was observed by Hattori \cite{hattori1929cause} in the deflection of one-sidedly treated rods.
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The earliest methods of quantifying the residual elastic strains within work pieces strategically remove material and observe the resulting deformation that occurs as the internal forces re-balance themselves \cite{sachs1927detection, mathar1934determination}.
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Today's implementations of this principle rely on the change in electrical resistance in strain gauge rosettes for high precision measurements around drilled holes \cite{schajer2013hole}.
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A more direct but technologically more involved approach is the measurement of local elastic strain within a crystalline material by X-ray diffraction (XRD) and subsequent evaluation of residual stresses\cite{murray2013applied}.
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The crystal lattice will scatter an incoming X-ray beam according to Bragg's law, where the only material parameter is the distance between the diffracting lattice planes, which is in turn directly influences by elastic strains.
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In the case of high energy XRD (HEXRD) of poly-crystalline material performed in transmission mode, Debye-Scherrer rings are usually recorded by a two dimensional (2D) detector and represent diffraction on favorably oriented crystallites of various diffraction vector orientations.
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Due to the presence of residual stresses, the rings may change their diameter and/or transform into ellipses, whereby the ratio of the principal axes holds information about the two dimensional residual elastic strain state normal to the beam \cite{withers2013applied}.
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By scanning over the area of a thin slice of a sample, a two dimensional impression of the internal stress gradient can be imaged\cite{todt2018gradient,bodner2021correlative}.
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In heat treatment, the various strains whose elastic response these methods intend to measure are often differentiated into thermal and transformation strains during cooling \cite{bhadeshia2017steels}.
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The former are caused simply by a sample's thermal contraction and a temperature gradient within, whereas the latter are rooted in metallurgical processes that depend not only on the material composition but also the specific heating and cooling regimes.
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The resulting interactions are quite complicated and have been subject to study for some time \cite{arimoto2016thermally}.
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