Surface heat treatment enhances the longevity of hardenable steel parts by affecting the treated volume in multiple ways:

On the one hand, the material becomes harder and thus experiences less wear and surface damage during operation.
The hardness is quite easy to measure, in essence requiring only a polished surface and a hardness testing machine.
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.
Thus, it is a preferred measure for quality control in a great many heat treatment facilities.

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.
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.
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.
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}.
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}.

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}.
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.
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.
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}.
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}.

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}.
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.
The resulting interactions are quite complicated and have been subject to study for some time \cite{arimoto2016thermally}.
