A microstructure analysis can tell you what heat treatment a specific material (in this case 4130) received, or, if the heat treatment is known, perhaps it can tell if the heat treatment was effective.
A lot depends on the section size, particularly with 4130. With thin sections, say less than 0.5 inch, achieving a predominately martensitic structure on water quenching after fully austenitizing. With heavier sections, say > 2 inches, the as-quenched structure will have a higher percentage of non-martensitic transformation products. These non-martensitic transformation products are lower in hardness than martensite and, like martensite, will soften with tempering, but not as much as martensite will. It can be difficult to distinguish between tempered bainite and tempered martensite.
To be able to determine if the material was heat treated properly, you have to know what the heat treat specification requirements are. In general, the heat treatment is performed to achieve specific mechanical properties. If those properties are not met, the microstructure can explain why, but the microstructure is not going to be able to tell you if the material will meet any specific mechanical properties.
For example, with 4130, a normalizing treatment can be expected to produce a microstructure composed of ferrite and pearlite, and result in a hardness of, say 262 HBW maximum. If the hardness checks out to be 363 HBW and the microstructure reveals a large percentage of untempered martensite, it would be likely the material was not allowed to cool in still air from the austentitzing temperature. To be able to draw this conclusion, however, you have to know both the required heat treatment (normalizing) and required mechanical properties (262 HBW max). Just knowing the surface hardness is out of spec does not tell you the problem and just looking at the microstructure is not going to tell you what the hardness is.
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