For wall thicknesses from 25 mm to roughly 100 mm in carbon and low-alloy steel castings, X-ray tube voltages in the 130-400 kV band cover the bulk of practical radiographic work, with published guidance mapping 13-25 mm to 90-130 kV and 25-50 mm to 130-160 kV [S1].
Thicker sections cross energy bands fast: 50-100 mm falls into the 200-400 kV window, 100-200 mm requires radioisotope sources (Ir-192, Co-60), and beyond roughly 250 mm a linear accelerator running in the MeV range is the only way to get usable film density on stainless steel and alloy steel castings [S2][S5].
Why kV selection is driven by steel thickness, not part size
Radiographic contrast on a casting comes from the local thickness difference a void, inclusion, or shrinkage cavity introduces against the sound base metal, so the correct kV is the one that lets the beam pass through the thickest sound region with measurable but not total film exposure [S2]. At 25-50 mm steel, the published chart sits at 130-160 kV; at 50-100 mm, jump to 200-400 kV; at 100-200 mm, isotope sources replace X-ray tubes entirely [S1][S5]. Pick the kV to the thickest sound section, not the average, or the thickest area will burn through and the defect contrast collapses.
The same logic explains why foundries and pressure-equipment shops running ASTM E446, E186, and E280 reference radiographs pair their X-ray set with a gamma projector and, on the heaviest jobs, a linac: each tool covers a thickness band where it gives the cleanest contrast-to-penetration balance [S2][S4].
X-ray tube kV bands for steel castings up to 100 mm
For 13-25 mm steel castings, 90-130 kV is the working window; for 25-50 mm, 130-160 kV; for 50-100 mm, 200-400 kV is the documented band [S1][S5]. The US Army TM-1-1500-335-23 table confirms a parallel set: 100-200 kV for moderate sections, 200-400 kV for thicker steel, and a step up to 1000 kV once you cross into heavy cast sections [S5].
On carbon steel castings with relatively smooth surfaces (e.g. machined or ground weld-prep areas), staying in the middle of each band maximises defect sensitivity; on rough as-cast surfaces, the same kV will produce higher unsharpness, and many shops drop the kV 10-15% to recover contrast at the cost of longer exposure time [S1].
Beyond 100 mm: gamma sources and linear accelerators
Ir-192 sources cover roughly the 10-100 mm steel band effectively, while Co-60, with its higher ~1.17-1.33 MeV photon energy, is the isotope of choice from about 50 mm up to 150-200 mm sections; for very thick wall sections, a linear accelerator is the only practical way to maintain usable film density [S2][S4]. Investment-casting radiography confirms the same source menu: Ir-192 and Co-60 for thick or dense cast parts, with X-ray machines for thinner sections and CT work [S4].
Wall thickness inside this thickness range is one of the few NDT parameters where direct ultrasonic gauging and radiographic thickness correlation are routinely cross-checked; a 39DL PLUS or 45MG gauge with High Penetration software past 12.7 mm and a 500 kHz M101 transducer past 50 mm will read the same wall that the radiographer is trying to shoot through, which lets you set kV off a measured number rather than a nominal [S3]. For shop-side thickness checks on rough sand-cast surfaces, high-viscosity couplants (gel, glycerin) are mandatory to push ultrasound into the metal [S3].
Matching kV to defect type and acceptance standard
ASTM E446 covers steel castings up to 2 in (50.8 mm), E186 covers 2-4.5 in (50.8-114 mm), and E280 covers 4.5-12 in (114-305 mm), so the kV/source decision is also an acceptance-standard decision: the wrong energy band can leave the radiograph out of scope before the image is even graded [S4]. Defect severity grades run 1 (best) to 5 (worst), and the image quality indicator (IQI) must be visible on the radiograph for the section thickness being graded; if your kV hides the IQI, the entire exposure is non-conforming regardless of how clean the part looks [S2].
Selection criteria summarised as a quick reference: carbon or low-alloy steel up to 25 mm, 90-130 kV; 25-50 mm, 130-160 kV; 50-100 mm, 200-400 kV X-ray or Ir-192; 100-200 mm, Co-60 gamma; above ~250 mm, linear accelerator (MeV) [S1][S2][S5]. For foundries sourcing heavy castings such as stainless steel valve bodies and large alloy steel housings, the same band map applies, but be ready to escalate to gamma and linac capability rather than pushing a 400 kV tube past its useful range.
Common pitfalls when sizing kV to thick castings
Three failure modes show up repeatedly in thick-casting radiography. First, picking kV to the average wall instead of the thickest sound section: the thickest area burns through, the radiograph fails the density spec, and the job is re-shoot. Second, assuming a higher kV always improves penetration: past the optimal band you lose contrast on small gas porosity and slag, and the image gets flatter and harder to grade against E186/E280 reference films. Third, ignoring surface condition: rough as-cast surfaces and sand inclusions scatter the beam and demand either lower kV (with longer exposure) or surface grinding before exposure [S1][S3].
Where the foundry or job shop has access to both modalities, pairing radiographic kV selection with ultrasonic thickness gauging from the opposite face removes most of the guesswork: the UT gauge tells you the actual wall the beam has to penetrate, the radiographer picks the band, and the IQI is sized to that measured wall [S3]. For reference, comparable product spec decisions on raw-material cost and casting metallurgy are covered in the grey cast iron vs cast steel price data for 2026, which is useful when a job is on the boundary between a cast iron and a cast steel radiographic scope.
Sourcing, standards, and what to track next
The governing documents for radiographic kV/source selection on steel castings are the ASTM E446/E186/E280 reference-radiograph set, the IQI and density rules inside ASTM E94, and the source-energy guidance in the US Army TM-1-1500-335-23 table that maps thickness to the 50-150 kV, 100-200 kV, 200-400 kV, 1000 kV, and 2-6 MeV bands [S4][S5]. The two trackable signals from 2026 NDT vendor output are continued displacement of low-energy film radiography by digital detector arrays (DR) at the 130-400 kV band, and broader adoption of portable MeV-band linear accelerators for in-field heavy-section casting work [S1][S2].
For engineers sizing a new thick-casting inspection cell, the next concrete step is to lock the thickness range, pick the dominant ASTM reference standard (E446, E186, or E280), then match kV or source against the band map above and verify the chosen IQI is visible at full wall before the first production part is shot.