Both AISI 410 (UNS S41000) and AISI 420 (UNS S42000) are hardenable straight-chromium martensitic grades in the 11.5-14% Cr family, but the carbon split is what drives their post-quench hardness apart, with 410 capped near 0.15% C and 420 reaching 0.15-0.40% C in standard and modified compositions [S1][S4][S5].
On a hardened bar, the practical split lands at roughly 35-45 HRC for 410 versus 50-55 HRC for 420 [S1], a gap that maps directly to carbon content, retained chromium carbide fraction, and the resulting tempering response [S2][S4]. For a working spec on wear surfaces, that 5-15 HRC delta is the single most important variable to lock in before any other consideration.
Carbon and Chromium: The Compositional Split
410 contains 0.08-0.15% C and 11.5-13.5% Cr, the lower bound of the martensitic family, with Mn and Si each capped at 1.0% and P/S at 0.040%/0.030% maximum [S1]. 420 starts at a 0.15% C minimum and 12.0-14.0% Cr, with the same 1.0% Mn/Si and 0.04% P / 0.03% S ceilings per ASTM A276 [S4].
More carbon means more chromium carbide volume fraction after austenitising and quenching, which is the metallurgical reason 420 reaches 50 HRC while 410 plateaus near 42-45 HRC under identical heat treatment [S4][S5]. For general stainless steel selection context, this carbon-versus-chromium balance is the lever that defines the entire martensitic family, from 410 at the soft end through 420HC, 425 Mod, and 440A at the hard end.
Hardness After Quench and Temper: The Numbers
ASTM A276 tempering data for grade 420 shows tensile strength climbing from 655 MPa annealed to 1580-1620 MPa in the 204-427°C temper range, with Brinell hardness peaking at 444-461 HB, roughly equivalent to 45-48 HRC, at the 204-316°C temper window before dropping to 262 HB (~26 HRC) at a 650°C temper [S4]. The Allegheny Ludlum typical-property table cross-checks this: 410 lands at 38-45 HRC as-quenched, standard 420 at 53-57 HRC, 420HC at 56-59 HRC, and 425 Mod at 57-60 HRC [S5].
For an engineer writing a print, the conservative spec band is 410 at 35-45 HRC and 420 at 50-55 HRC after oil quench from 982-1066°C (1800-1950°F) plus a low-temperature temper [S1][S5]. The Vishwa Stainless comparison corroborates the 35-45 / 50-55 HRC split and frames 420 as the "cutlery grade" because of its edge retention, while 410 stays the "general-purpose" pick for stressed structural parts [S1].
Why 420 Hardens Harder: Carbide Volume and Retained Austenite
At the same austenitising temperature, 420 dissolves more carbon into the austenite phase, then precipitates a higher density of chromium carbides (M23C6 type) on tempering, which is the dominant strengthening contributor above ~500 HV [S3][S5]. The 2019 Moradi et al. diode-laser study quantified the gap on a single sample set: a laser-hardened surface layer reached about 620 HV at 1.8 mm depth on 410 versus 720 HV at 1.2 mm depth on 420, using a continuous-wave diode laser at up to 1600 W, with the 420 layer being thinner but harder because its higher carbon concentrated the martensite transformation in a shallower zone [S3].
Practically, that means two process outcomes: a furnace-hardened 420 bar hits ~50-55 HRC through the full cross-section, while a laser-hardened 420 surface hits ~720 HV (roughly 60 HRC equivalent) but only over 1.2 mm of case depth, and 410 laser-hardened reaches ~620 HV (~55 HRC) over a deeper 1.8 mm case [S3]. The trade-off for laser users is case depth versus peak hardness, and the trade-off for furnace users is the ductility/toughness tax that comes with the higher carbon in 420.
Comparison Matrix: 410 vs 420 on Decision Criteria
Lining the two grades against four spec criteria gives a clean procurement picture: (1) achievable hardness after Q&T: 410 at 35-45 HRC, 420 at 50-55 HRC, a 5-15 HRC delta in 420's favour [S1][S4][S5]; (2) wear resistance: 410 is adequate for sliding contact under moderate load, 420 is the better pick for abrasive or edge-loaded service because of higher carbide volume [S1][S5]; (3) ductility and weldability: 410 wins, because lower carbon reduces hard martensite in the HAZ and lowers cold-cracking risk during PWHT, while 420 is generally avoided in as-welded heavy sections [S1]; (4) corrosion resistance: both are moderate, in the same 12% Cr family bracket, and both perform best on a polished or hardened surface rather than a rough machined finish [S1][S4].
On cost, 410 typically runs lower per kilogram than 420 because of the lower carbon and easier hot-working, but the price gap is usually small relative to machining, heat-treatment, and rejection costs, so it should not drive the spec on its own [S1]. For context on adjacent austenitic picks in high-temperature service, the 321 vs 347 stainless comparison walks through stabilised grades that are not hardenable, useful when weldability and oxidation resistance matter more than hardness.
Heat-Treatment Process Window and What to Spec
Oil quenching from 982-1066°C (1800-1950°F) is the standard austenitising range for both 410 and 420, followed by tempering selected to balance hardness against toughness [S5]. The Allegheny Ludlum blue sheet recommends the 204-316°C temper for 420 to hold peak hardness (444-461 HB, ~45-48 HRC), and a 538-650°C temper when impact toughness or ductility is more important than peak hardness, with Charpy V values climbing from 19-20 J at the hard end to 42 J at 650°C [S4][S5].
For 410, the same 1800-1950°F austenitise works, and a 316-427°C temper lands the part in the 38-45 HRC band, which is the typical callout for pump shafts, valve stems, and fasteners [S1][S5]. Avoid specifying 420 for any part that will see significant post-weld heat treatment without a deliberate re-austenitise, because the as-welded HAZ in 420 can crack on cooling, while 410 is far more forgiving in welded assemblies. For stainless pipe applications, 410 is the more common martensitic pick because of this weldability margin, even though it costs some hardness.
Use-Case Recommendations and Failure Modes
Pick 420 when the application is wear- or edge-driven: surgical and dental instruments, shear blades, needle valves, plastic moulds, and cutlery, where the 50-55 HRC window plus higher carbide content delivers edge retention and abrasion life that 410 cannot match [S1][S4][S5]. The 2019 laser-hardening data confirms the same ranking at the surface, with 420 reaching 720 HV versus 410 at 620 HV under identical laser parameters, the hardness difference that drives knife and blade makers to 420 [S3].
Pick 410 when the application is strength- and corrosion-driven with welding or forming in the mix: pump shafts, valve stems, turbine blades, fasteners, and exhaust components, where the 35-45 HRC window is sufficient, ductility is preserved, and the 0.08-0.15% C range avoids HAZ cracking [S1]. The standard failure mode to watch is temper embrittlement in 420 if the part is held in the 400-500°C range during service, which can drop impact energy sharply; the mitigation is to either temper above 500°C and accept lower hardness, or to spec a vacuum-melted 420 variant with tighter impurity control.
For laser surface hardening specifically, expect a thinner but harder case on 420 (1.2 mm at 720 HV) and a thicker, softer case on 410 (1.8 mm at 620 HV), and size the laser overlap and scan strategy accordingly, because 420's faster transformation kinetics leave less margin for heat-affected softening on adjacent tracks [S3]. For adjacent material selection work in high-temperature oxidation service, the 310S vs 309S stainless comparison is a useful counterpoint, as those austenitic grades trade hardness for scaling resistance above 900°C.
Detailed specification references: hardness tester.