Quenched and tempered (Q&T) alloy steels span a minimum tensile strength range of roughly 470–1250 N/mm² (MPa) in the Q&T condition, with the upper end dominated by low-alloy Cr-Mo and Cr-Ni-Mo grades such as 42CrMoS4 and 25NiCrMo13-5 [S1]. At the extreme, corrosion-resistant chromium variants push ultimate tensile strength (UTS) past 290 ksi (~2000 MPa) while keeping fracture toughness K_IC above 65 ksi√in [S5].
Carbon sits at 0.20–0.50% in most engineering Q&T grades, and the heat-treat window allows the specifier to slide UTS across a wide band by changing tempering temperature and as-quenched martensite content [S5]. For dynamic parts, automotive drivelines, and high-strength fasteners, this adjustability is exactly why Q&T steels dominate over normalized or annealed alternatives, a point we cover in the related comparison of normalized vs annealed alloy steel machining behavior.
Baseline ranges: 470 MPa to 1250 MPa for the standard Q&T envelope
Swiss Steel's engineering Q&T portfolio lists 11 standard grades including 42CrMoS4 (1.7227), 33MnCrB5-2 (1.7185), 51CrV4 (1.8159), 25NiCrMo13-5 (1.6657), C22R (1.1149) and C60 (1.0601), all defined to deliver a minimum tensile strength band of approximately 470 to 1250 N/mm² in the Q&T condition [S1]. The lower bound (470 MPa) lines up with plain-carbon grades such as C22R after a high-temperature temper, and the upper bound (1250 MPa) corresponds to heavily alloyed 42CrMoS4 or 51CrV4 tempered at 400–500°C to preserve hardness.
A separate engineering Q&T reference puts the yield strength window at 490–1200 MPa for medium-carbon alloy Q&T steels, matching the 470–1250 MPa UTS band closely and confirming that Q&T specs are typically written around yield, not just UTS [S8]. For spec engineers, the operative rule of thumb is that 1000 MPa UTS is the most commonly specified point in this envelope, because it corresponds to ISO 898-1 property class 10.9 fasteners with 0.25–0.35% C and impact energy above 27 J at −40°C [S5].
Low-temperature temper pushes UTS above 1.5 GPa in 0.20–0.45% C steels
Tempering at low temperature (below roughly 500°C) on plain 0.20–0.45% C low-alloy steels delivers yield strength above 1.2 GPa and UTS above 1.5 GPa with ductile fracture, according to Tkachev et al. (2023, cited 98 times) [S2]. The mechanism is lath martensite with controlled retained austenite, stabilised by Si and Mn partitioning, and tempered just enough to relieve quench stress without coarsening carbides.
Two caveats come with that 1.5 GPa territory. First, AISI 4320 and AISI 4340 tempered below 400°C cap impact toughness at 60 J/cm² and 30 J/cm² respectively, which is too low for many dynamic-loading components [S2]. Second, the 200–500°C band is exactly the temper martensite embrittlement (TME) window driven by carbide precipitation and substitutional-element segregation, so the steel composition must be controlled to avoid brittle intergranular fracture [S2]. For thicker sections the same alloy, tempered to the same hardness, will also show a 25–50°C rise in the ductile-to-brittle transition temperature if the as-quenched microstructure contains only 50% martensite instead of full martensite [S3].
Alloying levers: Cr, Ni, Mo, V, B, and the 4140 benchmark
Chromium at 0.80–2.50% is the workhorse hardenability element; it shifts the TTT diagram rightward and enables through-hardening of round bars up to 75 mm diameter at lower cost than equivalent nickel-molybdenum grades [S5]. Nickel at 0.50–1.50% refines grain size and lifts sub-zero toughness, and at 3.0–5.5% it carries corrosion-resistant aerospace variants above 2000 MPa UTS [S5]. Molybdenum at 0.30–2.0% provides temper resistance, suppresses softening during post-tempering exposure, and improves creep strength for elevated-temperature service [S5]. Boron appears in leaner grades such as 33MnCrB5-2 (1.7185) for the same hardenability at lower carbon, and vanadium in 51CrV4 (1.8159) and 58CrV4 (1.8162) for grain pinning and secondary hardening.
4140 (≈42CrMoS4) is the canonical reference grade. In the as-quenched condition it can hit 55–57 HRC, but practical working hardness after tempering is 28–50 HRC, with the upper end reserved for high-stress shafts and the lower end for parts that still need meaningful impact energy [S7]. Engineering teams sourcing these grades should expect longer lead times when nickel and chromium surcharges bite, a pattern documented in nickel and chromium surcharges stretching alloy steel lead times.
Martensite fraction, cooling after temper, and what actually moves UTS
Tartaglia's SAE 8822 and SAE 8622 work shows that 35%, 50%, 80%, 97%, and 100% as-quenched martensite contents can all be reached by changing quench media and section size, but tensile strength after tempering below 425°C is essentially unchanged across that range; it is toughness and fatigue life that move, not UTS [S3]. At 1240 MPa UTS the high-cycle fatigue life increases with martensite content, while low-cycle fatigue is relatively insensitive, and the cyclic yield strength becomes independent of martensite content once the UTS is fixed [S3].
Cooling rate after tempering does not change UTS either. Tests reported by Thermal Processing (2023) varied post-temper cooling from still air to forced water quench and found no difference in tensile strength; toughness is where divergence appears, with faster post-temper cooling reducing the time spent in the TME window [S6]. This matters for shops running large bar heats: water-quenching off the temper can recover several joules of Charpy impact versus slow cooling, with no penalty on the tensile certificate.
Comparison: how the main Q&T alloy families stack up on UTS, toughness, and cost
The table below summarises the four families a spec engineer is most likely to compare head-to-head. All values are taken from the research and rounded to the closest band the source explicitly states. [S3]
Family / typical grade / typical UTS (MPa) / tempering temperature / reported toughness limit / cost tier. Plain-carbon Q&T (C22R, C35Pb, C60): 470–900 MPa, 450–650°C, moderate Charpy, low cost [S1][S8]. Cr-Mo low-alloy (42CrMoS4, 4140): 850–1250 MPa, 400–600°C, 28–50 HRC working hardness, moderate cost [S1][S7]. Cr-Ni-Mo high-alloy (25NiCrMo13-5, 4340-class): 1200–1500+ MPa, 200–500°C with TME risk, 30–60 J/cm² impact, high cost [S1][S2]. Corrosion-resistant Cr-rich (9–14.5% Cr variants): above 2000 MPa UTS with K_IC above 65 ksi√in, premium cost, mainly aerospace [S5].
The takeaway is that moving from plain-carbon Q&T to Cr-Mo low-alloy roughly doubles the maximum UTS at a moderate cost step, while pushing into the 1.5 GPa+ band with Cr-Ni-Mo chemistry costs impact toughness and requires tight control of the tempering window. Plate-product sourcing is a related decision, and the practical thickness limits are laid out in alloy steel plate thickness range for structural fabrication; for sub-zero service, the link between nickel content and Charpy transition is quantified in nickel content vs low-temperature toughness in alloy steel.
Selection criteria: who Q&T alloy steel is for, and who should look elsewhere
Q&T alloy steel is the right call for dynamically loaded shafts, axles, gears, connecting rods, high-strength fasteners (ISO 898-1 class 10.9 and 12.9), drill collars, and any part where through-section mechanical properties must be repeatable bar to bar [S1][S5]. Single-bar Q&T processing is the default for safety-relevant automotive components, because it keeps the property scatter lower than coil tempering and avoids the "high scattering of steel properties" failure mode that drives recalls [S1].
Q&T is the wrong call for parts that need maximum corrosion resistance (specify a stainless or a 9–14.5% Cr corrosion-resistant Q&T), for welded fabrications where 0.3%+ chromium without V/Mo balancing raises cracking risk [S5], or for ultra-low-temperature service where a lean chemistry simply cannot reach the required Charpy transition. In those cases austenitic stainless, nickel-rich cryogenic alloys, or quenched-and-partitioned (Q&P) third-generation AHSS are the better fit, not a Q&T grade pushed beyond its design window.
Testing and standards: how the 470–2000 MPa range is actually verified
Tensile strength is measured on a standardised specimen pulled to fracture in a universal testing machine, with yield strength, UTS, and elongation at break taken off the stress-strain curve [S4]. The same machine class covers everything from 4140 coupons at 1000 MPa to corrosion-resistant Cr-rich specimens above 2000 MPa, which is why the laboratory tensile testing machine and its grips, extensometers, and load-frame stiffness matter when you are characterising Q&T grades near the high end of the range. Process-side controls rely on checking and testing apparatus for hardness mapping, surface condition, and case-depth verification, all of which feed back into the Q&T recipe.
Standards governing the chemistry and property classes include ISO 898-1 for fastener property classes 10.9 and 12.9 (0.25–0.35% C, 1000–1200 MPa UTS, 27 J at −40°C) [S5] and ASTM B117/G61 for corrosion testing of the Cr-rich variants [S5]. For alloy steel grades in plate form the thickness range and through-thickness property guarantees are typically tied to the relevant structural-fabrication standard, and they share a construction-equipment service environment covered under construction machinery and equipment specifications. Dimensional verification on finished bar and plate typically routes through standard lamps and light fittings-adjacent visual-inspection cells using calibrated lighting equipment and electric lamps on automated surface-defect lines.
Limits and failure modes in the high-UTS regime
Three failure modes dominate when Q&T alloy steel is pushed past 1250 MPa. TME in the 200–500°C tempering window causes intergranular brittle fracture; AISI 4320/4340 tempered below 400°C have shown impact energies capped at 60 and 30 J/cm² respectively, which is a hard ceiling for many dynamic applications [S2]. Slack-quenched microstructures (only 50% martensite) shift the ductile-to-brittle transition temperature up by 25–50°C versus a fully martensitic structure tempered to the same hardness [S3]. High-carbon, high-chromium compositions with chromium above 0.3% and no V/Mo balancing become weld-crack sensitive, which eliminates welded fabrication as a downstream option [S5].
To stay inside the safe window, a spec engineer should fix the tempering temperature based on the required Charpy transition, not on a target hardness alone, and treat UTS as the consequence of that decision rather than the primary input. A practical next step is to pull the as-quenched microstructure requirement (full martensite vs slack-quenched) into the purchase specification, since that single clause is what separates a 50% martensite heat from a 100% martensite heat at the same UTS certificate [S3]. Trackable signals to watch over the next procurement cycle: ISO 898-1 class 12.9 fastener demand, nickel and chromium surcharge movements on alloy steel lead times, and any revision of ASTM A370 tensile-specimen geometries that would re-baseline the 470–1250 MPa envelope.