Steel O6 QT-55 vs O6-SR
Steel · O6 temper comparison
O6 QT-55 and O6-SR are two conditions of the same steel grade, O6: QT-55 (hardened & Tempered 54–57 HRC) versus SR (stress relieved). The chemistry is identical, so every difference below comes from the condition. O6 QT-55 is the stronger of the two: yield strength 1,650 MPa (239 ksi) against 400 MPa (58 ksi) for O6-SR, about 4.1 times as much; tensile strength follows at 1,900 MPa (276 ksi) vs 710 MPa (103 ksi). In exchange, O6-SR is the more ductile: elongation 23% versus 4% for O6 QT-55, and O6 QT-55 is harder (610 HV vs 210 HV). O6-SR runs hotter in service (400°C vs 300°C continuous). On the shop floor, O6-SR machines more easily (good vs not recommended).
O6 QT-55Choose it when strength drives the design.
O6-SRChoose it when the part is bent, formed or impact-loaded, or when the part sees heat.
O6 QT-55 vs O6-SR: propertiesHow to read these bars
- One bar per material. A full bar is the highest value among all steel grades in the FabDigit library; each bar is drawn as a share of that.
- The better value (by the usual engineering direction) is in bold. Rows with no direction — density, thermal conductivity — have no bold.
- Values are typical room-temperature figures for the condition named on each card.
- Confirm against the certified mill or resin datasheet before design release.
How to read these bars
- One bar per material. A full bar is the highest value among all steel grades in the FabDigit library; each bar is drawn as a share of that.
- The better value (by the usual engineering direction) is in bold. Rows with no direction — density, thermal conductivity — have no bold.
- Values are typical room-temperature figures for the condition named on each card.
- Confirm against the certified mill or resin datasheet before design release.
Mechanical
Strength, stiffness, ductility and hardness at room temperature.
Tensile Strength (Ultimate)
Yield Strength (0.2% offset)
Elongation at Break
Reduction in Area
Compressive Strength
Shear Strength
Fatigue Strength (Endurance Limit)
Charpy V-Notch Impact (RT)
Hardness, Vickers
Same in both: Elastic (Young's) Modulus 203 GPa29.4 Msi · Shear Modulus 79 GPa11.5 Msi · Bulk Modulus 160 GPa23.2 Msi · Poisson's Ratio 0.29 · Hardness, Brinell 200 HB · Hardness, Rockwell B 93 HRB
Physical
Density and transition temperatures.
Same in both: Density 7.7 g/cm³0.28 lb/in³ · Melting Point (Solidus) 1,290°C2,354 °F · Liquidus Temperature 1,420°C2,588 °F
Thermal
How each material carries heat and how hot it can run.
Max Service Temperature (continuous)
Same in both: Thermal Conductivity 32 W/m·K18.5 BTU/hr·ft·°F · Specific Heat Capacity 460 J/kg·K0.11 BTU/lb·°F · Thermal Expansion (CTE, 20–100 °C) 12.2 µm/m·K6.8 µin/in·°F · Latent Heat of Fusion 250 J/g107 BTU/lb · Min Service Temperature -30°C-22 °F
Electrical
Conductivity and insulation.
Same in both: Electrical Conductivity 3.4 % IACS · Electrical Resistivity 5×10⁻⁷ Ω·m19.7 µΩ·in
Chemical & Environmental
Corrosion and environmental resistance (0–100 scores).
Same in both: Galvanic Potential (seawater, vs SCE) -0.6 V · Corrosion Resistance 9 / 100
Manufacturability
FabDigit workshop ratings (0–100 scores) — how each material behaves on the machine.
Machinability
Machinability Rating (AISI 1212 = 100 %)
Weldability
Formability (cold)
Same in both: Castability 20 / 100 · Brazeability / Solderability 40 / 100 · Polishability 45 / 100
Sustainability
Primary-production footprint per kilogram.
Same in both: Embodied Energy (primary production) 34 MJ/kg14,617 BTU/lb · Embodied Carbon (primary production) 2.5 kg CO₂/kg · Embodied Water 60 L/kg7.2 gal/lb · Typical Recycled Content 40%
Strength versus weight
Yield strength against density for every steel grade in the FabDigit library; the compared grades are highlighted. Up and left is stronger for its weight.
O6 QT-55 vs O6-SR — frequently asked
Is O6 QT-55 stronger than O6-SR?
Yes. O6 QT-55 has a typical yield strength of 1,650 MPa (239 ksi) against 400 MPa (58 ksi) for O6-SR — about 4.1 times as much. Tensile strength is 1,900 MPa (276 ksi) vs 710 MPa (103 ksi).
Which condition does FabDigit quote by default, QT-55 or SR?
ANN is the default O6 condition in the FabDigit quote flow (neither QT-55 nor SR). The other condition is available on request or by drawing note — specify it in the material field or the part notes and the quote is priced for it.
Which is easier to machine, O6 QT-55 or O6-SR?
O6-SR. Its machinability rates good (62/100) against not recommended (14/100) for O6 QT-55; for welding the order is O6 QT-55 first (poor, 20/100) then O6-SR (poor, 15/100).
Ready to make it?
Upload a CAD file and FabDigit prices the part in O6 QT-55 and O6-SR side by side — material, machining time, tolerance and finish in one quote. Certified material data ships with the order on request.
Typical handbook values for the conditions named above, normalised for comparison. Hardness on non-Brinell scales is converted approximately for charting only. Request certified data with your order for design-critical properties.