Steel O6
AISI/SAE O6 · UNS T31506
O6 (UNS T31506) is a silicon-graphitic, oil-hardening cold-work tool steel in which free graphite in the matrix improves machinability and galling resistance.
Reference datasheet — O6 is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the ANN condition
O6 tempers and conditions
O6 is supplied in 6 conditions, soft to hard. ANN is the default FabDigit quotes; the values on this page are for it.
Compare all 6 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Stress relieved | Intermediate condition used on large or asymmetric parts to minimise distortion during subsequent hardening. | 400 | 710 | 23 | 13 HRC | 3.4 % IACS |
ANNDefault Annealed | Standard as-supplied condition for machining, turning and rough forming before hardening. | 415 | 720 | 22 | 200 HB | — |
Cold Drawn Annealed | Pick for close-tolerance drill rod / precision-ground stock that will be machined then hardened. | 550 | 780 | 14 | 17 HRC | 3.4 % IACS |
Hardened & Tempered 54–57 HRC | Choose when shock loading or heavier sections demand extra toughness at some cost in wear life. | 1,650 | 1,900 | 4 | 56 HRC | — |
Hardened & Tempered 58–61 HRC | Best general balance of wear resistance and toughness for rolls, cams, forming dies and slides. | 1,800 | 2,050 | 2 | 60 HRC | — |
Hardened & Tempered 62–65 HRC | Maximum hardness and wear/galling resistance for gauges, bushings and arbor surfaces in small sections. | 1,900 | 2,200 | 1 | 63 HRC | 30 W/m·K |
What is O6?
O6 (UNS T31506) is a silicon-graphitic, oil-hardening cold-work tool steel in which free graphite in the matrix improves machinability and galling resistance. It is used for arbors, rolls, bushings and gauges, supplied as annealed or cold-drawn annealed bar and hardened to 55-63 HRC; annealed properties are about 415 MPa yield, 720 MPa tensile, 22% elongation, 200 HB and 203 GPa modulus, with service to roughly 400 °C. Corrosion resistance is minimal (score 9), weldability is poor (score 20), and high-polish surfaces pit at the graphite nodules. O1 is the usual alternative where polish matters more than machinability. O6 has a density of 7.7 g/cm³ (0.278 lb/in³), lighter than 95% of steel grades. It melts at 1,290°C (2,354 °F). O6 has a yield strength of 415 MPa (60.2 ksi) and a tensile strength of 720 MPa (104 ksi) — weaker than 56% of steel grades. Elongation at break is 22% and the elastic modulus is 203 GPa (29.4 Msi).
What is O6 used for?
- arbors
- rolls
- bushings
- gauges
Advantages
- Easy to machine — 62/100, better than 89% of steel grades
Limitations
- Low electrical conductivity — 3.4 % IACS, worse than 86% of steel grades
- Difficult to weld — 20/100, worse than 84% of steel grades
- Hard to polish — 45/100, worse than 84% of steel grades
- Poor heat conductor — 32 W/m·K (18.5 BTU/hr·ft·°F), worse than 82% of steel grades
- High embodied carbon — 2.5 kg CO₂/kg, worse than 82% of steel grades
O6 properties
Typical room-temperature values for O6-ANN; bars are the position among steel grades.
Physical3
O6 has a density of 7.7 g/cm³ (0.278 lb/in³), lighter than 95% of steel grades. It melts at 1,290°C (2,354 °F).
Mechanical15
O6 has a yield strength of 415 MPa (60.2 ksi) and a tensile strength of 720 MPa (104 ksi) — weaker than 56% of steel grades. Elongation at break is 22% and the elastic modulus is 203 GPa (29.4 Msi).
Thermal6
O6 is rated for continuous service to 400°C (752 °F). It conducts heat at 32 W/m·K (18.5 BTU/hr·ft·°F), worse than 82% of steel grades. Thermal expansion is 12.2 µm/m·K (6.78 µin/in·°F).
Electrical3
O6 conducts electricity at 3.4 % IACS, worse than 86% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (9/100), worse than 98% of steel grades.
Sustainability4
Producing a kilogram of O6 takes about 34 MJ of energy and emits 2.5 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 40%.
Manufacturability8
O6's machinability is good (62/100, better than 89% of steel grades); weldability poor (20/100); formability poor (25/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. O6 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with O6
Is O6 easy to machine?
Best-machining O-series grade — free graphite acts as a chip breaker and lubricant; annealed stock turns, drills and taps easily with HSS or carbide, while hardened parts require grinding or CBN.
Can O6 be welded?
Not recommended structurally; if repair is unavoidable preheat to 300–370 °C, use matching or low-hydrogen filler, and temper immediately at the part's tempering temperature.
Can O6 be formed, bent or molded?
Cold forming is limited to gentle bends in the annealed state; hot work at 900–1050 °C followed by slow cooling and spheroidize anneal at 760–790 °C.
O6 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| Cpart present as free graphite | 1.25 – 1.55 | 1.45 |
| Mn | 0.3 – 1.1 | 0.8 |
| Sipromotes graphitization | 0.55 – 1.5 | 1.05 |
| Crmax per ASTM A681 | ≤ 0.3 | 0.15 |
| Mo | 0.2 – 0.3 | 0.25 |
| Ni | ≤ 0.3 | |
| V | ≤ 0.3 | |
| Cu | ≤ 0.25 | |
| P | ≤ 0.03 | |
| S | ≤ 0.03 | |
| Fe | balance |
What surface finishes work on O6?
Takes a good but not mirror finish (graphite nodules pit at high polish); protect with black oxide, phosphate or light oil, and nitride or hard-chrome plate where extra wear resistance is needed.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Organic Coating
Black / Phosphate
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Steel O6 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- T31506
How much does O6 cost?
O6 is a premium-cost steel ($$$), cheaper than about 51% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
O6 — frequently asked
What is O6 used for?
O6 is typically used for arbors, rolls, bushings and gauges. In short: best-machining O-series grade; built-in graphite lubricity; low-cost galling resistance.
What is the yield strength of O6?
O6 has a typical yield strength of 415 MPa (60.2 ksi) and a tensile strength of 720 MPa (104 ksi) — weaker than 56% of steel grades. Strength varies by condition: see the 6 listed tempers.
Is O6 easy to machine?
Reasonably — machinability is rated good (62/100, better than 89% of steel grades). Best-machining O-series grade — free graphite acts as a chip breaker and lubricant; annealed stock turns, drills and taps easily with HSS or carbide, while hardened parts require grinding or CBN.
Can O6 be welded?
Not readily — weldability is rated poor (20/100). Not recommended structurally; if repair is unavoidable preheat to 300–370 °C, use matching or low-hydrogen filler, and temper immediately at the part's tempering temperature.
What surface finishes work on O6?
Takes a good but not mirror finish (graphite nodules pit at high polish); protect with black oxide, phosphate or light oil, and nitride or hard-chrome plate where extra wear resistance is needed.
Can O6 be formed, bent or molded?
Cold forming is limited to gentle bends in the annealed state; hot work at 900–1050 °C followed by slow cooling and spheroidize anneal at 760–790 °C.
What is the maximum service temperature of O6?
O6 is rated for continuous use to about 400°C (752 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between O6-ANN and O6 QT-63?
ANN is the default condition — standard as-supplied condition for machining, turning and rough forming before hardening. QT-63: maximum hardness and wear/galling resistance for gauges, bushings and arbor surfaces in small sections. Yield strength is 415 MPa in ANN versus 1,900 MPa in QT-63.
How much does O6 cost?
O6 is a premium-cost steel ($$$), cheaper than about 51% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Sources
- ASTM A681 — Standard Specification for Tool Steels, Alloy — ASTM International (2015)
- SAE J437/J438 — Tool and Die Steels — SAE International (2015)
- ASM Handbook Vol.1: Properties and Selection — Irons, Steels, and High-Performance Alloys (Tool Steels) — ASM International (1990)
- Tool Steels, 5th ed. (Roberts, Krauss, Kennedy) — ASM International (1998)
- Graphitic oil-hardening tool steel (O6) product data — Tool-steel producers/distributors (2023)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.