Steel SK4
JIS SK4
SK4 is a Japanese carbon tool steel at about 0.9–1.0 % C, a tougher variant of the SK series that trades peak hardness for impact resistance.
Reference datasheet — SK4 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 A condition
SK4 tempers and conditions
SK4 is supplied in 10 conditions, soft to hard. A is the default FabDigit quotes; the values on this page are for it.
Compare all 10 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
ADefault Solution annealed | Standard stocked condition for bar and strip: softest state for machining, blanking and forming before hardening. | 400 | 670 | 18 | 190 HB | — |
Hot rolled | As-rolled bar and flats, usually re-annealed before machining; cheapest input form. | 450 | 780 | 12 | 22 HRC | 9.5 % IACS |
Normalized | Air-cooled from ~800–850 °C to break up network carbide and refine grain before hardening. | 500 | 850 | 10 | 25 HRC | 9.5 % IACS |
Cold drawn | Cold-drawn round or flat bar with tight size and clean surface for pin, punch and shaft blanks. | 620 | 800 | 9 | 22 HRC | 9.5 % IACS |
CR ½H | Partially cold-rolled strip for parts that still need moderate bending before hardening. | 750 | 900 | 6 | 30 HRC | 9.3 % IACS |
Cold rolled strip, full hard (spring temper) | Fully work-hardened strip supplied ready-to-use for flat springs, clips and shim blades without heat treatment. | 1,100 | 1,250 | 2 | 43 HRC | 9.2 % IACS |
Q&T 44–48 HRC | Toughest heat-treated condition, used for coil and leaf springs and shock-loaded tooling. | 1,320 | 1,500 | 6 | 46 HRC | 9.2 % IACS |
Q&T 50–55 HRC | Impact-tolerant tool condition for cold chisels, striking punches and shear blades. | 1,500 | 1,750 | 3 | 52 HRC | 9 % IACS |
Q&T 56–60 HRC | Slightly tougher edge condition for punches, chisels and woodworking blades that see impact. | 1,650 | 1,950 | 1 | 58 HRC | 8.5 % IACS |
Q&T 60–63 HRC | Maximum-hardness condition for knife edges, razors, files, scrapers and wear inserts in thin sections. | 1,800 | 2,100 | 1 | 61 HRC | — |
What is SK4?
SK4 is a Japanese carbon tool steel at about 0.9–1.0 % C, a tougher variant of the SK series that trades peak hardness for impact resistance. It is used for springs, blades, saws and hand tools and is stocked as annealed bar and as half-hard to hard cold-rolled strip. Annealed properties are 400 MPa yield, 670 MPa tensile and 18 % elongation at 190 HB, with quench-and-temper conditions from 44–48 HRC up to 60–63 HRC and a 300 °C service limit. It is effectively non-weldable and rusts without a coating; choose SK3 when higher hardness and edge retention matter more than toughness. SK4 has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,400°C (2,552 °F). SK4 has a yield strength of 400 MPa (58 ksi) and a tensile strength of 670 MPa (97.2 ksi) — weaker than 60% of steel grades. Elongation at break is 18% and the elastic modulus is 205 GPa (29.7 Msi).
What is SK4 used for?
- springs
- blades
- saws
- hand tools
Advantages
- Polishes to a fine finish — 75/100, better than 94% of steel grades
- Low embodied carbon — 1.9 kg CO₂/kg, better than 92% of steel grades
- Conducts heat well — 48 W/m·K (27.7 BTU/hr·ft·°F), better than 79% of steel grades
- High electrical conductivity — 9.5 % IACS, better than 75% of steel grades
Limitations
- Difficult to weld — 15/100, worse than 91% of steel grades
- Limited service temperature — 300°C (572 °F), worse than 78% of steel grades
SK4 properties
Typical room-temperature values for SK4-A; bars are the position among steel grades.
Physical3
SK4 has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,400°C (2,552 °F).
Mechanical15
SK4 has a yield strength of 400 MPa (58 ksi) and a tensile strength of 670 MPa (97.2 ksi) — weaker than 60% of steel grades. Elongation at break is 18% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
SK4 is rated for continuous service to 300°C (572 °F). It conducts heat at 48 W/m·K (27.7 BTU/hr·ft·°F), better than 79% of steel grades. Thermal expansion is 11.5 µm/m·K (6.39 µin/in·°F).
Electrical3
SK4 conducts electricity at 9.5 % IACS, better than 75% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (10/100), worse than 86% of steel grades.
Sustainability4
Producing a kilogram of SK4 takes about 25 MJ of energy and emits 1.9 kg of CO₂ — less than 82% of steel grades. Typical recycled content is 25%.
Manufacturability8
SK4's machinability is fair (45/100, worse than 65% of steel grades); weldability poor (15/100); formability fair (40/100).
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. SK4 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with SK4
Is SK4 easy to machine?
Machine only in the spheroidize-annealed state (~45 % of AISI 1212) with sharp carbide or HSS tooling and good coolant; after hardening, only grinding and EDM are practical.
Can SK4 be welded?
Effectively non-weldable for structural joints — high carbon gives hard, crack-prone martensite; if unavoidable, preheat 250–350 °C, use low-hydrogen filler and post-weld temper immediately.
Can SK4 be formed, bent or molded?
Anneal before blanking or bending; cold-rolled strip work-hardens rapidly, so use generous bend radii, and hot-forge at 1050–850 °C followed by slow cooling and spheroidize annealing.
SK4 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 |
|---|---|---|
| CJIS G 4401 SK95 (old SK4) | 0.9 – 1 | 0.95 |
| Si | 0.1 – 0.35 | 0.22 |
| Mn | 0.1 – 0.5 | 0.35 |
| P | ≤ 0.03 | |
| S | ≤ 0.03 | |
| Curesidual | ≤ 0.25 | |
| Niresidual | ≤ 0.25 | |
| Crresidual | ≤ 0.3 | |
| Fe | balance |
What surface finishes work on SK4?
Grinds and laps to a fine, high-polish edge; protect with oil, black oxide, phosphate or thin plating (bake after plating to avoid hydrogen embrittlement) since bare surfaces rust quickly.
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
How much does SK4 cost?
SK4 is a budget-cost steel ($), cheaper than about 90% 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.
SK4 — frequently asked
What is SK4 used for?
SK4 is typically used for springs, blades, saws and hand tools. In short: tougher than SK3; spring and blade tool steel; hard strip supplied ready to form.
What is the yield strength of SK4?
SK4 has a typical yield strength of 400 MPa (58 ksi) and a tensile strength of 670 MPa (97.2 ksi) — weaker than 60% of steel grades. Strength varies by condition: see the 10 listed tempers.
Is SK4 easy to machine?
Reasonably — machinability is rated fair (45/100, worse than 65% of steel grades). Machine only in the spheroidize-annealed state (~45 % of AISI 1212) with sharp carbide or HSS tooling and good coolant; after hardening, only grinding and EDM are practical.
Can SK4 be welded?
Not readily — weldability is rated poor (15/100). Effectively non-weldable for structural joints — high carbon gives hard, crack-prone martensite; if unavoidable, preheat 250–350 °C, use low-hydrogen filler and post-weld temper immediately.
What surface finishes work on SK4?
Grinds and laps to a fine, high-polish edge; protect with oil, black oxide, phosphate or thin plating (bake after plating to avoid hydrogen embrittlement) since bare surfaces rust quickly.
Can SK4 be formed, bent or molded?
Anneal before blanking or bending; cold-rolled strip work-hardens rapidly, so use generous bend radii, and hot-forge at 1050–850 °C followed by slow cooling and spheroidize annealing.
What is the maximum service temperature of SK4?
SK4 is rated for continuous use to about 300°C (572 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between SK4-A and SK4 QT60-63?
A is the default condition — standard stocked condition for bar and strip: softest state for machining, blanking and forming before hardening. QT60-63: maximum-hardness condition for knife edges, razors, files, scrapers and wear inserts in thin sections. Yield strength is 400 MPa in A versus 1,800 MPa in QT60-63.
How much does SK4 cost?
SK4 is a budget-cost steel ($), cheaper than about 90% 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
- JIS G 4401 — Carbon tool steels (SK95, formerly SK4) — JSA (2009)
- ASM Handbook Vol. 1 — Properties and Selection: Irons, Steels and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol. 4 — Heat Treating (carbon tool steels W-series) — ASM International (1991)
- ASM Handbook Vol. 16 — Machining (machinability ratings) — ASM International (1989)
- GB/T 1298 — Carbon tool steels (T9A/T10A comparison) — SAC (2008)
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.