Steel 708M40-S (Q&T condition S)
Heat-treated state
BS 708M40 · AISI/SAE 4140 · EN 42CrMo4
Tough medium-strength Q&T grade for heavy shafts and general engineering.
Values for the S condition
Other 708M40 tempers and conditions
708M40 is listed in 13 conditions; see the grade overview at 708M40.
Compare all 13 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Solution annealed | Softest condition for deep machining, cold forming or before final heat treatment. | 420 | 655 | 25 | 190 HB | — |
Hot rolled | Cheapest mill stock, bought to be heat treated by the customer after rough machining. | 450 | 750 | 20 | 19 HRC | 7.8 % IACS |
Q&T condition R | Lightly tempered condition for large sections or maximum toughness/ductility. | 525 | 775 | 20 | 22 HRC | — |
Cold drawn | Close-tolerance bright bar for shafts and pins where full Q&T strength is not needed. | 620 | 760 | 14 | 20 HRC | 7.8 % IACS |
SThis page Q&T condition S | Tough medium-strength Q&T grade for heavy shafts and general engineering. | 620 | 850 | 18 | 26 HRC | — |
Normalized | Uniform fine grain structure for forgings prior to machining or hardening; moderate strength off the shelf. | 655 | 1,000 | 17 | 30 HRC | — |
TDefault Q&T condition T | The standard stock condition of 708M40 bar — best all-round strength/toughness/machinability balance. | 700 | 925 | 17 | 29 HRC | — |
Q&T (condition T/U) + gas nitrided case | Choose for wear-resistant, low-distortion surfaces (spindles, rams, gears) while keeping a tough Q&T core. | 720 | 950 | 15 | 55 HRC | — |
Q&T condition V | Use where higher yield is required and section size is modest; toughness starts to fall. | 850 | 1,075 | 13 | 34 HRC | — |
Solution heat treated (unstable) | High-strength condition for small/medium sections such as bolts, pins and torsion parts. | 930 | 1,150 | 12 | 37 HRC | — |
Q&T condition X | Very high strength Q&T for small sections; check notch sensitivity and hydrogen embrittlement risk. | 1,000 | 1,225 | 11 | 40 HRC | — |
Q&T condition Y | Near the practical strength ceiling for 708M40; only for small diameters and low-notch designs. | 1,080 | 1,300 | 10 | 42 HRC | — |
Q&T condition Z | Maximum-strength low-temper condition for small parts; low toughness, high distortion and cracking sensitivity. | 1,380 | 1,600 | 8 | 48 HRC | — |
What is 708M40-S?
708M40-S is 708M40 heat treated to S — tough medium-strength Q&T grade for heavy shafts and general engineering. 708M40-S has a yield strength of 620 MPa (89.9 ksi) and a tensile strength of 850 MPa (123 ksi) — stronger than 78% of steel grades. Elongation at break is 18% and the elastic modulus is 210 GPa (30.5 Msi). 708M40-S has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,416°C (2,581 °F). T is the default condition FabDigit quotes; S is available on request or by drawing note.
Advantages
- High strength — 620 MPa (89.9 ksi), better than 78% of steel grades
- Polishes to a fine finish — 65/100, better than 78% of steel grades
708M40-S properties
Typical room-temperature values for 708M40-S; 10 are specific to this condition. Other listed values are 708M40 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
708M40-S has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,416°C (2,581 °F).
Mechanical15
708M40-S has a yield strength of 620 MPa (89.9 ksi) and a tensile strength of 850 MPa (123 ksi) — stronger than 78% of steel grades. Elongation at break is 18% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
708M40-S is rated for continuous service to 400°C (752 °F). It conducts heat at 42.6 W/m·K (24.6 BTU/hr·ft·°F), worse than 51% of steel grades. Thermal expansion is 12.3 µm/m·K (6.83 µin/in·°F).
Electrical3
708M40-S conducts electricity at 7.8 % IACS, worse than 57% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (12/100), worse than 53% of steel grades.
Sustainability4
Producing a kilogram of 708M40-S takes about 29 MJ of energy and emits 2 kg of CO₂ — less than 53% of steel grades. Typical recycled content is 35%.
Manufacturability7
708M40-S's machinability is good (55/100, better than 62% of steel grades); weldability fair (40/100); formability poor (28/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with 708M40-S
Is 708M40 easy to machine?
Machines well up to ~300 HB with carbide at 100–180 m/min; above ~40 HRC use coated carbide/CBN, rigid setups and reduced feeds — annealed or ≤277 HB stock is preferred for heavy cutting.
Can 708M40 be welded?
Weldable with care only: preheat 250–350 °C, low-hydrogen consumables, interpass control and immediate stress-relief/temper at 600–650 °C; avoid welding in the high-strength (X/Y/Z) conditions.
Can 708M40 be formed, bent or molded?
Hot forge 1100–1200 °C, finish above 900 °C and normalize afterwards; cold forming limited to annealed bar with generous bend radii.
What surface finishes work on 708M40?
No anodizing — protect by oil, phosphate, paint, zinc/hard-chrome plating, black oxide or nitriding; bake 190–220 °C after plating to avoid hydrogen embrittlement.
708M40 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 |
|---|---|---|
| C | 0.36 – 0.44 | 0.4 |
| Si | 0.1 – 0.35 | 0.25 |
| Mn | 0.65 – 1 | 0.8 |
| P | ≤ 0.04 | — |
| Smax (708A42/709M40 variants have tighter S) | ≤ 0.04 | — |
| Cr | 0.9 – 1.2 | 1.05 |
| Mo | 0.15 – 0.25 | 0.2 |
| Fe | balance | — |
708M40-S — frequently asked
What is 708M40-S used for?
708M40-S is typically used for shafts, gears, fasteners and tooling. In short: benchmark Cr-Mo engineering steel; widest availability in family; Q&T to R–Z classes.
What is the yield strength of 708M40-S?
708M40-S has a typical yield strength of 620 MPa (89.9 ksi) and a tensile strength of 850 MPa (123 ksi) — stronger than 78% of steel grades. Strength varies by condition: see the 13 listed tempers.
Is 708M40-S easy to machine?
Reasonably — machinability is rated good (55/100, better than 62% of steel grades). Machines well up to ~300 HB with carbide at 100–180 m/min; above ~40 HRC use coated carbide/CBN, rigid setups and reduced feeds — annealed or ≤277 HB stock is preferred for heavy cutting.
Can 708M40-S be welded?
With care — weldability is rated fair (40/100). Weldable with care only: preheat 250–350 °C, low-hydrogen consumables, interpass control and immediate stress-relief/temper at 600–650 °C; avoid welding in the high-strength (X/Y/Z) conditions.
What surface finishes work on 708M40-S?
No anodizing — protect by oil, phosphate, paint, zinc/hard-chrome plating, black oxide or nitriding; bake 190–220 °C after plating to avoid hydrogen embrittlement.
Can 708M40-S be formed, bent or molded?
Hot forge 1100–1200 °C, finish above 900 °C and normalize afterwards; cold forming limited to annealed bar with generous bend radii.
What is the maximum service temperature of 708M40-S?
708M40-S 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 708M40-T and 708M40-Z?
T is the default condition — the standard stock condition of 708M40 bar — best all-round strength/toughness/machinability balance. Z: maximum-strength low-temper condition for small parts; low toughness, high distortion and cracking sensitivity. Yield strength is 700 MPa in T versus 1,380 MPa in Z.
Sources
- BS 970-1: Wrought steels for mechanical and allied engineering purposes — BSI (1991)
- EN 10083-3 Quenched and tempered steels — alloy steels (42CrMo4) — CEN (2006)
- ASM Handbook Vol.1: Properties and Selection: Irons, Steels and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.19: Fatigue and Fracture — ASM International (1996)
- EN19 / 708M40 engineering steel datasheet — West Yorkshire Steel / Bohler-Uddeholm (2023)
- GB/T 3077 (42CrMo) — SAC (2015)
- 42CrMo — Mysteel / 1688 (2025)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.