Steel 708M40
BS 708M40 · AISI/SAE 4140 · EN 42CrMo4
708M40 is the British Standard designation for the general-purpose chromium-molybdenum engineering steel equivalent to 42CrMo4 and AISI 4140, supplied quenched and tempered to the R–Z tensile classes.
Reference datasheet — 708M40 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 T condition
708M40 tempers and conditions
708M40 is supplied in 13 conditions, soft to hard. T is the default FabDigit quotes; the values on this page are for it.
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 |
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 | — |
Supply forms, finishes and certifications
Ways 708M40 is supplied — form, process route, surface or paperwork — without a datasheet of their own. The property values above apply.
| Option | What it is | Type |
|---|---|---|
U | Q&T condition U | Coating |
What is 708M40?
708M40 is the British Standard designation for the general-purpose chromium-molybdenum engineering steel equivalent to 42CrMo4 and AISI 4140, supplied quenched and tempered to the R–Z tensile classes. It is the default choice for shafts, gears and fasteners because it combines good hardenability in medium sections with reliable machinability, and it is widely available as hot-rolled and cold-drawn bar, forgings and tempered-plus-nitrided stock. Default-condition values are 700 MPa yield, 925 MPa tensile strength, 17% elongation and 277 HB (29 HRC), with service to 400 °C. It is weldable only with preheat and post-weld temper and has no corrosion resistance; 817M40 (4340) is the step up when higher strength in heavy sections is needed. 708M40 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). 708M40 has a yield strength of 700 MPa (102 ksi) and a tensile strength of 925 MPa (134 ksi) — stronger than 81% of steel grades. Elongation at break is 17% and the elastic modulus is 210 GPa (30.5 Msi).
What is 708M40 used for?
- shafts
- gears
- fasteners
- tooling
Advantages
- High strength — 700 MPa (102 ksi), better than 81% of steel grades
- Polishes to a fine finish — 65/100, better than 78% of steel grades
708M40 properties
Typical room-temperature values for 708M40-T; bars are the position among steel grades.
Physical3
708M40 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 has a yield strength of 700 MPa (102 ksi) and a tensile strength of 925 MPa (134 ksi) — stronger than 81% of steel grades. Elongation at break is 17% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
708M40 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 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 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 machinability is fair (52/100, better than 52% of steel grades); weldability fair (40/100); formability poor (28/100).
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. 708M40 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with 708M40
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.
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 |
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.
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 708M40 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
How much does 708M40 cost?
708M40 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.
708M40 — frequently asked
What is 708M40 used for?
708M40 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?
708M40 has a typical yield strength of 700 MPa (102 ksi) and a tensile strength of 925 MPa (134 ksi) — stronger than 81% of steel grades. Strength varies by condition: see the 13 listed tempers.
Is 708M40 easy to machine?
Reasonably — machinability is rated fair (52/100, better than 52% 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 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?
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 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?
708M40 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.
How much does 708M40 cost?
708M40 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
- 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; 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.