708M40 open-die forgings

Chemical composition of 708M40

The table below gives the specified composition range for 708M40 in BS 970-3:1991, in weight percent. Each heat is analysed on the ladle, and product analysis is carried out where the order requires it. Both analyses are recorded on the mill certificate.

Chromium raises hardenability and wear resistance. Molybdenum adds strength at elevated temperature and suppresses temper embrittlement. The molybdenum range distinguishes 708M40 from 709M40, which is specified at 0.25 to 0.35% Mo.

Mechanical properties of 708M40

BS 970 specifies 708M40 by condition letter. Each letter denotes a tensile band achieved by quenching and tempering, and each band applies up to a stated limiting ruling section, since hardenability decreases as section size increases. Purchase orders should state the condition letter in addition to the grade.

Condition T is the most commonly ordered condition. The values above are the specified bands. Certified results on the mill certificate govern acceptance. Charpy V-notch impact values at +20 C are typically around 50 J longitudinal, and are tested when the order requires them.

Typical test results

Results recorded on a 708M40 T forging produced at our works: tensile strength 949 N/mm2, yield strength 820 N/mm2, elongation 18%, hardness 277 HB. Results vary with section size, test position and tempering temperature.

Forging and heat treatment of 708M40

Process notes

Cooling after forging

708M40 is sensitive to hydrogen flaking, also known as white spots, which produces internal cracks that ultrasonic testing will reject. Light sections are slow cooled in a sand pit. Heavy sections are transferred from the press to a controlled annealing or normalising cycle. This cooling practice is applied as standard on all 708M40 forgings.

Temper embrittlement range

Tempering between approximately 250 C and 375 C substantially reduces impact toughness in this grade. Where a drawing specifies a hardness that would require tempering within this range, the point is raised at the quotation stage.

Surface hardening

After quenching and tempering, 708M40 can be nitrided or flame and induction hardened to above 50 HRC on the surface while the core retains its toughness. This is the usual treatment for shaft journals and gear teeth.

708M40 equivalent grades

The grades listed below belong to the same 1% Cr-Mo family as 708M40 and are widely accepted against it. They are cross-references rather than automatic substitutes. Whether a substitution is valid depends on the standard, delivery condition, section size and test requirements stated on the purchase order.

708M40 forgings we manufacture

All forms listed below are forged to drawing at our Jiangyin works. Each part is upset, drawn, punched or ring rolled so that grain flow follows the contour of the finished component.

The ranges shown reflect regular production. Parts outside these ranges are quoted on enquiry.

Where 708M40 forgings are used

708M40 is specified where a component requires greater strength than plain carbon steel provides, without the cost of a nickel-bearing grade such as 817M40. It is used across the following sectors.

• Pumps and compressorsPump shafts for chemical and plunger pumps, compressor shafts, impeller hubs, industrial air compressor components.
• Pressure vessels and heat exchangersForged flanges, tube sheets, shell rings and nozzles for air receivers and shell and tube heat exchangers.
• Oil, gas and offshoreSubsea and deepwater production components, wellhead and Christmas tree parts, forged bodies and manifolds.
• Power transmissionGear blanks, pinions, spindles and rolls for gearboxes, turbines and gas compressor trains.
• ValvesValve stems, seat rings, blocks and bodies for ball, gate, globe, check and plug valves.
• Cement, mining and sugar millsMill shafts, eccentric shafts, crankshafts, forged rolls and crusher components.
• Shipbuilding and heavy machineryRudder stocks, forged wheels, rolls and press components for paper, pulp and pharmaceutical plant.
• Process towers and modulesForged rings and shafts for columns, towers, tanks, silos, preheaters and process skids.

Testing, inspection and certification

The following tests and documents are available on 708M40 forgings.

• Chemical analysis. Ladle analysis on every heat, product analysis where the order requires it.
• Mechanical testing. Tensile, yield, elongation, reduction of area and Brinell hardness. Charpy V-notch impact at ambient or low temperature on request.
• Ultrasonic testing. To EN 10228-3, SEP 1921 or ASTM A388, at the acceptance class stated on the order.
• Other NDT. Magnetic particle to ASTM E709 and dye penetrant to ASTM E165 on machined surfaces.
• Dimensional report. Against the drawing, with machining allowance recorded.
• Certification. EN 10204 3.1 as standard. EN 10204 3.2 witnessed by TUV, SGS, BV, LR or DNV on request.

The 3.1 certificate records chemical analysis, mechanical properties, hardness, non-metallic inclusion rating, heat treatment record, grain size, forging ratio and the NDT method and acceptance criteria.

How to order 708M40 forgings

Quotations are prepared from a drawing. Where no drawing is available, the following information is sufficient.

1. Shape and dimensions. Finished size, or rough size with the required machining allowance.
2. Quantity. Number of pieces, and whether the order is a one-off or a repeat.
3. Condition. Annealed, normalised, or quenched and tempered to condition R, S, T or U.
4. Ultrasonic testing. Standard and acceptance class, where UT is required.
5. Certificate. EN 10204 3.1, or 3.2 with the inspection body named.
6. Delivery. Port and required date.

Frequently asked questions about 708M40

Related grades we forge

Same 1% Cr-Mo family

AISI 4140, AISI 4140H, 42CrMo4, 42CrMo4V, 42CrMoS4, 1.7225, 1.7227, SCM440, 42CD4

Higher temperature and corrosive service

For service conditions beyond the range of 708M40 we forge precipitation-hardening stainless steels including 17-4PH, 15-5PH, 17-7PH and PH13-8Mo, and nickel alloys including Inconel 718, Inconel 625, Incoloy 825 and Hastelloy C-22.