709M40 is a 1% Cr-Mo medium hardenability general purpose high tensile steel.

709M40 is generally supplied in the Hardened and Tempered condition in the tensile range of 850 – 1000Mpa (T Condition) but can be heat treated to 700 – 1200Mpa dependent on section size (R – W Condition).
This alloy is used in most industry sectors for a wide variety of applications. Typical uses include Drive shafts, couplings, bolts and gears, together with many surface and sub-surface components in the oil and gas industries.
This grade also has reasonably good impact properties at low temperatures and can be good for some elevated temperature uses.

709M40 can be further surface hardened by either flame or induction hardening methods, giving case hardnesses in excess of 50 HRc.

Related Specifications 709M40 1% Cr-Mo
Also covers 708M40 – 1% Cr-Mo Steel BS970 – 1955 EN19
DIN 42CrMo4
W.Nr. 1.7225
AISI 4140
Typical Chemical composition
Carbon 0.4%
Silicon 0.25%
Manganese 0.85%
Phosphorous <0.040%
Sulphur <0.040%
Chromium 1.00%
Mechanical Property Requirements – BS970 Part 3 1991
Condition Rulling Section Tensile Strength Mpa Yield (0.2%) Mpa Elongation % Izod Ftlbs Charpy J Hardness HB
R 250mm 700/850 480 15 25 28 201/255
S 150mm 725/925 570 15 40 50 223/277
T 100mm 850/1000 665 13 40 50 248/302
U 63mm 925/1075 740 12 35 42 369/331
V 29mm 1000/1150 835 12 35 42 293/352
W 19mm 1075/1225 925 12 30 35 311/375
Forging
Forging Temperature for this material should be 850 – 1200oC

Soak times should be kept to a minimum to avoid heavy scaling, but sufficient time should be given to allow centre to achieve furnace temperature.

After forging pieces should be allowed to cool in still air.
Heat Treatment
Hardening – Heat to 840 – 875oC for a time commensurate with ruling section and quench in Oil, Water or Polymer.

Note: If water quench is to be used, although not recommended, care must be taken to ensure that all sharp corners are removed prior to heat treatment.Tempering – Re-heat to 550 – 700oC as required, dependent on final required properties. Hold for a time commensurate with the ruling section and cool in still in air.
Machining
709M40 has good to very good machinability, dependent on condition, and operations such as sawing, turning, broaching, milling etc can be accomplished satisfactorily using standard machine tool manufacturers recommended speeds and feeds.
Welding
Welding of 709M40 in the hardened and tempered condition is not advised and should be avoided if possible. If welding is required it should be done using low hydrogen electrodes, while the material is in the annealed condition, and the work piece should be stress relieved (595 – 620oC) immediately after cooling to hand warm, prior to hardening and tempering.