It's not so much the toughness or strength that it gets used for it is the fact it is unlikely to distort when cuts are not balanced hence it gets used for things like crankshafts.

The "stressproof" is due to it being heat treated (normalised) after rolling to remove any inbuilt stresses which are what cause other bright metals move when cut

Edited By JasonB on 31/07/2022 13:56:39

From what I gather 708M40 is the equivalent to 4140 and looking up 708M40 as you have said is equivalent to EN19

Be careful, It is possible to find an alternative steel with similar mechanical properties and composition, several have already been suggested. However, when you order 4140 "stressproof", you are specifying the material's post heat treatment condition.

As Jason pointed out; The "stressproof" part is due to the steel being carefully heat treated (normalised) after rolling to remove any inbuilt stresses which are what cause other bright metals move when cut. The"stressproof" treatment makes the material less likely to distort when cuts are not balanced; hence it gets used for things like crankshafts.

It's not so much the type of steel, it's all about the stress relief after rolling

Mike

4140 is the general, go-to material for shafts and axles in Australia when a higher strength material is required. It is normally supplied as bright or ground bar and machines OK but costs about 50% more than 1020/1030 grade (mild steel) bar.

1030 bright bar has a tensile strength of ~500MPa, 4140 ~900Mpa.

And you can "stressproof" steel yourself by heating to a dull red and letting it cool slowly. I bury mine in a bucket of powdered lime to let it cool slowly and normalise. Seems to work well, even with welded fabrications such as my Versatile Dividing Head body arc welded up from steel pieces. Was very easy to machine and several years later, no sign of distortion.

My local steel merchant sells two grades of steel only, 1020 and 4140. I rarely find any use for the latter in model type work, other than I might use it for an upcoming new spindle for my Drummond lathe.

Not sure anything about naming steels is normal! Most steel making nations had, and maybe still have, their own naming systems, and repeated attempts to rationalise them have mostly failed. AISI is well-known in the USA, less so abroad, but we all have tables of equivalents: if in Australia you want AISI 4140, BS970-659M15, or SAE J403 there's probably a local equivalent.

In the UK we still buy steel using the "Emergency Number" system introduced during WW2, even though the system was replaced in 1972! My local supplier looks blank if I ask for 220M07 but smiles if I ask EN1A. As no-one makes EN1A, he's selling me 220M07 or similar, and someone in the supply chain relabelled it to suit conservative Brits! The metal could have come from anywhere. More likely Chinese or EU made than American, but it might be Indian, Russian, German, South Korean, Turkish, Brazilian or whatever…

The naming problem got worse with the internet because folk around the world specify what they know, probably not realising their understanding doesn't travel well. Faced with an unfamiliar specification number, you have to find an equivalent that your local stockist keeps. My local metal retailers aren't switched on to equivalents so I do my own research. Fortunately the internet is good at identifying alternatives that a local shop can supply.

Frankly, it's a mess!

Dave