4140 and 4340 alloy steel 1

[bill8123]

Hi All,

I’m a slightly confused young mech engineer who has let his materials knowledge lapse.

I’m looking at the differences between 4140 and 4340 alloy steel and in particular their toughness proprieties.

With refinance to the web pages below I see 4140 has higher Impact Strength (J) (Izod), Elongation (%) and Reduction in Area (%).

To me this suggests that 4140 is tougher? But I have been told that 4340 is in actually tougher?

Can somebody please help? Thanks!!

http://www.efunda.com/materials/all...fm?id=aisi_4140&prop=all&page_title=aisi 4140

http://www.efunda.com/materials/all...fm?ID=AISI_4340&prop=all&Page_Title=AISI 4340

 

[swall]

The data in your efunda link is for annealed 4140 and 4340. In an actual application, you would be using quenched and tempered material. Consequently, your conclusions about the relative toughness of these two materials is not valid.

 

[ERob]

'Toughness' is used in a couple of contexts in metallurgy and materials science. One context is 'fracture toughness' which is defined as the ability of a material to resist propagation of cracks that inevitably exist in a material, as described by theories of fracture mechanics. Toughness, in a different context, is defined as the energy per unit volume that is dissipated during plastic deformation up to the point of failure. A precise definition, in this context, is the area under the true stress-true strain curve. The Charpy impact strength is one measure of a materials toughness (in this context). However, the Charpy impact test is performed at high strain rate, so if the material is sensitive to strain rate, then the relative toughness of the material may differ at a lower strain rate. Also, I believe that the degree of work hardening that the material has previously undergone will alter the measured toughness of the material. Unfortunately, I don't know the specifics of 4140 vs. 4340, but I hope this is helpful to you in thinking about toughness. Perhaps someone with more experience with these two alloys can offer more help. Can someone confirm that 4340 is tougher by some measure other than the Charpy impact test?

 

[redpicker]

You are not making a fair comparison. The properties for the 4140 are listed for material annealed at 815C (1500F)
Tensile: 655.0 MPa (95,000 PSI)
Yield: 417.1 MPa (60,500 PSI)
% El: 25.7
HB: 197
Izod Impact: 54.5 J (40.2 Ft-Lbs)

The properties for 4340 are listed for material annealed at 810C (1490F)
Tensile: 744.6 MPa (108,000 PSI)
Yield: 472.3 MPa (68,500 PSI)
% El: 22.0
HB: 217
Izod Impact: 51.1 J (37.7 Ft-Lbs)

So, the 4340 is at a higher strength than the 4140, which is probably has a larger effect (particularly at these strength levels in the annealed condition) than the differences in chemical composition.

4340 is typically considered to be tougher than 4140 in the quenched and tempered condition. This result is more from the fact that 4340 has higher hardenability and is more consistent in the heat treat response, particularly in thicker sections, than the differences in chemical compositions. 4340 is also typically produced to higher cleanliness standards than 4140, which is another big contributor to the toughness.

In my experience, however, in sections where heat treatment results in >95% martensite, with comparable levels of cleanliness, and tempered to equal hardness and strength levels, 4140 will have a higher impact strength than 4340. This is contrary to “common wisdom”, but I have test data that supports it. This is mostly a trivial point, however, since in most applications where 4340 is preferred over 4140, the section thickness is such that the 4140 might not fully harden, but the 4340 will.

rp

 

[metengr]

The 4340 alloy steel by virtue of having nickel as an alloying element increases hardenability and increases toughness over 4140. Impact energy relates to toughness but when it comes to fracture toughness data, 4340 will have higher toughness over 4140 at similar strength levels (quench and temper). Impact energy comparison has more to do with response to rapid strain rate effects and notch sensitivity, and I would not be surprised by rp's statement.

http://www.dtic.mil/dtic/tr/fulltext/u2/a129837.pdf

 

[ERob]

rp & metengr,

It sounds like quenching and tempering both 4140 and 4340 will increase toughness (as well as hardness) compared to the annealed condition. Is this due to retained austenite?, reduction in pearlite?, grain size (i.e. Hall-Petch)? or some other factor? I understand that martensite is very hard and brittle. I don't understand why quench and temper increases toughness compared to the annealed, ferrite + pearlite condition. Can you help me understand?

- ERob

 

[EdStainless]

By using Q&T you are forming a tempered martensite structure.
The quench and temper are both critical.
If the heat treat is done correctly for these alloys there will be no retained austenite.

In thin sections 4140 would be my go-to grade. Easy to heat treat and a wide range of properties depending on tempering temp.
But when you get to thick sections (a few inches) or parts with lots of variation in cross section then the Ni addition shows its value. I used to make high strength shafting from 4335V. It was a life saver for large sizes.

While the lower C in 4330 will give you slightly lower strength than 4140 the properties through thicker sections will more than make up for it. In order to use 4140 in these applications you would need to temper at higher temp (lower strength) in order to get acceptable core properties.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[redpicker]

No, it is not due to retained austenite. There is negligible retained austenite in either 4140 or 4340 in the quenched and tempered condition.

The increase in strength and toughness results from the fine (and equal) dispersion of carbides in the structure. This is a pretty basic heat treating concept, so a bit of googling on-line should get you more than enough references if you really want to know the exact mechanisms involved in heat treating.

The strengthening mechanisms are a bit more complex. An analogy a long-since dead professor once told me was gravel and clay. A road bed made entirely of clay would be durable for many uses, but would not perform well when wet and would become too soft. Now, one entirely of gravel would do fine when wet, but would not support heavy loads very well. Mix the two, and you have a very durable roadbed. Now, consider such a surface made with rocks the size of a grapefruit. Compare that with one with the same weight of rocks, but instead of grapefruit size, golf ball size. Now, marble size. The finer the rock, the tougher and more durable the roadbed will be.

That is similar to what is happening with a quenched and tempered steel. The alloying elements make it easier to achieve the fine dispersion of the carbides (and, help maintain strength during tempering, as well as having other effects). The benefits of normalizing and annealing treatments are they can produce very predictable mechanical properties with few process controls on the heat treating process. The quench and temper process can produce higher strengths and toughness, but requires much tighter controls to maintain consistency in the result.

rp

 

[bill8123]

Thanks everyone for the responses. Much appreciated.

Does anyone know a good book on Metallurgy for a Mechanical engineer who designs mining machinery? A book from a more practical perspective would be nice.

 

[TVP]

Metallurgy for the Non-Metallurgist, Second Edition

http://www.asminternational.org/portal/site/

http://www/AsmStore/ProductDetails/?vgnextoid=eb0b76d47ff2f210VgnVCM100000621e010aRCRD

Metals Handbook Desk Edition, 2nd Edition

http://www.asminternational.org/portal/site/

http://www/AsmStore/ProductDetails/?vgnextoid=863685a10f0f8110VgnVCM100000701e010aRCRD