Charpy Values for AISI 4140 @ -50F 3

[valvesarefun]

Hello All,

I am trying to establish minimum acceptable and minimum average Charpy impact values (ft-lb) for AISI 4140 at -50F.

The material will be Normalized, austenitized, quenched, and tempered. It should have approx. UTS of 115 ksi, YS of 99ksi, and hardness around 21 HRC.

The material will be supplied in bar form and machined into solid valve stems.

Also looking for input as to whether a transverse or longitudinal test is preferable, or if these terms even apply to product supplied in bar form.

Im sure im probably leaving some important info out, so please feel free to ask for additional information.

Thanks in advance!

Best Regards,

VRF

 

[metengr]

The acceptable average or minimum CVN value at -50 deg F needs to be established by you, and you should base the value on service application and consequences of brittle fracture.

If you are trying to determine what the expected minimum or average CVN values are at -50 deg F, run test specimens using transverse orientation (less favorable orientation).

 

[salmon2]

Do you have a standard to comply with? It should define the Charpy requirement for you, if any. If not, it can be time-consuming and expensive as you have to evalute your application and risk of failure per metengr and probably have to do a lot of functional test.

Please check out the below link on page 14, you can find 4142 (not 4140 but close enough) properties vs tempering temperature. You can see at your target strength level, it has pretty decent impact toughness, but note it is Izod, not V-notch Charpy, and those values are probably at longitudinal direction and room temperature.

http://www.emjmetals.com/pdf/bluebook-r.pdf

Longitudinal and tranverse Charpies apply to bar stock perfectly. Usually they are both acceptable depending on the size of bar stocks, but if failure loss is high, ask for transverse Charpy because it is the critical direction. Longitudinal requirement is often times 50% higher than tranverse direction.

From different steel mills, 4140s can exhibit quite different Chapry results even if everything esle is the same, especially transverse Charpy because it is more sensitive to chemical composition and process.

 

[mrfailure]

If you want to make sure the material is not mostly brittle at -50 deg. F, then you also may want to have a % ductility requirement based on visual assessment of your Charpy's per ASTM A370. This is a fast and dirty way to confirm testtemperature is above ductile-brittle transition temperature (DBTT).

Aaron Tanzer

http://www.lehightesting.com

 

[redpicker]

Your properties are not consistent. With a hardenss of "around 21 HRC", your "approximate" tenisle strength will only be about 110 KSI. To get 115 KSI, you will have to be above 23 HRC with a working range of 23-28 HRC, perhaps wider, averaging closer to 25 HRC than 21. Maybe this difference is not significant to you, maybe it is, but you need to be aware of it.

For a valve stem application, I would think a longitudinal CVN would be what you are interested in, since the principle stresses will be in that direction. But, as mentioned above, your expected loading conditions should be what dictates that.

What properties you can expect with 4140 are widely dependent on size, specific heat treating conditions, and specific chemistry. From personal experience, I know you can get over 50 Ft-Lbs at -50 F with controlled heat treatment and slight modifications to the base 4140 chemistry ranges, but you can also get less than 10 Ft-Lbs at this temperature and strength level.

The proper way to procede is to determine the properties you need for the application, find a material and processing that will achieve those properties, and finally, write a specifcation that will insure you will get the properties you need.

rp

 

[kingsley47]

If this is for an oilfield application, a 31 ft*lb average with a single specimen breaking at a minimum of 21 ft*lbs tends to be the norm. This is for charpy specimens taken from the longitudinal direction.

I am only throwing this out there because it is what I generally see in specifications concerning 4140 for subsea oilfield applications. Seeing as I do not know your application, I don't know if this would meet your requirements or not.

It would be a good idea to modify the chemistry requirements to keep the phosphorus and sulfur content at a max of .025% each. This should help keep your impact values fairly high.

If you require impact strength well above 31/21 at -50 F, you may want to consider changing your material to 4340.

 

[MECHGU]

I have a similar issue with 4140 material in the same strength range. As mentioned above my application is oil/gas related and part of the ABS standard. The issue is my results are all over the map. Can someone give me guidance on what exactly needs to be controlled? I don't have a materials lab to use a trial an error method or the time. Any assistance would be greatly appreciated. My requirement is 33 ft/lb @ -20 deg C.

I often make the change to 4340 but see similar inconsistencies.

 

[salmon2]

MECHGU,

Does variance happen btween three specimens or beteen different heats or even different steel sources? If it is from differnt steel source, it is totally understandable. If it is other two, then it is either a steel supplier with poor quality control or a incapable matl lab. Every step together give you the final mechanical properties, so it is hard to say unless more details are provided.

 

[MECHGU]

It is between different heat lots and sources as you suspect. There is occasional variation between the 3 legs of the testing. We might see one poor result which I have always written off poor/inconsistent material in that area. I guess what I am trying to understand is what elements of the chemistry do I need to control and what specific parts of the heat treat are crucial. In this case the material is in the 269-321 BHN range.

 

[redpicker]

As you have observed, 4140 is capable of meeting the properties you are interested in, but you really have to control the material and heat treat process to achieve them regularly.

With the chemistry, you will want to limit P & S to as low as you can afford. The cost of the material goes up with lower P & S, so you shouldn't be specifying .005 max if you don't need it that low. For what you have said, I would think 0.025 max P & S would be needed and perhaps 0.020 Max P and 0.015 Max S, but the lower limits will have an effect on cost as well as availability.

Heat treatment is also very important, but closely related to the chemistry, too. You might consider having a minimum DI requirement based on the OD size. This requiement depends on the quench (oil or water) and cross section size at the time of heat treatment.

4140 is a very good material when used within it's capabilities.
rp

 

[metengr]

If you specify a normalize and temper heat treatment, this will reduce variability and help to meet low temperature notch toughness.

 

[valvesarefun]

MECHGU,

Since my original post, I have learned that there are many different things that can affect the notch toughness of 41xx series steels, probably more than can really be mentioned in any kind of detail in this type of post. The two main things I have found so far are as follows:

First, as redpicker mentioned, keep your impurities (P&S) as low as possible (his % given above are right on the money).

Second, as redpicker also mentioned, you need to make sure the heat treatment process is well controlled and tailored for your specific chemistry range. If your desired YS & TS is like mine, then you will be using a quenched and tempered product. The tempering temperature and cooling rate from that tempering temperature will both affect your notch toughness dramatically. I think if the cooling rate is the problem(like if the material is furnace cooled slowly, which is just bad process all together), you will know because the steel will exhibit bad mechanical properties all around due to temper embrittlement. The main thing is the tempering temperature and time. The temperature must be appropriate for the chemistry and the time must be appropriate for the thickness of the part.

Where the balancing act comes in, is trying to achieve both the moderately high yield and tensile strength AND the desired charpy values. It is realitively easy to temper at a high temperature, restore alot of ductility to the part, and get good CVN numbers, but you will loose a considerable ammount of strength in doing that.

I think the ideal way to proceed is to: consult with a metallurgist, work to define a modified chemistry range, find a source that will supply it, define a specific heat treatment procedure, and then test the product to make sure its good. Once you have the process lined out, your results should all begin to fall within an acceptable variance.

Also, it might not be a half bad idea to take some of the good and bad samples that you have, and have them analyzed under a microscope at a metallurgical lab. If it is an experienced lab, you should just be able to tell them what the problem is and they can look for clues as to the cause.

Best Regards,

VRF

 

[TVP]

MECHGU,

The replies from redpicker and metengr are excellent, and should guide you toward reducing the variation and consistently achieving the target of 33 ft lbs at -20 C. I suggest using the lower values of 0.020 max for P and 0.015 max for S that redpicker mentioned, based on the allowable hardness as high as 321 HB (35 HRC). Whether or not normalizing should be implemented depends on several factors including the initial austenitic grain size size and the size of the original billet/bar. Small diameter bars (say less than 2" in diameter) usually have consistent fine grain size (all grains with ASTM number 5 or higher). Large bars do not, and thus benefit enormously from normalizing prior to quench and temper treatments.

 

[salmon2]

As others said, control P and S as low as possible. Make sure autenitizing temperature not too high or holding period too long, then you can have finer grain. Specifying location for testing coupons as large diameter can vary dramatically from OD to core. Same supplier, same heat but different bar size can vary also and this is why redpicker suggested DI requirement. Also, the high end of your hardness range seems in suspecible temper embrittlement range.