AISI 1045 Brittle Fracture 1

[meca]

I am using AISI 1045 for the shaft of a Sheave. The shaft will be outdoors in Canada, and so brittle fracture is a consideration. Does anyone know where I can find the low temperature characteristics of this material?

 

[diamondjim]

Meca,
I would not even bother to go there.
Use 4340 or 8620 which has enough nickel
in it to make it take the cold temperatures.

 

[Metalguy]

meca,

Different heats of 1045 can fracture in a brittle manner at widely different temps., just like most other steel alloys-with nickel or not.

You need to find out the chem. composition and the grain size of what you have. If you haven't already bought it, you can specify that certain elements be kept very low to lower the "brittle/ductile" transition temp.

Let me know the chem. and GS and I'll figure out what you can expect.

 

[meca]

Thank you Metalguy and diamondjim for your helpful responses. My understanding was that the material was already purchased, and so I was trying to determine the Chemical Composition and Grain size as you recommended; however, I have now confirmed that the material has not been ordered.

So diamondjim, I will take your advice and recommend 4340 or 8620. Can you please advise of a reference that would verify the selection of these materials for low temp, so that I may defend my selection of materials to my client?

 

[CoryPad]

meca,

According to ASM Handbook Volume 1 Properties and Selection: Irons, Steels, and High-Performance Alloys, 4340 heat treated to 35 HRC has Charpy V-notch impact energy over 100 J at -70 &[ignore]deg[/ignore];C. This indicates high fracture resistance at low temperatures.

 

[Philrock]

Brittleness at low temperatures can be a serious and sometimes insurmountable problem. I heard a true story about a large ship stationed in the Arctic whose anchor chain winch shaft broke whenever they dropped anchor in water deeper than the length of the chain. This meant the loss of many thousands of dollars worth of shaft, drum, anchor and chain every time it happened, not to mention the loss of anchoring capability. They tried many different solutions, including premium materials, larger shafts, and stress reduction fillets. The only thing that worked in the long run was putting a large heater in the winch compartment to keep the shaft from getting too cold.

Your application may be a little less severe, and a good alloy steel may do the job. It's my understanding that while 8620 can perform well, it is difficult to predict impact strength, and extensive experiments are often required to get optimum performance from this material. 4340 will be more expensive, but more predictable - you'll have a much better chance of getting it right the first time with 4340.

 

[Metalguy]

meca,

What is the diameter of the shaft? It is a very simple process to determine just what temp. will cause brittle fracture. We determine this in the nuke industry all the time, then add 60 deg F for a safety margin.

There have been heats of 8620, 4340 and just about every other low-alloy steel which will fail via brittle fracture at temps. higher than what you need. Every heat and heat-treated condition must be tested, but it's possible to guess pretty close if you have the right info for your steel.

 

[meca]

Metalguy,

The diameter of my shaft is 1 1/2". Other than 4340, what else do I need to specify when purchasing?

 

[TVP]

meca,

You need to specify the material, manufacturing method, and heat treating process with sufficient detail so that factors that affect brittle fracture are carefully controlled. There are a number of things that you need to consider:

1. Desired mechanical properties-- presumably you are investigating a nominal carbon composition because the shaft reqires a strong and/or hard surface. What is the strength and hardness reqirement?

2. Desired low temperature performance-- you may not have a design requirement for this, other than "shaft must not break at X temperature". Fracture toughness is one factor to consider, and fatigue strength is another.

3. Fracture toughness can be approximately understood using the conventional notched bar impact testing (usually Charpy). ASTM E 23 - 02 Standard Test Methods for Notched Bar Impact Testing of Metallic Materials is one standard that covers this test. This should probably be a requirement for the material to be certified for your application.

4. In addition to the standard chemical composition, you may need to place restrictions on the elements that are detrimental to fracture toughness: S, P, O, N, etc. S & P are usually specified to levels below 0.010 % (by weight) when fracture toughness is a requirement.

5. Cleanliness as determined by inclusion analysis (microscopic and macroscopic) is frequently specified for fracture-critical components. ASTM E 45 - 97Standard Test Methods for Determining the Inclusion Content of Steel is one method used, which categorizes inclusions by type (A = sulfides, B = oxides, C = silicates, & D = globular oxides), size, and distribution. Worst field views for thin and heavy series should be 0.5 maximum for all of these categories when low-temperature fracture toughness is needed. This may require that the material is produced using a degassing step before casting, or that the material be produced by a consumable electrode type process (VIM, ESR, VAR, etc.).

6. Heat treating process-- Is this part going to be through hardened? What is the section size? Hardenability needs to be appropriate when through hardening larger sections. The heat treating process must be carefully controlled so that no decarburization or internal oxidation occurs. Cracking after quenching is another defect that must be eliminated. Dye penetrant testing or magnetic particle inspection are commonly used to inspect heat treated parts for surface cracks.

These are some of the variables that you should consider when specifying the material for this application. Just writing "4340, HRC = 35" is not sufficient to guarantee adequate performance.

 

[Metalguy]

The first thing you should do is realize that 4340 is not anything special for a relatively small dia. like this.

First of all, you need to determine what strength you need. If the orig. shaft is soft, and the loads are low, you could simply order a 304 stainless shaft (annealed) and not have to give low-temp. brittle fracture another thought.

If you need something stronger, you should simply ask your steel supplier what he has avail. for real low-temp. service.

For carbon/low-alloy steels, the first thing you want is a fine grain size-the finer the better, something around 7 minimum. Then DEMAND that the P (phosp.) be VERY low-less than 0.01%. Also DEMAND that the nitrogen be low, less than ~.004%. Finally, use as low a carbon content as you can for the strength you need.

If this is really critical, require Charpy V-notch impact testing, and make sure you have at least ~ 25 ft. lb. at the lowest temp. you'll have. 50 ft. lb. is even better.

 

[meca]

Thank you for the advise, this is very helpful. All of this discussion makes me wish I would have paid better attention in my Materials Class!!

 

[Metalguy]

Glad some of this helped. Let us all know what you end up with.

 

[diamondjim]

Meca,
This 4340 material is used often in API bearing
designs and also DNV specs in the North Sea
area which can see extreme temperatures.
BHN range 269/311 is normally the range.

API American (Petroleum Industry)
DNV (Deutch Norse Veritas)

It is my understanding that the 8600 steels were
developed by the railway industry to solve the
impact of the couplers shattering in the extreme
temperature in Canada.

 

[nickelkid]

Metalguy,

What about a 400 series SS?

 

[Metalguy]

410 SS would probably be OK, IF it was hardened and then tempered above 1100 deg F. A few Charpy V notch tests would confirm what temp. will handle.

A slightly better version is 403 (turbine quality).

However, I would not use any martensitic SS if the shaft runs in sleeve bearings. There is a unique problem in this situation (wire-wooling), and the shaft can be destroyed very quickly.

 

[FourJawChuck]

You could use a good quality 1020 carbon steel instead of the higher carbon 1045, these steels work much better at low temps assuming you don't need to heatreat it to a higher hardness. I live in Canada and most outdoor applications require a low carbon steel to survive sudden load impacts at -40C, if you need a journal surface have a heat treated sleeve shrunk onto the shaft. There is no need to use exotic alloys to survive Canadian winters, mild steel works just fine. Do not use Superior shafting or any of the "shafting" type materials, the grain size and material properties do not lend themselves well to extreme low temps. Your working loads should determine your material diameter and if mild steel doesn't provide the strength required at that size then I would look at the alloys to increase strength.

As for stainless I would 304 as my first choice with 316L second and then 416SE if you need higher strength.

What is the load this shaft will see, a little more info would be nice?