8640 Material Substitution

You mention that you have the parts made of 8640 forged, and then heat treated?

If so, you can actually use 4340 or 4140 and make the same process. You can heat treat the parts after machining the teeth, so machining would not also be a problem. Before forging, you'll have to have it normalized, or it won't be forged without taking the die itself with it. Your hardness corresponds to 37-39 HRC after heat treat, not a big deal with these steel series. If you have it in stock, use it.

By the way, do you do some surface hardening, to use an 86XX series? If so, can you just bypass it? If you have your teeth in a fixed arrangement, not a rotating gear, for example, you could rely on the part hardness itself.

My final tip, If you are going to heat treat anyway, is that even the 1045 could do you a great job. And pretty cheaper too. 170 Brinell cold drawn and annealed as supplied (easy machining and good to forging too), the surface may be induction hardened up to 50 HRC if essential, can be quenched and tempered to 37-39 HRC with similar strengh. If you didn't need to machine teeth, you probably would not need post heat treatment.

As you mentioned some impact load, the less alloy and hardness, the better.

We used to do some 1045 with water quench, we got away from it because some times it would not quench high enough for a good temper. The concern with the 4340 is that the broaches used to cut these teeth are very expensive. Changing the tooth load or how the chip comes off the cutter can make the teeth load up and break. Makes me a little nervous. I know 4140 will machine okay I was worried about the lower toughness of the 4140 material at that hardness. The part is a lifting pawl that goes into a mechanical jack. They get used at all different ambient temperatures. It is very possible for the parts to be used at -30F or -40F.

By the way, do you know the nominal design stress in this part, so we can better evaluate the question? And, did you have any experience with parts with lower hardness, I mean, some report from the field about failed parts?

We have used 8620, had a lot of good luck with it and it is very avaiable...

You don't say why the forging house wants to substitute.

You don't say what the depth of hardness is on your part.

You don't say what is the part diameter and maximum/ minimum section.

Thus, well meaning people have given you suggestions for through hardening grades (4340) probable through hardening grades (4140) depending on section, and shallow surface hardening grades (1045) with absolutely no idea of the actual part requirements and geometry and depth of section and or case hardness... all critical factors to understand for the fit and function of your heat treated "stress properties way up there Part"

Gears typically are heat treated to a high surface case hardness over a tougher less hard, more resilient core. The .40 mean carbon of 8640 gives different properties than would 8620.

You don't say why broaching is a problem with any of these grades. These grades are all "broachable" with chipload per tooth and sfm varied slightly. certainly not a show stopper issue.

If the original engineer designer actually did engineering and determined that 8640 to a particular heat treat is suitable for the application, you would be well advised to stay with that unless you can demonstrate a greater need than you have provided.

When the substituted parts fail in service, Lawyers will ask "who is the engineer of record? and show us your engineering work and calculations." A copy of these emails will not even qualify as misfeasance in that instance...

Stick with the original design, perhaps find a forging house that can work with the specified material.

Milo

The forge house is having trouble finding the 8640 material to run our parts with. The sections on the part are not large, (7/8 of an inch or smaller) the part will through harden during heat treat. The tools we use to cut the teeth on these pawls are $6-8K each. They are custom tools for cutting the unique tooth form on our parts. If we break one it can take 12-16 weeks to get a new one. That is why I am being cautious with higher nickel content in the material. We have bent pawls on test in the past when the parts were not properly heat treated so I know the factors of safety can not be excessive. I have not gone to the trouble of modeling the part to do an fea, I was hoping to get some insight about other materials or variations in the heat treat to increase the toughness of alternate materials based on experience. My company has been making this product for more than 50 years. These are very old designs that date back to the late 20's that probably were all made with 1045 or 1050 steel back in the early part of the 20th century when that was the material available to choose from.

You mentioned before that you could have problems in some parts due to not achieving the desired hardness. As it indeed seems that the original parts were made on 1045, I'd go toward it, but improving the heat treatment process to have some more consistency, avoiding loosing parts. It's not so hard to do and, after done, you could save some money from material.

Of course, due to this problem, the forging will have a good discount for its services, right?

Well you don't say if the broaching takes place before or after the heat treat, but it really doesn't matter as the 4140 and 8640 will both be in about the same hardness after annealling, forging or after heat treatment. This opinion will be borne out by the following FACTS:

At 7/8 of an inch the j-2 position of the Jominy hardenability band provides the achievable properties for both 8640 and 4140 for surface hardness using mild oil quench. At J-2 both harden to a 53-60 Hrc.

J-6 on the H-band gives center hardness properties for 1" diameter 8640, my read of the band is 46-58 Hrc.

4140 at J-6 similarly mirrors 1" diameter center hardness achieved. for 4140, the band at J-6 is 50-58HRC. It is a subset of the 8640 properties...

Machinability wise, both 4140 LP annealed (Lammellar pearlitic annealed) and 8640 LP Annealled run at 110 sfm or 66% of 1212 using HSS tooling. In fact their tooling recommendations are in the same table, #33 in the republic cold finished bar machining handbook.

Knowing the process sequence would be helpful, but since these grades will be equivalent mnechanical property wise as long as they are in the same condition- (For the 7/8 diameter that we are discussing)- The broach won't know any difference. Watch the load meter or amps drawn...

Having said that, it WILL make a difference if your new material supplier runs low sulfur compared to your present supplier, or if suddenly you get Bearing Quality or Aircraft Quality. (low volume fraction of manganese sulfides in these steels means lousy chip separation from the work, and increased load.)

Otherwise with the facts you've provided, I don't think that your process will notice the change.

Milo

Milo, thanks for the input. Of course the machining is done in the annealed or normalized state. I don't have any concerns about machining the 4140 material, I had concerns with machining 4340 material, and I had separate concerns about the toughness of 4140 material compared to the 8640 at low temperatures.

Well, 8640 was designed to be best combination of properties with minimum alloying addition, something your suppliers might be missing in todays expensive materials world. Hhowever, its the ugly step sister of both 4140 and 4340 in terms of low temp charpy impact as the following data indicate. Charpy impact for oil hardened 8640 at 0 F is 19-24 ft lbs; 19-23 at - 25F;

Similar data (not same) Charpy impact for 4140 available for 1575 quench in oil temper 1240 F to 288 BHN is 109-124 ft lbs; 1575 quench in oil temper 1075 to 286 Bhn should give you 75-90 ft lbs; 80-92 at - 40.

Personally, my opinion is 4340 is overkill for this small diameter; the rich alloying is to improve depth of hardness; since you can get to the center with less alloy, why go there?

The above data is from alloy digest march 1954.

Milo

So I went with the 4140 material.

In the conditions you stated so far, 4140 would be the choice.

Slightly off the subject; are you having your tooling PVD Coated? If not, is suggest you do.

Thanks for the help, I tried to find the Alloy Book you referred to in the previous posts I guess it has gone out of print.