How to estimate the maximum hardenability of a steel?

[coreman73]

I am currently working with some AISI 51B60 grade steel. Engineering has asked that we make 1/4" hex bit stock using this steel grade. The bit is to be used as part of an impact bit socket.

The internal specification requires heat treating to a hardness range of 52-55 HRC. How would I determine if this hardness range is acceptable for 51B60 steel? In other words, is it safe to harden this particular grade to this range and have it attain the required properties (impact resistance, toughness without being too brittle) as it relates to being used in an impact environment?

Would there possibly be a better steel grade to achieve this hardness range and desired properties?

 

[blacksmith37]

I suggest that your question is "What are good steels for a product 1/4" thick exposed to impact loads and requiring a hardness of HRC 52/55"?.Typically a steel containing nickel is recommended for impact, and as low a C as necessary. Many years ago ,we made small impact wrench parts with 8620 with a carburized case of 10 mils.

 

[swall]

51B60 at Rc 52-55 is not a good candidate for an impact bit.
Take a look at one of the S series tool steels instead.

 

[Guest102023]

Buy a similar sized impact socket made by a reputable maker and reverse engineer it.
Chemical analysis will tell you the grade, and metallographic examination with show you what surface treatment was done. Rockwell hardness testing will tell you the core hardness. Your local heat treater will be able to reproduce all of that pretty easily. Total cost of testing should not exceed $500.

And no, 51B60 is not a good candidate for good toughness properties.

 

[dinjin]

Almost any steel with at least 20 points of carbon should allow you to reach that hardness range. I agree with the blacksmith's comments about using nickel to make it more
ductile. I would also recommend case hardening instead of thru hardening. The harder the steel, the more brittle it becomes. A soft core will help. You can afford to experiment with both. How do you intend to harden the steel?

 

[hydtools]

We made impact parts with S7 tool steel.

You may have other issues to deal with if what you are making is 'part' of an impact socket.

Ted

 

[unclesyd]

I went back and looked at some notes from many years ago to check on some work we did with sockets. As the time there were essential 3 mainstream suppliers at the time. All there used dropped forged AISI-4140 for the impact sockets that we tested. For some reason that I didn't record the hardness. At the time we also used a lot hex drives and adapters and all were made from AISI S2. All the smaller wrench drive sockets were made from many different alloys.
As we had an impact repair shop we did make a lot of replacement parts from Bearcat (S-7).
If this item is exposed to personnel there are standards, both EU and American. At the time only the sockets were covered.

 

[coreman73]

Thanks for all the advice everyone. I really appreciate it. It's pretty evident that 51B60 isn't the best choice for an impact bit. What is the reason though? Is it simply that it doesn't contain more of an elemental profile as a tool steel, which I see recommended a few times (S tool steel)?

In theory, is there anything that could be done during heat treatment of this 51B60 material to make it better suited for impact loading?

Just out of curiosity, S tool steel doesn't specify Ni but it was suggested twice as being an element that really contributes to increased material ductility. Why would S tool steel be recommended then?

 

[hydtools]

Compare Jominy end hardenablilty and Charpy strength.

Ted

 

[Metalguy]

Nickel can add lots of toughness in steels such as HY130, but they have very low C and will not harden anywhere near what something like S7 will.

As tool steels go, S7 is on the low side of C content, and uses vanadium to promote fine grain size--both of which aid toughness.

Keep in mind that while ductility is a component of toughness they measure two different things. Also, the "hardenability" in the thread title is a measure of how thick a piece of steel will harden (transform to martensite/bainite), it does not indicate how hard the steel will be.

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