Heat treatment of 17-4 PH Stainless - Effect on Impact Loading

[MechEgr]

I am designing a surgical instrument which will be impacted many times over its life cycle. We are trying to determine the best heat treating option for the steel (17-4 precipitation hardened) so that we maximize its usable life. Typically, we heat treat 17-4 to H900. However, I'm not sure that this gives us the best mechanical properties for something which will be impact loaded. I was thinking something more along the lines of H1075 or H1150-M. Both of those options have higher Charpy Impact values, but both also have lower ultimate tensile strengths. We've had one device fail already (it was treated to H900).

Any help would be appreciated.

Thanks!

 

[Q_Goest]

It might help to provide details around the impact loading your part is under, perhaps a sketch and an explanation of what the failures are you're seeing.

 

[Bandit127]

What was the failure mode of the device?

If it was a brittle fracture I would seek a tougher material. 17-4 PH annealed is around 33 HRc, so soft for impact resistance. H900 is the lowest hardened state. The other states would be more brittle.

I don't know if 440 is appropriate for medical devices but I do know that it is very tough, even at 62 HRc.

Other than that, a high chrome content tool steel might be appropriate.

 

[MechEgr]

Thanks for the suggestions.

The failure mode is likely a combination of bending moment (from a compression impact load) and tensile stress (from a tensile impact load).

I am surprised that H900 is the lowest hardened state though. I don't know a lot about heat treating, but from the table (attached, table for heat treating 17-4), it looks like H900 is harder (Rockwell) than H1075 and is also tougher (in terms of Charpy impact value). Am I not reading that correctly, or are those not the metrics I should be looking for?

 

[Bandit127]

I am surprised that H900 is the lowest hardened state though. I don't know a lot about heat treating, but from the table (attached, table for heat treating 17-4), it looks like H900 is harder (Rockwell) than H1075 and is also tougher (in terms of Charpy impact value). Am I not reading that correctly, or are those not the metrics I should be looking for?

Apologies. You are correct about the hardness values.

If your failure was due to a lack of tensile strength, it seems you have the best condition for maximum strength already.

I would be looking around for other materials. For example, 440 seems to be used for medical applications and it can be hardened to high 50s HRC to give a UTS of 1900 MPa. It is tougher to machine in the annealed state though.

 

[256bits]

Do you want hardness or toughness for the material?
A higher charpy value indicates the material absorbs more energy before breaking.
A higher Brinell, or Rockwell number means that the surface will indent less under a localized force.

Q_Goest asked about the type of loading and failure, which was not particularly answered.

Perhaps you are utilizing the instrument with strike blows such as hitting it with a hammer during use. Or perhaps, the instrument is quickly being tossed amongst other tools after use and failure means indentations and scratches.

 

[MechEgr]

Thanks for your replies, everyone.

The loading will be performed with a mallet (hammer), so the most important feature is device toughness. Form the comments above, that leads me to believe that Charpy impact value is of some importance. However, the material must be harder than its counterpart, mating material, which is a TI-6 (HRc 36). In thinking about this problem more, I think the best solution for us is to keep the heat treat we're currently using, but change the design slightly to dissipate bending and tensile stresses better.

Thanks again, this was helpful.