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

In summary, the author is trying to determine if a higher impact value, such as H1075 or H1150-M, is better for a surgical instrument which will be impacted many times over its life cycle. They are not sure that the higher impact value provides the best mechanical properties for the steel. However, they are considering a material with a higher hardness and tougher surface.
  • #1
MechEgr
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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!
 
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  • #2
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.
 
  • #3
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.
 
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  • #4
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?
 

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  • #5
MechEgr said:
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.
 
  • #6
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.
 
  • #7
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.
 

1. What is the purpose of heat treatment in 17-4 PH Stainless?

The purpose of heat treatment in 17-4 PH Stainless is to improve its mechanical properties, such as strength, toughness, and corrosion resistance. It involves heating the metal to a specific temperature and then cooling it at a controlled rate to achieve the desired properties.

2. How does heat treatment affect the impact loading of 17-4 PH Stainless?

Heat treatment can significantly improve the impact loading of 17-4 PH Stainless. This is because it alters the microstructure of the metal, making it more resistant to cracks and fractures. The precise heat treatment process used can determine the exact impact loading improvement.

3. What is the recommended heat treatment process for 17-4 PH Stainless?

The recommended heat treatment process for 17-4 PH Stainless is a two-step process, involving solution treatment and aging. Solution treatment involves heating the metal to a high temperature, followed by rapid cooling to create a homogeneous microstructure. Aging involves heating the metal at a lower temperature for a specific time to achieve the desired mechanical properties.

4. Is there a specific temperature range for heat treatment of 17-4 PH Stainless?

Yes, there is a specific temperature range for heat treatment of 17-4 PH Stainless. The solution treatment temperature range is typically between 1040-1100°C, and the aging temperature range is usually between 480-620°C. These temperatures may vary depending on the specific properties desired for the metal.

5. Are there any potential downsides to heat treatment of 17-4 PH Stainless?

While heat treatment can improve the mechanical properties of 17-4 PH Stainless, it can also lead to distortion and reduced corrosion resistance. The high temperatures involved in the process can cause the metal to expand and contract, leading to warping or cracking. Additionally, the aging process can reduce the corrosion resistance of the metal in certain environments. These factors should be considered when deciding on the appropriate heat treatment for 17-4 PH Stainless.

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