BiPolarMoment
Mechanical
- Mar 28, 2006
- 621
Not sure if Metallurgy or this forum would have the best resource but here goes:
I’m not an expert so please correct any inconsistencies I have in my theory.
I have a few questions regarding the effect of titanium nitriding and/or aluminum titanium nitriding that I'll refer to as their acronyms from here on out.
Traditionally we manufacture medical instrumentation from a fairly narrow band of stainless materials. For cutting instrumentation I prefer to use 420/440A/440C due to the edge retention properties, reasonably good corrosion resistance, and availability. However, we have a new requirement that some instruments I would use this material for have to be AlTiN’d for the purposes of reducing reflectivity—my understanding is that this is performed at temperatures greatly exceeding the temper temperature generally used to achieve a 50+ HRC in any of these materials. Does this not adversely effect the material properties in a way that would compromise its strength and long term function? I believe I have some historical evidence of instrument fractures in TiN coated 440A that were analyzed and found microstructure changes—though I wasn’t involved so I can’t speak to the veracity.
On the flip side we often use precipitation hardened steels (mostly 17-4/455/465) for almost all other instruments that need to have more strength than 304/316. In this case sometimes they are TiN coated for cosmetic or labeling purposes (e.g. “push the gold button”). It is my understanding that there are low and high temperature TiN coatings available but that generally they are all done below the age hardening temperature of these steels (usually 800/850 F). As long as the temperature remains below the age hardening temperature should you expect any change to the microstructure/mechanical properties?
One last inquiry to tie this together—in the case of edge retention I’ve always been of the opinion that PH steels are not as optimal because while their hardness on a macroscopic level is usually very high, the matrix the martensitic precipitates is in is generally not so much (I’ve certainly seen evidence for this in damaged 17-4 instruments). This is why in order to solve my question #1 above I’m hesitant to switch to a PH grade and AlTiN coat it because ultimately the coating is thin and subject to the limits of its substrate. Am I wrong in this line of thinking? Because, if so, I imagine that would be simplest solution.
Thanks for any direction you can give me, it would be nice to be able to advise others at my workplace as I think there is some misconception about how TiN and AlTiN coatings are performed and how it might affect the effectiveness of their parts.
I’m not an expert so please correct any inconsistencies I have in my theory.
I have a few questions regarding the effect of titanium nitriding and/or aluminum titanium nitriding that I'll refer to as their acronyms from here on out.
Traditionally we manufacture medical instrumentation from a fairly narrow band of stainless materials. For cutting instrumentation I prefer to use 420/440A/440C due to the edge retention properties, reasonably good corrosion resistance, and availability. However, we have a new requirement that some instruments I would use this material for have to be AlTiN’d for the purposes of reducing reflectivity—my understanding is that this is performed at temperatures greatly exceeding the temper temperature generally used to achieve a 50+ HRC in any of these materials. Does this not adversely effect the material properties in a way that would compromise its strength and long term function? I believe I have some historical evidence of instrument fractures in TiN coated 440A that were analyzed and found microstructure changes—though I wasn’t involved so I can’t speak to the veracity.
On the flip side we often use precipitation hardened steels (mostly 17-4/455/465) for almost all other instruments that need to have more strength than 304/316. In this case sometimes they are TiN coated for cosmetic or labeling purposes (e.g. “push the gold button”). It is my understanding that there are low and high temperature TiN coatings available but that generally they are all done below the age hardening temperature of these steels (usually 800/850 F). As long as the temperature remains below the age hardening temperature should you expect any change to the microstructure/mechanical properties?
One last inquiry to tie this together—in the case of edge retention I’ve always been of the opinion that PH steels are not as optimal because while their hardness on a macroscopic level is usually very high, the matrix the martensitic precipitates is in is generally not so much (I’ve certainly seen evidence for this in damaged 17-4 instruments). This is why in order to solve my question #1 above I’m hesitant to switch to a PH grade and AlTiN coat it because ultimately the coating is thin and subject to the limits of its substrate. Am I wrong in this line of thinking? Because, if so, I imagine that would be simplest solution.
Thanks for any direction you can give me, it would be nice to be able to advise others at my workplace as I think there is some misconception about how TiN and AlTiN coatings are performed and how it might affect the effectiveness of their parts.
![[tongue]](/data/assets/smilies/tongue.gif "[tongue] [tongue]")
. The instrument fracturing was not 440C but 440A so I researched and found out the temper temperature was 325 F to achieve the spec 55-58 HRC. However, it was then TiN coated at temperatures exceeding 350 C (662 F). Tests of the fracture sites at the core measured 44/47 HRC and microstructure showed carbide precipitation at the grain boundaries. So basically it appears the wrong grade was chosen as it appears that 440C could have used a temper temperature above the PVD process to achieve the same hardness.