Precipitation hardened steels, does hardness equal wear resistance?? 1

[PeterCharles]

I’ve been meaning to write this for a while.

With ordinary steels they can be hardened, to a greater or lesser extent depending on chemical composition, by a suitable heat treatment process. With increasing hardness comes increasing strength and wear resistance, and it’s the wear resistance I’m interested in.

Precipitation hardening steels can also be hardened by heat treatment and with hardness comes strength. But what about wear resistance?

Should I expect a ‘normal’ steel hardened to say Rc40 to have the same wear resistance as a precipitation hardened steel also Rc40?

Can anyone comment on this?

 

[TVP]

Martensitic microstructures have better wear resistance than other types (austenitic or ferritic). Medium to high carbon steels that have a tempered martensite microstructure (small, uniformly dispersed carbide particles) have better wear resistance than low-carbon precipitation hardened steels, since the latter have a less uniform microstructure consisting of intermetallic particles (Ni₃Al, etc.) dispersed through a low carbon martensite.

 

[metengr]

I agree with TVP. In addition, wear is a complex process that is difficult, at best, to make general statements or conclusions because of the number of variables. In addition to the chemical composition and microstructure of the substrate, you have the wear process itself (i.e. metal to metal contact or metal to nonmetal contact or gas or fluid process streams against a particular surface).

 

[unclesyd]

As stated above wear is quite complicate and the PH alloys lead the list.

Using 17/4 PH as an example it's wear resistance decreases until around 37 Rc then increases until a maximum at 43 RC. I have some unpublished data that shows a further increase in wear resistance as the hardness increases due to cyclic service at temperature.

Using the cross cylinder (.5") test 17/4 PH and several other PH steels show a marked increase in as the RPM of the tester is increased. The minimum wear resistance is at 100 RPM while the max wear resistance is at 400 RPM. The weight loss at 400 is 1/4 the weight loss at 100.

The wear resistance of self mated 17/4 PH shows a increase as the temperature is increased temperature.

PH alloys self mated are complicated while PH alloys mated with other materials are very complicated.

If you can give some idea of the possible end use I have some data of tests with various materials both published and unpublished.

While the above information is somewhat confusing you will have to add in galling, abrasion, and corrosion.

If you are interested in one of the most wear resistant metals you can get, then you should have the 17/4 PH SS component Liquid Nitrided.

 

[arunmrao]

I agree with TVP and metengr,that equal hardness in 2 different materials will produce dissimilar wear behaviour. In PH steels the carbon content in martensite is low as compared to medium carbon steels. This is very important.

Also affecting the properties will be type of carbides,size and distribution. A careful analysis needs to be done to understand wear pattern

 

[Tmoose]

I try not to ask hard steel to fight my wear battles, especially when rubbing or rolling against a similar material. There are applications in which steel must battle abrasives, but for me Improved surface finish, sealing, and lubrication have been heroes much more often than jacked up hardness. Sometimes a little dry lube performs a miracle.

 

[PeterCharles]

Firstly, thanks for the responses.

Lets go back to where I started.
We use case hardened steel forged links with similar connecting pins to convey bulk solids, the chain being immersed in the bulk solid. The basic material is a 0.2% carbon, low alloy steel (sometimes boron treated) core hardness values typically Rc40-44. Sometimes to combat corrosive conditions we have used 440B connecting pins, but on other occasions a precipitation hardening stainless steel has been used hardened to typically Rc42.

My experience is that although the ph stainless steel appeared to have the same hardness as the core of the case hardened pins they appeared to wear more rapidly. (Case hardened links and pins are not discarded just because the case has been worn through!!).

I reasoned that with the case hardened steel hardening took place to the grains themselves. With a precipitation hardening steel hardening is as a result of precipitates at the grain boundary. The grains themselves therefore may not be subject to hardening and hence the wear resistance would be less. I'm not trained metallurgy so I just wanted to see what others thought/knew about the subject.

 

[metengr]

PeterCharles;
In your last statement a correction needs to be made in that the intermetallic compounds of Ni/Al that TVP mentioned above for precipitation hardening stainless steels are not just confined to grain boundaries, they are dispersed throughout the low carbon martensitic structure or matrix. It just so happens that these intermetallic compounds don't have the same affect in improving wear resistance as the tempered medium to high carbon steels that contain more uniformly dispersed carbides in a ferrite matrix versus a low carbon martensite matrix.

 

[kclim]

Unclesyd:

I'm interested to see what wear data/references you have on 17/4 PH steel. A little bit of a suprise to me that there is an optimum hardness (in terms of wear resistance).

Recently was involved in a project where they were trying to determine what materials to use to manufacture a balance piston and bush for a boiler feed pump (The piston rotated relative to the bush).

A number of suggestions came up, including 440, 17/4 PH and stellite overlays.

In the end we stuck with 17/4 for both components with different hardnesses for the piston and bush (to avoid galling). 17/4 also allowed us to machine in the annealed stage, followed by heat treating to high hardness without distortion.

We specified that they should treat the piston to as high hardness as possible (48 HRC from what I gather), although based on what you've said, we probably should have settled for a lower hardness.

 

[rd400guy]

PeterCharles,
Based on my work with tool steels, I can say that, although they are related, we never treated hardness and wear resistance in the same manner. So quick answer, with ANY steel, hardness does not equal wear resistance. With that said, please read on.

Typically, the first property addressed was toughness, or resistance to breaking. As you're not noticing a problem with connecting pins breaking, the toughness of the materials you're using is sufficient.

Next, we would address hardness, or resistance to deformation. This is typically confused with wear resistance. In the case of punches or dies, which I have more experience with, mushrooming is a common ailment when the hardness is less than required. The same material can be used with different heat treatment to increase hardness, but one must realize that in doing so, you'll *almost* always give up toughness.

Breaking and deformation are unpredictable failure methods. If you have a tool (or part) that breaks or deforms, you need to specify a new material (or heat treatment) to address the lack of toughness or hardness respectively.

After you have a material that doesn't break or deform through normal use, you address the issue of wear resistance. Unlike breaking or deformation, wear IS a predictable failure method.

As metengr and TVP stated, the "normal" steels have some type of carbide that is *hopefully* uniformly distributed throughout the matrix. Precipitation hardening steels have an intermetallic phase that is again, *hopefully* uniformly distributed throughout the matrix. However, the PH steel's intermetallic "particles" usually DO NOT have the same hardness as normal steel's carbide particle "counterparts".

Traditionally, when the primary failure mode is actual wear and not deformation, the solution employed is to go to a steel with a harder carbide. Carbide hardness increases from chromium to molybdenum, tungsten, and finally vanadium (vanadium carbide would have a Rc of 80+ if the scale went there). Not changing the type of carbide former, but increasing the percentage of carbon and carbide former will also increase the wear resistance as the volume percent of carbide increases.

With respect to actual wear resistance and not deformation, I do not know of any PH steels that will out perform a "normal" carbide containing steel. However, if your failure method is deformation, a PH steel treated to a higher hardness may outperform a "normal" steel.

I realize I haven't really answered your question, but I wanted to share my thoughts on wear resistance. Hope it helps, and sorry for the long post.

 

[EdStainless]

to provide a small added comment to rd400's nice discussion.

Wear can come in many forms. Is there an abrasive? Is corrosion a factor? how much? What is the loading, both force and frequency? Is galling a concern?

If there is an abrasive involved then you need to be very concerned about the relitive hardness of the abrasive and the 'hard particles' in your metal.

My favorite source for information on this is Friction and Wear by Rabinowicz. It is old, but small and thorough


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Corrosion, every where, all the time.
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