Tool Steel Selection 5

[JCReynolds79]

Hi All,

I am after a bit of advice please. I am designing a simple tool for inspection purposes. It is basically a hollow cylinder (approx. 40mm OD, 30mm ID, 50mm long) with a mating "plunger" that will slide in the middle of it (30mm OD, 50mm long).

I want the tool to be fairly hard wearing, but it is not going to see a hug amount of usage. Not like a plastic injection mould tool or anything. So I wanted to make it out of some sort of tool steel, just for durability.

Not sure what would be a simple, general purpose choice and also whether I should be heat treating after machining. My thoughts so far are (B)01 or P20.

Any advice, greatly appreciated, thanks.

Regards,

Jon Reynolds

 

[EdStainless]

It really depends on the size and thickness. Since all that you want is durability just about any tool steel will do, so pick one that is easy to heat treat.
If it is small and thin enough (I believe that 5" is the max thickness) use A-7 or another air hardening grade. Easy to HT with minimal distortion. That would let you rough machine close to size and just finish grind after HT.

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P.E. Metallurgy, Plymouth Tube

 

[Lyrl]

In a wear application you will see much better performance if you heat treat the steel.
In the described application (light usage) the best value might be 1045 or 4140 material. I believe either is cheaper than any tool steel, they're both very common (any heat treater will know what to do with it and have fast turnaround), and in the section sizes you describe (0.2"/5mm wall thickness) either will achieve hardness of 50-55 HRC which will give good wear performance. They both require oil quench (actually 4140 could harden with a high pressure gas quench, but that equipment is not very common) which is more likely to result in enough distortion to require some dimensional touchup after heat treat.

Otherwise I second Ed's suggestion of an A series (as a commercial heat treater I've seen A2 more than anything else) for lower distortion to minimize the risk of needing dimensional touchup post-heat treat.

 

[JCReynolds79]

Hi guys,

Thanks for the responses so far. I am just unsure if my application really needs anything this special and that I might be trying to over-spec it. It is a small piece that just helps someone measure something. So it does not get battered about, won't be used very often, will sit in a metrology draw most of the time and just get taken out when required to make the measurement coming into contact with the workpiece and a micrometer.

I just wanted it fairly wear resistant, so that if it does get dropped or rolls around in a draw bumping against other bits of kit, it doesn't get damaged too easily. Maybe something like EN24T wouild work. But again, I am unsure if I should heat treat it after to harden it up a bit. I don't want it to cost the world as it will have a short useful life cycle.

Cheers.

Regards,

Jon Reynolds

 

[EdStainless]

You will have more money in grinding it than the metal and HT cost.
Go ahead and do it in A2.
The problem with 1040 or 4140 is that they quench in liquid and there will be more distortion.
Since this a shell having strength is a good thing.

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P.E. Metallurgy, Plymouth Tube

 

[Maui]

If this is going to be part of a gage set-up then you will want this component to be dimensionally stable over time. If you make it out of an air hardening tool steel like A2 (which is a good idea), it will normally have about 20% retained austenite in the microstructure after going through the standard austenitizing and tempering operations. Over time as the part is sitting in your metrology drawer the retained austenite in the microstructure can transform into untempered martensite. Since there is a volumetric change associated with this phase transformation, the dimensions of the part will change. So if you're using it as part of a gage set-up, this would be bad. Very bad. To protect yourself from this undesirable outcome, be certain to include a cryogenic or cold treatment as part of the heat treating process. The cryo treatment is usually performed after completion of the first temper cvycle, and it helps to complete the transformation from austenite to martensite. ALWAYS follow up the cryo treatment with at least one normal tempering cycle in order to temper the freshly transformed martensite.

Maui

 

[tbuelna]

Since this application is for an inspection gauge, I agree with the others that the most important criteria for material selection is dimensional stability over time. For something like a matching cylindrical plug and ring gauge set I don't see hardness being a concern, since precision inspection components are usually treated with great care. As EdStainless pointed out, the HT, grinding and honing costs will far exceed the cost of the small amount of steel material required. You need less than 5lbs of raw material for both parts, and even if you use the highest quality steel available, costing say $30/lb, that would only come to $150 for raw material. On the other hand, the cost for the multiple cycle HT process described by Maui would easily run $1000 or more, since you would have to pay a lot charge for each step even though you are only processing 2 parts. The finish grinding and honing will also be expensive, since the set-up charges will only be spread over on a single part.

 

[JCReynolds79]

Many thanks for all the informative answers.

Regards,

Jon Reynolds

 

[EdStainless]

I'll back Maui up on the cryo treatment.
We actually have a lot of D2 gages (we use it for other tooling as well).
We found that it didn't really change size over time, but gages tended to distort )go out of round).

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P.E. Metallurgy, Plymouth Tube

 

[arunmrao]

Ed,

I agree about D2 being a god choice. Always, if I am in doubt, I go for D2 and it has never failed.

A cheaper option could be EN24 .


"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[tbuelna]

Maui-

Thanks for the info about retained austenite in tool steel like A2. I have some experience with heat treating gear and bearing steels, but not tool steels. 20% retained austenite is higher than I am used to seeing. With bearing steels we might see 5% retained austenite after the initial HT cycle and before the material is given a sub-zero treatment. Is the high percentage of retained austenite due to A2 using air quench? Or is there some other reason?

Appreciate any input from you on the subject.

 

[Lyrl]

Both the ability to harden in air and the tendency to have high levels are retained austenite are functions of the steel's chemistry.

All steels have a certain time frame in which they have to cool from the austenitizing temperature (~1300°F) to the martensite start temperature (~900°F) in order to harden. Steels where this time frame is short (i.e. their austenite is unstable) have to be cooled very quickly, with water or brine. Steels where this time frame is a medium length can be hardened in oil. Steel chemistries where this time frame is longer (i.e. their austenite is more stable), like A2, can harden in air. A side effect of the chemistry that allows the slower quench rate is high levels of retained austenite.

 

[Maui]

Tbuelna, the chemical composition of A2 includes

Carbon 1.00%
Manganese 0.85%
Silicon 0.35%
Chromium 5.25%
Molybdenum 1.10%
Vanadium 0.25%

The amount and type of alloy additions combined with high carbon levels are what allow A2 to air harden. The high carbon and chrome contents promote the stabilization of austenite, and these are the main reasons why A2 possesses relatively high levels of retained austenite after undergoing standard heat treatment. The amount of retained austenite that is present depends strongly on the austenitizing temperature that is used and the rate of quenching. Higher austenitizing temperatures and slower quench rates result in more retained austenite (see the "Austenitizing Temperature vs Retained Austenite" chart for A2 on page 546 of ASM's Heat Treater's Guide, 2nd Edition). According to this chart, if A2 is air quenched from an austenitizing temperature of 1700F it should possess approximately 16% retained austenite. If 1800F is used instead, the amount of retained austenite increases to over 30%.

It's in the nature of the beast.

The normal austenitizing temperature range for A2 is 1750 - 1800F, and the martensite start temperature is approximately 355F.

Maui

 

[mrfailure]

This reminds me of a question that is in the sample PE exam distributed by TMS for Metallurgy. The one constituent you need to avoid in tool steel used for gaging is retained austenite, as slow phase change over time takes away the dimensional stability that is the essential component for such use.

 

[tbuelna]

Thanks for the explanation Maui.