Set ring gages - 8620 or Graph-mo at what HRC? 4

[dbooker630]

I recently evaluated some prematurely worn and some new set rings used to set up dial bore gages. Print calls out Timken Graph-Mo tool steel hardened, deep frozen to 63-64 HRC. My analysis found the gages to be case hardened 8620 steel; the new ones were 61-62 HRC and the worn ones 53-58 HRC.

While it's unclear how long they've been coming in this way, our discussion with our gage source is focusing on what is acceptable for this type of gage. The key attribute is dimensional stability over time, and these gages are expected to last many, many years.

Our gage source feels 60-62 works as well as 63-64 for hardness and it appears they feel 8620 works as well as Graph-mo. I'm interested in other's experiences with gage materials and performance as I have to write a new specification to properly define what is expected. Thanks for your input.

 

[NickE]

No, 8620 is not acceptable for gauges that are called out to be constructed of through hardening tool steel. There is a significant difference in carbides and micro-structure. Also I would ask your supplier who gave them a deviation from the blueprint call out. Then I would find a new supplier. It is COMPLETELY UNACCEPTABLE to not make gauges EXACTLY to the print. I have only designed 2 gauges, however the materials and tolerances on those gauges had to be perfect or the gauge was garbage.

(And though your supplier may not see a difference btw 60-62 and 63-64 HRc there is going to be a difference especially after many many years.)

 

[tripleZ]

I've never worked with 8620 before, but when looking it up on the web, it seems to be consistently associated with 4140, 4340, etc. Personally I've found those grades to be garbage for gage design.

I guess my first question would pertain to what you're willing to pay for ring gages? The second would be what kind of environment the gages see (e.g. QA Lab or shop floor)? The gages I design are used by our floor operators frequently. There is a lot of abrasive action, so wear is always an issue. For a normal slide gage for some of our parts, we'll use air-hardened D2 @ 60-62 HRc. Fairly stable over time...but then we can live with a ±.0002" range. I may have to replace them every year or two. But they aren't that expensive.

If this is going to be a frequently used gage which is going to see a lot of wear and tear, I may lean towards something like an M2 or CPM-M4/10V grade (both oil quenched). But you'll pay a heck of a lot more to make it out of those grades vs. D2. Cryo treating them will also help to get you the stability you're looking for. I was always amazed at what a cryo treatment would do to help relieve instability. We used to grind some complex core rods out of 10V/15V, and cryo treating them really helped to keep the warpage-in-grinding down to a bare minimum.

Again, I'm dealing with abrasive wear, sintered parts, and a lot of metal powder floating around, so my applications may not relate to yours.

 

[arunmrao]

8620 is truly a garbage material as it is a case hardened steel with a case depth of 0.6-1 mm. Also the type of carbides and their distribution varies from the conventional grades of tool steels used.

Maybe it was considered as a one time use and throw away to justify the low material costs.

 

[unclesyd]

I concur with tripleZ on the use of D2 for gages. We have made our gages for many years from D2 with never any problem holding 0.00005" (air gauge) tolerances over a period of years with very no measurable wear.

To stabilize D2 we use at least a triple temper with a cryo treatment. We finish our ring gauges with a Helical hone.