MicroHardness Vs Superficial

[mfgenggear]

Question please advise?

Data :the below is an example only not an actual part.
this for my information only & I am doing a test.

Parts that are carburized on gear teeth
Lets say 20 teeth 20 Diametral Pitch 20 Deg Pressure Angle
External Spur gear
Material AISI 9310 Steel
Specification AMS 2759/7 Carburizing & Heat Treatment
Class 2(Requirements) Type 1 (except Endo Gas not Vacuum )
Effective case depth requirement is ~ .010-.025
parts are to be carburized to .022-.027 to allow for grind allowance.
Case Hardness Required 90 15N Minimum @ .004 depth.
core hardness 33-43 HRc

stock removal from gear teeth shall be .004 per surface from
profiles.

Parts are Pre heat, Carburized (1750 deg F for 2 hours),austenitized(1525 deg F for 2 hours )Quenched in salt(400 deg F), subzero stabilized, & tempered to 300 deg F a sample part is sectioned & mounted. then verified with micro hardness (Vickers or Knoop)

a suprficial hardness verification of finished ground gear
90.2 15N min passes.

how ever with the micro hardness check it checks two points under @ .002 & .006 depth.
( should be 732 HK 500 GM (60 HRc)
(Actuall is 690 HK500 GM (58 HRc)

can some one please advise

if the superficial passes then parts are acceptable or should it be reheat treated thanks.

 

[dbooker630]

I would subject the parts to a rework carb & quench cycle to raise the surface carbon potential, but it would help if you have the capability to firstview a microstructure.

If you go strictly by your superficial result, yes you do have a part that meets spec but I would question that all parts meet the minimum since your result was only 0.2 pts above 15N spec.

Then you may have to consider GR&R for a superficial tester which at least in my experience I have found to be higher than standard or micro scales.

 

[swall]

To follow up on dbooker630's point about the likelyhood that some parts in the same batch will be out of spec, it should be a standard practice in your heat treat plant to periodically sample parts from different locations in the load, to see where the high case parts are and where the low case parts are. Then, your regular checks can be established to pull parts from that part of the load that would represent the worse case scenario. One other thought--with a tempering temperature of 300F, if you are mounting the microhardness samples in a thermoplastic or thermoset media, you could actually be tempering them back. You might lose as much as one Rc point on the micr0hardness test. Use a cold set mounting media.

 

[mfgenggear]

DBooker630 & Swall thank you for your replies.

Dbooker can you please explain the GR&R proceedure
for the superficial tester. and the Micro tester?

I run into this situation wnen lab techs are verifing the core hardness as well.

again micro was verifing 2 points under
the superficial
so you have found the superficial to be higher than the calibration standards?

so are you saying the micro is more accurate?

Swall hmm I will verify the temperature of the mounts
thanks for that tip.

 

[dbooker630]

mfgdnggear,

There are precautions to take with microhardness samples also. As swall has stated the temperature of the mounting press is a factor. Also the sectioning of the sample, ensuring that it was not tempered on the cutoff machine.

GR&R stands for Gage Repeatability and Reproducibility. It can be done for attributes as well as variables like superficial hardness. It is a measure of the equipment variation combined with the appraiser variation.

What you will do is get ten different samples, and have three different people (appraisers) test each sample three different times. You can reduce the number of appraisers or trials, but it may not be as accurate.

If you enter the data into a spreadsheet the total variation of the gage can be calculated. Less than 10% is considered good; above 30% the gage or procedures should be examined. Click on the link to upload an Excel spreadsheet that you can use. The auto industry through the AIAG publishes manuals on how to conduct a GR&R.

 

[salmon2]

1) When the indentation is shallow, there is a depth effect. The shallower the penatration, the higher hardness rating.

2) Also, the part may loose some carbon during heat treatment on the surface.

Both of them are possible.

 

[tripleZ]

If you have access to Minitab, you can use that to set up the GR&R experiment. The tutorial will pretty much walk you through the process.