parts contract during carburize!!! 7

[mfgenggear]

small parts,the material is AMS6265 (AISI 9310) steel
has an internal flat root spline & ext spur gear,roughly 1.350" od , .973 long,
spline is finished before gas carburize. the procedure is customer propriety but is similar to the AMS2759/7 spec..
this is a 17 tooth , 24/48 dp 30 deg pa int spline.
the clearance between the major dia to the total involute form (which would be the go gage size + .005".
is extremely close, so it's a major pain trying to re-work
the spline after heat treat.
other than that the part is simple geometry & simple to manufacture. the best I can do is increase the major diameter, during the broach operation prior to heat treat.

the spline major diameter contracts during heat treat
what is generally the procedure that causes for this contraction?
is there there a method to minimize contraction?

before I discuss it with my vendor I would like some suggestions from this board.

Thanks in Advance

Mfgenggear

 

[redpicker]

The distortion occurs during quenching. This distortion is a complex phenomenon and my efforts at trying to deal with by giving the part some "windage" (i.e., making the OD larger and hoping it will shrink into spec) is usually met by the next batch growing, not shrinking.

Steps taken to minimize distortion will help. Normalizing the parts prior to carburizing could help. Making sure the parts are at the same uniform temperature prior to quenching also helps. Giving the parts a carburize and slow cool, then do your sizing, then quench and temper to develop the case is another option.

Sometimes these will control the distortion enough to be practical on a production basis, sometimes you have to just deal with grinding the parts to size after hardening, or change the design to permit a wider tolerance.

rp

 

[mfgenggear]

Redpicker

I want to thank you for your response.

I do Normalize, Harden & Temper for merchantability & to control distortion. prior to semi finishing & heat treat.

for some weird reason this supplier shrinks every thing,
other suppliers actually control their parts better.

is it in the quenching oil maybe?
the temperature of the oil not being controlled properly

or at the hardening cycle maybe not preheating?

Thanks
Mfgenggear

 

[TVP]

Any time you are concerned with distortion of oil quenched parts you definitely need to understand the following details:

Oil temperature, and not just the initial temperature but how hot does it get during quenching, and do subsequent parts/batches experience higher initial temperature?

Volume of oil (if volume is small and size/amount of parts is large, quench rate is negatively impacted).

Quench speed of the oil, slow vs. fast, which are obviously inexact terms for describing the heat extraction properties, but it's what the industry uses.

Agitation-- is it used, is it uniform, etc.?

 

[mfgenggear]

TVP

thank you for your post

the spelling on my last post should be machinability
dam spell checker.

any way the oil speed, temperture & agitation is a concern.
and addition to the age of the equipment?

Thanks
Mfgengear

 

[gearcutter]

We've never experienced a situation where too high a quench oil temperature increases the rate of distortion; it's always been the opposite for us.

We have found that using a 'slower' quench speed, by increasing the oil temperature using a pre-heat, can and does help to reduce the risk of distortion.

Using a quench press is probably the best way of helping keep distortion to a minimum.

We never finish machine critical diameters or gear profiles before case hardening.......this invites too many problems.

If we need to spline a bore; it's done after case hardening.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

gearcutter

your response is most welcome

I do use a quench press when the parts are complicated or
are gear rings, or require flatness.

these are short stubby little parts.
with a gear on the OD, & internal spline in the ID.
the spline & gear require the same case depth by the customer.

What oil type & how is the speed specified for quenching oils? sorry for NOB question.

but never the less I got leads to work on.

Thank You

Mfgenggear

 

[TVP]

Fast oils cause more distortion. Take a look at some of the products from Houghton-- not all of them report quench speed, but you can see a fast oil in the range of 8-10 seconds while a normal oil is 14-16 seconds.

http://www.houghton.ca/products.asp?Cat=1&SubCat=9

 

[tbuelna]

I agree with gearcutter. Mask the bore and do the spline after HT. The teeth on that 24/48 pitch spline are thin enough that they will likely through harden from carburizing unless masked anyway. Core hardness for 9310 after carburize and HT will be around Rc 40 max.

Wherever you have large changes in cross section, like small thin spline teeth on a thick rim, the large disparity in local heat transfer rates will produce quench distortions.

Good luck.
Terry

 

[gearcutter]

mfgenggear;

You need to discuss quenching rates with your heat treater as the advice they give will be based around the equipment they are using.

An issue often overlooked, while dealing with distortion, is the level of retained austenite. Performing a further freeze quench cycle can help reduce distortion as it encourages the conversion of any retained austenite into martensite.
We regularly perform a freeze quench on parts for the sole purpose of minimizing distortion.

In my opinion; aside from the distortion issue, a spec which asks for a pitch this small to be case carburized is a spec which has been created by someone with minimal practical and design experience. In a situation like this I would advise my customer to look at alternatives such as using a nitriding material.

There is an excellent chapter in an ASM book which talks about issues with distortion as a result of case carburizing: GEAR MATERIALS, PROPERTIES & MANUFACTURE is the book.




Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

Redpicker, TVP , Tbuelna, Gear Cutter

Thanks for your help.

Sometimes a person can get tunnel vision. I have been making gears for a long time but I needed a little help
from my Friends here on Eng Tips. Brain storming really helps.

Parts are to be broached after Carburize & Harden
the customer has made it optional to Carburize the splines.

Problem solved far as this part.
I will continue to Investigate with my supplier on the Quenching mediums for Carburize & hardening.

Thanks a bunch

Mfgenggear