How to avoid being accused of wrecking machined parts through HT? 1

[ZXTDustin]

Hi, guys,
I myself is a material& HT engineer for a heavy-duty truck transmission company in China.
The most commonly encountered problem is that we HT engineers are accused by machine engineers
of wrecking the parts through HT process, especially concerning the dimensional tolerance
deformation after HT.
Since the big bosses are always with very limited HT background, they take sides easily
with machining, we've been suffering from this for a very long time.
1.Do you have the same issues on this?
2.How do you work with machine engineers to avoid disputes?
3.Are there rule-of-thumb on the dimensional tolerance allocation between machining and HT
process you apply?


Thank you very much in advance.

 

[BrianPetersen]

I presume "HT" = "heat treating".

How much process monitoring do you have?

Are the material specifications correct? How do you know they're correct?

Are you checking the critical dimensions in question prior to heat-treat to ensure that you aren't sending parts through the heat-treat process that are out of specification before the process starts?

Are you monitoring and logging the process-critical factors of the heat treatment process, and checking the calibration of instruments at suitable intervals?

Is the problem the dimensional changes, or is it inconsistent dimensional changes?

Is the process as a whole correct, and being done in the correct order? It's very common that heat treating process causes dimensional changes and degrades the surface quality of the parts in question. Some parts inherently have to be rough-machined, then heat treated, then ground to size.

 

[monkeydog]

Never final machine before heat treat.

 

[IRstuff]

If dimensional changes were inevitable, the designers should have designed to accommodate them.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[DVWE]

Never final machine before heat treat.

My thoughts exactly.

ZXTDustin,

Is there a constraint in production that requires HT after machining? Ideally, HT first, then machine. If not, perhaps rough machine, HT, then final machine.

The devil is in the details; she also wears prada.

 

[3DDave]

Heat treat produces changes in the shapes of the metal crystals, which changes the properties. When the crystals change shape they also change size. The people machining the parts should know this and make plans for these changes, which are a necessary part of the heat treating process.

One annoying factor is that if the material is not uniform, the orientation and character of the crystals in the as-machined parts will produce unpredictable changes in the crystal conversion. In addition, heat treat will also allow the effects of cold-work strain to be released, also causing unpredictable changes. There may be a material selection or purchasing problem. The result will be warping and twisting of parts.

 

[miningman]

sounds as if the big bosses and machine engineers know a thing or two that the OP still needs to learn

 

[EdStainless]

If the changes are consistent then your machinists should be able to accommodate them.
If they are are inconsistent then you need to figure out if it is the HT at fault (variations in methods), or the machining, or the material.
This is a huge issue in many industries.
Solution usually requires addressing all three components.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Metalfixer]

Most of our parts are automotive related. We use a Production Part Approval Process (PPAP).
You should require process drawings, such as OP 40 "Before Heat Treat" and OP 50
"After Heat Treat", or something similar, with dimensions and description of heat treat.
Quality Control Department is responsible for checking these parts, including the sample
parts that should have originally been sent to heat treat.
As an engineer, be careful what you sign off on, and you can point the finger to the
responsible parties.

 

[EdStainless]

The one that I have seen people trip up on many times is that the initial qualifying run worms fine.
Then in full production they change raw material, often because they can save a couple of cents/ton by skipping a normalize HT on the RM. Then they end up getting more distortion when the HT after machining.
If the problem is variable and people swear that 'they didn't change anything', what it really means is that they changed things that they didn't think about measuring or controlling.
Solving this really takes everybody in the chain paying attention to details, documenting and controlling every step.
If your QA system isn't that tight and robust, then you may never really be able to solve the issue.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube