Slack quenching 1

[arash66]

I have been always told that if you get proof failure one of the main reasons can be slack quenching.
my questions are:

- is this true?
- what else can be involved?
- can we explain the reasons scinetifically (by dislocation theory or...)?

thank you evryone for your help.

 

[metengr]

arash66;

- is this true?

As a generalization, no. A slack quenched microstructure can be a contributor to failure. It depends on the type of failure, material and conditions of service.

- what else can be involved?

One major factor unrelated to a slack quenched microstructure which can cause failure is poor design.

- can we explain the reasons scinetifically (by dislocation theory or...)?

Unclear what you are asking.

 

[arash66]

Thanks metengr.
material is low alloy steel and it is hardened and temepred and as i mentioned it is 0.2% proof stress failure.

the location of the test is fixed and sometimes we get test failure on these parts not always, so we rule out the design issue.

what i am looking for, is scinetific explanation for correlation between slack quench and 0.2% proof stress failure.

 

[metengr]

arash66;
Thanks for the follow-up. If you are observing failures related to inadequate strength of the material after heat treatment, a slack quenched microstructure could be the cause.

Have you performed a metallurgical evaluation of one or more of the failed test parts to confirm slack quenching? It would be wise to confirm if the heat treated low alloy steel contains phase constituents other than tempered martensite.

 

[Maui]

There are any number of reasons why your test samples fail to meet the 0.2% proof stress criterion that you have established. It could be that your heat treating furnaces are not functioning within the specified tolerances within the working zone of the furnace (usually tolerances of +/- 25F or better are required). If this is the case then your heat treated test pieces will not achieve consistent properties, even though you may believe that your heat treating practice is identical from piece to piece. Your tempering temperature may be too high. Your melt chemistry may vary from heat to heat enough to occasionally cause parts to fail your test. Your test piece fabrication practices may contribute to the problem by leaving too rough of a surface finish, or your gripping mechanism may cause problems. The list goes on and on...

Maui

http://www.EngineeringMetallurgy.com

 

[arash66]

the reason that we consider it something other than normal causes like furnace uniformity and ... is the fact that material passes the UTS comfortably but fails 0.2%PS. so PS/UTS ratio changes when we get failure.

 

[Maui]

Did you have samples from the same heat where some failed while others passed? By how much did they typically fail or pass? You'll need to post some actual numbers. What I'd like to see are values for 0.2% proof stress and UTS on heats that passed as well as heats that failed. These values should be posted together with the actual heat chemistries.

Maui

http://www.EngineeringMetallurgy.com

 

[Maui]

And if you could post microstructures too with the requested data, that would be very helpful.

Maui

http://www.EngineeringMetallurgy.com

 

[redpicker]

I have been always told that if you get proof failure one of the main reasons can be slack quenching.

In my opinion, this is false. At least, the "one of the main reasons" part. What I understand "slack quenching" to mean is "removal from the quench medium before the parts have fully cooled to the temperature of the quenching medium". While this may result in low yield strength, there are many cases where it does not (and it can be useful in prevention of cracking). Not only that, but there are many other reasons for low yield strength.

There are a lot of reasons, some mentioned above:
Low austenitizing temperature
Short austenitizing time
Surface condition of the material (ie, heavy forging scale)
Too large section
Poor quench agitation
Wrong quenchant (ie, slow oil when a fast oil should have been used)
Poor steel chemistry
Too large a quenching load
Too long delay after leaving austenitizing furnace before quenching
and many others...

The fact the UTS is within range but the Yield Strength is not COULD suggest that inadequate transformation to martensite occurred during quenching, however, there are other reasons as well. But, even then, slack quenching is only one reason for inadequate transformation to martensite.

rp