Failed Champy impact tests 2

[tio21122]

I have some engineering componments made of low carbon low alloy steels (0.13%C,0.25Si, 1.1Mn,0.6Ni,0.15Cr)

The materials are cutted from forged bar and machined into different size and shape. This is followed by heat treatments of normalising, austenlising(940C), water quenching and tempering at 650C. Time for each heat treatment is 1hr/inch.

The problem is that for componments larger then 6 inchs, the thinner sections (only 1 inch) always failed the impact tests.

Any idea of why is that?

Thanks in advance!

 

[arunmrao]

What is the impact value desired and obtained? I suppose you are referring to room temperature testing values.

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[tio21122]

the tests were carried out at -30C, required 40J, but get 22J,25J,28J. It seems not far.

In the thick positions results are so good as: 200J, 204J, and 218J.

 

[arunmrao]

Can you change the quench media to oil for thin sections. I think you will get over the problem.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[tio21122]

It is a complex shape compoment with thick and thin section al together. I can not put them into two quenching media. thanks

 

[arunmrao]

Your statement is now complete that you have a job with varying thicknesses and that in the thin regions the impact properties are lower. You need to use a slower quenching media or insulate thin sections so that during water quenching it cools slower. alternately select an alloy with better hardenability.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[tio21122]

Why the tempering does not bring the impact properties up at the thinner section?

Using slow quench method may not be good, as the thick position still need a high strength.

Can you give me an example of how thinner sction can be insulated?

Many thanks

 

[salmon2]

You did machining first, resulting in non-uniform thickness, then followed by Q&T, right?

So I doubt that you got uniform microstructure. To me, the Charpy at thin sections seems reasonable to me for a tempered martensite, but not the Charpy of thick section which are more like from a majority of ferrite.

So can you heat treat the bar first and then follow the machining. Actually this way you can have better dimensional control.

 

[leizhou]

Yes I agreed that the dimensional control is better and
microstructure is more uniform. But the tesile strength at the thinner section is not high enough as required.

thanks

 

[metengr]

tio21122;
I would strongly suggest before you make any process changes to have the CVN samples that exhibited low toughness and some of the CVN samples that exhibited high toughness sent to a metallurgical lab for metallographic evaluation (hardness and microstructure).

You can't afford to speculate on this, and the microstructure/hardness evaluation will be key to identifying any process changes.

 

[stanweld]

Agree with metengr. Microstructure is the key which will provide info for Q&T paramters. I suspect that the 1-inch section may be a mixed martensitic/bainitic or mixed bainitic/ferritic structure. You may also find that grain size is somewhat larger in the 1-inch section due to longer exposure to the austenitizing temp. If so, I suggest shortening the austenitizing hold time and decreasing the austinitizing temp by 15 to 20 C.

 

[TVP]

I agree with both stanweld and metengr about obtaining a proper metallurgical evaluation, which needs to include a test for prior austenite grain size in the thick and thin sections. The austenitizing temperature of 940 C is way too high to obtain good fracture toughness properties, as it will result in a large prior austenite grain size. A low C grade with Ni and Cr would typically be austenitized in the range of 845-870 C, with lower temperatures producing more favorable austenite grain size and therefore improved fracture toughness.

 

[metengr]

Not to re-consider what has been mentioned, the problem with having such large transitions in the Q&T heat treatment is the time at temperature hold time for proper austenitizing. Even utilizing the correct lower austenitizing temperature for this steel stated by TVP, the time at temperature will be governed by nominal thickness (6 "), and in this case I believe the 1 hr/inch soak time may be the real problem.

If it is established by proper metallurgical evaluation that the prior austenite grain size is indeed too large in the thinner sections of the this component, the austenitizing hold time will either need to be optimzed or possibly as mentioned above, Q&T the forged bar and machine the components after final heat treatment.

 

[redpicker]

What is the hardness of the thin section?
Of the thick section?
What is the required hardness?

It is possible that a 40 C increase in tempering temperature will increase the impacts enough while still maintaining the required hardness.

rp