Fracture in quenched and tempered low alloy steels 3

[priyam07]

If I have a fracture surface showing some pockets of intergranular cracking and dimple mode after the tempering process, is it possible the part has undergone temper embrittlement or is this related to hydrogen embrittlement (from the moisture absorbed possibly during flame cutting). Anyways is there a way one can differentiate intergranular cracking due to hydrogen and temper embrittlement in Q&T steels.
(P content is 0.011 wt %; tempering done at around 750F)

 

[arunmrao]

Are you discussing cracking during processing of low alloy steel or while in service? It would help, if you are more specific.

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[priyam07]

Sorry about that, cracks were found during straightening after tempering (which would explain the ductile overloading failure) and parts cracking even before service.

 

[arunmrao]

A little more description of the alloy, thermal processing history and any description of the part will be useful.

It has always been a trick during straightening operation after Q&T operation. If you search in this forum, you could find references to similar experiences.

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[priyam07]

Low alloy steel blade with carbon 0.20-0.30; and boron. Austenitized at 1650 F and quenched; tempered at 750 F.

 

[CoryPad]

No carburizing of the blade?

 

[weldstan]

What was the hardness? And what is the hardness and tensile properties required?

 

[metengr]

Review here for TE

http://www.twi-global.com/technical...r-embrittlement-and-how-can-it-be-controlled/

Here is a second paper for review that is descriptive regarding hydrogen embrittlement regarding failure analysis.

http://sti.srs.gov/fulltext/WSRC-STI-2008-00062.pdf

 

[priyam07]

No carburizing and the hardness is around 44-50 HRC.

 

[priyam07]

Thanks metengr for the references!

 

[CoryPad]

I do not think your parts were tempered long enough at 750 [°]F. Typical hardnesses for carbon steel like 1030 or low alloy steel like 4130 are:

Alloy, Tempering temp/[°]F, Hardness/HRC
1030, 700, 39
1030, 800, 31
4130, 700, 42
4130, 800, 38


Due to the poor tempering, you have produced hardness values that lead to embrittlement.

 

[priyam07]

Thanks Corypad for the info, I have to correct the hardness though: 44-50 HRC is the spec we shoot for and the actual hardness on the piece is just measured out to be 37HRC (below the spec). Although, if it is overloading that caused the failure during straightening what about the pockets of intergranular cracking?

 

[arunmrao]

Corypad, I agree. A poor tempering process and may be a delay after hardening might have caused the brittle failure. I do not know how the OP suggests Hydrogen embrittlement.

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[dbooker630]

How much distortion are you trying to remove? Also have you sectioned through the fractured area? It may be helpful to examine the microstructure adjacent to and beyond the fracture. This would include testing hardness at the surface with a superficial tester or near surface with a Knoop test.

 

[priyam07]

I was interested in knowing more about why and how the fracture surface shows brittle IG cracking, possible temper embrittlement in case if the part was held or slow cooled in at that high temperature.

Yes, I have sectioned the fracture area and the microstructure is tempered martensite near and far from the fracture. By hardness testing are we interested in finding any decarb layer?

 

[CoryPad]

Intergranular fracture appeared due to grain boundary weakening, which can be the result of multiple mechanisms. In this case, it could be Tempered Martensite Embrittlement (TME) due to incorrect tempering in the range of 300 to 350 [°]C (570 to 660 [°]F). TME is influenced by phosphorus and sulfur concentration, and the presence of thin cementite films at the grain boundaries. Temper Embrittlement (TE) occurs when steel is cooled slowly or isothermally heated in the range of 400 to 600 [°]C (750 to 1100 [°]F), and also sensitive to phosphorus concentration.

 

[priyam07]

Corypad, that's what I would think so too improper tempering. Just had a doubt though will it be just in some pockets of IG region or all over the fracture surface. I have seen another broken piece returned by the customer with the entire fracture surface showing IG mode.

 

[mrfailure]

Agree with Cory. I just wonder if you were able to identify origin in the intergranular region in optical and/or SEM examination as a confirmation.

 

[priyam07]

Mrfailure, it was hard to locate any point of origin in most cases. In one case the center of the cross section shows some segregation cracks (IG under SEM)to be possible location of origin.