Low alloy steel cracking after HT 3

[CdotS]

How do I differentiate fracture surfaces of low alloy steel plates (15B30) that failed after heat treatment to achieve 48HRC (austenitize at 1550F, water quench, temper at 850F,& straighten in a press to remove distortion, water quench to room temperature)? In others, how do I know whether the failure was due to quench cracking, hydrogen cracking, mechanical bending in the straightening stage, etc?

 

[swall]

You can rule out hydrogen cracking. I see nothing in your process that could cause this. If you have access to an SEM, you could quickly discern quench cracking from cracking during straightening--the former will be intergranular; the latter ductile dimple rupture. If no SEM, look for presence/absence of a shear lip and oxidation of the fracture. Tempering at 850F would cause a quench crack to be darkened. If a shear lip is present, that would tell you that the fracture was ductile in nature and happened during straightening.

 

[unclesyd]

Are you straightening at 850F?

Boron Steels don't like to be worked at these temperatures.

 

[metengr]

In addition to the above posts regarding examination of the fracture surfaces, you really should have a metallographic examination performed of the fractured pieces or better yet a crack-containing piece.

Metallographic examination would provide supplemental information to confirm the root cause of the cracks - improper quenching technique, improper ramp rates for austenitizing, susceptible microstructure, severe quenching medium….

 

[CdotS]

Thanks to all for your replies. They are useful. I have more specific questions.

UncleSyd: You have said that it is not a good idea to work when it is at 875F. Why? I thought it has more ductility/plasticity at 875F than at room temperature. Could you please explain your thoughts.

MetEngr: Yes, we have taken cross sections containing cracks. They reveal whether they are single crack or mutiple cracks, inter- or trans-granular, etc. My interest is to know how different types of fracture would appear in cross section metallographs. For example, if it is interganular, can we confidently say it is only due to hydrogen cracking? Transgranular - doe sit mean quench cracking? Am I missing some thing here?

Since the steel, after Q+T, has very little ductility, it appears to be brittle. Maybe SEM would indicate more details on microplasticty such as local dimples on the fracture surface.

 

[metengr]

CdotS;
In my opinion, metallographic examination provides a key source of information as you indicated. I would caution you that there are no simple rules to say that if the crack is transgranular it is.... or intergranular it is.... Transgranular cracking could be related to loss of ductility, quench cracking, hydrogen embrittlement, etc. Etching of the microstructure with the crack provides valuable information to determine if the crack occurred during heating, or during quenching, and of course during post heat treatment forming (based on the presence or lack of oxide along the crack interface, and the appearance of the surrounding microstructure (evidence of decarb)).

Intergranular cracking can be related to hydrogen embrittlement, reheat cracking and yes, even quench cracking in certain situations. This is why metallographic examination coupled with the SEM go hand-in-hand for root cause analysis.

I agree with swall that you could probably rule out hydrogen embrittlement, based on what you stated for your HT process. However, other factors are not as obvious for certain steels.

 

[unclesyd]

This comment was based on my experience with Boron containing steels.
We had a very costly incident involving Boron containing fasteners, SHCS. About 20,000 fasteners were purchased and several hundred were installed in our process which operates at 600F. During the startup of this production line several of these fasteners broke while being loosened to change out some failed components. On this production line we had to do a an emergency overhaul at about 90 days. As we disassembled the components at operating temperature we had several hundred failures, mostly first thread. we were fortunate that most failure occurred on removable components as they could be moved to the Elox for broken fastener removal. The ones on the fixed components required many hours of 24/7 work. There was another line that had the same fasteners installed and when it when it was shutdown for overhaul it was allowed to cool, freezing the polymer, which required an extra three days for the overhaul. About 5% of the fasteners broke during disassembly.


During the subsequent investigation as to what had occurred the following was revealed.

These fasteners were sold on RT strength values, our problem

There was an upper temperature limit of 400F-500F, not tested under highly stressed conditions.

This material should not be worked hot. The fasteners were made by cold heading, heat treating with thread rolling, cold.

At the time the only information about hot working the material was some work at 950F. The results were very poor. These results prompted some lab work at 900F since our cleaning procedure was carried out at this temperature. After 4 hr exposure at 900F the majority of the 96 bolts in the test component broke or were found to be cracked when inspected by wet MT.

Based on this we put a ban on Boron containing fasteners on on site to prevent the possibility of them being used in our manufacturing process. This included both UN and Metric. We had some problems with components used in the hot zone that were made in Europe using Boron containing fasteners.

Sorry I didn't have the material used for the fasteners but though involved in the investigation I was on the periphery as I was working on another project and didn't get in on all the meetings and was not copied in on the final report and there is no copy in the Metallurgical Laboratory files. There was apparently a lot of CMA on some of the initial decisions on purchasing the fasteners.

I haven't kept up with the use of Boron Steels but I have noticed in several articles concerning the working components containing these steels that heating for working, bending/straightening, shouldn't be attempted.