Brittle 715 HSLA Grade 50

[joelande]

I have a part that is stamped from 12 gauge 715 HSLA, grade 50 steel. It is then heat treated (QPQ). Occasionally when I inspect the parts they are ductile when bent and other times they are fairly brittle. Is this a material issue or does this have something to do with the heat treat process? It is completely flat so it's not breaking along a formed section.

 

[metengr]

Not enough information. I would believe this is a heat treatment process problem. Either way, you should have the failed components either sent back to the heat treater for evaluation or work with the heat treater and have an independent metallurgical lab perform a proper failure analysis. If this is a process problem the heat treater needs to know this information and feedback.

 

[redpicker]

I agree with metengr, sounds like a process problem. Express your concerns to the heat treater and see if they can help.

rp

 

[joelande]

Thank you for your input. I have talked to the heat treater and am sending parts for evaluation. His suggestion was the it was probably material related because the QPQ process wouldn't produce a brittle part with only a case depth of .0003-.0008; the part is .104 thick. His other thought was that since this is 715 HSLA, grade 50 and not a specific grade of material like 1008 that the chemistry could vary a lot. My concern is that we have been using this material for years without a problem with zinc plating, and have used it in formed applications. If it was a material problem I would think it would show up in the forming operation. I know that cold working can affect this but the part in question is completely flat.

 

[metengr]

You need to have an independent failure analysis performed. This is the only way to get to the bottom of this problem.

 

[TVP]

joe,

As part of your investigation with a suitable test lab, here are some things to consider:

1. HSLA steels do have variation in composition, with microalloy elements V, Nb, and Ti together with Al and B all forming stable nitride particles.

2. Cooling after nitriding (ferritic nitrocarburizing) must be carefully controlled.

3. Excessive nitride networks can be formed that are detrimental to toughness.

 

[joelande]

TVP,

Thank you for your input. I have been doing some other research and that info is consistent with what I've discovered. Again, thanks for the help; much appreciated. Anything else you can think of is welcome.

 

[Keraj]

TVP wrote: “Cooling after nitriding (ferritic nitrocarburizing) must be carefully controlled.”
Indeed. Cooling rate from the nitriding temperature (1075F) has tremendous effect on thin sections of soft steels.
For example, 1010 has core hardness of about 70 HRB in annealed condition. When 0.2 inch thick plate made of this steel is water quenched from the nitriding temperature after 90 minutes of salt bath nitriding hardness at 0.003” below the surface increased to about 30 HRC equivalent. Even when the same steel is cooled slowly, in the still air, there is still increase in hardness to about 90 HRB at 0.003” depth. Depth of 0.003” is about the depth to which you may observe needles of precipitated nitrides (there is no precipitation of needles in water quench) and, thus, the visual total depth of the nitrided layer (diffusion layer). The higher-than-core hardness may persist much deeper regions, up to the 0.030” depth. Since it is impossible for nitrogen to penetrate that deep during the process, the higher hardness is presumably the result of stress induced hardening of the otherwise very soft steel (work hardening). It is well known that nitrided layers produce compressive stresses by the surface. Of course these stresses develop during the cooling from the nitriding temperature and are in direct relation to the thermal and structural differences between surface and the core material.
To sum it up, increase hardness has been found to be present over 30% of the plate thickness (counting both sides) and the hardness levels increased with increased cooling rate.
Increased hardness below the compound layer is not necessarily detrimental. It provides better support for the compound layer which may otherwise be crashed by loads when resting on the very soft substrate. However, like in your case, it also decreases ductility of the component. If this is not acceptable you will need to request slow cool form the nitriding temperature from your heat treater.
In traditional SBN process such as “Melonite” the cooling is always slow since the parts are transferred to 700F bath were destruction of cyanide takes place. However, since desludging of this bath is very work intensive and the sludge very toxic and hard to dispose, there has been a trend to replace the 700F salt bath with water bath. Water solution can be detoxified much easier. If slow cooling is requested or necessary, parts can be cooled in forced air before water rinse. Yet, the cooling rate of thin sections will still be higher than in the 700F bath. Besides, air cooling is an added cost since the frozen salt on the parts is much harder to dissolve.
In short, you must consider if the increased brittleness of your parts is unacceptable and, if yes, be ready to face increased costs of the heat treatment. Also, you may want to discuss the lot size with your heat treater because air cooling effectiveness will depend on the load configuration.