Cracking associated with less severe quench 1

[Lyrl]

Our company heat treats a 1/4 inch diameter bar with two bends. Material is 6150 steel. The part is austenitized and high pressure gas quenched.

Several months ago, a review of our processes for similar parts found this diameter is typically run with a less severe quench. (None of the other similar diameter parts have bends). I lowered the quench pressure for this part to standardize the process for similar diameter parts. We ran ~27,000 parts with the new lower quench pressure.

Then the customer notified us they were finding cracked parts (found after plating, but the plating was inside the crack indicating possible heat treat issue). They sorted and found ~9,000 parts with cracks, about 1 out of 3. I've attached a picture.

We ran several loads with the older, higher-pressure quench, and customer says no cracks. Customer has spent several months investigating their process and claims no changes to pre-heat treat operations occurred in this time period.

Our customer's conclusion of quench cracking due to the change in quench pressure is opposite of standard wisdom that less quench severity = less crack risk. Is it plausible that the reduction in quench severity is what triggered the cracks? Are there any tests you'd recommend as due diligence before we close our investigation?

 

[AndrewFinAustralia]

Just thinking aloud and knowing full well I'm singing to the choir.

Might be case depth Lyrl.

With the higher pressure quench, I assume that the quench rate is quicker, meaning that the initially formed martensite is thicker than a lower pressure quench. As this expands, it places the outside of the bar in compression and the inside in tension. The strain across the diameter would therefore vary with the quench severity.

Couple this reduced depth of strain interface in the lower pressure quench with electroplating (potential for hydrogen embrittlement) and you may have hit the magic combination.

Just a thought that might be way off or may be worth considering if you cross this bridge again.

AF

 

[kumkumvijay]

Hi
The company crack must be checked under microscope to identify is it quench crack.
Quench crack passes through grain boundaries.

The crack zone is high stress area due to bending process. So there is a possibility for crack while bending it. It must be hot forged to avoid crack for this grade.

If you have quench delay for long time then there chances.

As it is hug stress area plating can crack in the plating process.

I suggest the following
1. First check is it crack on plating or Bar?
2. Check the crack propagation in microscope ( trams or inter granular)
3. Check the in coming bar with magnetic particle inspection for pre crack ?
4. Check for the crack in as quenched condition using MPI.

 

[dbooker630]

Not that I would not trust your supplier's testing, but it may be a good idea to check the as-bent bar. How are the bends created - localized flame hardening or cold bent? Also check if there is any variation in the 6150 chemistry.

 

[redpicker]

I agree that it doesn't make much sense that a lower pressure quench would increase cracking, and the extent the cracking has increased is no less than astonishing. The only thing that makes any sense is that with the lower pressure, you are removing the parts from the quench before transformation is complete and final cooling is not controlled. That and extended time spent in the as-quenched condition or allowing the as-quenched parts to cool to too low a temperature. Is it possible that with the lower pressure, you extended the quenching time?

At this rate, I think that I'd set-up a mag inspection (a simple AC coil with a water-based wet fluorescent solution) to inspect a 10% sample, both pre and post heat treatment. It wouldn't be hard to set-up a station that two people could get 300+ pieces per hour and with a scrap rate that high, it would almost certainly be cost effective.

 

[Maui]

Your customer may also be experiencing hydrogen assisted cracking during the plating process. Do they perform a hydrogen bake out after plating?

Maui

 

[Lyrl]

Thanks for the responses everyone, it gives us several avenues to pursue.