410 heat treated to B6 - Failures 2

[Stiff914]

Would like you to help me out with a question. We have 4" to 5" 410 bars we had heat treating to meet B6. These bars have failed multiple heat treatment to meet B6. I am told by the heat-treater you can re-heat 410 indefinetely without any issues, is this correct?

 

[WKTaylor]

RE 410 CRES...

What material specification and what heat treat spec?

Regards, Wil Taylor

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[Stiff914]

This procedure is for a SA479 gr 410 Cond. A bars - air tempered to meet ASME SA193 gr B6. (4" dia)

Ramp at 400F per hour to 1375+/- 25F
hold at 1375 +/- 25f fpr 4.5 hrs
Air cool to below 400F

We also tried Vacuum harden and air temper (4.75" dia)
Harden Cycle
pump down to 100 microns or lower
introduce and maintain 500 microns minimum of nitrogen partial pressure.
ramp at 600 per hour to 1425 +/- 25f
hold at 1425 +/- 25f to equalize
ramp at 300 per hour to 1775 +/- 25f
hold at 1775 +/- 25f for 1 hour + 15 /-0
nitrogen quench to below 250f
Air temper cycle
ramp at 400 per hour to 1375 +/- 25f
hold at 1375 +/- 25f for 5 hours +15 /-0
Air cool to below 400f

We have tried with different cycles and times / all tensile failures too low, some have too low yield as well. Any that has passed failed charpy impact. With the first procedure all results were at least 15 ksi too low on tensile and yield. With the second procedure they were less than 10 ksi too low. Tried different mills as well, Valbruna and Venus Wire Ind. Weird thing is, these procedures had worked in the past.
What do you think?

 

[metengr]

Check the complete chemical composition against the material specification. Also, focus on tramp elements related to low CVN values.

 

[EdStainless]

My guess is lower C than in the past.
Or people have changes their mill cooling practices.
What are your starting properties? That could be telling.

To check HT response try this:
Direct quenching a piece in oil from 1825F (allow a 4-5 hr soak for a 4" bar)
After you grind a flat on the surface and check hardness it should be >56RC
A temper of 2hr at 1000F should get you roughly 35HRC, 150ksi UTS and 125ksi Yield

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P.E. Metallurgy, Plymouth Tube

 

[Stiff914]

Thank you all for your thoughtful responses. Now the question is, can I use the same bars that have already been heat treated, or do I have to start with a fresh 410 cond. A. I know some grades cannot be heat treated multiple times, but I am told by the heat treater that 410 is capable of this. I'm skeptical.
Starting properties attached.

 

[metengr]

The 410 stainless steel can be re-heat treated. The chemical composition looks acceptable, as stated. I suspect besides Sn, which can reduce impact values after temper, you may have other tramp elements, like As and Sb.

 

[EdStainless]

Their reported HT response looks good.
You can re-heat treat provided that you are going to eh same or higher temps than the first time.
I noticed that Valbruna only used a 1700F austenitizing temp, that is lower than what I would have used (1775-1825).
If you are hardening 5" bar a 1 hr hold won't cut it, you need more soak time. Even if I give you 'credit' for the 3 hr of ramp time I still doubt that you are getting good soak.
A 1375F temper? A 1250F temper give you about 100ksi UTS. How low are you going?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Stiff914]

You caught my mistake up there. It's a one hour hold per inch.