Induction Temper - does it arrest cracks in IH'd shafts?

[dbooker630]

I'm having an issue with induction hardening a 2.5" dia/ 8" long shaft made from 4340H steel. What is unique about this job is that there is an axial drilled hole running through the length, there is a rolled spline at mid-length with a hole drilled from the surface to the axial hole.

My issue is quench cracks after IH that are visible along the spline. Opening the fracture shows the origin to be along the hole, in line with the case-core transition. In each case the surface finish of the hole is questionable. In shafts with no cracks the hole surface is acceptable.

The frequency of the condition is very low, but because it exists at all I have to 100% mag to ensure it does reach my customer. As far as the induction recipe I have reduced heat input and quench severity as low as possible without creating soft spots.

I receive the green part from an outside (outsourced!) machining source. The parts that I make in-house with a hole in it have not experienced any cracking issues. It sounded easy enough to approach the machining source with this issue, who also supplies another site in our company with this part.

The other site is not reporting any problems with cracking due to the addition of an induction temper between quenching and furnace temper. They agreed that hole surfaces were a problem and that induction tempering is the only way to prevent it. My practice is to furnace temper within one hour of quenching, which usually is sufficient. (We just had a thread about how long to wait after quenching).

But my question is can the induction temper work rapid enough to prevent the quench crack itself, or just arrest its propagation? I say this because I noticed in examining the fractures that there was a small circular subsurface zone surrounding the hole, followed by a zone that extended to the surface.

This suggests to me that there was an immediate quench crack that extended to the circle, then subsequent propagation while the shaft awaited tempering. My theory is that the induction temper prevents the propagation but the subsurface zone still could exist. What do you think?

 

[redpicker]

I suspect the induction temper works because it is performed before the cracks form, reliving the transformation stresses which eliminates the cracks in the first place.

Quench cracks usually don't form above 350 def F. If you can get the temper performed before the part drops below around 300 F, either furnace or induction temper, I suspect you won't get any cracking. With 4340, you have to get it below 600 F with the quench, but the cooling rate from 600 to 300 isn't that critical. That is, go back to your normal quench severity but time your quench so the hardened surface comes out at 500 F (+/- 50) and put the parts in a pile (covered with a fiber blanket, perhaps?) to cool to below 350 but above 250 before temper. I'd wager that would stop the cracking, although with 4340 and a liquid quench, I would still perform the mag inspection (unless your customer doesn't care about getting an occasional crack now and then).

Of course, quenching to 350F and performing a snap induction temper 100 degrees below your target tempering temperature would also work.

rp

 

[Yallapragada1022]

Very interesting problem. I had to go through the same issue (splines & hole cracking) last year on 4150H shafts and we were able to address the issue by:
1) Lowering the heat input
2) Reducing the quench severity (Houghton Aqua Quench 365) - Lowered the quenchant GPM and increasing the concentration to 6% (Refractometer).
3) In our case, the draw furnace is located next to the induction scanner and the part is placed in the draw furnace (400 deg.F) immediately.

Plugging the hole is an option that you may want to look at.

We allow less than 15 minutes after quench to have the parts in the draw.
I have not tried the induction temper that your sister plant has been doing. I am not really sure if induction temper for very short period of time is going to prevent the cracking.

We were induction tempering the thread end on another shaft after induction hardening cycle in the same induction scanner and experienced cracking in the thread radius and other locations along the length of the thread. The cracking was observed immediately after the hardening operation and also after tempering. Residual stress analysis showed that we were inducing tensile resiual stresses in the thread end radius because induction temper for short time only heats the surface and does not penetrate the core.
Thanks for posting a very interesting problem. Let us see what others have to say.


Rao Yallapragada

 

[swall]

You might try increasing your quench delay. Use as much delay as you can and still meet your hardness requirement.

 

[TVP]

I'm on the fence regarding whether or not the induction temper is preventing the crack or just arresting a small crack. I would need to know a little more about the entire process (power, scan speed, quench flow rate & concentration & temperature, typical surface hardness, typical case depth and hardness through the case, and actual surface roughness of the hole. If you are aggressively heating this part (fast speed with high power to produce a hard and/or deep case) and aggressively cooling this part (cold water, high flow rate, little or no polymer) then I think it is cracking immediately. How are the parts handled immediately after quenching? Always conveyed nicely, or is there manual movement, banging, dropping into tubs, etc.?

 

[swall]

Dbooker630--another question. Is the induction hardening process single shot or scanned?

 

[dbooker630]

Thanks for all of your responses. Let me provide some more process specifics:

Material: SAE 4340H, prehardened & tempered to 45HRC
Machine: Inductoscan 300kW/3kHz
Scan: 96kW, 2.88kHz, .20"/sec, total length 8"
Quench: 30gpm, 6psi, 18-20%Polyquench 15XND - was 12-18%
As-quenched: 62-64HRC
Case Depth: .250"(50HRC)- used to run closer to .350"
Final hardness: 55-58 HRC after 400F 1.5hr furnace draw

Parts are hand loaded in machine. There are two different scans on this part. The first scan is of the opposite end with no holes to deal with and no issues. Parts are flipped and the second (problem) scan begins as noted above.

After main scan and quench is complete an auxiliary quench remains on in order to cool the part enough (~130F) so that it can be hand removed. The hardened parts (~25lb) are then placed in a pan. The sister plant uses a robot to load/unload.

 

[Yallapragada1022]

What is the quench temperature that you try to maintain?

Rao Yallapragada

 

[dbooker630]

85 to 95 F. Machine will not cycle below 85. Typically is at 90 during continuous run.