D2 and water 1

[strand1]

I work in an OSB mill where 44 D2 knifes approx. 1/4" x 4" x 21", treated to 58-60, are used to flake or strand logs. The knives are held in a knife pack that has an outer retaining bar made of mild steel. Occasionally these knife packs load and friction creates heat to I assume, 500 plus degrees F as the edge of the mild steel bars that retain the D2 knifes, turns blue. Then, as part of the process, the cutting ring returns to home where the knife packs are sprayed with water and the cutting process repeats. My question is, what effects if any, does the combination of heating to that temperature and spraying with water have on the knife edges. Occasionally when we change the knife packs the edges are damaged, some appear to be softened and peened over and others seem to be brittle and fractured. Could the heat and water be the culprit.
Thanks-

 

[EdStainless]

What tempering temperature do you use on these?
Have you ever checked hardness on the failed blades?
My hunch is that the blades are getting a lot hotter than 500F.
If they are getting harder (and breaking) then must be reaching 1800F and then being quenched.
If they are soft you can look at a tempering curve to estimate the temp. For example to temper to 45 RC takes about 1100F.

Sound like you need to work on a system to minimize the packing of chips in the knife pack.

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Plymouth Tube

 

[arunmrao]

Use a IR pyrometer and check the temperature of the knife, before you follow any changes. Can you allow your knives to be air cooled, before spraying water. May be your inventory for the fresh ones will have to be increased.

 

[redpicker]

It sounds to me as if the knives are, in fact, overheating. You would have to get D2 well above 1000F (>1100F or so) to result in softening from short-term exposure. When these knives soften, they dull, which creates more heat, accelerating softening... Death spiral. Also, a dull blade is much more likely to fracture, since the force on a dull blade is much higher than on a sharp one.

The cooling with water is likely to help the situation, not aggravate it, since the blade would have to get well above the softening temperature for the water cooling to result in cracking. As cracked or nicked knives are scrap, it is beneficial to replace/re-sharpen the blades before they get to this point. And, avoid logs that have imbedded nails, barbed wire, or other metallic “inclusions”.

rp

 

[EdStainless]

You know it may be that knives overheat and temper first, then they become dull, then they heat more.
They could break because of the increased force from being dull, or the dull knives could actually heat enough so that when they are water quenched they become brittle.

Actually changing and resharpening knives more frequently will lead to increased productivity and blades lasting longer.
Can you build spare knife pack and change them quickly? If so then you would be better off stopping before they overheat as much as they do now.

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Plymouth Tube

 

[kumkumvijay]

I share my opinion from metallurgical point of view:

D2 material is high carbon low alloy steel. High carbon gives high hardness and wear resistance when quenched from austunitie zone. When it get very high amount of stress from cooling & high martensite % then it will tend to break.

soft:
When insufficient amount of cooling done from the austunite zone then huge amount of retained austunie is possible which is soft and will make poor edges.

to fix the problem:
1. control the cooling time, amount and flow
2. time heated to austunite zone.

 

[Maui]

strand1, I suggest having one of the used blades that exhibits a "softened and peened over" cutting edge sent to a lab for metallurgical analysis. This should give you some definitive results on which to base an appropriate decision.

Maui

http://www.EngineeringMetallurgy.com