Stress Relieving - AISI1074 4

[shankarviyer]

We make circular saw blades of 18" dia x 0.037" thick from AISI 1074 steel coil having a hardness of 26 to 28 HRc.

The circular shape is blanked on a die punch and then 150 to 200 pcs of saws are stacked together and sent for sress relieving (in a tub filled with cast iron shavings).

After stress relieving, we expect the saw blanks to have a flatness of 0.005". But some times we get up to 0.02" flatness. This is causing a huge rejection daily.

The cycle followed is:

Heat up to 1000 F in 4 Hours
Hold at 1000 F for 6 to 7 hours
Furnace cool to room temperature ( 6 to 7 hours)

My questions:

1. Can we use the stress relieving cycle twice? to get the desired result ? Will the hardness drop if we repeat the process.

2. Do we have to follow a different cycle to get better results.

 

[unclesyd]

What is your hardening procedure?
A 1000°F temperature is normally a tempering temperature.
250°F/hour might be a little fast.

Do you see any relation of flatness to position in stack?

Have you used thermocouples to check temperatures in stack.

I assume your getting your blanks from sheet steel and if so a metal sheet will sometime have very pronounced directional qualities which can make the stampings behave differently.

 

[shankarviyer]

Thanks UncleSyd. Let me answer your question:

1. We are not hardening the stampings. We get Steel coil with a hardness of 26 to 28 HRC. It is hardened in the mill itself (POSCO).

2. Do you recommend a slower heating cycle ?

3. We do not use thermocouples. We fill the stampings in a metal tub. Fill all the remaining space with cast iron shaving.

4. We use 1000 Deg F because, we want to do stress relieving only and regain flatness. We have tried higher than this and we lost the hardness.

5. We have also noticed sheet metal directional qualities are playing a big role. How to prevent it


Thanks

 

[unclesyd]

Question #2:
Yes i would until you have resolved the warpage problem, I would install a thermocouple in the stack of blanks and drop the heating above 400°F 100°/hour until the thermocouple reads the 1000°F as per your procedure. Once you reach the temperature of 1000°F you can cut you hold time to a couple of hours to help compensate for the longer heat up times. The reduction in holding time is possible due to the fact that 95% of the stress relief is accomplished in the first 10 minutes at temperature. Another possibility is drop your hold temperature to 900°F which is below the apparent tempering temperature.

#5:
There are amyriad of ways to mitigate some of the warpage due to using a coil.

Do you have a leveler in you process line?

Have you tried to get cut to length blanks that have been through a leveler?

What part of the blank shows the most warpage?

Do your tooling get good maintenance?

What is the process for stamping the blank?

Have you looked into correcting the failed blanks?

Your problem can be resolved since you have better rstio of good to bad.

 

[MetalGuy10]

Shank,

As previously mentioned, leveling and flatness are critical. Coils often have edge wave, center buckle, or quarter buckle that is removed with a tension leveler. ASTM A568 has typical flatness requirements for coil and cut the length (sheet), but your coil is definitely not typical because your coil’s hardness and strength is much greater than typical high strength low alloy product (HSLA).

Flatness, camber, and bowing are relatively easy to measure on coil product. On a flat surface, carefully unwind about 30’ feet or more from the coil’s OD wrap. When the coil is in a state of non tension, flatness and center buckle are easy to spot. The acceptance criteria for flatness and shape will likely depend upon the purchase order and what tolerances were requested; the criterion also depends upon your manufacturing capabilities regardless of spec.

What is the flatness and shape of the sheared blanks before stress relief?

The following are quality issues that you may want to track on an ongoing basis: When you are running the coil on your line, do you see a bow up, bow down, or neutral? Does the coil’s flatness and profile change when the very OD or ID wraps are being processed? OD and ID wraps can also have some handling damage that can cause problems of its own. Do the production issues occur on all coils, or just a few? What is the strip widths typical gauge profile? Gauge profile: measure thickness at 3/8” in from the edge for slit coils, then at ¼ of the width (quarter line), then at ½ width (Crown /center line), and finally the opposite quarter line and edge. All coils have a unique ID and heat number, that can be traced back to mill production.

You may also consider contacting your service center or whoever supplied the POSCO (Korean Steel Mill) Material so a mill metallurgist can visit your facility.

 

[unclesyd]

MetalGuy,

Appreciate your post concerning the terminology used in describing problems when working off a coil which will allow shankarviyer to talk specifics with his supplier.

 

[shankarviyer]

UncleSyd & MetalGuy(10)

Thanks a lot. Let me answer the questions of Metalguy:

1. The coils (across the width) have a flatness of 0.003" per inch. The coil width is 18.25" and 0.037" thick.

2. We purchased the coils from POSCO twice

First time 2.6T coils x 2 were received and the results were good. The stampings before stress relieving were within 0.080" flatness. After stress relieving, the stampings were within 0.008" flatness. We did not measure the coil flatness, bowing & centre buckle.

3. Next time we purchased 3.7T coils x 6 and the problem started.

The stampings before stress relieving had a flatness of 0.35" and they looked like a dish. I have uploaded some pictures. After stress relieving we get about 0.030" flatness. We have processed so far 3 coils and we could not get evan 5% of the jobs right. We are now struggling with the 4th coil.

4. We do not have a coil flattening machine.

5. Instead, we tried to thermally flatten the cut to size sheet by low temperature stress relieving (800 DeG F) and again after stamping at 1000 DegF.

6. The pictures are attached.

7. I will measure the gauge profile tomorrow and send it across. Our factory is on a weekly holiday today.

Look forward to hearing from you.

 

[TVP]

5. Instead, we tried to thermally flatten the cut to size sheet by low temperature stress relieving (800 DeG F) and again after stamping at 1000 DegF.

It is a good idea to thermally flatten the cut-to-size blank prior to forming, but there will be little or no stress relief at 800 F. I suggest increasing the temperature for this step to 1000-1100 F. For such a small blank, you could just place it into a furnace at the correct temperature for ~ 30 minutes rather than the long cycles you have been using for the coils.

 

[MetalGuy10]

Shankarviyer

Your picture of the stamping shows a large variation in “temper colors”. A variation in temper colors often indicates a variation in the cooling rate at both hot and cold rolling mills. Slight variations in temper colors are frequently seen on hot band from many mills. Hot mills utilize large quantities of water from spray banks to cool the strip before coiling. Water distribution during the coiling operation is often not uniform, especially if the strip has severe bowing up or down, center buckle or edge wave after exiting the last stand of a hot reduction mill.

Large temperature variations when heating or cooling metals is often a source of distortion.

From your picture, the steel supplied appears to be a hot band, but with the thickness .037 that would be more typical of a cold band, i.e, cold reduced and annealed after hot rolling and pickling. With the chemistry supplied, SAE 1074, and the hardness of the material 26-28 HRC, POSCO may have the capability of quench and tempering hot band strip before it is coiled.

On the first coils supplied you noted, “The stampings before stress relieving were within 0.080" flatness” and on the second set of coils supplied you commented, “The stampings before stress relieving had a flatness of 0.35" and they looked like a dish”. That large difference in flatness before stress relieving may be related to steel mill process variation. The coils supplied appear to be a contributing factor to the off flat condition, but that may not warrant a valid mill claim. The mill would look at the tolerances and other factors specified on the purchase order before they would consider the condition to be a valid claim. A mill may also specify that the end user has to have adequate leveling capabilities before accepting a claim for flatness.

There is a big difference between a flattener and a leveler. Good leveling often requires keeping the strip under tension while it is being leveled. Red Bud is a company that makes some of the best leveling equipment,

http://www.redbudstretcherlevelers.com/index.htm.

You may want to consider running a small trial on a single coil that is tension leveled by a service center. A trial on cut to length sheet may also be an option. If possible, you should visit the service center to watch the leveling process.

Using thermal flattening may have success like a “roller coaster”, up and down, hit and miss. Good luck and thanks for the sharing topic.

 

[shankarviyer]

Thanks a lot guys. Will keep you posted on the end results.

Thanks again.

 

[unclesyd]

Please do since most of the time members do receive any back.
When you correct or have any more problems, again please post.
If a thread has an ending it can be used as a reference to help others.

 

[shankarviyer]

Gentlemen

We implemented the suggestion of MetalGuy10. It worked. We sent the coil to a steel mill which has tesnion levelling & cut to length facility.

It took a long time to locate such a facility closer to us and get them to agree to our request.

The coil was passed throught tension leveller and then cut to length strips. We brought the strips and did the stampings at our end. It worked. Now the flatness before stress relieving is less than 3mm. It used to be 9 or 10mm earlier. We habe now sent them for stress relieving.

Thanks a lot all of you.

Shankar v iyer

 

[metengr]

Great follow-up on information from MetalGuy10.