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    Default Thermal Damage

    My M2 steel in some cases will heat up to a very light straw color. Visually my only frame of reference would be a color chart on how hot the steel got before cooling back down. Would this indicate that the temp only got up to 150-225C degrees or so? What would a loss of hardness be at that temp?

    What type of damage would this cause to the metal and how could I differentiate between stress cracking or pitting?

    In some cases I see both types of failures in different dies that we make. Some look like they have been chipped out along the tooth because of a push-pull work hardening and or a surface finish scratch that accumulated stress points along it until it failed. These types usually occur at the effective diameter or pitch diameter of the thread. Others have a sort of pitting where under magnification the cratering is apparent and uniform, and will occur at the crest of the thread.


    Thanks
    Jim

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    Can you post some pictures?
    Also, after having lurked here for several years, I don't recollect anything like your post.
    Maybe it's the wrong forum. Have you tried Abama.com or any other site for blacksmithing. Metallurgy?
    Good luck! fusker

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    You need to describe what your are doing more clearly. Is the heating as a result of heat treating or operation? Is is during grinding a part? What is the part and function? Push/pull? I take it you are not a metallurgist.

    Tom

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    I manufacture thread rolling dies, my job is to grind over 400+ thread forms in varying die widths.

    This link sums up what I make.

    YouTube

    This particular one if you were to roll out the circumference of the die onto a flat plane with the helix angle at 6 degrees the width of the die being 2.5" I get over 22" of thread length.

    So what I have for wheels is very limited at the moment its between a 5SG80 and a 38A180 on occasion some of my thread forms require a 500 grit wheel. The thread I am struggling with itself has a nose radius of about .014 so I can barely fit two grains of the 5SG into the nose radius. The surface finish suffers but I can generally go up to 1mm per pass without degrading the thread form too much and not burning.

    With the 38A white wheel I am not doing anything over a 1.5" width which drops the total travel to a manageable number and probably why I have not encountered the loading I have grinding the wider dies. I've still dealt with burning but typically I will take more passes and slow the wheel down so that it will breakdown quicker.

    One option on the table is to rough grind with the 80 then come in with the 180. But its so hard to make that jump as the cycle time increases substantially.


    As far as my original post, I would like to pinpoint the certain failures and see if the surface finish plays a bigger role or does the heating cooling cycles of the metal play a equal or greater role in the failure of the thread.

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    Jim- A better analysis of your problem would best come from a metallurgist, which I am not. But some thoughts do come to mind that may/may not be factors. First off is the material itself. Is your material coming from a reputable source? The quality of tool steel never used to be in question 20-30 years ago but I noticed anomalies started popping up in the late 1980's enough to warrant buying from select suppliers for critical or time intensive components.

    Heat treatment from a good source? I presume they know what they're doing or problems would show up on other dies. How hard are you getting these? Do you test the failed ones? Have you tried cryogenic treatment for the problematic dies? There's varying opinions on cryogenic treatment, who does it is critical to outcome. I've limited experience with it and have no qualified opinion.

    I wouldn't think that heating/cooling cycle of grinding is causing a problem for material like M2, even if it's getting up to 225°C (437°F) as the tempering range of M2 begins above 537°C (1000°F). Using an infrared heat gun, even a cheap one, should give a more accurate indication of temperatures. I doubt you're reaching overall piece temps with normal grinding. What I would consider is the very localized condition when burning. That indicates elevated sectional temperatures and thermal stress, which may/may not affect hardness and/or surface cracking in that section. I've seen this affect heat treated components in stamping dies but this is a question best answered by a metallurgist.

    Next there's the lube used in the rolling process. Is there a possibility that galling is occurring for those problematic dies resulting in tear-out? You've mentioned a "cratering" effect and this might be similar to other cold forming operations or BUE on cutting tools where the deposited material eventually breaks free. taking more with it. Another question best put to the metallurgist.

    Have you discussed this with your abrasive supplier as well as other abrasive brands. They know a whole lot more than I do about this and may be able to offer some alternatives to what your using now.

    Lastly, have you tried other tool steels for the problematic dies? There are many other powdered metal flavors I've not tried but have very good numbers. Yes they're more expensive and seem to offer greater potential while being more difficult to grind but if the problematic dies are costing you in component failure and downtime then perhaps it's worth investigating.

    When I've run out of my normal solutions I start trying to eliminate any variables or uncertainties. Sometimes that means verifying a process. When that's been done and the problem remains it's time to seek people smarter than I am and maybe try something new. Hope this was of some help, no experience with your process. Report if you find a solution.

    There's a lot of other members here more knowledgeable than I am, perhaps some of them will jump in on this.
    Last edited by AD Design; 03-01-2020 at 12:21 AM. Reason: Additional thought

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    Quote Originally Posted by Scra99tch View Post
    So what I have for wheels is very limited at the moment its between a 5SG80 and a 38A180 on occasion some of my thread forms require a 500 grit wheel. The thread I am struggling with itself has a nose radius of about .014 so I can barely fit two grains of the 5SG into the nose radius. The surface finish suffers but I can generally go up to 1mm per pass without degrading the thread form too much and not burning.

    As far as my original post, I would like to pinpoint the certain failures and see if the surface finish plays a bigger role or does the heating cooling cycles of the metal play a equal or greater role in the failure of the thread.
    As a guess you are crush dressing the grinding wheels and then grinding the thread form using 100% oil as a coolant. The die heating during grinding might be reduced by switching over to a high sulfur semi synthetic or soluble oil coolant.

    This may not work in all cases. Water evaporates at 100 deg C and a grinding oil evaporates at roughly 200 deg C depending on its viscosity. If the grinding interface temperature is above 100 deg C and a water based coolant is used, the grinding wheel will be operating dry. The goal in using a water based coolant is to improve the heat transfer rate from the part and keep the contact temperature below the coolant boiling point.

    Using a sulfur EP additive in a 100 % oil coolant will also lower grinding area contact temperatures. A example of this type of coolant is Castrol Variocut G600. The optimum oil viscosity is a function of wheel speed and abrasive grit size. The Benz oil website has a chart for selecting grinding oil viscosity.

    A thin layer on the surface of the thread forming die may be in tension and have surface cracks as a result of being heated during grinding and then flash cooled.

    This is a old (June 7, 2000) but still useful paper from Cincinnati Milacron explaining the advantages of using a high sulfur coolant mix when using either the white aluminum oxide abrasives or the seeded gel abrasives. They are showing wheel life increases of up to 20 times as well as reduced power consumption and improved surface finish when using the sulfur high pressure lubricant. The test results also show that there is a optimal metal removal rate for each abrasive to maximize grinding wheel life. This is the G ratio ( metal removed/abrasive lost ratio ) shown in the charts.

    https://www.me.psu.edu/chang/taiwan/sol_gel_2000.pdf
    Last edited by Robert R; 03-01-2020 at 12:19 PM.

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    Good question on the material. I would hope that our supplier is getting it from a source that is consistent with our other supplier of dies. I do know that 420 stainless from MSC has caused us problems in the past. I honestly do not know where they are getting the steel, our suppliers hardened, and Blanchard grind then we send it off to wire edm the hole and key way before it comes to me.


    There have been attempts to nitrogen quench dies in the past and we have applied a dynablue coating to the dies which will last longer, but not as long as our main supplier.

    Norton has a materials specialist that we will hopefully involve, she has been doing this type of thing for a long time. I am fairly confident that when shown the failures at certain magnification she'll say this is happen here and this is happening there. But for us to gather every variable might just be out of our scope at this moment.

    I have seen a component of the rolling process which gives me pause for concern. It s a heavy duty spring washer that allows one die to slide axially along its arbor, this can let one die drive the thread and another to follow and clean up. It causes problems on one end where starting the bars is tough and can lead to a messy blow out. But I wonder if it could be controlled hydraulically. Meaning it engages with hydraulics at the start, then after a set amount of rotations it loosens up and allows the one die to follow the master die. This would cut down on the push pull of the threads against each other and may limit the work hardened fractures I see.

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    Jim;
    The material specialist from Norton may very well offer the most revealing information you need right now. Whatever she has to say should create a starting point, working backwards from effect to determine possible solutions. She may just suggest a material and abrasive substitution for the problematic dies only. I would also like to hear what she has to say about this if you don't mind sharing the results.

    As for the suggestion of hydraulic controls on the "sliding die" I'm not familiar enough with your machine or the process to speculate on whether it would help or is even possible given the amount of available real estate in the machine and level of control required. Somebody here has more experience with motion control to offer a valid opinion and I'm hoping they'll chime in. I will say that several manufacturing companies I've worked at successfully exceeded OEM numbers with production machinery by modifying/adding components to improve the process.

    Seems like adding control to the movement just puts more stress on the die to control the thread alignment but I doubt I have a clear understanding of the process. Why is there sometimes difficulty starting? If you've seen bad results during hard starting then you may have greater insight into a possible solution.
    Last edited by AD Design; 03-07-2020 at 02:30 AM. Reason: Not enough coffee

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    By all means, the engineer that is working with me is leaving so this may be awhile quite awasy away. I think they are jsut happy content with the dies that are working and once everything is sorted out tackling the longevity.

    The link I posted above in one of my threads shows the two dies side by side (not our process). Was thinking as they are locked in, and getting threads to match is simple a lateral movement (I believe) of one of the spindles. So once match was in the ball park instead of the two dies fighting against each other allowing one to slide as one is controlling the form may relive a certain amount of the push pull these experience.

    Thread and Profile Rolling - Profiroll Technologies

    Again not the type of machine we use but similar operation.

    YouTube

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    I already understood the basics of the process at a level shown in the videos. Explaining progressive stamping by showing you a video of a press going up/down and the holes/perforations/bends in the material doesn't explain much either beyond a very basic level. There are still plenty of unanswered questions that remain and since the engineer on this is leaving the project any comments I make are pure speculation (WAG) at best. If your end of things is done for "In House" you might try a different material (one of the PM steels or even Ferro-TiC come to mind) for the problematic dies with a grinding wheel recommended for that particular material. If this is for a customer then it depends upon whether you want to spend the money to show initiative or not. Any of the steel suppliers like Krupp, Crucible, etc. ought to be willing to discuss your needs and make a recommendation. Wish I had a better answer for you but with the amount of supplied details I can't draw a better conclusion. May I ask what your specific role in this is?
    Last edited by AD Design; 03-07-2020 at 12:34 PM. Reason: Clarity

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    I grind the dies.

    Ferro-TiC looks interesting, wonder what the cost is and if I would be able to see any real benefit. Its' worth a try, but I suspect my failures are more in our material supply, with my grinding approach a close second.

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    I've made some parts with Ferro-TiC and I'll include a word of advice. There "should" be a data sheet with the material (used to be) that advised the speed/feed rates for machining. Do NOT deviate from the advised S/F or you run the risk of the material work hardening to ridiculous levels and all attempts at cutting after that are pointless. We always milled/turned/reamed to finished dimensions with new/sharp tooling. I have no idea what it's like to grind this. Any reason you're not wanting to try the powdered metals?

    Other than burning I wouldn't say your grinding is at fault. There's a few other suggestions I've made you never addressed so you may want to double check those before assuming your grinding approach is wrong or allowing somebody else to assume that.

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    M2 is a 'High Speed Steel' and as such is designed to keep it's full hardness even at elevated temperatures. M2 is usually tempered above 1000ºF so unless you're going above that temperature you are unlikely to be reducing the hardness. Straw color usually indicates around 400ºF, but if you're turning the body of the tool straw colored then localized heating around the area you're machining/grinding is likely much higher.

    The 'pitting' at the crest of the threads on the tool could be 'adhesive wear' which is caused by the metal you're forming sticking to the tool and then pulling bits off it. This could be helped by using a coating that has a low affinity for the material you're rolling. DLC (diamond like carbon) has a very low coefficient of friction and it's my understanding that it can help reduce adhesive wear.

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    Aarongough, most (if not all) of this was proposed in Post #5 but never replied to by the OP. Perhaps the weight of two will launch further investigation. Hope you're doing well.

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    Quote Originally Posted by AD Design View Post
    Aarongough, most (if not all) of this was proposed in Post #5 but never replied to by the OP. Perhaps the weight of two will launch further investigation. Hope you're doing well.
    Ah, sorry! Clearly I read through the thread too quickly! Thanks mate, same to you!

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    Quote Originally Posted by aarongough View Post
    I read through the thread too quickly!
    -That's ok, perhaps the OP did too. Sometimes opinions aren't considered valid until heard from more than one source. You did weigh in on the topic so how could I object to analysis that was the same as mine?


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