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Gareth Barry

Color change of steel during martensite formation

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Posted (edited)

Hello all. 

I am trying to come grips with heat treating en45 spring steel. I previously heated and quenched a piece in water, which snapped easily with some pressure, telling me it hardened. I then quench a piece in brine, which cracked in numerous places, also hardened. Both test pieces showed very fine grain at the breaking point. My main point, In both cases, the steel took a sort of dull gray color, almost white, a very visible color change. Is this martensite? 

Now my actual knife, which I making as a gift for a friend, I quenched in warm canola oil. The steel took on exactly the same dull gray color, but took longer to form, which makes sense due to the slow quench. However, the steel doesn't seem as hard asi was expecting. I tempered for 90 minutes at 200 celcius, got a nice straw color. 

At this stage I am thinking it is a surface decarb issue, as there was a lot of scaling of the steel. Anyway my main question was about being able to actually "see" steel harden from a color change. Thanks and sorry for the long post. 

Edited by Gareth Barry

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In short: No, there is no visible-to-the-naked-eye difference between a sanded/polished surface between martensite and any other state.  The fracture surface can tell you something, but destructive tests are not always very helpful.  Also, ignore that straw color after tempering.  Temper colors mean nothing of importance as they are too dependent on surface and ambient air chemistries.  

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Hi , what austenising temperature are you quenching from?  EN45 needs around 900C to harden (so needs to be done in a temperature controled oven to get any chance of a good result) and is not really suited to a water or brine quench. Its a deep hardening steel and the water is not necesary.

 iIs also suceptable to a relativly deep decarb, deeper than most carbon steels ...I have always thoufght that this was due to the High HT temp needed to harden it, but could be for other reasons.

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Thanks for the replies. Just to clarify, I am talking about the color of the steel almost immediately after the quench, around a second or so from lifting out the water and a couple of more seconds after lifting it from oil, taking on a very whitish gray color. This is before any sanding, grinding and cleanup. I did use my oven to temper, I wasn't really too concerned by the color. I have read that en45 requries a higher than normal austinising temp, so I tried to heat it as hot as my charcoal forge with a hair dryer can, perhaps its still not hot enough? However, why would the test pieces have cracked/snapped so easily? My basic knowledge tells me that means it did harden in water. The only purpose of that test was for me to detemine if I was reaching austinising temp, as I was aware of the difficulties with en45.

I guess en45 really isnt great for using with an open charcoal forge, although I see smiths like James raw uses it? hmmmm....

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Perhaps this color is just the clean steel where the scale blew off in the quench?  

900 C isn't all that hot in the forging temperature world, so I feel sure your charcoal can get there. That's 1651 F, or an orangey color.  Do you use the muffle pipe trick to watch for decalescence?  This will give you a visual confirmation you have reached the correct temperature for quenching.  A muffle can also prevent decarb.  And En45 is an oil-quench-only steel.  I'm surprised it didn't shatter in water.

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