Mixed up some identical S7 and 4140 parts -- How to tell apart?

colonelhogan44 · May 15, 2020

I have 8 of the same part all made on a manual lathe, 4 made from 4140 and 4 made from annealed S7. They look visually identical, and I don't have access to hardness testing or XRF or any specialized analysis equipment, but I do have a decent optical microscope. I'm going to break out my metals handbook to see if there is anything in that may provide guidance.

Any tips and/or tricks on how to distinguish the parts? I have 1 of each that are of a known material. Thoughts?

Milland · May 15, 2020

Did you use a common insert or lathe bit to turn or face each one? If so, there may be different surface finishes depending on material (very slight texture changes) due to material characteristics.

Make one more part from a known stock, then check whatever face you use as a reference on the microscope. Now compare the remaining eight "unknowns", and see if you can reliably catch similarities and variations to the test article.

Can't help you if you consistently wind up with five and three, though...

atomarc · May 15, 2020

Having a known sample might be the ticket. Granted, this is about a low-tech as a person can get, using a file, smack each control piece as identically as you can, note the indent and do the same to the piece in question. This crude method may ID your mystery pieces for you.

This is provided you can live with a small nick in the saleable pieces.

Stuart

trevj · May 15, 2020

I'd say it was time to start calling the metals yards to see if you can get them to point an XRF gun at them.

Or, start over and chock it up to the cost of an education.

Material costs and time are like to be the factors to decide which is cheaper and faster.

colonelhogan44 · May 15, 2020

I don't have access to the tooling they were made on, unfortunately. I attempted to measure a difference in magnetism with a 1/10 gram scale and neodimium magnet, but there was no detectable difference.

colonelhogan44 · May 15, 2020

atomarc said:
Having a known sample might be the ticket. Granted, this is about a low-tech as a person can get, using a file, smack each control piece as identically as you can, note the indent and do the same to the piece in question. This crude method may ID your mystery pieces for you.

This is provided you can live with a small nick in the saleable pieces.

Stuart

I'm going to get a piece of scrap of both and try cutting with a triangle file. The S7 is RC38 and the 4140 is RC10, I should be able to feel that I'd think.

atex57 · May 15, 2020

If the finish is not needed on all area's then use a spark test.

Ed.

Milland · May 15, 2020

colonelhogan44 said:
I'm going to get a piece of scrap of both and try cutting with a triangle file. The S7 is RC38 and the 4140 is RC10, I should be able to feel that I'd think.

Yes, you should. But clean the file well before each cut, and try hard to use the same force and speed with each pass.

colonelhogan44 · May 15, 2020

The file was inconclusive, but the spark test revealed all. I have both types sorted out now! Thanks gents!

D Nelson · May 15, 2020

Drop a ball bearing on them the hard one will bounce higher up. I think
Don

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Nerdlinger · May 15, 2020

I always feel so cool doing a spark test and then so dumb when I can never tell the difference haha. I know you already got it done but in my experience tapping them with something like a wrench will produce a dull thud on the soft part and a sharp ringing on the hard part. Oh well, congratulations, detective!

MattiJ · May 16, 2020

colonelhogan44 said:
The file was inconclusive, but the spark test revealed all. I have both types sorted out now! Thanks gents!

Given how close they are in carbon content I'm quite surprised that you were able to sort them out on spark test.
Or is it the chromium that makes difference?

cyanidekid · May 16, 2020

Congrats you got it with the spark test, just for potential future use, another way, when you have parts from the same stock in each different alloy, where the crystal structure will be consistent within each group, is to do an etch.

You can do a partial, and mask some surfaces off if that matters, say wrap electrical tape around the circumference and sand the end of the part, partially immerse in sodium bisulfate solution ( jewelry pickle, relatively safe and cheap). 1/2-2 hr should reveal some consistent pattern under magnification, or differences in color and texture that are noticeable, and the etch should be light enough to scotchbrite or sand out.

If you have parts from different lots of each alloy that wouldn’t work, but if it’s the same mill run in each different alloy, probably.

Does anyone else have a favorite etchant? Hydrochloric is the obvious, but I avoid that in the shop because of the vapors and the rusting problem.

dian · May 16, 2020

D Nelson said:
Drop a ball bearing on them the hard one will bounce higher up. I think
Don

Sent from my iPhone using Tapatalk Pro

leeb method: yes, i do that all the time, using a graduated glass tube.

https://www.kern-sohn.com/data/zusatzseiten/downloads/z-cs-gb-kp-hardness-testing-of-metals.pdf

surprisingly its very little known, its even hard to find on the net.

colonelhogan44 · May 16, 2020

MattiJ said:
Given how close they are in carbon content I'm quite surprised that you were able to sort them out on spark test.
Or is it the chromium that makes difference?

I'm not sure what it was that made the difference, but with two pieces of scrap of each material to play with first, there was a pretty significant difference -- The 4140 was bright, lots of stars and projected about 1 foot. The S7 was dull, projecting about 4-6 inches. I used a tiny dremel cutting disc to minimize the damage.

gbent · May 16, 2020

Next time I'm sure you will put a witness mark in an unobtrusive location to identify one material.

dana gear · May 16, 2020

Back in the stone age when I started working in the shop there was 2 giant safety pin looking things(Best way to describe them) that held probably 30 spark testing rods each, the rods had a small through hole on one end that the giant safety pin would retain them from. Each rod was about 10 inch's more or less long and about 5/8 inch in diameter. They were color coded as to the material/grade with a corresponding chart that explained the color code system. I did use them from time to time with good successes. In an attempt to prevent problems with material mix up's all new raw material was marked the moment it came in the door.

colonelhogan44 · May 17, 2020

gbent said:
Next time I'm sure you will put a witness mark in an unobtrusive location to identify one material.

Yep, I would have, but someone else made the parts and mixed them up it was up to me as the engineer to figure out a way to sort out the mess

dian · May 18, 2020

cyanidekid said:
Congrats you got it with the spark test, just for potential future use, another way, when you have parts from the same stock in each different alloy, where the crystal structure will be consistent within each group, is to do an etch.

You can do a partial, and mask some surfaces off if that matters, say wrap electrical tape around the circumference and sand the end of the part, partially immerse in sodium bisulfate solution ( jewelry pickle, relatively safe and cheap). 1/2-2 hr should reveal some consistent pattern under magnification, or differences in color and texture that are noticeable, and the etch should be light enough to scotchbrite or sand out.

If you have parts from different lots of each alloy that wouldn’t work, but if it’s the same mill run in each different alloy, probably.

Does anyone else have a favorite etchant? Hydrochloric is the obvious, but I avoid that in the shop because of the vapors and the rusting problem.

nital? message to short.

Mark Rand · May 18, 2020

Nital being 2% (some folks use 5%) nitric acid dissolved in denatured alcohol.

Mixed up some identical S7 and 4140 parts -- How to tell apart?

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