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I am attempting to determine which material has been used in a spring. I know that the spring is ferrous. However, I would like to know whether the spring would have been heat treated.

jore1
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  • That resistance seems high for a simple spring contact. – Solar Mike Mar 21 '21 at 22:06
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    If you want to know anything more than it is carbon or low alloy steel ( because of ferromagnetism) , get a chemical analysis and a hardness test. Probably not 316 SS which will cold work to significant levels of ferromagnetism. – blacksmith37 Mar 21 '21 at 22:20
  • Your "6 ohms" measurement doesn't make sense. If your spring was stainless steel with a cross section area of 1mm^2, you would need a length about 8 meters get 6 ohms of resistance. The resistance of a non-stainless steel would be 3 or 4 times lower than that. – alephzero Mar 21 '21 at 22:39
  • seems an unlikely application for 316, or any fancier stainless alloy really. 301 would be mildly magnetic ???. but non-magnetic when annealed so that could be a test. low-carbon steel w/ nickel plating (commonly over a very thin layer of copper) sounds plausible – Pete W Mar 22 '21 at 00:06
  • @petew Are there any other reasons low carbon steel would be chosen instead of stainless apart from price? Also would any heat treatment be necessary for this application? – jore1 Mar 22 '21 at 10:24
  • In don't know, but would guess the motivation is price, which in turn is whatever lets spring machine run at the highest speed with few defects. I think there are methods of estimating carbon content from grain structure, since you seem to have access to have some good images with magnification, but maybe someone else here can speak to that. A medium carbon spring steel would probably be hardened but again i would defer to others here. (perhaps @blacksmith37 ?) – Pete W Mar 22 '21 at 12:43
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    You did something and got a display of 6.0 on your meter. That doesn't prove you measured the resistance of the piece of spring wire. A battery terminal with a resistance of 6 ohms is not believable whatever it was made from. – alephzero Mar 22 '21 at 13:43

3 Answers3

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Spring stock is not extruded- it is cold rolled which greatly increases its yield point and severely elongates the grains. It is not low-carbon; it is low-alloy manganese or medium or high-carbon steel.

Had it been heat-treated after rolling, it would have recrystallized into bulky, ductile grains.

High-quality battery terminals/springs are subject to a very thin copper flash coating which serves as an adhesion layer for a topcoat which is either intended to increase corrosion resistance (nickel) or provide low contact resistance (gold; usually selective-area plated to minimize costs).

niels nielsen
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  • higher carbon means higher yield and more response to cold work. regarding the microstructure, that is the result of both mechanical and thermal processing & I'll trust the spring manufacturer to know what they need. – niels nielsen Mar 23 '21 at 15:38
  • Okay thanks for the response. – jore1 Mar 23 '21 at 17:40
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Since you apparently have a metallurgical lab to make micrographs for you, find our what chemical analysis kit they have. Maybe they have an SEM in a back room that can give you elemental compositions of both materials.

Even if they don't have full wet chemical analysis or SEM, they will have chemical and electroetch kit, which combined with an etch cookbook and some more microscope work should let you identify whether the steel is stainless, and what the plating is.

Have a look at flat spring and electrical contact springs for sale online. You will find what the common steels and platings are.

OpticalResonator
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Carpet_Diver
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Springs are steel because it is : 1- cheap, 2- available and, 3 - it makes excellent springs. It may contain various alloys and carbon levels. It may be cold drawn or heat-treated ( quench and tempered ) . Very likely the spring is cold drawn carbon steel because that is the cheapest. The main concern for most springs is hydrogen stress cracking . Very little hydrogen will cause this because of the high strength and high stress. So some protective coating may be applied , as simple as paint. Recently, hydrogen stress cracking caused a torsion spring in my garage door to fail . The present weather of cold nights and warm days caused moisture to condense on the cold spring , that introduced enough hydrogen ( few ppm ) from corrosion to fracture the spring . The spring was 25 years old and the original coating of oil/ grease was gone. I suggest oiling garage door springs as I have now done , to resist this corrosion / cracking.

blacksmith37
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