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Heat Treatment of Metal

11/21/2016 4:17 AM

Is it possible to detect if a steel is heat treated or not, only by its microstructure and chemical composition?

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#1

Re: Heat Treatment of Metal

11/21/2016 4:22 AM

The steel should come with a certificate that tells you what it is and how it was treated before it came to you.

Sometimes if you know what it is and you need to know how it was treated you might perform a hardness test.

http://www.anvilfire.com/article.php?bodyName=/FAQs/heattreating.htm

There is more in the internet dependent on what exactly you are trying to do.

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#2

Re: Heat Treatment of Metal

11/21/2016 9:28 AM

X-ray diffraction can perform non-destructive testing of a metals crystal structure (hardness) to determine if it was properly heat treated. The question is if the knowledge produced by this test is worth the cost of this test?

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#3

Re: Heat Treatment of Metal

11/21/2016 11:34 AM

Here is an article on the subject:

http://www.sciencedirect.com/science/article/pii/S1738573315301844

Fig. 2.

Optical Micrographs of 10Cr ODS Steels in Various Heat Treatments; (a) NT, (b) HR-T, and (c) FC

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#4

Re: Heat Treatment of Metal

11/21/2016 11:34 AM

For Carbon Steel there are different types of heat treatment.

Take a sample coupon, prepare it and put it under a microscope to identify the grain structure.

Heat treatment[edit]

Iron-carbon phase diagram, showing the temperature and carbon ranges for certain types of heat treatments. Main article: Heat treatment

The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. Note that the electrical and thermal conductivity are only slightly altered. As with most strengthening techniques for steel, Young's modulus (elasticity) is unaffected. All treatments of steel trade ductility for increased strength and vice versa. Iron has a higher solubility for carbon in the austenite phase; therefore all heat treatments, except spheroidizing and process annealing, start by heating the steel to a temperature at which the austenitic phase can exist. The steel is then quenched (heat drawn out) at a high rate causing cementite to precipitate and finally the remaining pure iron to solidify. The rate at which the steel is cooled through the eutectoid temperature affects the rate at which carbon diffuses out of austenite and forms cementite. Generally speaking, cooling swiftly will leave iron carbide finely dispersed and produce a fine grained pearlite (until the martensite critical temperature is reached) and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid steel (less than 0.77 wt% C) results in a lamellar-pearlitic structure of iron carbide layers with α-ferrite (pure iron) between. If it is hypereutectoid steel (more than 0.77 wt% C) then the structure is full pearlite with small grains (larger than the pearlite lamella) of cementite scattered throughout. The relative amounts of constituents are found using the lever rule. The following is a list of the types of heat treatments possible:

  • Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process. The result is a structure of rods or spheres of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel. The image to the right shows where spheroidizing usually occurs.[12]
  • Full annealing: Carbon steel is heated to approximately 40 °C above Ac3? or Acm? for 1 hour; this ensures all the ferrite transforms into austenite (although cementite might still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 20 °C (36 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the "bands" of pearlite are thick.[13] Fully annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more ductile.[14]
  • Process annealing: A process used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. The image rightward[clarification needed] shows the area where process annealing occurs.
  • Isothermal annealing: It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature. This method rids any temperature gradient.
  • Normalizing: Carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this ensures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and hardness.[15]
  • Quenching: Carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature. The critical temperature is dependent on the carbon content, but as a general rule is lower as the carbon content increases. This results in a martensitic structure; a form of steel that possesses a super-saturated carbon content in a deformed body-centered cubic (BCC) crystalline structure, properly termed body-centered tetragonal (BCT), with much internal stress. Thus quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approximately three to four (with more carbon) fold harder than normalized steel.[16]
  • Martempering (Marquenching): Martempering is not actually a tempering procedure, hence the term "marquenching". It is a form of isothermal heat treatment applied after an initial quench of typically in a molten salt bath at a temperature right above the "martensite start temperature". At this temperature, residual stresses within the material are relieved and some bainite may be formed from the retained austenite which did not have time to transform into anything else. In industry, this is a process used to control the ductility and hardness of a material. With longer marquenching, the ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and outer temperatures equalize. Then the steel is cooled at a moderate speed to keep the temperature gradient minimal. Not only does this process reduce internal stresses and stress cracks, but it also increases the impact resistance.[17]
  • Quench and tempering: This is the most common heat treatment encountered, because the final properties can be precisely determined by the temperature and time of the tempering. Tempering involves reheating quenched steel to a temperature below the eutectoid temperature then cooling. The elevated temperature allows very small amounts of spheroidite to form, which restores ductility, but reduces hardness. Actual temperatures and times are carefully chosen for each composition.[18]
  • Austempering: The austempering process is the same as martempering, except the steel is held in the molten salt bath through the bainite transformation temperatures, and then moderately cooled. The resulting bainite steel has a greater ductility, higher impact resistance, and less distortion. The disadvantage of austempering is it can only be used on a few steels, and it requires a special salt bath.[19]
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#5

Re: Heat Treatment of Metal

11/21/2016 12:24 PM

If it's not locked in ore then it's a pretty safe bet that there's been some heat treatment.

Whomever smelt it dealt it.

..you asked.. Don't loose your temper.

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#6
In reply to #5

Re: Heat Treatment of Metal

11/21/2016 1:19 PM

Yup, you hit the nail on the head with that one. The question is, who made the nail? Hopefully, it isn't Chinese "steel"

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#7

Re: Heat Treatment of Metal

11/21/2016 2:09 PM

This has all the earmarks of a commercial phishing expedition to me!

I call foul and will summarily report you to Admins.

You should have read the FAQs and Rules for Posting first, so you would know where this might be OK to post.

Do not pass go; Do not collect $200.00 commission!

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#9
In reply to #7

Re: Heat Treatment of Metal

11/22/2016 12:08 AM

ADMIN: Deleted Post

Vulgar/Rude/Improper Behavior: This post was deleted because it did not adhere to the behavioral policies of the site. Please review Section 14 of the CR4 Site FAQ and the Rules of Conduct.

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#11
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Re: Heat Treatment of Metal

12/27/2016 11:11 PM

I've been mulling this over since I got this email from one of our Moderators regarding my Anon post to spectrographic_uk and their question about heat treatment of metal.

I'm sorry I hurt their feelers. But, with a handle of spectrographic I would think they would know how to look at and analyze crystalline structure of metal! So I suggested that they should change their handle and I got spanked for it!! And that I wasn't playing nice in our sandbox!

If an OP is going to ask a question about a subject that should have been covered in school class for the name they choose for their handle, they are either a troll or they slept through/ cut the class that covered examing an analyzing crystalline structures of metals with a 10X or larger magifiying glass or microscrope.

I will try not to throw any more sand.

Dan

P.S. Happy New Year to All !

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#10
In reply to #7

Re: Heat Treatment of Metal

11/25/2016 3:19 AM

Indeed. It was reported on 21st; Admin has let it run since....

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#8

Re: Heat Treatment of Metal

11/21/2016 11:37 PM

BlueAssieBoy once quoted; "Men are like steel, a loss of temper and their usefulness is limited and they will probably crack at the most inconvenient time."

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