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I was wondering that if a proper process of heat treatment (heating to a temperature above upper critical temperature, then soaking and then quenching) is applied to a metal like steel, then what happens within the metal itself on microscopic level which makes it more strong and hard but decreases its toughness?

I mean I know this that when I increase temperature (or in other words, heat) of a metal like steel, then Elastic Modulus and strength decreases (because the metallic bonds become weaker and it is now easier to deform the metal and cause a slip between the layers of metal atoms) and the material becomes softer, but we are also applying heat to the metal in heat treatement but it becomes more hard and strong. How does these two things work in opposite ways?

Rameez Ul Haq
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    Steve Mould using steel balls to represent crystal structure and how dislocations in that make it harder: https://www.youtube.com/watch?v=xuL2yT-B2TM Heating just lets the crystal lattice change, but how it cools basically determines how the crystal structure forms. So it's not application of heat that is really deciding things. It's the progression crystal changing as it cools and you trying to freeze it at the right time to keep it frozen at some part in that progression. – DKNguyen Sep 23 '21 at 14:17
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    @DKNguyen With a little elaboration on the effects of cooling and tempering, this will be a valuable answer to the OP's concerns. – r13 Sep 23 '21 at 17:25
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    answers here are right on, but this is several weeks of learning in a Strength of Materials undergrad engineering class. – Tiger Guy Sep 23 '21 at 19:16
  • @r13 Unfortunately, that is almost everything I know and anything I would just basically be the difference between annealing, tempering, and hardening all of which involve heat...unless that is what the OP is actually asking. – DKNguyen Sep 23 '21 at 23:39
  • @DKNguyen I also agree with Tiger Guy's comment. Ley's see if OP will return and make it more clear after all these answers and comments. – r13 Sep 24 '21 at 00:12
  • I am just asking why would the steel or any other metal become stronger and harder when heat treated, when it becomes weaker and softer when it is only heated and operates at an increased temperature. I mean what difference does it make in the microsopic structure of the metal that it behaves differently after being heat treated than when it operates at a higher temperature only. – Rameez Ul Haq Sep 24 '21 at 07:41
  • My son studied this stuff last semester. He had a book about an inch thick that explained it all- or at least as much of it as a beginning mechanical engineer needs. – JRE Sep 24 '21 at 14:06
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    @RameezUlHaq All I have to say about that is that it has nothing to do with metallurgy but the fact you think being hot right now is the same as being hot in the past even if it is cold now. It should be should be evident why these are not the same now that I have pointed it out. If it still isn't clear, I will further point out that heat treated metal still gets weaker at elevated operating temperatures and that these operating temperatures are all lower than what you heat the metal up to during heat treatment. All operating temperatures are cold relative to the heat treatment temperature. – DKNguyen Sep 24 '21 at 15:44

4 Answers4

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In short the heat treatments in steel change the phase of iron between the following phases:

  • Austenite
  • Cementite
  • Martensite
  • Bainite
  • Ferrite
  • Perlite.

(Actually quenching does not allow low temperature phase changes to occur, so effectively the phases are sort of "frozen" in their high temperature equivalents).

enter image description here

Figure 1 : example of continuoous cooling transformation diagram of low alloy steel (sourceindustrialheating)

Of course this is also a function of the Fe-C phase diagram

enter image description here

Figure 2: FE-C phase diagram (source calphad.com)

Each phase has its own characteristic properties. There are several online resources that describe the differences (e.g. Interpretation of the microstructure of STeels).

NMech
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The effects of heating-quenching a metal is explained below

Transformation hardening is the heat-quench-tempering heat treatment cycle addressed earlier in this article. It's used to adjust strength and ductility to meet specific application requirements. There are three steps to transformation hardening:

  1. Cause the steel to become completely austenitic by heating it 50 to 100 degrees F above its A3-Acm transformation temperature (from that steel's iron-carbon diagram). This is called austenitizing.

  2. Quench the steel; that is, cool it so fast that the equilibrium materials of pearlite and ferrite (or pearlite and cementite) can't form, and the only thing left is the transitional structure martensite. The idea here is to form 100 percent martensite.

  3. Reduce brittleness by tempering the martensitic steel, which requires heating it, but keep temperatures below A1. Typically, this means temperatures are between 400 and 1,300 degrees F, which allows some of the martensite to turn into pearlite and cementite. Then allow the piece to air-cool slowly.

By using the proper heat treatment and choosing a steel with just the right amount of carbon, you can get just about any combination of hardness and ductility to meet a specific requirement. Remember, the more pearlite and cementite that forms, the more ductile and less brittle the steel will be. Conversely, more martensite means less ductility but more hardness.

One topic has been ignored up to this point is grain structure changes during precipitation hardening. A steel's grain size depends on the austenitizing temperature. When a steel that will transform is heated to slightly above its A3temperature and then cooled to room temperature, grain refinement takes place. Fine grain size offers better toughness and ductility.

Metallurgy Matters: Making steels stronger

r13
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You are mixing apples and oranges. Many steels harden by rapid cooling, but very few other metals do that; specifically, only aluminum bronze and certain titanium alloys. Many metals will strengthen by age hardening; Rapid cool softens, and then time at a lower temperature strengthens them.

There are a myriad of combinations, like HSS (high-speed steels). They harden by rapid cooling and then age harden during tempering. And many other secrets you can only learn by joining the secret order of metallurgists.

Peter Mortensen
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blacksmith37
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5

The other answers describe the "materials science" mechanisms of iron vs. temperature. I'm going to add this:

Matter "tries" to reach a minimum energy state whenever possible. In general, then, if you cool something as slowly as possible, you'll come closest to a solid which is a perfect crystalline structure. See "annealing." If you cool it a bit faster, dislocations and perhaps plane slips start to happen. And if you quench or otherwise cool it quickly, none of the molecules can settle into the low-energy state, and you get a glass. For fun, take a look at videos of Prince Rupert's Drops.

For iron, then, the overall strength in all 3 dimensions, flexibity vs. shatter point, etc., varies across the defined structures. You pick the cooling sequence you want based on the performance requirements.

Carl Witthoft
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