Heat Treat for Beginners

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Basic Rules

All you need is this book and follow the instructions. It also gives you the uses of different materials for you to choose which one best fits your need.

Just remember

Step 1- Heat treat

Step 2- Temper for minimum of 2 hours, some material requires 2 tempers so know your material!

LETS BEGIN!


Metal properties consist of: strength, ductility, conductivity, durability, and so on – Properties are controlled by, cooling, reheating, and retooling the metal at different rates of speed. This is what the science of metallurgy is all about. Heat treat is an important role in the machining world. Understanding this can control how a part is processed. Every tool you use gets a heat treatment of some sort, from end mills to drills. Machinists around the world should study on this subject at least once.

Metals are broken up into these categories: pure metals, compounds, and alloys. In the industrial world we generally talk about alloys which is a mixture of two metals or metal and another element.

Compound

A compound is several elements smelted together by chemical compounds. Two or more elements are preffered while making a compound. Only by chemical reactions can this be done.

Alloy

Alloys can be produced by mixing one metal with another, or by mixing other elements with metal. This is defined as a solid solution. Just know that the mixture is produced at high temperatures and left to cool down. Another cool fact is that the mixture can be separated by melting.

More Vocabulary

This is where heat treatments of different types come into play. Some of the different kinds are:

  •        Annealing – To cool slowly in order to reduce stress and hardness.
  •        Normalizing – A secondary reheating process undertaken after annealing. Aimed at producing a more uniform microstructure in the metal.
  •        Aging – This can occur naturally or be induced by heating. As the alloying agent precipitates out of the solid solution, the metal will harden.
  •        Quenching – The cooling process is rapid and achieved by immersion in liquids or gases. This process makes ferrous alloys (which have iron as the base metal) harder (and possibly more brittle). It makes non-ferrous alloys softer.
  •        Tempering – Metal that is quenched is reheated to a moderately high temperature to soften the material so it doesn’t shatter into pieces, or to make it tougher.
  •        Stress relieving – This is also a reheating process, but to a lower temperature than tempering, followed by slow cooling. Aimed at relieving the stresses induced in a metal by other processes.
  •        Case hardening – An alloying agent is diffused into the surface of a metal specifically, in order to increase its durability and wear resistance.

Using books and the internet anyone can use the above processes to heat treat and control specific metals to meet a hardness/toughness property.

Misc Information

Common uses of heat treatment in the industry involve steel, here are some examples:

Steel is an alloy of iron and a small amount of carbon, some alloys are added to change the properties. (Adding chromium, for example, produces the famous “stainless steel.”)

Iron without enough carbon is too soft and weak to be very useful; iron with too much carbon is too hard and difficult to work with. Precise percentages matter a lot!

Materials contain an “austinizing temperature” and at this temperature the molecules start to move around. Molecules will freeze in place if quenched in oil, air or water, depending on the material of course. Material is very brittle at this point, this is where tempering comes into play. Tempering a piece of material decreases the materials brittle state. (some material requires 2 tempers at a minimum of 2 hours each)

It makes a very great difference to the final metal whether austenite is cooled slowly or rapidly, because the speed creates different varieties of steel.

Slow cooling produces pearlite, an interpenetrating two-phased crystalline structure composed of ferrite (pure iron) and cementite (an iron-carbon compound with a carbon percentage of 6.67%).

Rapid cooling tends to produce martensite, which is much harder than pearlite but also more brittle. The strength of martensitic steel can be a great advantage in many applications, but the metal will need tempering to minimize its brittleness.

But whether the desired result is martensitic steel, pearlitic steel, bainitic steel (which falls in-between), or austenitic steel that retains its properties at room temperature, in metallurgy, many things are possible. All those results can be achieved with the proper heat treatments.

 

 

 

May 13, 2016

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