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Name: Rachel
Status: Student
Grade: 9-12
Location: FL
Country: N/A
Date: August 2005


Question:
What makes metal stronger, heat and then cool in air to room temperature, or heat the metal and then straight to water?



Replies:
Unfortunately, it is not that simple. Different properties arise from different cooling techniques. Also, when you say stronger, do you mean higher ultimate tensile strength (how much a metal can stand before pulled apart), better hardness (can take a hit better), or better fatigue strength (can stand to be pushed and pulled better), etc. There are many properties that designers want for a given application. A car designer usually cares more about strength and cost when designing a car whereas an airplane designer cares more about fatigue properties and weight. I digress a little, but it is important to know that "stronger" can mean different things to different engineers.

One of the main controlling factors to whether or not a metal is better at any particular property is its grain structure. When steel, for example, is made, a given recipe for the steel is used. Here in the US we tend to use ASTM standards to determine metal designations (i.e. 316 stainless steel, 6061-T6 Aluminum (see http://www.newton.dep.anl.gov/askasci/mats05/mats05009.htm for another explanation)). The steel is formed by heating it to a given temperature, which causes certain types of grains to form. This is why you might hear the term austenitic or martinsitic steel. These are types of grains that produce certain properties of the steel. Now, the way you cool it will determine what types of grains form as it cools down, once again changing the properties of the steel. By slow cooling the steel (cool in air for instance), certain grains will change from one form to another. Quenching (dropping it in water or oil), can cause the grains to be locked in, or even new types of grains to be formed. So, air cooling one designation of steel may make a certain property better, but air cooling of another type of steel may make that same property worse because you are changing the grain structure. Same goes for water or oil quenching or different metals. Sorry that the answer isn't so straight forward, but that is what makes materials engineering so interesting. Everyday material engineers are working around the world to see what affects certain manufacturing techniques have on certain materials. That is why today we have lighter weight materials that have higher tensile strengths for aircraft, plastics that are strong enough to bounce back after impact, and ceramics that can withstand going from very hot to very cold.

Thanks for using NEWTON.

Christopher Murphy, P.E.
Air Force Research Laboratory

Vince Calder



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