how is metal tempered to a certan streignth?

Question

how hot can metal be befor you can not hurt it by cooling it with water

Answer 1

Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. The nature of the grains (i.e. grain size and composition) determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling rate of diffusion, and the rate of cooling within the microstructure. In carbon and low alloy steels, fast rates of cooling result in a high degree of hardness. In precipitation hardened alloys like 2000 series, 6000 series, and 7000 series aluminum alloys, as well as some superalloys and some stainless steels, fast cooling rates result in a softer metal. At these fast cooling rates, the alloying elements are trapped in solution and require a tempering to precipitate intermetallic particles, there by achieving maximum strength and hardness

Annealing is a technique used to recover cold work and relax stresses within a metal. During annealing, small grains recrystallize to form larger grains, and precipitates dissolve into the matrix. The result is a soft, ducile metal. When an annealed part is allowed to cool in the furnace, it is called a "full anneal" heat treatment. When an annealed part is removed from the furnace and allowed to cool in air, it is called a "normalizing" heat treatment.

To harden by quenching, a metal (usually steel or cast iron) must be heated into the austenitic crystal phase and then quickly cooled in oil, water, or brine. Upon being rapidly cooled, the austentite will transform to martensite, a hard brittle crystalline structure. Often a quenched metal is too hard, and must be tempered (heat treated at a low temperature) to impart some ductility.

If a peripitation hardend alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Tempering a "solutionized" metal will alloy the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will fall out of solution and act as a reinforcing phase, there by increasing the strength of the alloy. This process is called "artificial aging". Conversely, some alloys may be "naturally aged" in which the intermetallic particles form at room temperature. Naturally aging alloys are often stored in a freezer to prevent them from aging until needed.

Complex heat treating schedules are often devised by metallurgists to optimize an alloy's mechanical properties. In the aerospace industry, a superalloy may undergo five or more different heat treating operations to develop the desired properties. This can lead to quality problems depending on the accuracy of the furnace's temperature controls and timer.

Case hardening is a type of quenching heat treatment in which the surface of a part is heated (either with a flame or inductive heating) and then quenched. This imparts a hard surface to a part, without robbing it of ductility. Tools, blades, and bearings are often case hardened

When the term is applied to aluminum alloys, however, its use frequently is restricted to the specific operations employed to increase strength and hardness of the precipitation-hardenable wrought and cast alloys. These usually are referred to as the "heat-treatable" alloys to distinguish them from those alloys in which no significant strengthening can be achieved by heating and cooling. The latter, generally referred to as "non heat-treatable" alloys depend primarily on cold work to increase strength. Heating to decrease strength and increase ductility (annealing) is used with alloys of both types; metallurgical reactions may vary with type of alloy and with degree of softening desired.

One essential attribute of a precipitation-hardening alloy system is a temperature-dependent equilibrium solid solubility characterized by increasing solubility with increasing temperature. The mayor aluminum alloy systems with precipitation hardening include:

Aluminum-copper systems with strengthening from CuAl2
Aluminum-copper-magnesium systems (magnesium intensifies precipitation)
Aluminum-magnesium-silicon systems with strengthening from Mg2Si
Aluminum-zinc-magnesium systems with strengthening from MgZn2
Aluminum-zinc-magnesium-copper systems

Heat treatment to increase strength of aluminum alloys is a three-step process:

Solution heat treatment: dissolution of soluble phases
Quenching: development of supersaturation
Age hardening: precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment).

Answer 2

Look in almost and Materials Science Engineering textbook. There are graphs of temperature, strength, cooling time, etc for different metals. A fast cool at a certain temp can produce something weak whereas a slow cool at the same temp can produce something strong.